1.\" $OpenBSD: ssh-keyscan.1,v 1.35 2014/03/12 13:06:59 naddy 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: March 12 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 , 102.Dq ecdsa , 103and 104.Dq ed25519 105keys. 106.It Fl v 107Verbose mode. 108Causes 109.Nm 110to print debugging messages about its progress. 111.El 112.Sh SECURITY 113If an ssh_known_hosts file is constructed using 114.Nm 115without verifying the keys, users will be vulnerable to 116.Em man in the middle 117attacks. 118On the other hand, if the security model allows such a risk, 119.Nm 120can help in the detection of tampered keyfiles or man in the middle 121attacks which have begun after the ssh_known_hosts file was created. 122.Sh FILES 123Input format: 124.Bd -literal 1251.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4 126.Ed 127.Pp 128Output format for RSA1 keys: 129.Bd -literal 130host-or-namelist bits exponent modulus 131.Ed 132.Pp 133Output format for RSA, DSA, ECDSA, and ED25519 keys: 134.Bd -literal 135host-or-namelist keytype base64-encoded-key 136.Ed 137.Pp 138Where 139.Ar keytype 140is either 141.Dq ecdsa-sha2-nistp256 , 142.Dq ecdsa-sha2-nistp384 , 143.Dq ecdsa-sha2-nistp521 , 144.Dq ssh-ed25519 , 145.Dq ssh-dss 146or 147.Dq ssh-rsa . 148.Pp 149.Pa /etc/ssh/ssh_known_hosts 150.Sh EXAMPLES 151Print the rsa host key for machine 152.Ar hostname : 153.Bd -literal 154$ ssh-keyscan hostname 155.Ed 156.Pp 157Find all hosts from the file 158.Pa ssh_hosts 159which have new or different keys from those in the sorted file 160.Pa ssh_known_hosts : 161.Bd -literal 162$ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \e 163 sort -u - ssh_known_hosts | diff ssh_known_hosts - 164.Ed 165.Sh SEE ALSO 166.Xr ssh 1 , 167.Xr sshd 8 168.Sh AUTHORS 169.An -nosplit 170.An David Mazieres Aq Mt dm@lcs.mit.edu 171wrote the initial version, and 172.An Wayne Davison Aq Mt wayned@users.sourceforge.net 173added support for protocol version 2. 174.Sh BUGS 175It generates "Connection closed by remote host" messages on the consoles 176of all the machines it scans if the server is older than version 2.9. 177This is because it opens a connection to the ssh port, reads the public 178key, and drops the connection as soon as it gets the key. 179