1//// 2 vim.syntax: asciidoc 3 4 Copyright (c) 2011 Thomas Graf <tgraf@suug.ch> 5//// 6 7Netlink Library (libnl) 8======================= 9Thomas Graf <tgraf@suug.ch> 103.2, May 9 2011: 11:numbered: 12 13== Introduction 14 15The core library contains the fundamentals required to communicate 16over netlink sockets. It deals with connecting and disconnectng of 17sockets, sending and receiving of data, construction and parsing of 18messages, provides a customizeable receiving state machine, and 19provides a abstract data type framework which eases the implementation 20of object based netlink protocols where objects are added, removed, or 21modified using a netlink based protocol. 22 23.Library Hierarchy 24 25The suite is split into multiple libraries: 26 27image:library_overview.png["Library Hierarchy"] 28 29link:core.html[Netlink Library] (libnl):: 30Socket handling, sending and receiving, message construction and parsing, ... 31 32link:route.html[Routing Family Library] (libnl-route):: 33Adresses, links, neighbours, routing, traffic control, neighbour tables, ... 34 35Netfilter Library (libnl-nf):: 36Connection tracking, logging, queueing 37 38Generic Netlink Library (libnl-genl):: 39Controller API, family and command registration 40 41 42=== How To Read This Documentation 43 44The libraries provide a broad set of APIs of which most applications only 45require a small subset of it. Depending on the type of application, some 46users may only be interested in the low level netlink messaging API while 47others wish to make heavy use of the high level API. 48 49In any case it is recommended to get familiar with the netlink protocol 50first. 51 52- <<core_netlink_fundamentals>> 53 54The low level APIs are described in: 55 56- <<core_sockets>> 57- <<core_send_recv>> 58 59 60=== Linking to this Library 61 62.Checking the presence of the library using autoconf 63 64Projects using autoconf may use +PKG_CHECK_MODULES()+ to check if 65a specific version of libnl is available on the system. The example 66below also shows how to retrieve the +CFLAGS+ and linking dependencies 67required to link against the library. 68 69The following example shows how to check for a specific version of libnl. If 70found, it extends the `CFLAGS` and `LIBS` variable appropriately: 71 72[source] 73---- 74PKG_CHECK_MODULES(LIBNL3, libnl-3.0 >= 3.1, [have_libnl3=yes], [have_libnl3=no]) 75if (test "${have_libnl3}" = "yes"); then 76 CFLAGS+="$LIBNL3_CFLAGS" 77 LIBS+="$LIBNL3_LIBS" 78fi 79---- 80 81NOTE: The pkgconfig file is named +libnl-3.0.pc+ for historic reasons, it also 82 covers library versions >= 3.1. 83 84.Header Files 85 86The main header file is `<netlink/netlink.h>`. Additional headers may need to 87be included in your sources depending on the subsystems and components your 88program makes use of. 89 90[source,c] 91----- 92#include <netlink/netlink.h> 93#include <netlink/cache.h> 94#include <netlink/route/link.h> 95----- 96 97.Version Dependent Code 98 99If your code wishes to be capable to link against multiple versions of libnl 100you may have direct the compiler to only include portions on the code depending 101on the version of libnl that it is compiled against. 102 103[source,c] 104----- 105#include <netlink/version.h> 106 107#if LIBNL_VER_NUM >= LIBNL_VER(3,1) 108 /* include code if compiled with libnl version >= 3.1 */ 109#endif 110----- 111 112.Linking 113----- 114$ gcc myprogram.c -o myprogram $(pkgconfig --cflags --libs libnl-3.0) 115----- 116 117=== Debugging 118 119The library has been compiled with debugging statements enabled it will 120print debug information to +stderr+ if the environment variable +NLDBG+ 121is set to > 0. 122 123----- 124$ NLDBG=2 ./myprogram 125----- 126 127.Debugging Levels 128[options="header", width="80%", cols="1,5", align="center"] 129|=============================================================== 130| Level | Description 131| 0 | Debugging disabled (default) 132| 1 | Warnings, important events and notifications 133| 2 | More or less important debugging messages 134| 3 | Repetitive events causing a flood of debugging messages 135| 4 | Even less important messages 136|=============================================================== 137 138.Debugging the Netlink Protocol 139 140It is often useful to peek into the stream of netlink messages exchanged 141with other sockets. Setting the environment variable +NLCB=debug+ will 142cause the debugging message handlers to be used which in turn print the 143netlink messages exchanged in a human readable format to to +stderr+: 144 145----- 146$ NLCB=debug ./myprogram 147-- Debug: Sent Message: 148-------------------------- BEGIN NETLINK MESSAGE --------------------------- 149 [HEADER] 16 octets 150 .nlmsg_len = 20 151 .nlmsg_type = 18 <route/link::get> 152 .nlmsg_flags = 773 <REQUEST,ACK,ROOT,MATCH> 153 .nlmsg_seq = 1301410712 154 .nlmsg_pid = 20014 155 [PAYLOAD] 16 octets 156 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 157--------------------------- END NETLINK MESSAGE --------------------------- 158-- Debug: Received Message: 159-------------------------- BEGIN NETLINK MESSAGE --------------------------- 160 [HEADER] 16 octets 161 .nlmsg_len = 996 162 .nlmsg_type = 16 <route/link::new> 163 .nlmsg_flags = 2 <MULTI> 164 .nlmsg_seq = 1301410712 165 .nlmsg_pid = 20014 166 [PAYLOAD] 16 octets 167 00 00 04 03 01 00 00 00 49 00 01 00 00 00 00 00 ........I....... 168 [ATTR 03] 3 octets 169 6c 6f 00 lo. 170 [PADDING] 1 octets 171 00 . 172 [ATTR 13] 4 octets 173 00 00 00 00 .... 174 [ATTR 16] 1 octets 175 00 . 176 [PADDING] 3 octets 177 00 00 00 ... 178 [ATTR 17] 1 octets 179 00 . 180 [...] 181--------------------------- END NETLINK MESSAGE --------------------------- 182 183----- 184 185[[core_netlink_fundamentals]] 186== Netlink Protocol Fundamentals 187 188The netlink protocol is a socket based IPC mechanism used for 189communication between userspace processes and the kernel or between 190userspace processes themselves. The netlink protocol is based on BSD 191sockets and uses the +AF_NETLINK+ address family. Every netlink 192protocol uses its own protocol number (e.g. +NETLINK_ROUTE+, 193+NETLINK_NETFILTER+, etc). Its addressing schema is based on a 32 bit 194port number, formerly referred to as PID, which uniquely identifies 195each peer. 196 197[[core_addressing]] 198=== Addressing 199 200The netlink address (port) consists of a 32bit integer. Port 0 (zero) 201is reserved for the kernel and refers to the kernel side socket of each 202netlink protocol family. Other port numbers usually refer to user space 203owned sockets, although this is not enforced. 204 205NOTE: In the beginning, it was common practice to use the process 206 identifier (PID) as the local port number. This became unpractical 207 with the introduction of threaded netlink applications and 208 applications requiring multiple sockets. Therefore libnl generates 209 unique port numbers based on the process identifier and adds an 210 offset to it allowing for multiple sockets to be used. The initial 211 socket will still equal to the process identifier for backwards 212 compatibility reasons. 213 214image:addressing.png["Addressing Example"] 215 216The above figure illustrates three applications and the kernel side 217exposing two kernel side sockets. It shows the common netlink use 218cases: 219 220 * User space to kernel 221 * User space to user space 222 * Listening to kernel multicast notifications 223 224.User Space to Kernel 225 226The most common form of netlink usage is for a user space application 227to send requests to the kernel and process the reply which is either 228an error message or a success notification. 229 230["mscgen"] 231-------- 232msc { 233 App1,App2,Kernel; 234 App1=>Kernel [label="request (src=11, dst=0)"]; 235 App1<=Kernel [label="reply (src=0, dst=11)"]; 236 ...; 237 App2=>Kernel [label="request (src=21, dst=0)"]; 238 App2<=Kernel [label="reply (src=0, dst=21)"]; 239} 240-------- 241 242.User Space to User Space 243 244Netlink may also be used as an IPC mechanism to communicate between user 245space applications directly. Communication is not limited to two peers, 246any number of peers may communicate with each other and multicasting 247capabilities allow to reach multiple peers with a single message. 248 249In order for the sockets to be visible to each other, both sockets must 250be created for the same netlink protocol family. 251 252["mscgen"] 253-------- 254msc { 255 App2,App3; 256 App2=>App3 [label="request (src=22, dst=31)"]; 257 App2<=App3 [label="reply (src=31, dst=22)"]; 258 ...; 259} 260-------- 261 262.User space listening to kernel notifications 263 264This form of netlink communication is typically found in user space 265daemons that need to act on certain kernel events. Such daemons will 266typically maintain a netlink socket subscribed to a multicast group that 267is used by the kernel to notify interested user space parties about 268specific events. 269 270["mscgen"] 271-------- 272msc { 273 Kernel,App3; 274 Kernel=>App3 [label="notification (src=0, group=foo)"]; 275 ...; 276} 277-------- 278 279Use of multicasting is preferred over direct addressing due to the 280flexibility in exchanging the user space component at any time without 281the kernel noticing. 282 283[[core_msg_format]] 284=== Message Format 285 286A netlink protocol is typically based on messages and consists of the 287netlink message header (+struct nlmsghdr+) plus the payload attached 288to it. The payload can consist of arbitrary data but usually contains 289a fixed size protocol specific header followed by a stream of 290attributes. 291 292.Netlink message header (struct nlmsghdr) 293 294image:nlmsghdr.png[align="center", alt="Netlink Message Header"] 295 296Total Length (32bit):: 297Total length of the message in bytes including the netlink message header. 298 299Message Type (16bit):: 300The message type specifies the type of payload the message is carrying. 301Several standard message types are defined by the netlink protocol. 302Additional message types may be defined by each protocol family. See 303<<core_msg_types>> for additional information. 304 305Message Flags (16bit):: 306The message flags may be used to modify the behaviour of a message type. 307See section <<core_msg_flags>> for a list of standard message flags. 308 309Sequence Number (32bit):: 310The sequence number is optional and may be used to allow referring to 311a previous message, e.g. an error message can refer to the original 312request causing the error. 313 314Port Number (32bit):: 315The port number specifies the peer to which the message should be delivered 316to. If not specified, the message will be delivered to the first matching 317kernel side socket of the same protocol family. 318 319[[core_msg_types]] 320=== Message Types 321 322Netlink differs between requests, notifications, and replies. Requests 323are messages which have the +NLM_F_REQUEST+ flag set and are meant to 324request an action from the receiver. A request is typically sent from 325a userspace process to the kernel. While not strictly enforced, requests 326should carry a sequence number incremented for each request sent. 327 328Depending on the nature of the request, the receiver may reply to the 329request with another netlink message. The sequence number of a reply 330must match the sequence number of the request it relates to. 331 332Notifications are of informal nature and no reply is expected, therefore 333the sequence number is typically set to 0. 334 335["mscgen"] 336-------- 337msc { 338 A,B; 339 A=>B [label="GET (seq=1, NLM_F_REQUEST)"]; 340 A<=B [label="PUT (seq=1)"]; 341 ...; 342 A<=B [label="NOTIFY (seq=0)"]; 343} 344-------- 345 346 347The type of message is primarly identified by its 16 bit message type set 348in the message header. The following standard message types are defined: 349 350- +NLMSG_NOOP+ - No operation, message must be discarded 351- +NLMSG_ERROR+ - Error message or ACK, see <<core_errmsg>> 352 respectively <<core_msg_ack>> 353- +NLMSG_DONE+ - End of multipart sequence, see <<core_multipart>> 354- +NLMSG_OVERRUN+ - Overrun notification (Error) 355 356Every netlink protocol is free to define own message types. Note that 357message type values +< NLMSG_MIN_TYPE (0x10)+ are reserved and may 358not be used. 359 360It is common practice to use own message types to implement RPC schemas. 361Suppose the goal of the netlink protocol you are implementing is allow 362configuration of a particular network device, therefore you want to 363provide read/write access to various configuration options. The typical 364"netlink way" of doing this would be to define two message types 365+MSG_SETCFG+, +MSG_GETCFG+: 366 367[source,c] 368-------- 369#define MSG_SETCFG 0x11 370#define MSG_GETCFG 0x12 371-------- 372 373Sending a +MSG_GETCFG+ request message will typically trigger a reply 374with the message type +MSG_SETCFG+ containing the current configuration. 