1.\" $OpenBSD: ssh-keyscan.1,v 1.15 2003/03/28 10:11:43 jmc 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 January 1, 1996 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 v46 19.Op Fl p Ar port 20.Op Fl T Ar timeout 21.Op Fl t Ar type 22.Op Fl f Ar file 23.Op Ar host | addrlist namelist 24.Op 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 p Ar port 50Port to connect to on the remote host. 51.It Fl T Ar timeout 52Set the timeout for connection attempts. 53If 54.Pa timeout 55seconds have elapsed since a connection was initiated to a host or since the 56last time anything was read from that host, then the connection is 57closed and the host in question considered unavailable. 58Default is 5 seconds. 59.It Fl t Ar type 60Specifies the type of the key to fetch from the scanned hosts. 61The possible values are 62.Dq rsa1 63for protocol version 1 and 64.Dq rsa 65or 66.Dq dsa 67for protocol version 2. 68Multiple values may be specified by separating them with commas. 69The default is 70.Dq rsa1 . 71.It Fl f Ar filename 72Read hosts or 73.Pa addrlist namelist 74pairs from this file, one per line. 75If 76.Pa - 77is supplied instead of a filename, 78.Nm 79will read hosts or 80.Pa addrlist namelist 81pairs from the standard input. 82.It Fl v 83Verbose mode. 84Causes 85.Nm 86to print debugging messages about its progress. 87.It Fl 4 88Forces 89.Nm 90to use IPv4 addresses only. 91.It Fl 6 92Forces 93.Nm 94to use IPv6 addresses only. 95.El 96.Sh SECURITY 97If a ssh_known_hosts file is constructed using 98.Nm 99without verifying the keys, users will be vulnerable to 100.I man in the middle 101attacks. 102On the other hand, if the security model allows such a risk, 103.Nm 104can help in the detection of tampered keyfiles or man in the middle 105attacks which have begun after the ssh_known_hosts file was created. 106.Sh EXAMPLES 107.Pp 108Print the 109.Pa rsa1 110host key for machine 111.Pa hostname : 112.Bd -literal 113$ ssh-keyscan hostname 114.Ed 115.Pp 116Find all hosts from the file 117.Pa ssh_hosts 118which have new or different keys from those in the sorted file 119.Pa ssh_known_hosts : 120.Bd -literal 121$ ssh-keyscan -t rsa,dsa -f ssh_hosts | \e\ 122 sort -u - ssh_known_hosts | diff ssh_known_hosts - 123.Ed 124.Sh FILES 125.Pa Input format: 126.Bd -literal 1271.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4 128.Ed 129.Pp 130.Pa Output format for rsa1 keys: 131.Bd -literal 132host-or-namelist bits exponent modulus 133.Ed 134.Pp 135.Pa Output format for rsa and dsa keys: 136.Bd -literal 137host-or-namelist keytype base64-encoded-key 138.Ed 139.Pp 140Where 141.Pa keytype 142is either 143.Dq ssh-rsa 144or 145.Dq ssh-dsa . 146.Pp 147.Pa /etc/ssh/ssh_known_hosts 148.Sh BUGS 149It generates "Connection closed by remote host" messages on the consoles 150of all the machines it scans if the server is older than version 2.9. 151This is because it opens a connection to the ssh port, reads the public 152key, and drops the connection as soon as it gets the key. 153.Sh SEE ALSO 154.Xr ssh 1 , 155.Xr sshd 8 156.Sh AUTHORS 157David Mazieres <dm@lcs.mit.edu> 158wrote the initial version, and 159Wayne Davison <wayned@users.sourceforge.net> 160added support for protocol version 2. 161