ssh-keyscan.1   [plain text]

.\"	$OpenBSD: ssh-keyscan.1,v 1.5 2001/04/18 16:21:05 ian Exp $
.\" Copyright 1995, 1996 by David Mazieres <>.
.\" Modification and redistribution in source and binary forms is
.\" permitted provided that due credit is given to the author and the
.\" OpenBSD project (for instance by leaving this copyright notice
.\" intact).
.Dd January 1, 1996
.Nm ssh-keyscan
.Nd gather ssh public keys
.Nm ssh-keyscan
.Op Fl t Ar timeout
.Op Ar -- | host | addrlist namelist
.Op Fl f Ar files ...
is a utility for gathering the public ssh host keys of a number of
hosts.  It was designed to aid in building and verifying
.Pa ssh_known_hosts
provides a minimal interface suitable for use by shell and perl
uses non-blocking socket I/O to contact as many hosts as possible in
parallel, so it is very efficient.  The keys from a domain of 1,000
hosts can be collected in tens of seconds, even when some of those
hosts are down or do not run ssh.  You do not need login access to the
machines you are scanning, nor does the scanning process involve
any encryption.
If you make an ssh_known_hosts file using
without verifying the keys, you will be vulnerable to
.I man in the middle
On the other hand, if your security model allows such a risk,
can help you detect tampered keyfiles or man in the middle attacks which
have begun after you created your ssh_known_hosts file.
.Bl -tag -width Ds
.It Fl t
Set the timeout for connection attempts.  If 
.Pa timeout
seconds have elapsed since a connection was initiated to a host or since the
last time anything was read from that host, then the connection is
closed and the host in question considered unavailable.  Default is 5
.It Fl f
Read hosts or 
.Pa addrlist namelist
pairs from this file, one per line.
.Pa -
is supplied instead of a filename,
will read hosts or 
.Pa addrlist namelist
pairs from the standard input.
Print the host key for machine
.Pa hostname :
.Bd -literal
ssh-keyscan hostname
Find all hosts from the file
.Pa ssh_hosts
which have new or different keys from those in the sorted file
.Pa ssh_known_hosts :
.Bd -literal
$ ssh-keyscan -f ssh_hosts | sort -u - ssh_known_hosts | \e\ 
	diff ssh_known_hosts -
.Pa Input format:,,name,,n,,
.Pa Output format:
host-or-namelist bits exponent modulus
.Pa /etc/ssh_known_hosts
It generates "Connection closed by remote host" messages on the consoles
of all the machines it scans.
This is because it opens a connection to the ssh port, reads the public
key, and drops the connection as soon as it gets the key.
.Xr ssh 1 ,
.Xr sshd 8
David Mazieres <>