375In object oriented terms one would describe this as "the kernel sets 376the local copy of the configuration in userspace". 377 378["mscgen"] 379-------- 380msc { 381 A,B; 382 A=>B [label="MSG_GETCFG (seq=1, NLM_F_REQUEST)"]; 383 A<=B [label="MSG_SETCFG (seq=1)"]; 384} 385-------- 386 387The configuration may be changed by sending a +MSG_SETCFG+ which will 388be responded to with either a ACK (see <<core_msg_ack>>) 389or a error message (see <<core_errmsg>>). 390 391["mscgen"] 392-------- 393msc { 394 A,B; 395 A=>B [label="MSG_SETCFG (seq=1, NLM_F_REQUEST, NLM_F_ACK)"]; 396 A<=B [label="ACK (seq=1)"]; 397} 398-------- 399 400Optionally, the kernel may send out notifications for configuration 401changes allowing userspace to listen for changes instead of polling 402frequently. Notifications typically reuse an existing message type 403and rely on the application using a separate socket to differ between 404requests and notifications but you may also specify a separate message 405type. 406 407["mscgen"] 408-------- 409msc { 410 A,B; 411 A<=B [label="MSG_SETCFG (seq=0)"]; 412} 413-------- 414 415[[core_multipart]] 416==== Multipart Messages 417 418Although in theory a netlink message can be up to 4GiB in size. The socket 419buffers are very likely not large enough to hold message of such sizes. 420Therefore it is common to limit messages to one page size (PAGE_SIZE) and 421use the multipart mechanism to split large pieces of data into several 422messages. A multipart message has the flag +NLM_F_MULTI+ set and the 423receiver is expected to continue receiving and parsing until the special 424message type +NLMSG_DONE+ is received. 425 426Multipart messages unlike fragmented ip packets must not be reassmbled 427even though it is perfectly legal to do so if the protocols wishes to 428work this way. Often multipart message are used to send lists or trees 429of objects were each multipart message simply carries multiple objects 430allow for each message to be parsed independently. 431 432["mscgen"] 433-------- 434msc { 435 A,B; 436 A=>B [label="GET (seq=1, NLM_F_REQUEST)"]; 437 A<=B [label="PUT (seq=1, NLM_F_MULTI)"]; 438 ...; 439 A<=B [label="PUT (seq=1, NLM_F_MULTI)"]; 440 A<=B [label="NLMSG_DONE (seq=1)"]; 441} 442-------- 443 444[[core_errmsg]] 445==== Error Message 446 447Error messages can be sent in response to a request. Error messages must 448use the standard message type +NLMSG_ERROR+. The payload consists of a 449error code and the original netlink mesage header of the request. 450 451image:nlmsgerr.png["Netlink Errror Message header"] 452 453Error messages should set the sequence number to the sequence number 454of the request which caused the error. 455 456["mscgen"] 457-------- 458msc { 459 A,B; 460 A=>B [label="GET (seq=1, NLM_F_REQUEST)"]; 461 A<=B [label="NLMSG_ERROR code=EINVAL (seq=1)"]; 462} 463-------- 464 465[[core_msg_ack]] 466==== ACKs 467 468A sender can request an ACK message to be sent back for each request 469processed by setting the +NLM_F_ACK+ flag in the request. This is typically 470used to allow the sender to synchronize further processing until the 471request has been processed by the receiver. 472 473["mscgen"] 474-------- 475msc { 476 A,B; 477 A=>B [label="GET (seq=1, NLM_F_REQUEST | NLM_F_ACK)"]; 478 A<=B [label="ACK (seq=1)"]; 479} 480-------- 481 482ACK messages also use the message type +NLMSG_ERROR+ and payload 483format but the error code is set to 0. 484 485[[core_msg_flags]] 486==== Message Flags 487 488The following standard flags are defined 489 490[source,c] 491-------- 492#define NLM_F_REQUEST 1 493#define NLM_F_MULTI 2 494#define NLM_F_ACK 4 495#define NLM_F_ECHO 8 496-------- 497 498- `NLM_F_REQUEST` - Message is a request, see <<core_msg_types>>. 499- `NLM_F_MULTI` - Multipart message, see <<core_multipart>> 500- `NLM_F_ACK` - ACK message requested, see <<core_msg_ack>>. 501- `NLM_F_ECHO` - Request to echo the request. 502 503The flag +NLM_F_ECHO+ is similar to the `NLM_F_ACK` flag. It can be 504used in combination with `NLM_F_REQUEST` and causes a notification 505which is sent as a result of a request to also be sent to the sender 506regardless of whether the sender has subscribed to the corresponding 507multicast group or not. See <<core_multicast>> 508 509Additional universal message flags are defined which only apply for 510+GET+ requests: 511 512[source,c] 513-------- 514#define NLM_F_ROOT 0x100 515#define NLM_F_MATCH 0x200 516#define NLM_F_ATOMIC 0x400 517#define NLM_F_DUMP (NLM_F_ROOT|NLM_F_MATCH) 518-------- 519 520- `NLM_F_ROOT` - Return based on root of tree. 521- `NLM_F_MATCH` - Return all matching entries. 522- `NLM_F_ATOMIC` - Obsoleted, once used to request an atomic operation. 523- `NLM_F_DUMP` - Return a list of all objects 524 (`NLM_F_ROOT`|`NLM_F_MATCH`). 525 526Use of these flags is completely optional and many netlink protocols only 527make use of the `NLM_F_DUMP` flag which typically requests the receiver 528to send a list of all objects in the context of the message type as a 529sequence of multipart messages (see <<core_multipart>>). 530 531Another set of flags exist related to `NEW` or `SET` requests. These 532flags are mutually exclusive to the `GET` flags: 533 534[source,c] 535-------- 536#define NLM_F_REPLACE 0x100 537#define NLM_F_EXCL 0x200 538#define NLM_F_CREATE 0x400 539#define NLM_F_APPEND 0x800 540-------- 541 542- `NLM_F_REPLACE` - Replace an existing object if it exists. 543- `NLM_F_EXCL` - Do not update object if it exists already. 544- `NLM_F_CREATE` - Create object if it does not exist yet. 545- `NLM_F_APPEND` - Add object at end of list. 546 547Behaviour of these flags may differ slightly between different netlink 548protocols. 549 550[[core_seq_num]] 551=== Sequence Numbers 552 553Netlink allows the use of sequence numbers to help relate replies to 554requests. It should be noted that unlike in protocols such as TCP 555there is no strict enforcment of the sequence number. The sole purpose 556of sequence numbers is to assist a sender in relating replies to the 557corresponding requests. See <<core_msg_types>> for more information. 558 559Sequence numbers are managed on a per socket basis, see 560<<core_sk_seq_num>> for more information on how to use sequence numbers. 561 562[[core_multicast]] 563=== Multicast Groups 564 565TODO 566 567See <<core_sk_multicast>> 568 569[[core_sockets]] 570== Netlink Sockets 571 572In order to use the netlink protocol, a netlink socket is required. 573Each socket defines an independent context for sending and receiving of 574messages. An application may make use multiple sockets, e.g. a socket to 575send requests and receive the replies and another socket subscribed to a 576multicast group to receive notifications. 577 578=== Socket structure (struct nl_sock) 579 580The netlink socket and all related attributes including the actual file 581descriptor are represented by +struct nl_sock+. 582 583[source,c] 584-------- 585#include <netlink/socket.h> 586 587struct nl_sock *nl_socket_alloc(void) 588void nl_socket_free(struct nl_sock *sk) 589-------- 590 591The application must allocate an instance of +struct nl_sock+ for each 592netlink socket it wishes to use. 593 594[[core_sk_seq_num]] 595=== Sequence Numbers 596 597The library will automatically take care of sequence number handling 598for the application. A sequence number counter is stored in the 599socket structure which is used and incremented automatically when a 600message needs to be sent which is expected to generate a reply such as 601an error or any other message type that needs to be related to the 602original message. 603 604Alternatively, the counter can be used directly via the function 605nl_socket_use_seq(). It will return the current value of the counter 606and increment it by one afterwards. 607 608[source,c] 609-------- 610#include <netlink/socket.h> 611 612unsigned int nl_socket_use_seq(struct nl_sock *sk); 613-------- 614 615Most applications will not want to deal with sequence number handling 616themselves though. When using nl_send_auto() the sequence number is 617filled in automatically and matched again when a reply is received. See 618section <<core_send_recv>> for more information. 619 620This behaviour can and must be disabled if the netlink protocol 621implemented does not use a request/reply model, e.g. when a socket is 622used to receive notification messages. 623 624[source,c] 625-------- 626#include <netlink/socket.h> 627 628void nl_socket_disable_seq_check(struct nl_sock *sk); 629-------- 630 631For more information on the theory behind netlink sequence numbers, 632see section <<core_seq_num>>. 633 634[[core_sk_multicast]] 635=== Multicast Group Subscriptions 636 637Each socket can subscribe to any number of multicast groups of the 638netlink protocol it is connected to. The socket will then receive a 639copy of each message sent to any of the groups. Multicast groups are 640commonly used to implement event notifications. 641 642Prior to kernel 2.6.14 the group subscription was performed using a 643bitmask which limited the number of groups per protocol family to 32. 644This outdated interface can still be accessed via the function 645nl_join_groups() even though it is not recommended for new code. 646 647[source,c] 648-------- 649#include <netlink/socket.h> 650 651void nl_join_groups(struct nl_sock *sk, int bitmask); 652-------- 653 654Starting with 2.6.14 a new method was introduced which supports subscribing 655to an almost infinite number of multicast groups. 656 657[source,c] 658-------- 659#include <netlink/socket.h> 660 661int nl_socket_add_memberships(struct nl_sock *sk, int group, ...); 662int nl_socket_drop_memberships(struct nl_sock *sk, int group, ...); 663-------- 664 665==== Multicast Example 666 667[source,c] 668-------- 669#include <netlink/netlink.h> 670#include <netlink/socket.h> 671#include <netlink/msg.h> 672 673/* 674 * This function will be called for each valid netlink message received 675 * in nl_recvmsgs_default() 676 */ 677static int my_func(struct nl_msg *msg, void *arg) 678{ 679 return 0; 680} 681 682struct nl_sock *sk; 683 684/* Allocate a new socket */ 685sk = nl_socket_alloc(); 686 687/* 688 * Notifications do not use sequence numbers, disable sequence number 689 * checking. 690 */ 691nl_socket_disable_seq_check(sk); 692 693/* 694 * Define a callback function, which will be called for each notification 695 * received 696 */ 697nl_socket_modify_cb(sk, NL_CB_VALID, NL_CB_CUSTOM, my_func, NULL); 698 699/* Connect to routing netlink protocol */ 700nl_connect(sk, NETLINK_ROUTE); 701 702/* Subscribe to link notifications group */ 703nl_socket_add_memberships(sk, RTNLGRP_LINK, 0); 704 705/* 706 * Start receiving messages. The function nl_recvmsgs_default() will block 707 * until one or more netlink messages (notification) are received which 708 * will be passed on to my_func(). 709 */ 710while (1) 711 nl_recvmsgs_default(sock); 712-------- 713 714[[core_sk_cb]] 715=== Modifiying Socket Callback Configuration 716 717See <<core_cb>> for more information on 718callback hooks and overwriting capabilities. 719 720Each socket is assigned a callback configuration which controls the 721behaviour of the socket. This is f.e. required to have a separate 722message receive function per socket. It is perfectly legal to share 723callback configurations between sockets though. 724 725The following functions can be used to access and set the callback 726configuration of a socket: 727 728[source,c] 729-------- 730#include <netlink/socket.h> 731 732struct nl_cb *nl_socket_get_cb(const struct nl_sock *sk); 733void nl_socket_set_cb(struct nl_sock *sk, struct nl_cb *cb); 734-------- 735 736Additionaly a shortcut exists to modify the callback configuration 737assigned to a socket directly: 738 739[source,c] 740-------- 741#include <netlink/socket.h> 742 743int nl_socket_modify_cb(struct nl_sock *sk, enum nl_cb_type type, enum nl_cb_kind kind, 744 nl_recvmsg_msg_cb_t func, void *arg); 745-------- 746 747.Example: 748[source,c] 749-------- 750#include <netlink/socket.h> 751 752// Call my_input() for all valid messages received in socket sk 753nl_socket_modify_cb(sk, NL_CB_VALID, NL_CB_CUSTOM, my_input, NULL); 754-------- 755 756=== Socket Attributes 757 758.Local Port 759 760The local port number uniquely identifies the socket and is used to 761address it. A unique local port is generated automatically when the 762socket is allocated. It will consist of the Process ID (22 bits) and a 763random number (10 bits) thus allowing up to 1024 sockets per process. 764 765[source,c] 766-------- 767#include <netlink/socket.h> 768 769uint32_t nl_socket_get_local_port(const struct nl_sock *sk); 770void nl_socket_set_local_port(struct nl_sock *sk, uint32_t port); 771-------- 772 773See section <<core_addressing>> for more information on port numbers. 774 775CAUTION: Overwriting the local port is possible but you have to ensure 776that the provided value is unique and no other socket in any other 777application is using the same value. 