man/man8/tc-pedit.8

     "Generic packet editor action in tc" 8 "12 Jan 2015" "iproute2" "Linux"

 NAME
pedit - generic packet editor action
 SYNOPSIS
 tc " ... " "action pedit [ex] munge " {  RAW_OP " | " LAYERED_OP " | " EXTENDED_LAYERED_OP " } [ " CONTROL " ]"
 RAW_OP " := "  offset " OFFSET"  "{ " u8 " | " u16 " | " u32 " } ["  AT_SPEC " ] " CMD_SPEC
 AT_SPEC " := "  at " AT " offmask " MASK " shift " SHIFT"
 LAYERED_OP " := { "  ip " IPHDR_FIELD" |
 ip " BEYOND_IPHDR_FIELD"  } " CMD_SPEC"
 EXTENDED_LAYERED_OP " := { "  eth " ETHHDR_FIELD" |
 ip " IPHDR_FIELD" |
 ip " EX_IPHDR_FIELD" |
 ip6 " IP6HDR_FIELD" |
 tcp " TCPHDR_FIELD" |
 udp " UDPHDR_FIELD"  } " CMD_SPEC"
 ETHHDR_FIELD " := { "  src " | " dst " | " type " }"
 IPHDR_FIELD " := { "  src " | " dst " | " tos " | " dsfield " | " ihl " | " protocol " |"  precedence " | " nofrag " | " firstfrag " | " ce " | " df " }"
 BEYOND_IPHDR_FIELD " := { "  dport " | " sport " | " icmp_type " | " icmp_code " }"
 EX_IPHDR_FIELD " := { "  ttl " }"

 IP6HDR_FIELD " := { "  src " | " dst " | " flow_lbl " | " payload_len " | " nexthdr " |"  hoplimit " }"
 TCPHDR_FIELD " := { "  sport " | " dport " | " flags " }"
 UDPHDR_FIELD " := { "  sport " | " dport " }"
 CMD_SPEC " := {"  clear " | " invert " | " set  VAL " | "  add  VAL " | "  preserve " } [ " retain  RVAL " ]"
 CONTROL " := {"  reclassify " | " pipe " | " drop " | " shot " | " continue " | " pass " | " goto " " chain " " CHAIN_INDEX " }"  DESCRIPTION
The
 pedit action can be used to change arbitrary packet data. The location of data to
change can either be specified by giving an offset and size as in
 RAW_OP , or for header values by naming the header and field to edit the size is then
chosen automatically based on the header field size. Currently this is supported
only for IPv4 headers.
 OPTIONS

 ex Use extended pedit.
 EXTENDED_LAYERED_OP and the add
 CMD_SPEC are allowed only in this mode.

 offset " OFFSET " "{ u32 | u16 | u8 }" Specify the offset at which to change data.
 OFFSET is a signed integer, it's base is automatically chosen (e.g. hex if prefixed by
 0x or octal if prefixed by
 0 ). The second argument specifies the length of data to change, that is four bytes
 ( u32 ), two bytes
 ( u16 ) or a single byte
 ( u8 ).
 at " AT " offmask " MASK " shift " SHIFT" This is an optional part of
 RAW_OP which allows to have a variable
 OFFSET depending on packet data at offset
 AT , which is binary ANDed with
 MASK and right-shifted by
 SHIFT before adding it to
 OFFSET .
 eth " ETHHDR_FIELD" Change an ETH header field. The supported keywords for
 ETHHDR_FIELD are:


 src .TQ
 dst Source or destination MAC address in the standard format: XX:XX:XX:XX:XX:XX

 type Ether-type in numeric value


 ip " IPHDR_FIELD" Change an IPv4 header field. The supported keywords for
 IPHDR_FIELD are:


 src .TQ
 dst Source or destination IP address, a four-byte value.

 tos .TQ
 dsfield .TQ
 precedence Type Of Service field, an eight-bit value.

 ihl Change the IP Header Length field, a four-bit value.

 protocol Next-layer Protocol field, an eight-bit value.

 nofrag .TQ
 firstfrag .TQ
 ce .TQ
 df .TQ
 mf Change IP header flags. Note that the value to pass to the
 set command is not just a bit value, but the full byte including the flags field.
Though only the relevant bits of that value are respected, the rest ignored.


