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NAME | SYNOPSIS | DESCRIPTION | FILES | EXAMPLES | SEE ALSO | AUTHORS | COLOPHON |
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SSH-KEYSCAN(1) General Commands Manual SSH-KEYSCAN(1)
ssh-keyscan — gather SSH public keys from servers
ssh-keyscan [-46cDHqv] [-f file] [-O option] [-p port] [-T
timeout] [-t type] [host | addrlist namelist]
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, the format of which is documented
in sshd(8). ssh-keyscan provides a minimal interface suitable 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 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.
Hosts to be scanned may be specified by hostname, address or by
CIDR network range (e.g. 192.168.16/28). If a network range is
specified, then all addresses in that range will be scanned.
The options are as follows:
-4 Force ssh-keyscan to use IPv4 addresses only.
-6 Force ssh-keyscan to use IPv6 addresses only.
-c Request certificates from target hosts instead of plain
keys.
-D Print keys found as SSHFP DNS records. The default is to
print keys in a format usable as a ssh(1) known_hosts
file.
-f file
Read hosts or “addrlist namelist” pairs from file, one per
line. If ‘-’ is supplied instead of a filename,
ssh-keyscan will read from the standard input. Names read
from a file must start with an address, hostname or CIDR
network range to be scanned. Addresses and hostnames may
optionally be followed by comma-separated name or address
aliases that will be copied to the output. For example:
192.168.11.0/24
10.20.1.1
happy.example.org
10.0.0.1,sad.example.org
-H Hash all hostnames and addresses in the output. Hashed
names may be used normally by ssh(1) and sshd(8), but they
do not reveal identifying information should the file's
contents be disclosed.
-O option
Specify a key/value option. At present, only a single
option is supported:
hashalg=algorithm
Selects a hash algorithm to use when printing
SSHFP records using the -D flag. Valid algorithms
are “sha1” and “sha256”. The default is to print
both.
-p port
Connect to port on the remote host.
-q Quiet mode: do not print server host name and banners in
comments.
-T timeout
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, the connection is closed and the host in question
considered unavailable. The default is 5 seconds.
-t type
Specify the type of the key to fetch from the scanned
hosts. The possible values are “ecdsa”, “ed25519”,
“ecdsa-sk”, “ed25519-sk”, or “rsa”. Multiple values may
be specified by separating them with commas. The default
is to fetch all the above key types.
-v Verbose mode: print debugging messages about 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 created.
/etc/ssh/ssh_known_hosts
Print the RSA host key for machine hostname:
$ ssh-keyscan -t rsa hostname
Search a network range, printing all supported key types:
$ ssh-keyscan 192.168.0.64/25
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,ecdsa,ed25519 -f ssh_hosts | \
sort -u - ssh_known_hosts | diff ssh_known_hosts -
ssh(1), sshd(8) Using DNS to Securely Publish Secure Shell (SSH)
Key Fingerprints, RFC 4255, 2006.
David Mazieres <[email protected]> wrote the initial version, and
Wayne Davison <[email protected]> added support for
protocol version 2.
This page is part of the openssh (Portable OpenSSH) project.
Information 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-9.9p2.tar.gz
fetched from
⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
GNU June 17, 2024 SSH-KEYSCAN(1)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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