778 779.Peer Port 780 781A peer port can be assigned to the socket which will result in all 782unicast messages sent over the socket to be addresses to the peer. If 783no peer is specified, the message is sent to the kernel which will try 784to automatically bind the socket to a kernel side socket of the same 785netlink protocol family. It is common practice not to bind the socket 786to a peer port as typically only one kernel side socket exists per 787netlink protocol family. 788 789[source,c] 790-------- 791#include <netlink/socket.h> 792 793uint32_t nl_socket_get_peer_port(const struct nl_sock *sk); 794void nl_socket_set_peer_port(struct nl_sock *sk, uint32_t port); 795-------- 796 797See section <<core_addressing>> for more information on port numbers. 798 799.File Descriptor 800 801Netlink uses the BSD socket interface, therefore a file descriptor is 802behind each socket and you may use it directly. 803 804[source,c] 805-------- 806#include <netlink/socket.h> 807 808int nl_socket_get_fd(const struct nl_sock *sk); 809-------- 810 811If a socket is used to only receive notifications it usually is best 812to put the socket in non-blocking mode and periodically poll for new 813notifications. 814 815[source,c] 816-------- 817#include <netlink/socket.h> 818 819int nl_socket_set_nonblocking(const struct nl_sock *sk); 820-------- 821 822.Send/Receive Buffer Size 823 824The socket buffer is used to queue netlink messages between sender and 825receiver. The size of these buffers specifies the maximum size you 826will be able to write() to a netlink socket, i.e. it will indirectly 827define the maximum message size. The default is 32KiB. 828 829[source,c] 830-------- 831#include <netlink/socket.h> 832 833int nl_socket_set_buffer_size(struct nl_sock *sk, int rx, int tx); 834-------- 835 836[[core_sk_cred]] 837.Enable/Disable Credentials 838 839TODO 840 841[source,c] 842-------- 843#include <netlink/socket.h> 844 845int nl_socket_set_passcred(struct nl_sock *sk, int state); 846-------- 847 848.Enable/Disable Auto-ACK Mode 849 850The following functions allow to enable/disable Auto-ACK mode on a socket. 851See <<core_auto_ack>> for more information on what implications that has. 852Auto-ACK mode is enabled by default. 853 854[source,c] 855-------- 856#include <netlink/socket.h> 857 858void nl_socket_enable_auto_ack(struct nl_sock *sk); 859void nl_socket_disable_auto_ack(struct nl_sock *sk); 860-------- 861 862.Enable/Disable Message Peeking 863 864If enabled, message peeking causes nl_recv() to try and use MSG_PEEK 865to retrieve the size of the next message received and allocate a 866buffer of that size. Message peeking is enabled by default but can be 867disabled using the following function: 868 869[source,c] 870-------- 871#include <netlink/socket.h> 872 873void nl_socket_enable_msg_peek(struct nl_sock *sk); 874void nl_socket_disable_msg_peek(struct nl_sock *sk); 875-------- 876 877.Enable/Disable Receival of Packet Information 878 879If enabled, each received netlink message from the kernel will include 880an additional struct nl_pktinfo in the control message. The following 881function can be used to enable/disable receival of packet information. 882 883[source,c] 884-------- 885#include <netlink/socket.h> 886 887int nl_socket_recv_pktinfo(struct nl_sock *sk, int state); 888-------- 889 890CAUTION: Processing of NETLINK_PKTINFO has not been implemented yet. 891 892[[core_send_recv]] 893== Sending and Receiving of Messages / Data 894 895[[core_send]] 896=== Sending Messages 897 898The standard method of sending a netlink message over a netlink socket 899is to use the function nl_send_auto(). It will automatically complete 900the netlink message by filling the missing bits and pieces in the 901netlink message header and will deal with addressing based on the 902options and address set in the netlink socket. The message is then 903passed on to nl_send(). 904 905If the default sending semantics implemented by nl_send() do not suit 906the application, it may overwrite the sending function nl_send() by 907specifying an own implementation using the function 908nl_cb_overwrite_send(). 909 910[source,c] 911-------- 912 nl_send_auto(sk, msg) 913 | 914 |-----> nl_complete_msg(sk, msg) 915 | 916 | 917 | Own send function specified via nl_cb_overwrite_send() 918 |- - - - - - - - - - - - - - - - - - - - 919 v v 920 nl_send(sk, msg) send_func() 921-------- 922 923.Using nl_send() 924 925If you do not require any of the automatic message completion 926functionality you may use nl_send() directly but beware that any 927internal calls to nl_send_auto() by the library to send netlink 928messages will still use nl_send(). Therefore if you wish to use any 929higher level interfaces and the behaviour of nl_send() is to your 930dislike then you must overwrite the nl_send() function via 931nl_cb_overwrite_send() 932 933The purpose of nl_send() is to embed the netlink message into a iovec 934structure and pass it on to nl_send_iovec(). 935 936[source,c] 937-------- 938 nl_send(sk, msg) 939 | 940 v 941 nl_send_iovec(sk, msg, iov, iovlen) 942-------- 943 944.Using nl_send_iovec() 945 946nl_send_iovec() expects a finalized netlink message and fills out the 947struct msghdr used for addressing. It will first check if the struct 948nl_msg is addressed to a specific peer (see nlmsg_set_dst()). If not, 949it will try to fall back to the peer address specified in the socket 950(see nl_socket_set_peer_port(). Otherwise the message will be sent 951unaddressed and it is left to the kernel to find the correct peer. 952 953nl_send_iovec() also adds credentials if present and enabled 954(see <<core_sk_cred>>). 955 956The message is then passed on to nl_sendmsg(). 957 958[source,c] 959-------- 960 nl_send_iovec(sk, msg, iov, iovlen) 961 | 962 v 963 nl_sendmsg(sk, msg, msghdr) 964-------- 965 966.Using nl_sendmsg() 967 968nl_sendmsg() expects a finalized netlink message and an optional 969struct msghdr containing the peer address. It will copy the local 970address as defined in the socket (see nl_socket_set_local_port()) into 971the netlink message header. 972 973At this point, construction of the message finished and it is ready to 974be sent. 975 976[source,c] 977-------- 978 nl_sendmsg(sk, msg, msghdr) 979 |- - - - - - - - - - - - - - - - - - - - v 980 | NL_CB_MSG_OUT() 981 |<- - - - - - - - - - - - - - - - - - - -+ 982 v 983 sendmsg() 984-------- 985 986Before sending the application has one last chance to modify the 987message. It is passed to the NL_CB_MSG_OUT callback function which 988may inspect or modify the message and return an error code. If this 989error code is NL_OK the message is sent using sendmsg() resulting in 990the number of bytes written being returned. Otherwise the message 991sending process is aborted and the error code specified by the 992callback function is returned. See <<core_sk_cb>> for more information 993on how to set callbacks. 994 995.Sending Raw Data with nl_sendto() 996 997If you wish to send raw data over a netlink socket, the following 998function will pass on any buffer provided to it directly to sendto(): 999 1000[source,c] 1001-------- 1002#include <netlink/netlink.h> 1003 1004int nl_sendto(struct nl_sock *sk, void *buf, size_t size); 1005-------- 1006 1007.Sending of Simple Messages 1008 1009A special interface exists for sending of trivial messages. The function 1010expects the netlink message type, optional netlink message flags, and an 1011optional data buffer and data length. 1012[source,c] 1013-------- 1014#include <netlink/netlink.h> 1015 1016int nl_send_simple(struct nl_sock *sk, int type, int flags, 1017 void *buf, size_t size); 1018-------- 1019 1020The function will construct a netlink message header based on the message 1021type and flags provided and append the data buffer as message payload. The 1022newly constructed message is sent with nl_send_auto(). 1023 1024The following example will send a netlink request message causing the 1025kernel to dump a list of all network links to userspace: 1026 1027[source,c] 1028-------- 1029#include <netlink/netlink.h> 1030 1031struct nl_sock *sk; 1032struct rtgenmsg rt_hdr = { 1033 .rtgen_family = AF_UNSPEC, 1034}; 1035 1036sk = nl_socket_alloc(); 1037nl_connect(sk, NETLINK_ROUTE); 1038 1039nl_send_simple(sock, RTM_GETLINK, NLM_F_DUMP, &rt_hdr, sizeof(rt_hdr)); 1040-------- 1041 1042[[core_recv]] 1043=== Receiving Messages 1044 1045The easiest method to receive netlink messages is to call nl_recvmsgs_default(). 1046It will receive messages based on the semantics defined in the socket. The 1047application may customize these in detail although the default behaviour will 1048probably suit most applications. 1049 1050nl_recvmsgs_default() will also be called internally by the library whenever 1051it needs to receive and parse a netlink message. 1052 1053The function will fetch the callback configuration stored in the socket and 1054call nl_recvmsgs(): 1055 1056[source,c] 1057-------- 1058 nl_recvmsgs_default(sk) 1059 | 1060 | cb = nl_socket_get_cb(sk) 1061 v 1062 nl_recvmsgs(sk, cb) 1063-------- 1064 1065.Using nl_recvmsgs() 1066 1067nl_recvmsgs() implements the actual receiving loop, it blocks until a 1068netlink message has been received unless the socket has been put into 1069non-blocking mode. 1070 1071For the unlikely scenario that certain required receive characteristics 1072can not be achieved by fine tuning the internal recvmsgs function using 1073the callback configuration (see <<core_sk_cb>>) the application may provide 1074a complete own implementation of it and overwrite all calls to nl_recvmsgs() 1075with the function nl_cb_overwrite_recvmsgs(). 1076 1077[source,c] 1078-------- 1079 nl_recvmsgs(sk, cb) 1080 | 1081 | Own recvmsgs function specified via nl_cb_overwrite_recvmsgs() 1082 |- - - - - - - - - - - - - - - - - - - - 1083 v v 1084 internal_recvmsgs() my_recvmsgs() 1085-------- 1086 1087[[core_recv_character]] 1088.Receive Characteristics 1089 1090If the application does not provide its own recvmsgs() implementation 1091with the function nl_cb_overwrite_recvmsgs() the following characteristics 1092apply while receiving data from a netlink socket: 1093 1094[source,c] 1095-------- 1096 internal_recvmsgs() 1097 | 1098+-------------->| Own recv function specified with nl_cb_overwrite_recv() 1099| |- - - - - - - - - - - - - - - - 1100| v v 1101| nl_recv() my_recv() 1102| |<- - - - - - - - - - - - - - -+ 1103| |<-------------+ 1104| v | More data to parse? (nlmsg_next()) 1105| Parse Message | 1106| |--------------+ 1107| v 1108+------- NLM_F_MULTI set? 1109 | 1110 v 1111 (SUCCESS) 1112-------- 1113 1114The function nl_recv() is invoked first to receive data from the 1115netlink socket. This function may be overwritten by the application 1116by an own implementation using the function nl_cb_overwrite_recv(). 1117This may be useful if the netlink byte stream is in fact not received 1118from a socket directly but is read from a file or another source. 1119 1120If data has been read, it will be attemped to parse the data. This 1121will be done repeately until the parser returns NL_STOP, an error was 1122returned or all data has been parsed. 1123 1124In case the last message parsed successfully was a multipart message 1125(see <<core_multipart>>) and the parser did not 1126quit due to either an error or NL_STOP nl_recv() respectively the 1127applications own implementation will be called again and the parser 1128starts all over. 1129 1130See <<core_parse_character>> for information on how to extract valid 1131netlink messages from the parser and on how to control the behaviour 1132of it. 1133 1134[[core_parse_character]] 1135.Parsing Characteristics 1136 1137The internal parser is invoked for each netlink message received from 1138a netlink socket. It is typically fed by nl_recv() (see 1139<<core_recv_character>>). 1140 1141The parser will first ensure that the length of the data stream 1142provided is sufficient to contain a netlink message header and that 1143the message length as specified in the message header does not exceed 1144it. 1145 1146If this criteria is met, a new struct nl_msg is allocated and the 1147message is passed on to the the callback function NL_CB_MSG_IN if one 1148is set. Like any other callback function, it may return NL_SKIP to 1149skip the current message but continue parsing the next message or 1150NL_STOP to stop parsing completely. 1151 1152The next step is to check the sequence number of the message against 1153the currently expected sequence number. The application may provide 1154its own sequence number checking algorithm by setting the callback 1155function NL_CB_SEQ_CHECK to its own implementation. In fact, calling 1156nl_socket_disable_seq_check() to disable sequence number checking will 1157do nothing more than set the NL_CB_SEQ_CHECK hook to a function which 1158always returns NL_OK. 1159 1160Another callback hook NL_CB_SEND_ACK exists which is called if the 1161message has the NLM_F_ACK flag set. Although I am not aware of any 1162userspace netlink socket doing this, the application may want to send 1163an ACK message back to the sender (see <<core_msg_ack>>). 