1.\" $OpenBSD: ssh-keyscan.1,v 1.14 2002/02/13 08:33:47 mpech 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.Op Fl v46 18.Op Fl p Ar port 19.Op Fl T Ar timeout 20.Op Fl t Ar type 21.Op Fl f Ar file 22.Op Ar host | addrlist namelist 23.Op Ar ... 24.Sh DESCRIPTION 25.Nm 26is a utility for gathering the public ssh host keys of a number of 27hosts. It was designed to aid in building and verifying 28.Pa ssh_known_hosts 29files. 30.Nm 31provides a minimal interface suitable for use by shell and perl 32scripts. 33.Pp 34.Nm 35uses non-blocking socket I/O to contact as many hosts as possible in 36parallel, so it is very efficient. The keys from a domain of 1,000 37hosts can be collected in tens of seconds, even when some of those 38hosts are down or do not run ssh. For scanning, one does not need 39login access to the machines that are being scanned, nor does the 40scanning process involve any encryption. 41.Pp 42The options are as follows: 43.Bl -tag -width Ds 44.It Fl p Ar port 45Port to connect to on the remote host. 46.It Fl T Ar timeout 47Set the timeout for connection attempts. If 48.Pa timeout 49seconds have elapsed since a connection was initiated to a host or since the 50last time anything was read from that host, then the connection is 51closed and the host in question considered unavailable. Default is 5 52seconds. 53.It Fl t Ar type 54Specifies the type of the key to fetch from the scanned hosts. 55The possible values are 56.Dq rsa1 57for protocol version 1 and 58.Dq rsa 59or 60.Dq dsa 61for protocol version 2. 62Multiple values may be specified by separating them with commas. 63The default is 64.Dq rsa1 . 65.It Fl f Ar filename 66Read hosts or 67.Pa addrlist namelist 68pairs from this file, one per line. 69If 70.Pa - 71is supplied instead of a filename, 72.Nm 73will read hosts or 74.Pa addrlist namelist 75pairs from the standard input. 76.It Fl v 77Verbose mode. 78Causes 79.Nm 80to print debugging messages about its progress. 81.It Fl 4 82Forces 83.Nm 84to use IPv4 addresses only. 85.It Fl 6 86Forces 87.Nm 88to use IPv6 addresses only. 89.El 90.Sh SECURITY 91If a ssh_known_hosts file is constructed using 92.Nm 93without verifying the keys, users will be vulnerable to 94.I man in the middle 95attacks. 96On the other hand, if the security model allows such a risk, 97.Nm 98can help in the detection of tampered keyfiles or man in the middle 99attacks which have begun after the ssh_known_hosts file was created. 100.Sh EXAMPLES 101.Pp 102Print the 103.Pa rsa1 104host key for machine 105.Pa hostname : 106.Bd -literal 107$ ssh-keyscan hostname 108.Ed 109.Pp 110Find all hosts from the file 111.Pa ssh_hosts 112which have new or different keys from those in the sorted file 113.Pa ssh_known_hosts : 114.Bd -literal 115$ ssh-keyscan -t rsa,dsa -f ssh_hosts | \e\ 116 sort -u - ssh_known_hosts | diff ssh_known_hosts - 117.Ed 118.Sh FILES 119.Pa Input format: 120.Bd -literal 1211.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4 122.Ed 123.Pp 124.Pa Output format for rsa1 keys: 125.Bd -literal 126host-or-namelist bits exponent modulus 127.Ed 128.Pp 129.Pa Output format for rsa and dsa keys: 130.Bd -literal 131host-or-namelist keytype base64-encoded-key 132.Ed 133.Pp 134Where 135.Pa keytype 136is either 137.Dq ssh-rsa 138or 139.Dq ssh-dsa . 140.Pp 141.Pa /etc/ssh/ssh_known_hosts 142.Sh BUGS 143It generates "Connection closed by remote host" messages on the consoles 144of all the machines it scans if the server is older than version 2.9. 145This is because it opens a connection to the ssh port, reads the public 146key, and drops the connection as soon as it gets the key. 147.Sh SEE ALSO 148.Xr ssh 1 , 149.Xr sshd 8 150.Sh AUTHORS 151David Mazieres <dm@lcs.mit.edu> 152wrote the initial version, and 153Wayne Davison <wayned@users.sourceforge.net> 154added support for protocol version 2. 155