1.\" $OpenBSD: ssh-keyscan.1,v 1.34 2014/01/28 14:13:39 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 $Mdocdate: January 28 2014 $ 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.Dq addrlist namelist 60pairs from 61.Ar file , 62one per line. 63If 64.Pa - 65is supplied instead of a filename, 66.Nm 67will read hosts or 68.Dq addrlist namelist 69pairs from the standard input. 70.It Fl H 71Hash all hostnames and addresses in the output. 72Hashed names may be used normally by 73.Nm ssh 74and 75.Nm sshd , 76but they do not reveal identifying information should the file's contents 77be disclosed. 78.It Fl p Ar port 79Port to connect to on the remote host. 80.It Fl T Ar timeout 81Set the timeout for connection attempts. 82If 83.Ar timeout 84seconds have elapsed since a connection was initiated to a host or since the 85last time anything was read from that host, then the connection is 86closed and the host in question considered unavailable. 87Default is 5 seconds. 88.It Fl t Ar type 89Specifies the type of the key to fetch from the scanned hosts. 90The possible values are 91.Dq rsa1 92for protocol version 1 and 93.Dq dsa , 94.Dq ecdsa , 95.Dq ed25519 , 96or 97.Dq rsa 98for protocol version 2. 99Multiple values may be specified by separating them with commas. 100The default is to fetch 101.Dq rsa 102and 103.Dq ecdsa 104keys. 105.It Fl v 106Verbose mode. 107Causes 108.Nm 109to print debugging messages about its progress. 110.El 111.Sh SECURITY 112If an ssh_known_hosts file is constructed using 113.Nm 114without verifying the keys, users will be vulnerable to 115.Em man in the middle 116attacks. 117On the other hand, if the security model allows such a risk, 118.Nm 119can help in the detection of tampered keyfiles or man in the middle 120attacks which have begun after the ssh_known_hosts file was created. 121.Sh FILES 122Input format: 123.Bd -literal 1241.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4 125.Ed 126.Pp 127Output format for rsa1 keys: 128.Bd -literal 129host-or-namelist bits exponent modulus 130.Ed 131.Pp 132Output format for rsa, dsa and ecdsa keys: 133.Bd -literal 134host-or-namelist keytype base64-encoded-key 135.Ed 136.Pp 137Where 138.Ar keytype 139is either 140.Dq ecdsa-sha2-nistp256 , 141.Dq ecdsa-sha2-nistp384 , 142.Dq ecdsa-sha2-nistp521 , 143.Dq ssh-ed25519 , 144.Dq ssh-dss 145or 146.Dq ssh-rsa . 147.Pp 148.Pa /etc/ssh/ssh_known_hosts 149.Sh EXAMPLES 150Print the rsa host key for machine 151.Ar hostname : 152.Bd -literal 153$ ssh-keyscan hostname 154.Ed 155.Pp 156Find all hosts from the file 157.Pa ssh_hosts 158which have new or different keys from those in the sorted file 159.Pa ssh_known_hosts : 160.Bd -literal 161$ ssh-keyscan -t rsa,dsa,ecdsa -f ssh_hosts | \e 162 sort -u - ssh_known_hosts | diff ssh_known_hosts - 163.Ed 164.Sh SEE ALSO 165.Xr ssh 1 , 166.Xr sshd 8 167.Sh AUTHORS 168.An -nosplit 169.An David Mazieres Aq Mt dm@lcs.mit.edu 170wrote the initial version, and 171.An Wayne Davison Aq Mt wayned@users.sourceforge.net 172added support for protocol version 2. 173.Sh BUGS 174It generates "Connection closed by remote host" messages on the consoles 175of all the machines it scans if the server is older than version 2.9. 176This is because it opens a connection to the ssh port, reads the public 177key, and drops the connection as soon as it gets the key. 178