1164 1165[source,c] 1166-------- 1167 parse() 1168 | 1169 v 1170 nlmsg_ok() --> Ignore 1171 | 1172 |- - - - - - - - - - - - - - - v 1173 | NL_CB_MSG_IN() 1174 |<- - - - - - - - - - - - - - -+ 1175 | 1176 |- - - - - - - - - - - - - - - v 1177 Sequence Check NL_CB_SEQ_CHECK() 1178 |<- - - - - - - - - - - - - - -+ 1179 | 1180 | Message has NLM_F_ACK set 1181 |- - - - - - - - - - - - - - - v 1182 | NL_CB_SEND_ACK() 1183 |<- - - - - - - - - - - - - - -+ 1184 | 1185 Handle Message Type 1186-------- 1187 1188[[core_auto_ack]] 1189=== Auto-ACK Mode 1190 1191TODO 1192 1193== Message Parsing & Construction 1194 1195=== Message Format 1196 1197See <<core_netlink_fundamentals>> for an introduction to the netlink 1198protocol and its message format. 1199 1200.Alignment 1201 1202Most netlink protocols enforce a strict alignment policy for all 1203boundries. The alignment value is defined by NLMSG_ALIGNTO and is 1204fixed to 4 bytes. Therefore all netlink message headers, begin of 1205payload sections, protocol specific headers, and attribute sections 1206must start at an offset which is a multiple of NLMSG_ALIGNTO. 1207 1208[source,c] 1209-------- 1210#include <netlink/msg.h> 1211 1212int nlmsg_size(int payloadlen); 1213int nlmsg_total_size(int payloadlen); 1214-------- 1215 1216The library provides a set of function to handle alignment 1217requirements automatically. The function nlmsg_total_size() returns 1218the total size of a netlink message including the padding to ensure 1219the next message header is aligned correctly. 1220 1221[source,c] 1222-------- 1223 <----------- nlmsg_total_size(len) ------------> 1224 <----------- nlmsg_size(len) ------------> 1225 +-------------------+- - -+- - - - - - - - +- - -+-------------------+- - - 1226 | struct nlmsghdr | Pad | Payload | Pad | struct nlsmghdr | 1227 +-------------------+- - -+- - - - - - - - +- - -+-------------------+- - - 1228 <---- NLMSG_HDRLEN -----> <- NLMSG_ALIGN(len) -> <---- NLMSG_HDRLEN --- 1229-------- 1230 1231If you need to know if padding needs to be added at the end of a 1232message, nlmsg_padlen() returns the number of padding bytes that need 1233to be added for a specific payload length. 1234 1235[source,c] 1236-------- 1237#include <netlink/msg.h> 1238int nlmsg_padlen(int payloadlen); 1239-------- 1240 1241=== Parsing a Message 1242 1243The library offers two different methods of parsing netlink messages. 1244It offers a low level interface for applications which want to do all 1245the parsing manually. This method is described below. Alternatively 1246the library also offers an interface to implement a parser as part of 1247a cache operations set which is especially useful when your protocol 1248deals with objects of any sort such as network links, routes, etc. 1249This high level interface is described in <<core_cache>>. 1250 1251.Splitting a byte stream into separate messages 1252 1253What you receive from a netlink socket is typically a stream of 1254messages. You will be given a buffer and its length, the buffer may 1255contain any number of netlink messages. 1256 1257The first message header starts at the beginning of message stream. 1258Any subsequent message headers are access by calling nlmsg_next() on 1259the previous header. 1260 1261[source,c] 1262-------- 1263#include <netlink/msg.h> 1264 1265struct nlmsghdr *nlmsg_next(struct nlmsghdr *hdr, int *remaining); 1266-------- 1267 1268The function nlmsg_next() will automatically substract the size of the 1269previous message from the remaining number of bytes. 1270 1271Please note, there is no indication in the previous message whether 1272another message follows or not. You must assume that more messages 1273follow until all bytes of the message stream have been processed. 1274 1275To simplify this, the function nlmsg_ok() exists which returns true if 1276another message fits into the remaining number of bytes in the message 1277stream. nlmsg_valid_hdr() is similar, it checks whether a specific 1278netlink message contains at least a minimum of payload. 1279 1280[source,c] 1281-------- 1282#include <netlink/msg.h> 1283 1284int nlmsg_valid_hdr(const struct nlmsghdr *hdr, int payloadlen); 1285int nlmsg_ok(const struct nlmsghdr *hdr, int remaining); 1286-------- 1287 1288A typical use of these functions looks like this: 1289 1290[source,c] 1291-------- 1292#include <netlink/msg.h> 1293 1294void my_parse(void *stream, int length) 1295{ 1296 struct nlmsghdr *hdr = stream; 1297 1298 while (nlmsg_ok(hdr, length)) { 1299 // Parse message here 1300 hdr = nlmsg_next(hdr, &length); 1301 } 1302} 1303-------- 1304 1305CAUTION: nlmsg_ok() only returns true if the *complete* message including 1306 the message payload fits into the remaining buffer length. It will 1307 return false if only a part of it fits. 1308 1309The above can also be written using the iterator nlmsg_for_each(): 1310 1311[source,c] 1312-------- 1313#include <netlink/msg.h> 1314 1315struct nlmsghdr *hdr; 1316 1317nlmsg_for_each(hdr, stream, length) { 1318 /* do something with message */ 1319} 1320-------- 1321 1322.Message Payload 1323 1324The message payload is appended to the message header and is guranteed 1325to start at a multiple of +NLMSG_ALIGNTO+. Padding at the end of the 1326message header is added if necessary to ensure this. The function 1327nlmsg_data() will calculate the necessary offset based on the message 1328and returns a pointer to the start of the message payload. 1329 1330[source,c] 1331-------- 1332#include <netlink/msg.h> 1333 1334void *nlmsg_data(const struct nlmsghdr *nlh); 1335void *nlmsg_tail(const struct nlmsghdr *nlh); 1336int nlmsg_datalen(const struct nlmsghdr *nlh); 1337-------- 1338 1339The length of the message payload is returned by nlmsg_datalen(). 1340 1341[source,c] 1342-------- 1343 <--- nlmsg_datalen(nlh) ---> 1344 +-------------------+- - -+----------------------------+- - -+ 1345 | struct nlmsghdr | Pad | Payload | Pad | 1346 +-------------------+- - -+----------------------------+- - -+ 1347nlmsg_data(nlh) ---------------^ ^ 1348nlmsg_tail(nlh) --------------------------------------------------^ 1349-------- 1350 1351The payload may consist of arbitary data but may have strict alignment 1352and formatting rules depening on the actual netlink protocol. 1353 1354[[core_msg_attr]] 1355.Message Attributes 1356 1357Most netlink protocols use netlink attributes. It not only makes the 1358protocol self documenting but also gives flexibility in expanding the 1359protocol at a later point. New attributes can be added at any time and 1360older attributes can be obsoleted by newer ones without breaking 1361binary compatibility of the protocol. 1362 1363[source,c] 1364-------- 1365 <---------------------- payload -------------------------> 1366 <----- hdrlen ----> <- nlmsg_attrlen(nlh, hdrlen) -> 1367 +-------------------+- - -+----- ------------+- - -+--------------------------------+- - -+ 1368 | struct nlmsghdr | Pad | Protocol Header | Pad | Attributes | Pad | 1369 +-------------------+- - -+-------------------+- - -+--------------------------------+- - -+ 1370nlmsg_attrdata(nlh, hdrlen) -----------------------------^ 1371-------- 1372 1373The function nlmsg_attrdata() returns a pointer to the begin of the 1374attributes section. The length of the attributes section is returned 1375by the function nlmsg_attrlen(). 1376 1377[source,c] 1378-------- 1379#include <netlink/msg.h> 1380 1381struct nlattr *nlmsg_attrdata(const struct nlmsghdr *hdr, int hdrlen); 1382int nlmsg_attrlen(const struct nlmsghdr *hdr, int hdrlen); 1383-------- 1384 1385See <<core_attr>> for more information on how to use netlink attributes. 1386 1387.Parsing a Message the Easy Way 1388 1389The function nlmsg_parse() validate a complete netlink message in one 1390step. If +hdrlen > 0+ it will first call nlmsg_valid_hdr() to check 1391if the protocol header fits into the message. If there is more payload 1392to parse, it will assume it to be attributes and parse the payload 1393accordingly. The function behaves exactly like nla_parse() when 1394parsing attributes, see <<core_attr_parse_easy>>. 1395 1396[source,c] 1397-------- 1398int nlmsg_parse(struct nlmsghdr *hdr, int hdrlen, struct nlattr **attrs, 1399 int maxtype, struct nla_policy *policy); 1400-------- 1401 1402The function nlmsg_validate() is based on nla_validate() and behaves 1403exactly the same as nlmsg_parse() except that it only validates and 1404will not fill a array with pointers to each attribute. 1405 1406[source,c] 1407-------- 1408int nlmsg_validate(struct nlmsghdr *hdr, int hdrlen, intmaxtype, 1409 struct nla_policy *policy); 1410-------- 1411 1412See <<core_attr_parse_easy>> for an example and more information on 1413attribute parsing. 1414 1415=== Construction of a Message 1416 1417See <<core_msg_format>> for information on the netlink message format 1418and alignment requirements. 1419 1420Message construction is based on struct nl_msg which uses an internal 1421buffer to store the actual netlink message. struct nl_msg +does not+ 1422point to the netlink message header. Use nlmsg_hdr() to retrieve a 1423pointer to the netlink message header. 1424 1425At allocation time, a maximum message size is specified. It defaults 1426to a page (PAGE_SIZE). The application constructing the message will 1427reserve space out of this maximum message size repeatedly for each 1428header or attribute added. This allows construction of messages across 1429various layers of code where lower layers do not need to know about 1430the space requirements of upper layers. 1431 1432+Why is setting the maximum message size necessary?+ This 1433question is often raised in combination with the proposed solution of 1434reallocating the message payload buffer on the fly using realloc(). 1435While it is possible to reallocate the buffer during construction 1436using nlmsg_expand() it will make all pointers into the message buffer 1437become stale. This breaks usage of nlmsg_hdr(), nla_nest_start(), and 1438nla_nest_end() and is therefore not acceptable as default behaviour. 1439 1440.Allocating struct nl_msg 1441 1442The first step in constructing a new netlink message it to allocate a 1443`struct nl_msg` to hold the message header and payload. Several 1444functions exist to simplify various tasks. 1445 1446[source,c] 1447-------- 1448#include <netlink/msg.h> 1449 1450struct nl_msg *nlmsg_alloc(void); 1451void nlmsg_free(struct nl_msg *msg); 1452-------- 1453 1454The function nlmsg_alloc() is the default message allocation function. 1455It allocates a new message using the default maximum message size which 1456equals to one page (PAGE_SIZE). The application can change the default 1457size for messages by calling nlmsg_set_default_size(): 1458 1459[source,c] 1460-------- 1461void nlmsg_set_default_size(size_t); 1462-------- 1463 1464CAUTION: Calling nlmsg_set_default_size() does not change the maximum 1465 message size of already allocated messages. 1466 1467[source,c] 1468-------- 1469struct nl_msg *nlmsg_alloc_size(size_t max); 1470-------- 1471 1472Instead of changing the default message size, the function 1473nlmsg_alloc_size() can be used to allocate a message with a individual 1474maximum message size. 1475 1476 1477If the netlink message header is already known at allocation time, the 1478application may sue nlmsg_inherit(). It will allocate a message using 1479the default maximum message size and copy the header into the message. 1480Calling nlmsg_inherit with +set+ to NULL is equivalent to calling 1481nlmsg_alloc(). 1482 1483[source,c] 1484-------- 1485struct nl_msg *nlmsg_inherit(struct nlmsghdr *hdr); 1486-------- 1487 1488Alternatively nlmsg_alloc_simple() takes a netlink message type and 1489netlink message flags. It is equivalent to nlmsg_inherit() except that it 1490takes the two common header fields as arguments instead of a complete 1491header. 1492 1493[source,c] 1494-------- 1495#include <netlink/msg.h> 1496 1497struct nl_msg *nlmsg_alloc_simple(int nlmsg_type, int flags); 1498-------- 1499 1500.Appending the netlink message header 1501 1502After allocating struct nl_msg, the netlink message header needs to be 1503added unless one of the function nlmsg_alloc_simple() or nlmsg_inherit() 1504have been used for allocation in which case this step will replace the 1505netlink message header already in place. 1506 1507[source,c] 1508-------- 1509#include <netlink/msg.h> 1510 1511struct nlmsghdr *nlmsg_put(struct nl_msg *msg, uint32_t port, uint32_t seqnr, 1512 int nlmsg_type, int payload, int nlmsg_flags); 1513-------- 1514 1515The function nlmsg_put() will build a netlink message header out of 1516+nlmsg_type+, +nlmsg_flags+, +seqnr+, and +port+ and copy it into the 1517netlink message. +seqnr+ can be set to +NL_AUTO_SEQ+ to indiciate 1518that the next possible sequence number should be used automatically. 