1.\" $OpenBSD: ssh-keyscan.1,v 1.29 2010/08/31 11:54:45 djm Exp $ 2.\" 3.\" Copyright 1995, 1996 by David Mazieres <dm@lcs.mit.edu>. 4.\" 5.\" Modification and redistribution in source and binary forms is 6.\" permitted provided that due credit is given to the author and the 7.\" OpenBSD project by leaving this copyright notice intact. 8.\" 9.Dd $Mdocdate: August 31 2010 $ 10.Dt SSH-KEYSCAN 1 11.Os 12.Sh NAME 13.Nm ssh-keyscan 14.Nd gather ssh public keys 15.Sh SYNOPSIS 16.Nm ssh-keyscan 17.Bk -words 18.Op Fl 46Hv 19.Op Fl f Ar file 20.Op Fl p Ar port 21.Op Fl T Ar timeout 22.Op Fl t Ar type 23.Op Ar host | addrlist namelist 24.Ar ... 25.Ek 26.Sh DESCRIPTION 27.Nm 28is a utility for gathering the public ssh host keys of a number of 29hosts. 30It was designed to aid in building and verifying 31.Pa ssh_known_hosts 32files. 33.Nm 34provides a minimal interface suitable for use by shell and perl 35scripts. 36.Pp 37.Nm 38uses non-blocking socket I/O to contact as many hosts as possible in 39parallel, so it is very efficient. 40The keys from a domain of 1,000 41hosts can be collected in tens of seconds, even when some of those 42hosts are down or do not run ssh. 43For scanning, one does not need 44login access to the machines that are being scanned, nor does the 45scanning process involve any encryption. 46.Pp 47The options are as follows: 48.Bl -tag -width Ds 49.It Fl 4 50Forces 51.Nm 52to use IPv4 addresses only. 53.It Fl 6 54Forces 55.Nm 56to use IPv6 addresses only. 57.It Fl f Ar file 58Read hosts or 59.Pa addrlist namelist 60pairs from this file, one per line. 61If 62.Pa - 63is supplied instead of a filename, 64.Nm 65will read hosts or 66.Pa addrlist namelist 67pairs from the standard input. 68.It Fl H 69Hash all hostnames and addresses in the output. 70Hashed names may be used normally by 71.Nm ssh 72and 73.Nm sshd , 74but they do not reveal identifying information should the file's contents 75be disclosed. 76.It Fl p Ar port 77Port to connect to on the remote host. 78.It Fl T Ar timeout 79Set the timeout for connection attempts. 80If 81.Pa timeout 82seconds have elapsed since a connection was initiated to a host or since the 83last time anything was read from that host, then the connection is 84closed and the host in question considered unavailable. 85Default is 5 seconds. 86.It Fl t Ar type 87Specifies the type of the key to fetch from the scanned hosts. 88The possible values are 89.Dq rsa1 90for protocol version 1 and 91.Dq dsa , 92.Dq ecdsa 93or 94.Dq rsa 95for protocol version 2. 96Multiple values may be specified by separating them with commas. 97The default is 98.Dq rsa . 99.It Fl v 100Verbose mode. 101Causes 102.Nm 103to print debugging messages about its progress. 104.El 105.Sh SECURITY 106If an ssh_known_hosts file is constructed using 107.Nm 108without verifying the keys, users will be vulnerable to 109.Em man in the middle 110attacks. 111On the other hand, if the security model allows such a risk, 112.Nm 113can help in the detection of tampered keyfiles or man in the middle 114attacks which have begun after the ssh_known_hosts file was created. 115.Sh FILES 116.Pa Input format: 117.Bd -literal 1181.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4 119.Ed 120.Pp 121.Pa Output format for rsa1 keys: 122.Bd -literal 123host-or-namelist bits exponent modulus 124.Ed 125.Pp 126.Pa Output format for rsa, dsa and ecdsa keys: 127.Bd -literal 128host-or-namelist keytype base64-encoded-key 129.Ed 130.Pp 131Where 132.Pa keytype 133is either 134.Dq ecdsa-sha2-nistp256 , 135.Dq ecdsa-sha2-nistp384 , 136.Dq ecdsa-sha2-nistp521 , 137.Dq ssh-dss 138or 139.Dq ssh-rsa . 140.Pp 141.Pa /etc/ssh/ssh_known_hosts 142.Sh EXAMPLES 143Print the 144.Pa rsa 145host key for machine 146.Pa hostname : 147.Bd -literal 148$ ssh-keyscan hostname 149.Ed 150.Pp 151Find all hosts from the file 152.Pa ssh_hosts 153which have new or different keys from those in the sorted file 154.Pa ssh_known_hosts : 155.Bd -literal 156$ ssh-keyscan -t rsa,dsa,ecdsa -f ssh_hosts | \e 157 sort -u - ssh_known_hosts | diff ssh_known_hosts - 158.Ed 159.Sh SEE ALSO 160.Xr ssh 1 , 161.Xr sshd 8 162.Sh AUTHORS 163.An -nosplit 164.An David Mazieres Aq dm@lcs.mit.edu 165wrote the initial version, and 166.An Wayne Davison Aq wayned@users.sourceforge.net 167added support for protocol version 2. 168.Sh BUGS 169It generates "Connection closed by remote host" messages on the consoles 170of all the machines it scans if the server is older than version 2.9. 171This is because it opens a connection to the ssh port, reads the public 172key, and drops the connection as soon as it gets the key. 173