1.\" $OpenBSD: ssh-keyscan.1,v 1.17 2003/06/10 09:12:11 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 FILES 107.Pa Input format: 108.Bd -literal 1091.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4 110.Ed 111.Pp 112.Pa Output format for rsa1 keys: 113.Bd -literal 114host-or-namelist bits exponent modulus 115.Ed 116.Pp 117.Pa Output format for rsa and dsa keys: 118.Bd -literal 119host-or-namelist keytype base64-encoded-key 120.Ed 121.Pp 122Where 123.Pa keytype 124is either 125.Dq ssh-rsa 126or 127.Dq ssh-dss . 128.Pp 129.Pa /etc/ssh/ssh_known_hosts 130.Sh EXAMPLES 131Print the 132.Pa rsa1 133host key for machine 134.Pa hostname : 135.Bd -literal 136$ ssh-keyscan hostname 137.Ed 138.Pp 139Find all hosts from the file 140.Pa ssh_hosts 141which have new or different keys from those in the sorted file 142.Pa ssh_known_hosts : 143.Bd -literal 144$ ssh-keyscan -t rsa,dsa -f ssh_hosts | \e 145 sort -u - ssh_known_hosts | diff ssh_known_hosts - 146.Ed 147.Sh SEE ALSO 148.Xr ssh 1 , 149.Xr sshd 8 150.Sh AUTHORS 151.An David Mazieres Aq dm@lcs.mit.edu 152wrote the initial version, and 153.An Wayne Davison Aq wayned@users.sourceforge.net 154added support for protocol version 2. 155.Sh BUGS 156It generates "Connection closed by remote host" messages on the consoles 157of all the machines it scans if the server is older than version 2.9. 158This is because it opens a connection to the ssh port, reads the public 159key, and drops the connection as soon as it gets the key. 160