1519To use this feature, the message must be sent using the function 1520nl_send_auto(). Like +port+, the argument +seqnr+ can be set to 1521+NL_AUTO_PORT+ indicating that the local port assigned to the socket 1522should be used as source port. This is generally a good idea unless 1523you are replying to a request. See <<core_netlink_fundamentals>> 1524for more information on how to fill the header. 1525 1526NOTE: The argument +payload+ can be used by the application to reserve 1527 room for additional data after the header. A value of > 0 is 1528 equivalent to calling +nlmsg_reserve(msg, payload, NLMSG_ALIGNTO)+. 1529 See <<core_msg_reserve>> for more information on reserving room for 1530 data. 1531 1532.Example 1533[source,c] 1534-------- 1535#include <netlink/msg.h> 1536 1537struct nlmsghdr *hdr; 1538struct nl_msg *msg; 1539struct myhdr { 1540 uint32_t foo1, foo2; 1541} hdr = { 10, 20 }; 1542 1543/* Allocate a message with the default maximum message size */ 1544msg = nlmsg_alloc(); 1545 1546/* 1547 * Add header with message type MY_MSGTYPE, the flag NLM_F_CREATE, 1548 * let library fill port and sequence number, and reserve room for 1549 * struct myhdr 1550 */ 1551hdr = nlmsg_put(msg, NL_AUTO_PORT, NL_AUTO_SEQ, MY_MSGTYPE, sizeof(hdr), NLM_F_CREATE); 1552 1553/* Copy own header into newly reserved payload section */ 1554memcpy(nlmsg_data(hdr), &hdr, sizeof(hdr)); 1555 1556/* 1557 * The message will now look like this: 1558 * +-------------------+- - -+----------------+- - -+ 1559 * | struct nlmsghdr | Pad | struct myhdr | Pad | 1560 * +-------------------+-----+----------------+- - -+ 1561 * nlh -^ / \ 1562 * +--------+---------+ 1563 * | foo1 | foo2 | 1564 * +--------+---------+ 1565 */ 1566-------- 1567 1568[[core_msg_reserve]] 1569.Reserving room at the end of the message 1570 1571Most functions described later on will automatically take care of 1572reserving room for the data that is added to the end of the netlink 1573message. In some situations it may be requried for the application 1574to reserve room directly though. 1575 1576[source,c] 1577-------- 1578#include <netlink/msg.h> 1579 1580void *nlmsg_reserve(struct nl_msg *msg, size_t len, int pad); 1581-------- 1582 1583The function nlmsg_reserve() reserves +len+ bytes at the end of the 1584netlink message and returns a pointer to the start of the reserved area. 1585The +pad+ argument can be used to request +len+ to be aligned to any 1586number of bytes prior to reservation. 1587 1588The following example requests to reserve a 17 bytes area at the end of 1589message aligned to 4 bytes. Therefore a total of 20 bytes will be 1590reserved. 1591 1592[source,c] 1593-------- 1594#include <netlink/msg.h> 1595 1596void *buf = nlmsg_reserve(msg, 17, 4); 1597-------- 1598 1599NOTE: `nlmsg_reserve()` will *not* align the start of the buffer. Any 1600 alignment requirements must be provided by the owner of the 1601 previous message section. 1602 1603.Appending data at the end of the message 1604 1605The function `nlmsg_append()` appends `len` bytes at the end of the 1606message, padding it if requested and necessary. 1607 1608[source,c] 1609-------- 1610#include <netlink/msg.h> 1611 1612int nlmsg_append(struct nl_msg *msg, void *data, size_t len, int pad); 1613-------- 1614 1615It is equivalent to calling `nlmsg_reserve()` and `memcpy()`ing the 1616data into the freshly reserved data section. 1617 1618NOTE: `nlmsg_append()` will *not* align the start of the data. Any 1619 alignment requirements must be provided by the owner of the 1620 previous message section. 1621 1622.Adding attribtues to a message 1623 1624Construction of attributes and addition of attribtues to the message is 1625covereted in section <<core_attr>>. 1626 1627[[core_attr]] 1628== Attributes 1629 1630Any form of payload should be encoded as netlink attributes whenever 1631possible. Use of attributes allows to extend any netlink protocol in 1632the future without breaking binary compatibility. F.e. Suppose your 1633device may currently be using 32 bit counters for statistics but years 1634later the device switches to maintaining 64 bit counters to account 1635for faster network hardware. If your protocol is using attributes the 1636move to 64 bit counters is trivial and only involves in sending an 1637additional attribute containing the 64 bit variants while still 1638providing the old legacy 32 bit counters. If your protocol is not using 1639attributes you will not be able to switch data types without breaking 1640all existing users of the protocol. 1641 1642The concept of nested attributes also allows for subsystems of your 1643protocol to implement and maintain their own attribute schemas. Suppose 1644a new generation of network device is introduced which requires a 1645completely new set of configuration settings which was unthinkable when 1646the netlink protocol was initially designed. Using attributes the new 1647generation of devices may define a new attribute and fill it with its 1648own new structure of attributes which extend or even obsolete the old 1649attributes. 1650 1651Therefore, _always_ use attributes even if you are almost certain that 1652the message format will never ever change in the future. 1653 1654[[core_attr_format]] 1655=== Attribute Format 1656 1657Netlink attributes allow for any number of data chunks of arbitary 1658length to be attached to a netlink message. See <<core_msg_attr>> 1659for more information on where attributes are stored in the message. 1660 1661The format of the attributes data returned by nlmsg_attrdata() is as 1662follows: 1663 1664[source,c] 1665-------- 1666 <----------- nla_total_size(payload) -----------> 1667 <---------- nla_size(payload) -----------> 1668 +-----------------+- - -+- - - - - - - - - +- - -+-----------------+- - - 1669 | struct nlattr | Pad | Payload | Pad | struct nlattr | 1670 +-----------------+- - -+- - - - - - - - - +- - -+-----------------+- - - 1671 <---- NLA_HDRLEN -----> <--- NLA_ALIGN(len) ---> <---- NLA_HDRLEN --- 1672-------- 1673 1674Every attribute must start at an offset which is a multiple of 1675+NLA_ALIGNTO+ (4 bytes). If you need to know whether an attribute needs 1676to be padded at the end, the function nla_padlen() returns the number 1677of padding bytes that will or need to be added. 1678 1679image:attribute_hdr.png["Netlink Attribute Header"] 1680 1681Every attribute is encoded with a type and length field, both 16 bits, 1682stored in the attribute header (struct nlattr) preceding the attribute 1683payload. The length of an attribute is used to calculate the offset to 1684the next attribute. 1685 1686[[core_attr_parse]] 1687=== Parsing Attributes 1688 1689[[core_attr_parse_split]] 1690.Splitting an Attributes Stream into Attributes 1691 1692Although most applications will use one of the functions from the 1693nlmsg_parse() family (See <<core_attr_parse_easy>>) an interface exists 1694to split the attributes stream manually. 1695 1696As described in <<core_attr_format>> the attributes section contains a 1697infinite sequence or stream of attributes. The pointer returned by 1698nlmsg_attrdata() (See <<core_msg_attr>>) points to the first attribute 1699header. Any subsequent attribute is accessed with the function nla_next() 1700based on the previous header. 1701 1702[source,c] 1703-------- 1704#include <netlink/attr.h> 1705 1706struct nlattr *nla_next(const struct nlattr *attr, int *remaining); 1707-------- 1708 1709The semantics are equivalent to nlmsg_next() and thus nla_next() will also 1710subtract the size of the previous attribute from the remaining number of 1711bytes in the attributes stream. 1712 1713Like messages, attributes do not contain an indicator whether another 1714attribute follows or not. The only indication is the number of bytes left 1715in the attribute stream. The function nla_ok() exists to determine whether 1716another attribute fits into the remaining number of bytes or not. 1717 1718[source,c] 1719-------- 1720#include <netlink/attr.h> 1721 1722int nla_ok(const struct nlattr *attr, int remaining); 1723-------- 1724 1725A typical use of nla_ok() and nla_next() looks like this: 1726 1727.nla_ok()/nla_next() usage 1728[source,c] 1729-------- 1730#include <netlink/msg.h> 1731#include <netlink/attr.h> 1732 1733struct nlattr *hdr = nlmsg_attrdata(msg, 0); 1734int remaining = nlmsg_attrlen(msg, 0); 1735 1736while (nla_ok(hdr, remaining)) { 1737 /* parse attribute here */ 1738 hdr = nla_next(hdr, &remaining); 1739}; 1740-------- 1741 1742NOTE: `nla_ok()` only returns true if the *complete* attributes 1743 including the attribute payload fits into the remaining number 1744 of bytes. 1745 1746.Accessing Attribute Header and Payload 1747 1748Once the individual attributes have been sorted out by either splitting 1749the attributes stream or using another interface the attribute header 1750and payload can be accessed. 1751 1752[source,c] 1753-------- 1754 <- nla_len(hdr) -> 1755 +-----------------+- - -+- - - - - - - - - +- - -+ 1756 | struct nlattr | Pad | Payload | Pad | 1757 +-----------------+- - -+- - - - - - - - - +- - -+ 1758nla_data(hdr) ---------------^ 1759-------- 1760 1761The functions nla_len() and nla_type() can be used to access the attribute 1762header. nla_len() will return the length of the payload not including 1763eventual padding bytes. nla_type returns the attribute type. 1764 1765[source,c] 1766-------- 1767#include <netlink/attr.h> 1768 1769int nla_len(const struct nlattr *hdr); 1770int nla_type(const struct nlattr *hdr); 1771-------- 1772 1773The function nla_data() will return a pointer to the attribute 1774payload. Please note that due to +NLA_ALIGNTO+ being 4 bytes it may 1775not be safe to cast and dereference the pointer for any datatype 1776larger than 32 bit depending on the architecture the application is 1777run on. 1778 1779[source,c] 1780-------- 1781#include <netlink/attr.h> 1782 1783void *nla_data(const struct nlattr *hdr); 1784-------- 1785 1786[NOTE] 1787Never rely on the size of a payload being what you expect it to be. 1788_Always_ verify the payload size and make sure that it matches your 1789expectations. See <<core_attr_validation>> 1790 1791[[core_attr_validation]] 1792.Attribute Validation 1793 1794When receiving netlink attributes, the receiver has certain expections 1795on how the attributes should look like. These expectations must be 1796defined to make sure the sending side meets our expecations. For this 1797purpose, a attribute validation interface exists which must be used 1798prior to accessing any payload. 1799 1800All functions providing attribute validation functionality are based 1801on struct nla_policy: 1802 1803[source,c] 1804-------- 1805struct nla_policy { 1806 uint16_t type; 1807 uint16_t minlen; 1808 uint16_t maxlen; 1809}; 1810-------- 1811 1812The +type+ member specifies the datatype of the attribute, e.g. 1813+NLA_U32+, +NLA_STRING+, +NLA_FLAG+. The default is +NLA_UNSPEC+. The 1814+minlen+ member defines the minmum payload length of an attribute to 1815be considered a valid attribute. The value for +minlen+ is implicit 1816for most basic datatypes such as integers or flags. The +maxlen+ 1817member can be used to define a maximum payload length for an 1818attribute to still be considered valid. 1819 1820NOTE: Specyfing a maximum payload length is not recommended when 1821 encoding structures in an attribute as it will prevent any 1822 extension of the structure in the future. Something that is 1823 frequently done in netlink protocols and does not break 1824 backwards compatibility. 1825 1826One of the functions which use struct nla_policy is nla_validate(). 1827The function expects an array of struct nla_policy and will access the 1828array using the attribute type as index. If an attribute type is out 1829of bounds the attribute is assumed to be valid. This is intentional 1830behaviour to allow older applications not yet aware of recently 1831introduced attributes to continue functioning. 1832 1833[source,c] 1834-------- 1835#include <netlink/attr.h> 1836 1837int nla_validate(struct nlattr *head, int len, int maxtype, struct nla_policy *policy); 1838-------- 1839 1840The function nla_validate() returns 0 if all attributes are valid, 1841otherwise a validation failure specific error code is returned. 1842 1843Most applications will rarely use nla_validate() directly but use 1844nla_parse() instead which takes care of validation in the same way but 1845also parses the the attributes in the same step. See 1846<<core_attr_parse_easy>> for an example and more information. 1847 1848The validation process in detail: 1849 1850. If attribute type is 0 or exceeds +maxtype+ attribute is 1851 considered valid, 0 is returned. 1852. If payload length is < +minlen+, +-NLE_ERANGE+ is returned. 1853. If +maxlen+ is defined and payload exceeds it, +-NLE_ERANGE+ 1854 is returned. 