1.\" $OpenBSD: ssh-keyscan.1,v 1.40 2017/05/02 17:04:09 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: May 2 2017 $ 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 dsa , 94.Dq ecdsa , 95.Dq ed25519 , 96or 97.Dq rsa . 98Multiple values may be specified by separating them with commas. 99The default is to fetch 100.Dq rsa , 101.Dq ecdsa , 102and 103.Dq ed25519 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 RSA, DSA, ECDSA, and Ed25519 keys: 128.Bd -literal 129host-or-namelist keytype base64-encoded-key 130.Ed 131.Pp 132Where 133.Ar keytype 134is either 135.Dq ecdsa-sha2-nistp256 , 136.Dq ecdsa-sha2-nistp384 , 137.Dq ecdsa-sha2-nistp521 , 138.Dq ssh-ed25519 , 139.Dq ssh-dss 140or 141.Dq ssh-rsa . 142.Pp 143.Pa /etc/ssh/ssh_known_hosts 144.Sh EXAMPLES 145Print the rsa host key for machine 146.Ar hostname : 147.Bd -literal 148$ ssh-keyscan hostname 149.Ed 150.Pp 151Find all hosts from the file 152.Pa ssh_hosts 153which have new or different keys from those in the sorted file 154.Pa ssh_known_hosts : 155.Bd -literal 156$ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \e 157 sort -u - ssh_known_hosts | diff ssh_known_hosts - 158.Ed 159.Sh SEE ALSO 160.Xr ssh 1 , 161.Xr sshd 8 162.Sh AUTHORS 163.An -nosplit 164.An David Mazieres Aq Mt dm@lcs.mit.edu 165wrote the initial version, and 166.An Wayne Davison Aq Mt wayned@users.sourceforge.net 167added support for protocol version 2. 168.Sh BUGS 169It generates "Connection closed by remote host" messages on the consoles 170of all the machines it scans if the server is older than version 2.9. 171This is because it opens a connection to the ssh port, reads the public 172key, and drops the connection as soon as it gets the key. 173