ssh-keyscan is a utility for gathering the public ssh host keys of a
number of hosts. It was designed to aid in building and verifying
ssh_known_hosts files. ssh-keyscan provides a minimal interface suit‐
able for use by shell and perl scripts.
ssh-keyscan 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:
-4 Forces ssh-keyscan to use IPv4 addresses only.
-6 Forces ssh-keyscan to use IPv6 addresses only.
-c Request certificates from target hosts instead of plain keys.
Read hosts or “addrlist namelist” pairs from file, one per
line. If - is supplied instead of a filename, ssh-keyscan will
read hosts or “addrlist namelist” pairs from the standard
-H Hash all hostnames and addresses in the output. Hashed names
may be used normally by ssh and sshd, but they do not reveal
identifying information should the file's contents be dis‐
Port to connect to on the remote host.
Set the timeout for connection attempts. If 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 seconds.
Specifies the type of the key to fetch from the scanned hosts.
The possible values are “rsa1” for protocol version 1 and
“dsa”, “ecdsa”, “ed25519”, or “rsa” for protocol version 2.
Multiple values may be specified by separating them with com‐
mas. The default is to fetch “rsa”, “ecdsa”, and “ed25519”
-v Verbose mode. Causes ssh-keyscan to print debugging messages
about its progress.
If an ssh_known_hosts file is constructed using ssh-keyscan without
verifying the keys, users will be vulnerable to man in the middle
attacks. On the other hand, if the security model allows such a risk,
ssh-keyscan can help in the detection of tampered keyfiles or man in
the middle attacks which have begun after the ssh_known_hosts file was
Output format for RSA1 keys:
host-or-namelist bits exponent modulus
Output format for RSA, DSA, ECDSA, and Ed25519 keys:
host-or-namelist keytype base64-encoded-key
Where keytype is either “ecdsa-sha2-nistp256”, “ecdsa-sha2-nistp384”,
“ecdsa-sha2-nistp521”, “ssh-ed25519”, “ssh-dss” or “ssh-rsa”.
Print the rsa host key for machine hostname:
$ ssh-keyscan hostname
Find all hosts from the file ssh_hosts which have new or different keys
from those in the sorted file ssh_known_hosts:
$ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \
sort -u - ssh_known_hosts | diff 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
This page is part of the openssh (Portable OpenSSH) project. Informa‐
tion about the project can be found at
http://www.openssh.com/portable.html. If you have a bug report for
this manual page, see http://www.openssh.com/report.html. This page
was obtained from the tarball openssh-7.4p1.tar.gz fetched from
http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/ on 2017-03-13.
If you discover any rendering problems in this HTML version of the
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BSD November 8, 2015 BSD