1.\" $NetBSD: ssh-keyscan.1,v 1.11 2016/03/11 01:55:00 christos Exp $ 2.\" $OpenBSD: ssh-keyscan.1,v 1.38 2015/11/08 23:24:03 jmc Exp $ 3.\" 4.\" Copyright 1995, 1996 by David Mazieres <dm@lcs.mit.edu>. 5.\" 6.\" Modification and redistribution in source and binary forms is 7.\" permitted provided that due credit is given to the author and the 8.\" OpenBSD project by leaving this copyright notice intact. 9.\" 10.Dd November 8 2015 11.Dt SSH-KEYSCAN 1 12.Os 13.Sh NAME 14.Nm ssh-keyscan 15.Nd gather ssh public keys 16.Sh SYNOPSIS 17.Nm ssh-keyscan 18.Bk -words 19.Op Fl 46cHv 20.Op Fl f Ar file 21.Op Fl p Ar port 22.Op Fl T Ar timeout 23.Op Fl t Ar type 24.Op Ar host | addrlist namelist 25.Ar ... 26.Ek 27.Sh DESCRIPTION 28.Nm 29is a utility for gathering the public ssh host keys of a number of 30hosts. 31It was designed to aid in building and verifying 32.Pa ssh_known_hosts 33files. 34.Nm 35provides a minimal interface suitable for use by shell and perl 36scripts. 37.Pp 38.Nm 39uses non-blocking socket I/O to contact as many hosts as possible in 40parallel, so it is very efficient. 41The keys from a domain of 1,000 42hosts can be collected in tens of seconds, even when some of those 43hosts are down or do not run ssh. 44For scanning, one does not need 45login access to the machines that are being scanned, nor does the 46scanning process involve any encryption. 47.Pp 48The options are as follows: 49.Bl -tag -width Ds 50.It Fl 4 51Forces 52.Nm 53to use IPv4 addresses only. 54.It Fl 6 55Forces 56.Nm 57to use IPv6 addresses only. 58.It Fl c 59Request certificates from target hosts instead of plain keys. 60.It Fl f Ar file 61Read hosts or 62.Dq addrlist namelist 63pairs from 64.Ar file , 65one per line. 66If 67.Pa - 68is supplied instead of a filename, 69.Nm 70will read hosts or 71.Dq addrlist namelist 72pairs from the standard input. 73.It Fl H 74Hash all hostnames and addresses in the output. 75Hashed names may be used normally by 76.Nm ssh 77and 78.Nm sshd , 79but they do not reveal identifying information should the file's contents 80be disclosed. 81.It Fl p Ar port 82Port to connect to on the remote host. 83.It Fl T Ar timeout 84Set the timeout for connection attempts. 85If 86.Ar timeout 87seconds have elapsed since a connection was initiated to a host or since the 88last time anything was read from that host, then the connection is 89closed and the host in question considered unavailable. 90Default is 5 seconds. 91.It Fl t Ar type 92Specifies the type of the key to fetch from the scanned hosts. 93The possible values are 94.Dq rsa1 95for protocol version 1 and 96.Dq dsa , 97.Dq ecdsa , 98.Dq ed25519 , 99or 100.Dq rsa 101for protocol version 2. 102Multiple values may be specified by separating them with commas. 103The default is to fetch 104.Dq rsa , 105.Dq ecdsa , 106and 107.Dq ed25519 108keys. 109.It Fl v 110Verbose mode. 111Causes 112.Nm 113to print debugging messages about its progress. 114.El 115.Sh SECURITY 116If an ssh_known_hosts file is constructed using 117.Nm 118without verifying the keys, users will be vulnerable to 119.Em man in the middle 120attacks. 121On the other hand, if the security model allows such a risk, 122.Nm 123can help in the detection of tampered keyfiles or man in the middle 124attacks which have begun after the ssh_known_hosts file was created. 125.Sh FILES 126Input format: 127.Bd -literal 1281.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4 129.Ed 130.Pp 131Output format for RSA1 keys: 132.Bd -literal 133host-or-namelist bits exponent modulus 134.Ed 135.Pp 136Output format for RSA, DSA, ECDSA, and Ed25519 keys: 137.Bd -literal 138host-or-namelist keytype base64-encoded-key 139.Ed 140.Pp 141Where 142.Ar keytype 143is either 144.Dq ecdsa-sha2-nistp256 , 145.Dq ecdsa-sha2-nistp384 , 146.Dq ecdsa-sha2-nistp521 , 147.Dq ssh-ed25519 , 148.Dq ssh-dss 149or 150.Dq ssh-rsa . 151.Pp 152.Pa /etc/ssh/ssh_known_hosts 153.Sh EXAMPLES 154Print the rsa host key for machine 155.Ar hostname : 156.Bd -literal 157$ ssh-keyscan hostname 158.Ed 159.Pp 160Find all hosts from the file 161.Pa ssh_hosts 162which have new or different keys from those in the sorted file 163.Pa ssh_known_hosts : 164.Bd -literal 165$ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \e 166 sort -u - ssh_known_hosts | diff ssh_known_hosts - 167.Ed 168.Sh SEE ALSO 169.Xr ssh 1 , 170.Xr sshd 8 171.Sh AUTHORS 172.An -nosplit 173.An David Mazieres Aq Mt dm@lcs.mit.edu 174wrote the initial version, and 175.An Wayne Davison Aq Mt wayned@users.sourceforge.net 176added support for protocol version 2. 177.Sh BUGS 178It generates "Connection closed by remote host" messages on the consoles 179of all the machines it scans if the server is older than version 2.9. 180This is because it opens a connection to the ssh port, reads the public 181key, and drops the connection as soon as it gets the key. 182