 ip " BEYOND_IPHDR_FIELD" Supported only for non-extended layered op. It is passed to the kernel as
offsets relative to the beginning of the IP header and assumes the IP header is
of minimum size (20 bytes). The supported keywords for
 BEYOND_IPHDR_FIELD are:


 dport .TQ
 sport Destination or source port numbers, a 16-bit value. Indeed, IPv4 headers don't
contain this information. Instead, this will set an offset which suits at least
TCP and UDP if the IP header is of minimum size (20 bytes). If not, this will do
unexpected things.

 icmp_type .TQ
 icmp_code Again, this allows to change data past the actual IP header itself. It assumes
an ICMP header is present immediately following the (minimal sized) IP header.
If it is not or the latter is bigger than the minimum of 20 bytes, this will do
unexpected things. These fields are eight-bit values.


 ip " EX_IPHDR_FIELD" Supported only when
 ex is used. The supported keywords for
 EX_IPHDR_FIELD are:


 ttl


 ip6 " IP6HDR_FIELD" The supported keywords for
 IP6HDR_FIELD are:


 src .TQ
 dst .TQ
 flow_lbl .TQ
 payload_len .TQ
 nexthdr .TQ
 hoplimit


 tcp " TCPHDR_FIELD" The supported keywords for
 TCPHDR_FIELD are:


 sport .TQ
 dport Source or destination TCP port number, a 16-bit value.

 flags


 udp " UDPHDR_FIELD" The supported keywords for
 UDPHDR_FIELD are:


 sport .TQ
 dport Source or destination TCP port number, a 16-bit value.


 clear Clear the addressed data (i.e., set it to zero).

 invert Swap every bit in the addressed data.

 set " VAL" Set the addressed data to a specific value. The size of
 VAL is defined by either one of the
 u32 ", " u16 " or " u8 keywords in
 RAW_OP , or the size of the addressed header field in
 LAYERED_OP .
 add " VAL" Add the addressed data by a specific value. The size of
 VAL is defined by the size of the addressed header field in
 EXTENDED_LAYERED_OP . This operation is supported only for extended layered op.

 preserve Keep the addressed data as is.

 retain " RVAL" This optional extra part of
 CMD_SPEC allows to exclude bits from being changed. Supported only for 32 bits fields
or smaller.

 CONTROL The following keywords allow to control how the tree of qdisc, classes,
filters and actions is further traversed after this action.


 reclassify Restart with the first filter in the current list.

 pipe Continue with the next action attached to the same filter.

 drop .TQ
 shot Drop the packet.

 continue Continue classification with the next filter in line.

 pass Finish classification process and return to calling qdisc for further packet
processing. This is the default.

 EXAMPLES
Being able to edit packet data, one could do all kinds of things, such as e.g.
implementing port redirection. Certainly not the most useful application, but
as an example it should do:

First, qdiscs need to be set up to attach filters to. For the receive path, a simple
 ingress qdisc will do, for transmit path a classful qdisc
 ( HTB in this case) is necessary:


.EX
tc qdisc replace dev eth0 root handle 1: htb
tc qdisc add dev eth0 ingress handle ffff:


Finally, a filter with
 pedit action can be added for each direction. In this case,
 u32 is used matching on the port number to redirect from, while
 pedit then does the actual rewriting:


.EX
tc filter add dev eth0 parent 1: u32 \\
 match ip dport 23 0xffff \\
 action pedit pedit munge ip dport set 22
tc filter add dev eth0 parent ffff: u32 \\
 match ip sport 22 0xffff \\
 action pedit pedit munge ip sport set 23
tc filter add dev eth0 parent ffff: u32 \\
 match ip sport 22 0xffff \\
 action pedit ex munge ip dst set 192.168.1.199
tc filter add dev eth0 parent ffff: u32 \\
 match ip sport 22 0xffff \\
 action pedit ex munge ip6 dst set fe80::dacb:8aff:fec7:320e
tc filter add dev eth0 parent ffff: u32 \\
 match ip sport 22 0xffff \\
 action pedit ex munge eth dst set 11:22:33:44:55:66
tc filter add dev eth0 parent ffff: u32 \\
 match ip dport 23 0xffff \\
 action pedit ex munge tcp dport set 22


 SEE ALSO
 tc (8),  tc-htb (8),  tc-u32 (8)