1855. Datatype specific requirements rules, see <<core_attr_types>> 1856. If all is ok, 0 is returned. 1857 1858[[core_attr_parse_easy]] 1859.Parsing Attributes the Easy Way 1860 1861Most applications will not want to deal with splitting attribute 1862streams themselves as described in <<core_attr_parse_split>> 1863A much easier method is to use nla_parse(). 1864 1865[source,c] 1866-------- 1867#include <netlink/attr.h> 1868 1869int nla_parse(struct nlattr **attrs, int maxtype, struct nlattr *head, 1870 int len, struct nla_policy *policy); 1871-------- 1872 1873The function nla_parse() will iterate over a stream of attributes, 1874validate each attribute as described in <<core_attr_validation>> 1875If the validation of all attributes succeeds, a pointer to each attribute 1876is stored in the +attrs+ array at `attrs[nla_type(attr)]`. 1877 1878As an alernative to nla_parse() the function nlmsg_parse() can be used 1879to parse the message and its attributes in one step. See 1880<<core_attr_parse_easy>> for information on how to use these functions. 1881 1882.Example: 1883 1884The following example demonstrates how to parse a netlink message sent 1885over a netlink protocol which does not use protocol headers. The example 1886does enforce a attribute policy however, the attribute MY_ATTR_FOO must 1887be a 32 bit integer, and the attribute MY_ATTR_BAR must be a string with 1888a maximum length of 16 characters. 1889 1890[source,c] 1891--------- 1892#include <netlink/msg.h> 1893#include <netlink/attr.h> 1894 1895enum { 1896 MY_ATTR_FOO = 1, 1897 MY_ATTR_BAR, 1898 __MY_ATTR_MAX, 1899}; 1900 1901#define MY_ATTR_MAX (__MY_ATTR_MAX - 1) 1902 1903static struct nla_policy my_policy[MY_ATTR_MAX+1] = { 1904 [MY_ATTR_FOO] = { .type = NLA_U32 }, 1905 [MY_ATTR_BAR] = { .type = NLA_STRING, 1906 .maxlen = 16 }, 1907}; 1908 1909void parse_msg(struct nlmsghdr *nlh) 1910{ 1911 struct nlattr *attrs[MY_ATTR_MAX+1]; 1912 1913 if (nlmsg_parse(nlh, 0, attrs, MY_ATTR_MAX, my_policy) < 0) 1914 /* error */ 1915 1916 if (attrs[MY_ATTR_FOO]) { 1917 /* MY_ATTR_FOO is present in message */ 1918 printf("value: %u\n", nla_get_u32(attrs[MY_ATTR_FOO])); 1919 } 1920} 1921--------- 1922 1923.Locating a Single Attribute 1924 1925An application only interested in a single attribute can use one of the 1926functions nla_find() or nlmsg_find_attr(). These function will iterate 1927over all attributes, search for a matching attribute and return a pointer 1928to the corresponding attribute header. 1929 1930[source,c] 1931-------- 1932#include <netlink/attr.h> 1933 1934struct nlattr *nla_find(struct nlattr *head, int len, int attrtype); 1935-------- 1936 1937[source,c] 1938-------- 1939#include <netlink/msg.h> 1940 1941struct nlattr *nlmsg_find_attr(struct nlmsghdr *hdr, int hdrlen, int attrtype); 1942-------- 1943 1944NOTE: `nla_find()` and `nlmsg_find_attr()` will *not* search in nested 1945 attributes recursively, see <<core_attr_nested>>. 1946 1947==== Iterating over a Stream of Attributes 1948 1949In some situations it does not make sense to assign a unique attribute 1950type to each attribute in the attribute stream. For example a list may 1951be transferd using a stream of attributes and even if the attribute type 1952is incremented for each attribute it may not make sense to use the 1953nlmsg_parse() or nla_parse() function to fill an array. 1954 1955Therefore methods exist to iterate over a stream of attributes: 1956 1957[source,c] 1958-------- 1959#include <netlink/attr.h> 1960 1961nla_for_each_attr(attr, head, len, remaining) 1962-------- 1963 1964nla_for_each_attr() is a macro which can be used in front of a code 1965block: 1966 1967[source,c] 1968-------- 1969#include <netlink/attr.h> 1970 1971struct nalttr *nla; 1972int rem; 1973 1974nla_for_each_attr(nla, attrstream, streamlen, rem) { 1975 /* validate & parse attribute */ 1976} 1977 1978if (rem > 0) 1979 /* unparsed attribute data */ 1980-------- 1981 1982[[core_attr_constr]] 1983=== Attribute Construction 1984 1985The interface to add attributes to a netlink message is based on the 1986regular message construction interface. It assumes that the message 1987header and an eventual protocol header has been added to the message 1988already. 1989 1990[source,c] 1991-------- 1992struct nlattr *nla_reserve(struct nl_msg *msg, int attrtype, int len); 1993-------- 1994 1995The function nla_reserve() adds an attribute header at the end of the 1996message and reserves room for +len+ bytes of payload. The function 1997returns a pointer to the attribute payload section inside the message. 1998Padding is added at the end of the attribute to ensure the next 1999attribute is properly aligned. 2000 2001[source,c] 2002-------- 2003int nla_put(struct nl_msg *msg, int attrtype, int attrlen, const void *data); 2004-------- 2005 2006The function nla_put() is base don nla_reserve() but takes an additional 2007pointer +data+ pointing to a buffer containing the attribute payload. 2008It will copy the buffer into the message automatically. 2009 2010.Example: 2011 2012[source,c] 2013-------- 2014struct my_attr_struct { 2015 uint32_t a; 2016 uint32_t b; 2017}; 2018 2019int my_put(struct nl_msg *msg) 2020{ 2021 struct my_attr_struct obj = { 2022 .a = 10, 2023 .b = 20, 2024 }; 2025 2026 return nla_put(msg, ATTR_MY_STRUCT, sizeof(obj), &obj); 2027} 2028-------- 2029 2030See <<core_attr_types>> for datatype specific attribute construction 2031functions. 2032 2033.Exception Based Attribute Construction 2034 2035Like in the kernel API an exception based construction interface is 2036provided. The behaviour of the macros is identical to their regular 2037function counterparts except that in case of an error, the target 2038`nla_put_failure` is jumped. 2039 2040.Example: 2041[source,c] 2042-------- 2043#include <netlink/msg.h> 2044#include <netlink/attr.h> 2045 2046void construct_attrs(struct nl_msg *msg) 2047{ 2048 NLA_PUT_STRING(msg, MY_ATTR_FOO1, "some text"); 2049 NLA_PUT_U32(msg, MY_ATTR_FOO1, 0x1010); 2050 NLA_PUT_FLAG(msg, MY_ATTR_FOO3, 1); 2051 2052 return 0; 2053 2054nla_put_failure: 2055 /* NLA_PUT* macros jump here in case of an error */ 2056 return -EMSGSIZE; 2057} 2058-------- 2059 2060See <<core_attr_types>> for more information on the datatype specific 2061exception based variants. 2062 2063[[core_attr_types]] 2064=== Attribute Data Types 2065 2066A number of basic data types have been defined to simplify access and 2067validation of attributes. The datatype is not encoded in the 2068attribute, therefore bthe sender and receiver are required to use the 2069same definition on what attribute is of what type. 2070 2071[options="header", cols="1m,5"] 2072|================================================ 2073| Type | Description 2074| NLA_UNSPEC | Unspecified attribute 2075| NLA_U{8\|16\|32} | Integers 2076| NLA_STRING | String 2077| NLA_FLAG | Flag 2078| NLA_NESTED | Nested attribute 2079|================================================ 2080 2081Besides simplified access to the payload of such datatypes, the major 2082advantage is the automatic validation of each attribute based on a 2083policy. The validation ensures safe access to the payload by checking 2084for minimal payload size and can also be used to enforce maximum 2085payload size for some datatypes. 2086 2087==== Integer Attributes 2088 2089The most frequently used datatypes are integers. Integers come in four 2090different sizes: 2091[horizontal] 2092NLA_U8:: 8bit integer 2093NLA_U16:: 16bit integer 2094NLA_U32:: 32bit integer 2095NLA_U64:: 64bit integer 2096 2097Note that due to the alignment requirements of attributes the integer 2098attribtue +NLA_u8+ and +NLA_U16+ will not result in space savings in 2099the netlink message. Their use is intended to limit the range of 2100values. 2101 2102.Parsing Integer Attributes 2103 2104[source,c] 2105-------- 2106#include <netlink/attr.h> 2107 2108uint8_t nla_get_u8(struct nlattr *hdr); 2109uint16_t nla_get_u16(struct nlattr *hdr); 2110uint32_t nla_get_u32(struct nlattr *hdr); 2111uint64_t nla_get_u64(struct nlattr *hdr); 2112-------- 2113 2114Example: 2115 2116[source,c] 2117-------- 2118if (attrs[MY_ATTR_FOO]) 2119 uint32_t val = nla_get_u32(attrs[MY_ATTR_FOO]); 2120-------- 2121 2122.Constructing Integer Attributes 2123 2124[source,c] 2125-------- 2126#include <netlink/attr.h> 2127 2128int nla_put_u8(struct nl_msg *msg, int attrtype, uint8_t value); 2129int nla_put_u16(struct nl_msg *msg, int attrtype, uint16_t value); 2130int nla_put_u32(struct nl_msg *msg, int attrtype, uint32_t value); 2131int nla_put_u64(struct nl_msg *msg, int attrtype, uint64_t value); 2132-------- 2133 2134Exception based: 2135 2136[source,c] 2137-------- 2138NLA_PUT_U8(msg, attrtype, value) 2139NLA_PUT_U16(msg, attrtype, value) 2140NLA_PUT_U32(msg, attrtype, value) 2141NLA_PUT_U64(msg, attrtype, value) 2142-------- 2143 2144.Validation 2145 2146Use +NLA_U8+, +NLA_U16+, +NLA_U32+, or +NLA_U64+ to define the type of 2147integer when filling out a struct nla_policy array. It will 2148automatically enforce the correct minimum payload length policy. 2149 2150Validation does not differ between signed and unsigned integers, only 2151the size matters. If the appliaction wishes to enforce particular value 2152ranges it must do so itself. 2153 2154[source,c] 2155-------- 2156static struct nla_policy my_policy[ATTR_MAX+1] = { 2157 [ATTR_FOO] = { .type = NLA_U32 }, 2158 [ATTR_BAR] = { .type = NLA_U8 }, 2159}; 2160-------- 2161 2162The above is equivalent to: 2163[source,c] 2164-------- 2165static struct nla_policy my_policy[ATTR_MAX+1] = { 2166 [ATTR_FOO] = { .minlen = sizeof(uint32_t) }, 2167 [ATTR_BAR] = { .minlen = sizeof(uint8_t) }, 2168}; 2169-------- 2170 2171==== String Attributes 2172 2173The string datatype represents a NUL termianted character string of 2174variable length. It is not intended for binary data streams. 2175 2176The payload of string attributes can be accessed with the function 2177nla_get_string(). nla_strdup() calls strdup() on the payload and returns 2178the newly allocated string. 2179 2180[source,c] 2181-------- 2182#include <netlink/attr.h> 2183 2184char *nla_get_string(struct nlattr *hdr); 2185char *nla_strdup(struct nlattr *hdr); 2186-------- 2187 2188String attributes are constructed with the function +nla_put_string()+ 2189respectively +NLA_PUT_STRING()+. The length of the payload will be 2190strlen()+1, the trailing NUL byte is included. 2191 2192[source,c] 2193-------- 2194int nla_put_string(struct nl_msg *msg, int attrtype, const char *data); 2195 2196NLA_PUT_STRING(msg, attrtype, data) 2197-------- 2198 2199For validation purposes the type +NLA_STRING+ can be used in 2200+struct nla_policy+ definitions. It implies a minimum payload length 2201of 1 byte and checks for a trailing NUL byte. Optionally the +maxlen+ 2202member defines the maximum length of a character string (including the 2203trailing NUL byte). 2204 2205[source,c] 2206-------- 2207static struct nla_policy my_policy[] = { 2208 [ATTR_FOO] = { .type = NLA_STRING, 2209 .maxlen = IFNAMSIZ }, 2210}; 2211-------- 2212 2213==== Flag Attributes 2214 2215The flag attribute represents a boolean datatype. The presence of the 2216attribute implies a value of +true+, the absence of the attribute 2217implies the value +false+. Therefore the payload length of flag 2218attributes is always 0. 2219 2220[source,c] 2221-------- 2222int nla_get_flag(struct nlattr *hdr); 2223int nla_put_flag(struct nl_msg *msg, int attrtype); 2224-------- 2225 2226The type +NLA_FLAG+ is used for validation purposes. It implies a 2227+maxlen+ value of 0 and thus enforces a maximum payload length of 0. 2228 2229.Example: 2230[source,c] 2231-------- 2232/* nla_put_flag() appends a zero sized attribute to the message. */ 2233nla_put_flag(msg, ATTR_FLAG); 2234 2235/* There is no need for a receival function, the presence is the value. */ 2236if (attrs[ATTR_FLAG]) 2237 /* flag is present */ 2238-------- 2239 2240[[core_attr_nested]] 2241==== Nested Attributes 2242 2243As described in <<core_attr>>, attributes can be nested allowing for 2244complex tree structures of attributes. It is commonly used to delegate 2245the responsibility of a subsection of the message to a subsystem. 2246Nested attributes are also commonly used for transmitting list of objects. 2247 2248When nesting attributes, the nested attributes are included as payload 2249of a container attribute. 2250 2251NOTE: When validating the attributes using nlmsg_validate(), 2252 nlmsg_parse(), nla_validate(), or nla_parse() only the 2253 attributes on the first level are being validated. None of these 2254 functions will validate attributes recursively. Therefore you 2255 must explicitely call nla_validate() or use nla_parse_nested() 2256 for each level of nested attributes. 2257 2258The type +NLA_NESTED+ should be used when defining nested attributes 2259in a struct nla_policy definition. It will not enforce any minimum 2260payload length unless +minlen+ is specified explicitely. This is 2261because some netlink protocols implicitely allow empty container 2262attributes. 