1.\" $OpenBSD: ssh-keyscan.1,v 1.30 2012/04/11 13:34:17 djm Exp $ 2.\" $FreeBSD$ 3.\" 4.\" Copyright 1995, 1996 by David Mazieres <dm@lcs.mit.edu>. 5.\" 6.\" Modification and redistribution in source and binary forms is 7.\" permitted provided that due credit is given to the author and the 8.\" OpenBSD project by leaving this copyright notice intact. 9.\" 10.Dd April 11, 2012 11.Dt SSH-KEYSCAN 1 12.Os 13.Sh NAME 14.Nm ssh-keyscan 15.Nd gather ssh public keys 16.Sh SYNOPSIS 17.Nm ssh-keyscan 18.Bk -words 19.Op Fl 46Hv 20.Op Fl f Ar file 21.Op Fl p Ar port 22.Op Fl T Ar timeout 23.Op Fl t Ar type 24.Op Ar host | addrlist namelist 25.Ar ... 26.Ek 27.Sh DESCRIPTION 28.Nm 29is a utility for gathering the public ssh host keys of a number of 30hosts. 31It was designed to aid in building and verifying 32.Pa ssh_known_hosts 33files. 34.Nm 35provides a minimal interface suitable for use by shell and perl 36scripts. 37.Pp 38.Nm 39uses non-blocking socket I/O to contact as many hosts as possible in 40parallel, so it is very efficient. 41The keys from a domain of 1,000 42hosts can be collected in tens of seconds, even when some of those 43hosts are down or do not run ssh. 44For scanning, one does not need 45login access to the machines that are being scanned, nor does the 46scanning process involve any encryption. 47.Pp 48The options are as follows: 49.Bl -tag -width Ds 50.It Fl 4 51Forces 52.Nm 53to use IPv4 addresses only. 54.It Fl 6 55Forces 56.Nm 57to use IPv6 addresses only. 58.It Fl f Ar file 59Read hosts or 60.Pa addrlist namelist 61pairs from this file, one per line. 62If 63.Pa - 64is supplied instead of a filename, 65.Nm 66will read hosts or 67.Pa addrlist namelist 68pairs from the standard input. 69.It Fl H 70Hash all hostnames and addresses in the output. 71Hashed names may be used normally by 72.Nm ssh 73and 74.Nm sshd , 75but they do not reveal identifying information should the file's contents 76be disclosed. 77.It Fl p Ar port 78Port to connect to on the remote host. 79.It Fl T Ar timeout 80Set the timeout for connection attempts. 81If 82.Pa timeout 83seconds have elapsed since a connection was initiated to a host or since the 84last time anything was read from that host, then the connection is 85closed and the host in question considered unavailable. 86Default is 5 seconds. 87.It Fl t Ar type 88Specifies the type of the key to fetch from the scanned hosts. 89The possible values are 90.Dq rsa1 91for protocol version 1 and 92.Dq dsa , 93.Dq ecdsa 94or 95.Dq rsa 96for protocol version 2. 97Multiple values may be specified by separating them with commas. 98The default is to fetch 99.Dq rsa 100and 101.Dq ecdsa 102keys. 103.It Fl v 104Verbose mode. 105Causes 106.Nm 107to print debugging messages about its progress. 108.El 109.Sh SECURITY 110If an ssh_known_hosts file is constructed using 111.Nm 112without verifying the keys, users will be vulnerable to 113.Em man in the middle 114attacks. 115On the other hand, if the security model allows such a risk, 116.Nm 117can help in the detection of tampered keyfiles or man in the middle 118attacks which have begun after the ssh_known_hosts file was created. 119.Sh FILES 120.Pa Input format: 121.Bd -literal 1221.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4 123.Ed 124.Pp 125.Pa Output format for rsa1 keys: 126.Bd -literal 127host-or-namelist bits exponent modulus 128.Ed 129.Pp 130.Pa Output format for rsa, dsa and ecdsa keys: 131.Bd -literal 132host-or-namelist keytype base64-encoded-key 133.Ed 134.Pp 135Where 136.Pa keytype 137is either 138.Dq ecdsa-sha2-nistp256 , 139.Dq ecdsa-sha2-nistp384 , 140.Dq ecdsa-sha2-nistp521 , 141.Dq ssh-dss 142or 143.Dq ssh-rsa . 144.Pp 145.Pa /etc/ssh/ssh_known_hosts 146.Sh EXAMPLES 147Print the 148.Pa rsa 149host key for machine 150.Pa hostname : 151.Bd -literal 152$ ssh-keyscan hostname 153.Ed 154.Pp 155Find all hosts from the file 156.Pa ssh_hosts 157which have new or different keys from those in the sorted file 158.Pa ssh_known_hosts : 159.Bd -literal 160$ ssh-keyscan -t rsa,dsa,ecdsa -f ssh_hosts | \e 161 sort -u - ssh_known_hosts | diff ssh_known_hosts - 162.Ed 163.Sh SEE ALSO 164.Xr ssh 1 , 165.Xr sshd 8 166.Sh AUTHORS 167.An -nosplit 168.An David Mazieres Aq dm@lcs.mit.edu 169wrote the initial version, and 170.An Wayne Davison Aq wayned@users.sourceforge.net 171added support for protocol version 2. 172.Sh BUGS 173It generates "Connection closed by remote host" messages on the consoles 174of all the machines it scans if the server is older than version 2.9. 175This is because it opens a connection to the ssh port, reads the public 176key, and drops the connection as soon as it gets the key. 177