1.\" $OpenBSD: ssh-keyscan.1,v 1.30 2012/04/11 13:34:17 djm 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: April 11 2012 $ 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.Pa addrlist namelist 60pairs from this file, one per line. 61If 62.Pa - 63is supplied instead of a filename, 64.Nm 65will read hosts or 66.Pa addrlist namelist 67pairs from the standard input. 68.It Fl H 69Hash all hostnames and addresses in the output. 70Hashed names may be used normally by 71.Nm ssh 72and 73.Nm sshd , 74but they do not reveal identifying information should the file's contents 75be disclosed. 76.It Fl p Ar port 77Port to connect to on the remote host. 78.It Fl T Ar timeout 79Set the timeout for connection attempts. 80If 81.Pa timeout 82seconds have elapsed since a connection was initiated to a host or since the 83last time anything was read from that host, then the connection is 84closed and the host in question considered unavailable. 85Default is 5 seconds. 86.It Fl t Ar type 87Specifies the type of the key to fetch from the scanned hosts. 88The possible values are 89.Dq rsa1 90for protocol version 1 and 91.Dq dsa , 92.Dq ecdsa 93or 94.Dq rsa 95for protocol version 2. 96Multiple values may be specified by separating them with commas. 97The default is to fetch 98.Dq rsa 99and 100.Dq ecdsa 101keys. 102.It Fl v 103Verbose mode. 104Causes 105.Nm 106to print debugging messages about its progress. 107.El 108.Sh SECURITY 109If an ssh_known_hosts file is constructed using 110.Nm 111without verifying the keys, users will be vulnerable to 112.Em man in the middle 113attacks. 114On the other hand, if the security model allows such a risk, 115.Nm 116can help in the detection of tampered keyfiles or man in the middle 117attacks which have begun after the ssh_known_hosts file was created. 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, dsa and ecdsa keys: 130.Bd -literal 131host-or-namelist keytype base64-encoded-key 132.Ed 133.Pp 134Where 135.Pa keytype 136is either 137.Dq ecdsa-sha2-nistp256 , 138.Dq ecdsa-sha2-nistp384 , 139.Dq ecdsa-sha2-nistp521 , 140.Dq ssh-dss 141or 142.Dq ssh-rsa . 143.Pp 144.Pa /etc/ssh/ssh_known_hosts 145.Sh EXAMPLES 146Print the 147.Pa rsa 148host key for machine 149.Pa hostname : 150.Bd -literal 151$ ssh-keyscan hostname 152.Ed 153.Pp 154Find all hosts from the file 155.Pa ssh_hosts 156which have new or different keys from those in the sorted file 157.Pa ssh_known_hosts : 158.Bd -literal 159$ ssh-keyscan -t rsa,dsa,ecdsa -f ssh_hosts | \e 160 sort -u - ssh_known_hosts | diff ssh_known_hosts - 161.Ed 162.Sh SEE ALSO 163.Xr ssh 1 , 164.Xr sshd 8 165.Sh AUTHORS 166.An -nosplit 167.An David Mazieres Aq dm@lcs.mit.edu 168wrote the initial version, and 169.An Wayne Davison Aq wayned@users.sourceforge.net 170added support for protocol version 2. 171.Sh BUGS 172It generates "Connection closed by remote host" messages on the consoles 173of all the machines it scans if the server is older than version 2.9. 174This is because it opens a connection to the ssh port, reads the public 175key, and drops the connection as soon as it gets the key. 176