2263 2264[source,c] 2265-------- 2266static struct nla_policy my_policy[] = { 2267 [ATTR_OPTS] = { .type = NLA_NESTED }, 2268}; 2269-------- 2270 2271.Parsing of Nested Attributes 2272 2273The function nla_parse_nested() is used to parse nested attributes. 2274Its behaviour is identical to nla_parse() except that it takes a 2275struct nlattr as argument and will use the payload as stream of 2276attributes. 2277 2278[source,c] 2279-------- 2280if (attrs[ATTR_OPTS]) { 2281 struct nlattr *nested[NESTED_MAX+1]; 2282 struct nla_policy nested_policy[] = { 2283 [NESTED_FOO] = { .type = NLA_U32 }, 2284 }; 2285 2286 if (nla_parse_nested(nested, NESTED_MAX, attrs[ATTR_OPTS], nested_policy) < 0) 2287 /* error */ 2288 2289 if (nested[NESTED_FOO]) 2290 uint32_t val = nla_get_u32(nested[NESTED_FOO]); 2291} 2292-------- 2293 2294.Construction of Nested Attributes 2295 2296Attributes are nested by surrounding them with calls to nla_nest_start() 2297and nla_nest_end(). nla_nest_start() will add a attribute header to 2298the message but no actual payload. All data added to the message from 2299this point on will be part of the container attribute until nla_nest_end() 2300is called which "closes" the attribute, correcting its payload length to 2301include all data length. 2302 2303[source,c] 2304-------- 2305int put_opts(struct nl_msg *msg) 2306{ 2307 struct nlattr *opts; 2308 2309 if (!(opts = nla_nest_start(msg, ATTR_OPTS))) 2310 goto nla_put_failure; 2311 2312 NLA_PUT_U32(msg, NESTED_FOO, 123); 2313 NLA_PUT_STRING(msg, NESTED_BAR, "some text"); 2314 2315 nla_nest_end(msg, opts); 2316 return 0; 2317 2318nla_put_failure: 2319 nla_nest_cancel(msg, opts); 2320 return -EMSGSIZE; 2321} 2322-------- 2323 2324==== Unspecified Attribute 2325 2326This is the default attribute type and used when none of the basic 2327datatypes is suitable. It represents data of arbitary type and length. 2328 2329See <<core_addr_alloc, Address Allocation>> for a more information on 2330a special interface allowing the allocation of abstract address object 2331based on netlink attributes which carry some form of network address. 2332 2333See <<core_data_alloc, Abstract Data Allocation>> for more information 2334on how to allocate abstract data objects based on netlink attributes. 2335 2336Use the function nla_get() and nla_put() to access the payload and 2337construct attributes. See <<core_attr_constr, Attribute Construction>> 2338for an example. 2339 2340=== Examples 2341 2342==== Constructing a Netlink Message with Attributes 2343 2344[source,c] 2345-------- 2346struct nl_msg *build_msg(int ifindex, struct nl_addr *lladdr, int mtu) 2347{ 2348 struct nl_msg *msg; 2349 struct nlattr *info, *vlan; 2350 struct ifinfomsg ifi = { 2351 .ifi_family = AF_INET, 2352 .ifi_index = ifindex, 2353 }; 2354 2355 /* Allocate a default sized netlink message */ 2356 if (!(msg = nlmsg_alloc_simple(RTM_SETLINK, 0))) 2357 return NULL; 2358 2359 /* Append the protocol specific header (struct ifinfomsg)*/ 2360 if (nlmsg_append(msg, &ifi, sizeof(ifi), NLMSG_ALIGNTO) < 0) 2361 goto nla_put_failure 2362 2363 /* Append a 32 bit integer attribute to carry the MTU */ 2364 NLA_PUT_U32(msg, IFLA_MTU, mtu); 2365 2366 /* Append a unspecific attribute to carry the link layer address */ 2367 NLA_PUT_ADDR(msg, IFLA_ADDRESS, lladdr); 2368 2369 /* Append a container for nested attributes to carry link information */ 2370 if (!(info = nla_nest_start(msg, IFLA_LINKINFO))) 2371 goto nla_put_failure; 2372 2373 /* Put a string attribute into the container */ 2374 NLA_PUT_STRING(msg, IFLA_INFO_KIND, "vlan"); 2375 2376 /* 2377 * Append another container inside the open container to carry 2378 * vlan specific attributes 2379 */ 2380 if (!(vlan = nla_nest_start(msg, IFLA_INFO_DATA))) 2381 goto nla_put_failure; 2382 2383 /* add vlan specific info attributes here... */ 2384 2385 /* Finish nesting the vlan attributes and close the second container. */ 2386 nla_nest_end(msg, vlan); 2387 2388 /* Finish nesting the link info attribute and close the first container. */ 2389 nla_nest_end(msg, info); 2390 2391 return msg; 2392 2393nla_put_failure: 2394 nlmsg_free(msg); 2395 return NULL; 2396} 2397------ 2398 2399==== Parsing a Netlink Message with Attributes 2400 2401[source,c] 2402-------- 2403int parse_message(struct nlmsghdr *hdr) 2404{ 2405 /* 2406 * The policy defines two attributes: a 32 bit integer and a container 2407 * for nested attributes. 2408 */ 2409 struct nla_policy attr_policy[] = { 2410 [ATTR_FOO] = { .type = NLA_U32 }, 2411 [ATTR_BAR] = { .type = NLA_NESTED }, 2412 }; 2413 struct nlattr *attrs[ATTR_MAX+1]; 2414 int err; 2415 2416 /* 2417 * The nlmsg_parse() function will make sure that the message contains 2418 * enough payload to hold the header (struct my_hdr), validates any 2419 * attributes attached to the messages and stores a pointer to each 2420 * attribute in the attrs[] array accessable by attribute type. 2421 */ 2422 if ((err = nlmsg_parse(hdr, sizeof(struct my_hdr), attrs, ATTR_MAX, 2423 attr_policy)) < 0) 2424 goto errout; 2425 2426 if (attrs[ATTR_FOO]) { 2427 /* 2428 * It is safe to directly access the attribute payload without 2429 * any further checks since nlmsg_parse() enforced the policy. 2430 */ 2431 uint32_t foo = nla_get_u32(attrs[ATTR_FOO]); 2432 } 2433 2434 if (attrs[ATTR_BAR]) { 2435 struct *nested[NESTED_MAX+1]; 2436 2437 /* 2438 * Attributes nested in a container can be parsed the same way 2439 * as top level attributes. 2440 */ 2441 err = nla_parse_nested(nested, NESTED_MAX, attrs[ATTR_BAR], 2442 nested_policy); 2443 if (err < 0) 2444 goto errout; 2445 2446 // Process nested attributes here. 2447 } 2448 2449 err = 0; 2450errout: 2451 return err; 2452} 2453-------- 2454 2455[[core_cb]] 2456== Callback Configurations 2457 2458Callback hooks and overwriting capabilities are provided in various places 2459inside library to control the behaviour of several functions. All the 2460callback and overwrite functions are packed together in struct nl_cb which 2461is attached to a netlink socket or passed on to functions directly. 2462 2463=== Callback Hooks 2464 2465Callback hooks are spread across the library to provide entry points for 2466message processing and to take action upon certain events. 2467 2468Callback functions may return the following return codes: 2469[options="header", cols="1m,4"] 2470|======================================================================== 2471| Return Code | Description 2472| NL_OK | Proceed. 2473| NL_SKIP | Skip message currently being processed and continue 2474 parsing the receive buffer. 2475| NL_STOP | Stop parsing and discard all remaining data in the 2476 receive buffer. 2477|======================================================================== 2478 2479.Default Callback Implementations 2480 2481The library provides three sets of default callback implementations: 2482* +NL_CB_DEFAULT+ This is the default set. It implets the default behaviour. 2483 See the table below for more information on the return codes of each 2484 function. 2485* +NL_CB_VERBOSE+ This set is based on the default set but will cause an 2486 error message to be printed to stderr for error messages, invalid 2487 messages, message overruns and unhandled valid messages. The 2488 +arg+ pointer in nl_cb_set() and nl_cb_err() can be used to 2489 provide a FILE * which overwrites stderr. 2490* +NL_CB_DEBUG+ This set is intended for debugging purposes. It is 2491 based on the verbose set but will decode and dump each message sent 2492 or received to the console. 2493 2494.Message Processing Callbacks 2495 2496.nl_sendmsg() callback hooks: 2497[cols="2m,4e,1m", options="header"] 2498|============================================================================ 2499| Callback ID | Description | Default Return Value 2500| NL_CB_MSG_OUT | Each message sent | NL_OK 2501|============================================================================ 2502 2503Any function called by NL_CB_MSG_OUT may return a negative error code to 2504prevent the message from being sent and the error code being returned. 2505 2506nl_recvmsgs() callback hooks (ordered by priority): 2507[cols="2m,4e,1m", options="header"] 2508|============================================================================ 2509| Callback ID | Description | Default Return Value 2510| NL_CB_MSG_IN | Each message received | NL_OK 2511| NL_CB_SEQ_CHECK | May overwrite sequence check algo | NL_OK 2512| NL_CB_INVALID | Invalid messages | NL_STOP 2513| NL_CB_SEND_ACK | Messages with NLM_F_ACK flag set | NL_OK 2514| NL_CB_FINISH | Messages of type NLMSG_DONE | NL_STOP 2515| NL_CB_SKIPPED | Messages of type NLMSG_NOOP | NL_SKIP 2516| NL_CB_OVERRUN | Messages of type NLMSG_OVERRUN | NL_STOP 2517| NL_CB_ACK | ACK Messages | NL_STOP 2518| NL_CB_VALID | Each valid message | NL_OK 2519|============================================================================ 2520 2521Any of these functions may return NL_OK, NL_SKIP, or NL_STOP. 2522 2523Message processing callback functions are set with nl_cb_set(): 2524[source,c] 2525-------- 2526#include <netlink/handlers.h> 2527 2528int nl_cb_set(struct nl_cb *cb, enum nl_cb_type type, enum nl_cb_kind kind, 2529 nl_recvmsg_msg_cb_t func, void *arg); 2530 2531typedef int (*nl_recvmsg_msg_cb_t)(struct nl_msg *msg, void *arg); 2532-------- 2533 2534.Callback for Error Messages 2535 2536A special function prototype is used for the error message callback hook: 2537 2538[source,c] 2539-------- 2540#include <netlink/handlers.h> 2541 2542int nl_cb_err(struct nl_cb *cb, enum nl_cb_kind kind, nl_recvmsg_err_cb_t func, void *arg); 2543 2544typedef int(* nl_recvmsg_err_cb_t)(struct sockaddr_nl *nla, struct nlmsgerr *nlerr, void *arg); 2545-------- 2546 2547.Example: Setting up a callback set 2548[source,c] 2549-------- 2550#include <netlink/handlers.h> 2551 2552/* Allocate a callback set and initialize it to the verbose default set */ 2553struct nl_cb *cb = nl_cb_alloc(NL_CB_VERBOSE); 2554 2555/* Modify the set to call my_func() for all valid messages */ 2556nl_cb_set(cb, NL_CB_VALID, NL_CB_CUSTOM, my_func, NULL); 2557 2558/* 2559 * Set the error message handler to the verbose default implementation 2560 * and direct it to print all errors to the given file descriptor. 2561 */ 2562FILE *file = fopen(...); 2563nl_cb_err(cb, NL_CB_VERBOSE, NULL, file); 2564-------- 2565 2566=== Overwriting of Internal Functions 2567 2568When the library needs to send or receive netlink messages in high level 2569interfaces it does so by calling its own low level API. In the case the 2570default characteristics are not sufficient for the application, it may 2571overwrite several internal function calls with own implementations. 2572 2573.Overwriting recvmsgs() 2574 2575See <<core_recv, Receiving Netlink Messages>> for more information on 2576how and when recvmsgs() is called internally. 2577 2578[source,c] 2579-------- 2580#include <netlink/handlers.h> 2581 2582void nl_cb_overwrite_recvmsgs(struct nl_cb *cb, 2583 int (*func)(struct nl_sock *sk, struct nl_cb *cb)); 2584-------- 2585 2586The following criteras must be met if a recvmsgs() implementation is 2587supposed to work with high level interfaces: 2588 2589- MUST respect the callback configuration +cb+, therefore: 2590 - MUST call +NL_CB_VALID+ for all valid messages, passing on 2591 - MUST call +NL_CB_ACK+ for all ACK messages 2592 - MUST correctly handle multipart messages, calling NL_CB_VALID for 2593 each message until a NLMSG_DONE message is received. 2594- MUST report error code if a NLMSG_ERROR or NLMSG_OVERRUN mesasge is 2595 received. 2596 2597.Overwriting nl_recv() 2598 2599Often it is sufficient to overwrite `nl_recv()` which is responsible 2600from receiving the actual data from the socket instead of replacing 2601the complete `recvmsgs()` logic. 2602 2603See <<core_recv_character, Receive Characteristics>> for more 2604information on how and when `nl_recv()` is called internally. 2605 2606[source,c] 2607-------- 2608#include <netlink/handlers.h> 2609 2610void nl_cb_overwrite_recv(struct nl_cb *cb, 2611 int (*func)(struct nl_sock * sk, 2612 struct sockaddr_nl *addr, 2613 unsigned char **buf, 2614 struct ucred **cred)); 2615-------- 2616 2617The following criteras must be met for an own `nl_recv()` 2618implementation: 2619 2620 - *MUST* return the number of bytes read or a negative error code if 2621 an error occured. The function may also return 0 to indicate that 2622 no data has been read. 2623 - *MUST* set `*buf` to a buffer containing the data read. It must be 2624 safe for the caller to access the number of bytes read returned as 2625 return code. 2626 - *MAY* fill out `*addr` with the netlink address of the peer the 2627 data has been received from. 2628 - *MAY* set `*cred` to a newly allocated struct ucred containg 2629 credentials. 