1.\" $OpenBSD: ssh-keyscan.1,v 1.38 2015/11/08 23:24:03 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: November 8 2015 $ 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 46cHv 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 c 58Request certificates from target hosts instead of plain keys. 59.It Fl f Ar file 60Read hosts or 61.Dq addrlist namelist 62pairs from 63.Ar file , 64one per line. 65If 66.Pa - 67is supplied instead of a filename, 68.Nm 69will read hosts or 70.Dq addrlist namelist 71pairs from the standard input. 72.It Fl H 73Hash all hostnames and addresses in the output. 74Hashed names may be used normally by 75.Nm ssh 76and 77.Nm sshd , 78but they do not reveal identifying information should the file's contents 79be disclosed. 80.It Fl p Ar port 81Port to connect to on the remote host. 82.It Fl T Ar timeout 83Set the timeout for connection attempts. 84If 85.Ar timeout 86seconds have elapsed since a connection was initiated to a host or since the 87last time anything was read from that host, then the connection is 88closed and the host in question considered unavailable. 89Default is 5 seconds. 90.It Fl t Ar type 91Specifies the type of the key to fetch from the scanned hosts. 92The possible values are 93.Dq rsa1 94for protocol version 1 and 95.Dq dsa , 96.Dq ecdsa , 97.Dq ed25519 , 98or 99.Dq rsa 100for protocol version 2. 101Multiple values may be specified by separating them with commas. 102The default is to fetch 103.Dq rsa , 104.Dq ecdsa , 105and 106.Dq ed25519 107keys. 108.It Fl v 109Verbose mode. 110Causes 111.Nm 112to print debugging messages about its progress. 113.El 114.Sh SECURITY 115If an ssh_known_hosts file is constructed using 116.Nm 117without verifying the keys, users will be vulnerable to 118.Em man in the middle 119attacks. 120On the other hand, if the security model allows such a risk, 121.Nm 122can help in the detection of tampered keyfiles or man in the middle 123attacks which have begun after the ssh_known_hosts file was created. 124.Sh FILES 125Input format: 126.Bd -literal 1271.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4 128.Ed 129.Pp 130Output format for RSA1 keys: 131.Bd -literal 132host-or-namelist bits exponent modulus 133.Ed 134.Pp 135Output format for RSA, DSA, ECDSA, and Ed25519 keys: 136.Bd -literal 137host-or-namelist keytype base64-encoded-key 138.Ed 139.Pp 140Where 141.Ar keytype 142is either 143.Dq ecdsa-sha2-nistp256 , 144.Dq ecdsa-sha2-nistp384 , 145.Dq ecdsa-sha2-nistp521 , 146.Dq ssh-ed25519 , 147.Dq ssh-dss 148or 149.Dq ssh-rsa . 150.Pp 151.Pa /etc/ssh/ssh_known_hosts 152.Sh EXAMPLES 153Print the rsa host key for machine 154.Ar hostname : 155.Bd -literal 156$ ssh-keyscan hostname 157.Ed 158.Pp 159Find all hosts from the file 160.Pa ssh_hosts 161which have new or different keys from those in the sorted file 162.Pa ssh_known_hosts : 163.Bd -literal 164$ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \e 165 sort -u - ssh_known_hosts | diff ssh_known_hosts - 166.Ed 167.Sh SEE ALSO 168.Xr ssh 1 , 169.Xr sshd 8 170.Sh AUTHORS 171.An -nosplit 172.An David Mazieres Aq Mt dm@lcs.mit.edu 173wrote the initial version, and 174.An Wayne Davison Aq Mt wayned@users.sourceforge.net 175added support for protocol version 2. 176.Sh BUGS 177It generates "Connection closed by remote host" messages on the consoles 178of all the machines it scans if the server is older than version 2.9. 179This is because it opens a connection to the ssh port, reads the public 180key, and drops the connection as soon as it gets the key. 181