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.\" $OpenBSD: ssh-keyscan.1,v 1.14 2002/02/13 08:33:47 mpech 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 by leaving this copyright notice intact.
.Dd January 1, 1996
.Nm ssh-keyscan
.Nd gather ssh public keys
.Nm ssh-keyscan
.Op Fl v46
.Op Fl p Ar port
.Op Fl T Ar timeout
.Op Fl t Ar type
.Op Fl f Ar file
.Op Ar host | addrlist namelist
.Op Ar ...
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. For scanning, one does not need
login access to the machines that are being scanned, nor does the
scanning process involve any encryption.
The options are as follows:
.Bl -tag -width Ds
.It Fl p Ar port
Port to connect to on the remote host.
.It Fl T Ar timeout
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 t Ar type
Specifies the type of the key to fetch from the scanned hosts.
The possible values are
.Dq rsa1
for protocol version 1 and
.Dq rsa
.Dq dsa
for protocol version 2.
Multiple values may be specified by separating them with commas.
The default is
.Dq rsa1 .
.It Fl f Ar filename
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.
.It Fl v
Verbose mode.
to print debugging messages about its progress.
.It Fl 4
to use IPv4 addresses only.
.It Fl 6
to use IPv6 addresses only.
If a ssh_known_hosts file is constructed using
without verifying the keys, users will be vulnerable to
.I man in the middle
On the other hand, if the security model allows such a risk,
can help in the detection of tampered keyfiles or man in the middle
attacks which have begun after the ssh_known_hosts file was created.
Print the
.Pa rsa1
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 -t rsa,dsa -f ssh_hosts | \e\
sort -u - ssh_known_hosts | diff ssh_known_hosts -
.Pa Input format:
.Bd -literal,,name,,n,,
.Pa Output format for rsa1 keys:
.Bd -literal
host-or-namelist bits exponent modulus
.Pa Output format for rsa and dsa keys:
.Bd -literal
host-or-namelist keytype base64-encoded-key
.Pa keytype
is either
.Dq ssh-rsa
.Dq ssh-dsa .
.Pa /etc/ssh/ssh_known_hosts
It generates "Connection closed by remote host" messages on the consoles
of all the machines it scans if the server is older than version 2.9.
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 <>
wrote the initial version, and
Wayne Davison <>
added support for protocol version 2.