2630 2631.Overwriting nl_send() 2632 2633See <<core_send, Sending Netlink Messages>> for more information on 2634how and when nl_send() is called internally. 2635 2636[source,c] 2637-------- 2638#include <netlink/handlers.h> 2639 2640void nl_cb_overwrite_send(struct nl_cb *cb, int (*func)(struct nl_sock *sk, 2641 struct nl_msg *msg)); 2642-------- 2643 2644Own implementations must send the netlink message and return 0 on success 2645or a negative error code. 2646 2647[[core_cache]] 2648== Cache System 2649 2650=== Allocation of Caches 2651 2652Almost all subsystem provide a function to allocate a new cache 2653of some form. The function usually looks like this: 2654[source,c] 2655-------- 2656struct nl_cache *<object name>_alloc_cache(struct nl_sock *sk); 2657-------- 2658 2659These functions allocate a new cache for the own object type, 2660initializes it properly and updates it to represent the current 2661state of their master, e.g. a link cache would include all 2662links currently configured in the kernel. 2663 2664Some of the allocation functions may take additional arguments 2665to further specify what will be part of the cache. 2666 2667All such functions return a newly allocated cache or NULL 2668in case of an error. 2669 2670=== Cache Manager 2671 2672The purpose of a cache manager is to keep track of caches and 2673automatically receive event notifications to keep the caches 2674up to date with the kernel state. Each manager has exactly one 2675netlink socket assigned which limits the scope of each manager 2676to exactly one netlink family. Therefore all caches committed 2677to a manager must be part of the same netlink family. Due to the 2678nature of a manager, it is not possible to have a cache maintain 2679two instances of the same cache type. The socket is subscribed 2680to the event notification group of each cache and also put into 2681non-blocking mode. Functions exist to poll() on the socket to 2682wait for new events to be received. 2683 2684 2685---- 2686 App libnl Kernel 2687 | | 2688 +-----------------+ [ notification, link change ] 2689 | | Cache Manager | | [ (IFF_UP | IFF_RUNNING) ] 2690 | | | 2691 | | +------------+| | | [ notification, new addr ] 2692 <-------|---| route/link |<-------(async)--+ [ 10.0.1.1/32 dev eth1 ] 2693 | | +------------+| | | 2694 | +------------+| | 2695 <---|---|---| route/addr |<------|-(async)--------------+ 2696 | +------------+| 2697 | | +------------+| | 2698 <-------|---| ... || 2699 | | +------------+| | 2700 +-----------------+ 2701 | | 2702---- 2703 2704.Creating a new cache manager 2705 2706[source,c] 2707---- 2708struct nl_cache_mngr *mngr; 2709 2710// Allocate a new cache manager for RTNETLINK and automatically 2711// provide the caches added to the manager. 2712err = nl_cache_mngr_alloc(NULL, NETLINK_ROUTE, NL_AUTO_PROVIDE, &mngr); 2713---- 2714 2715.Keep track of a cache 2716 2717[source,c] 2718---- 2719struct nl_cache *cache; 2720 2721// Create a new cache for links/interfaces and ask the manager to 2722// keep it up to date for us. This will trigger a full dump request 2723// to initially fill the cache. 2724cache = nl_cache_mngr_add(mngr, "route/link"); 2725----- 2726 2727.Make the manager receive updates 2728 2729[source,c] 2730---- 2731// Give the manager the ability to receive updates, will call poll() 2732// with a timeout of 5 seconds. 2733if (nl_cache_mngr_poll(mngr, 5000) > 0) { 2734 // Manager received at least one update, dump cache? 2735 nl_cache_dump(cache, ...); 2736} 2737---- 2738 2739.Release cache manager 2740 2741[source,c] 2742---- 2743nl_cache_mngr_free(mngr); 2744---- 2745 2746== Abstract Data Types 2747 2748A few high level abstract data types which are used by a majority netlink 2749protocols are implemented in the core library. More may be added in the 2750future if the need arises. 2751 2752=== Abstract Address 2753 2754Most netlink protocols deal with networking related topics and thus 2755dealing with network addresses is a common task. 2756 2757Currently the following address families are supported: 2758 2759[options="compact"] 2760 * `AF_INET` 2761 * `AF_INET6` 2762 * `AF_LLC` 2763 * `AF_DECnet` 2764 * `AF_UNSPEC` 2765 2766[[core_addr_alloc]] 2767.Address Allocation 2768 2769The function nl_addr_alloc() allocates a new empty address. The 2770+maxsize+ argument defines the maximum length of an address in bytes. 2771The size of an address is address family specific. If the address 2772family and address data are known at allocation time the function 2773nl_addr_build() can be used alternatively. You may also clone 2774an address by calling nl_addr_clone() 2775 2776[source,c] 2777-------- 2778#include <netlink/addr.h> 2779 2780struct nl_addr *nl_addr_alloc(size_t maxsize); 2781struct nl_addr *nl_addr_clone(struct nl_addr *addr); 2782struct nl_addr *nl_addr_build(int family, void *addr, size_t size); 2783-------- 2784 2785If the address is transported in a netlink attribute, the function 2786nl_addr_alloc_attr() allocates a new address based on the payload 2787of the attribute provided. The +family+ argument is used to specify 2788the address family of the address, set to +AF_UNSPEC+ if unknown. 2789 2790[source,c] 2791-------- 2792#include <netlink/addr.h> 2793 2794struct nl_addr *nl_addr_alloc_attr(struct nlattr *attr, int family); 2795-------- 2796 2797If the address is provided by a user, it is usually stored in a human 2798readable format. The function nl_addr_parse() parses a character 2799string representing an address and allocates a new address based on 2800it. 2801 2802[source,c] 2803-------- 2804#include <netlink/addr.h> 2805 2806int nl_addr_parse(const char *addr, int hint, struct nl_addr **result); 2807-------- 2808 2809If parsing succeeds the function returns 0 and the allocated address 2810is stored in +*result+. 2811 2812NOTE: Make sure to return the reference to an address using 2813 `nl_addr_put()` after usage to allow memory being freed. 2814 2815.Example: Transform character string to abstract address 2816[source,c] 2817----- 2818struct nl_addr *a = nl_addr_parse("::1", AF_UNSPEC); 2819printf("Address family: %s\n", nl_af2str(nl_addr_get_family(a))); 2820nl_addr_put(a); 2821a = nl_addr_parse("11:22:33:44:55:66", AF_UNSPEC); 2822printf("Address family: %s\n", nl_af2str(nl_addr_get_family(a))); 2823nl_addr_put(a); 2824----- 2825 2826.Address References 2827 2828Abstract addresses use reference counting to account for all users of 2829a particular address. After the last user has returned the reference 2830the address is freed. 2831 2832If you pass on a address object to another function and you are not 2833sure how long it will be used, make sure to call nl_addr_get() to 2834acquire an additional reference and have that function or code path 2835call nl_addr_put() as soon as it has finished using the address. 2836 2837[source,c] 2838-------- 2839#include <netlink/addr.h> 2840 2841struct nl_addr *nl_addr_get(struct nl_addr *addr); 2842void nl_addr_put(struct nl_addr *addr); 2843int nl_addr_shared(struct nl_addr *addr); 2844-------- 2845 2846You may call nl_addr_shared() at any time to check if you are the only 2847user of an address. 2848 2849.Address Attributes 2850 2851The address is usually set at allocation time. If it was unknown at that 2852time it can be specified later by calling nl_addr_set_family() and is 2853accessed with the function nl_addr_get_family(). 2854 2855[source,c] 2856-------- 2857#include <netlink/addr.h> 2858 2859void nl_addr_set_family(struct nl_addr *addr, int family); 2860int nl_addr_get_family(struct nl_addr *addr); 2861-------- 2862 2863The same is true for the actual address data. It is typically present 2864at allocation time. For exceptions it can be specified later or 2865overwritten with the function `nl_addr_set_binary_addr()`. Beware that 2866the length of the address may not exceed `maxlen` specified at 2867allocation time. The address data is returned by the function 2868`nl_addr_get_binary_addr()` and its length by the function 2869`nl_addr_get_len()`. 2870 2871[source,c] 2872-------- 2873#include <netlink/addr.h> 2874 2875int nl_addr_set_binary_addr(struct nl_addr *addr, void *data, size_t size); 2876void *nl_addr_get_binary_addr(struct nl_addr *addr); 2877unsigned int nl_addr_get_len(struct nl_addr *addr); 2878-------- 2879 2880If you only want to check if the address data consists of all zeros 2881the function `nl_addr_iszero()` is a shortcut to that. 2882 2883[source,c] 2884-------- 2885#include <netlink/addr.h> 2886 2887int nl_addr_iszero(struct nl_addr *addr); 2888-------- 2889 2890==== Address Prefix Length 2891 2892Although this functionality is somewhat specific to routing it has 2893been implemented here. Addresses can have a prefix length assigned 2894which implies that only the first n bits are of importance. This is 2895f.e. used to implement subnets. 2896 2897Use set functions `nl_addr_set_prefixlen()` and 2898`nl_addr_get_prefixlen()` to work with prefix lengths. 2899 2900[source,c] 2901-------- 2902#include <netlink/addr.h> 2903 2904void nl_addr_set_prefixlen(struct nl_addr *addr, int n); 2905unsigned int nl_addr_get_prefixlen(struct nl_addr *addr); 2906-------- 2907 2908NOTE: The default prefix length is set to (address length * 8) 2909 2910.Address Helpers 2911 2912Several functions exist to help when dealing with addresses. The 2913function `nl_addr_cmp()` compares two addresses and returns an integer 2914less than, equal to or greater than zero without considering the 2915prefix length at all. If you want to consider the prefix length, use 2916the function `nl_addr_cmp_prefix()`. 2917 2918[source,c] 2919-------- 2920#include <netlink/addr.h> 2921 2922int nl_addr_cmp(struct nl_addr *addr, struct nl_addr *addr); 2923int nl_addr_cmp_prefix(struct nl_addr *addr, struct nl_addr *addr); 2924-------- 2925 2926If an abstract address needs to presented to the user it should be 2927done in a human readable format which differs depending on the address 2928family. The function `nl_addr2str()` takes care of this by calling the 2929appropriate conversion functions internaly. It expects a `buf` of 2930length `size` to write the character string into and returns a pointer 2931to `buf` for easy `printf()` usage. 2932 2933[source,c] 2934-------- 2935#include <netlink/addr.h> 2936 2937char *nl_addr2str(struct nl_addr *addr, char *buf, size_t size); 2938-------- 2939 2940If the address family is unknown, the address data will be printed in 2941hexadecimal format `AA:BB:CC:DD:...` 2942 2943Often the only way to figure out the address family is by looking at 2944the length of the address. The function `nl_addr_guess_family()` does 2945just this and returns the address family guessed based on the address 2946size. 2947 2948[source,c] 2949-------- 2950#include <netlink/addr.h> 2951 2952int nl_addr_guess_family(struct nl_addr *addr); 2953-------- 2954 2955Before allocating an address you may want to check if the character 2956string actually represents a valid address of the address family you 2957are expecting. The function `nl_addr_valid()` can be used for that, it 2958returns 1 if the supplised `addr` is a valid address in the context of 2959`family`. See `inet_pton(3)`, `dnet_pton(3)` for more information on 2960valid adddress formats. 2961 2962[source,c] 2963-------- 2964#include <netlink/addr.h> 2965 2966int nl_addr_valid(char *addr, int family); 2967-------- 2968 2969=== Abstract Data 2970 2971The abstract data type is a trivial datatype with the primary purpose 2972to simplify usage of netlink attributes of arbitary length. 2973 2974[[core_data_alloc]] 2975.Allocation of a Data Object 2976The function `nl_data_alloc()` alloctes a new abstract data object and 2977fill it with the provided data. `nl_data_alloc_attr()` does the same 2978but bases the data on the payload of a netlink attribute. New data 2979objects can also be allocated by cloning existing ones by using 2980`nl_data_clone()`. 2981 2982[source,c] 2983-------- 2984struct nl_data *nl_data_alloc(void *buf, size_t size); 2985struct nl_data *nl_data_alloc_attr(struct nlattr *attr); 2986struct nl_data *nl_data_clone(struct nl_data *data); 2987void nl_data_free(struct nl_data *data); 2988-------- 2989 2990.Access to Data 2991 2992The function `nl_data_get()` returns a pointer to the data, the size 2993of data is returned by `nl_data_get_size()`. 2994 2995[source,c] 2996-------- 2997void *nl_data_get(struct nl_data *data); 2998size_t nl_data_get_size(struct nl_data *data); 2999-------- 3000 3001.Data Helpers 3002 3003The function nl_data_append() reallocates the internal data buffers 3004and appends the specified `buf` to the existing data. 3005 3006[source,c] 3007-------- 3008int nl_data_append(struct nl_data *data, void *buf, size_t size); 3009-------- 3010 3011CAUTION: Any call to `nl_data_append()` invalidates all pointers 3012 returned by `nl_data_get()` of the same data object. 3013 3014[source,c] 3015-------- 3016int nl_data_cmp(struct nl_data *data, struct nl_data *data); 3017-------- 3018
