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.\" $OpenBSD: ssh-keyscan.1,v 1.44 2018/03/05 07:03:18 jmc 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 $Mdocdate: March 5 2018 $
.Nm ssh-keyscan
.Nd gather SSH public keys
.Nm ssh-keyscan
.Op Fl 46cDHv
.Op Fl f Ar file
.Op Fl p Ar port
.Op Fl T Ar timeout
.Op Fl t Ar type
.Op Ar host | addrlist namelist
is a utility for gathering the public SSH host keys of a number of
It was designed to aid in building and verifying
.Pa ssh_known_hosts
the format of which is documented in
.Xr sshd 8 .
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
.Xr sshd 8 .
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 4
to use IPv4 addresses only.
.It Fl 6
to use IPv6 addresses only.
.It Fl c
Request certificates from target hosts instead of plain keys.
.It Fl D
Print keys found as SSHFP DNS records.
The default is to print keys in a format usable as a
.Xr ssh 1
.Pa known_hosts
.It Fl f Ar file
Read hosts or
.Dq addrlist namelist
pairs from
.Ar file ,
one per line.
.Sq -
is supplied instead of a filename,
will read from the standard input.
Input is expected in the format:
.Bd -literal,,name,,n,,
.It Fl H
Hash all hostnames and addresses in the output.
Hashed names may be used normally by
.Xr ssh 1
.Xr sshd 8 ,
but they do not reveal identifying information should the file's contents
be disclosed.
.It Fl p Ar port
Connect to
.Ar port
on the remote host.
.It Fl T Ar timeout
Set the timeout for connection attempts.
.Ar timeout
seconds have elapsed since a connection was initiated to a host or since the
last time anything was read from that host, the connection is
closed and the host in question considered unavailable.
The default is 5 seconds.
.It Fl t Ar type
Specify the type of the key to fetch from the scanned hosts.
The possible values are
.Dq dsa ,
.Dq ecdsa ,
.Dq ed25519 ,
.Dq rsa .
Multiple values may be specified by separating them with commas.
The default is to fetch
.Dq rsa ,
.Dq ecdsa ,
.Dq ed25519
.It Fl v
Verbose mode:
print debugging messages about progress.
If an ssh_known_hosts file is constructed using
without verifying the keys, users will be vulnerable to
.Em 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.
.Pa /etc/ssh/ssh_known_hosts
Print the RSA host key for machine
.Ar hostname :
.Dl $ ssh-keyscan -t rsa 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 -offset indent
$ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \e
sort -u - ssh_known_hosts | diff ssh_known_hosts -
.Xr ssh 1 ,
.Xr sshd 8
.%D 2006
.%R RFC 4255
.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
.An -nosplit
.An David Mazieres Aq Mt
wrote the initial version, and
.An Wayne Davison Aq Mt
added support for protocol version 2.