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SSH-KEYSCAN(1) General Commands Manual SSH-KEYSCAN(1)

Gather secure shell public keys from servers

ssh-keyscan [-46DdHv] [-f file] [-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.

The options are as follows:

Force ssh-keyscan to use IPv4 addresses only.
Force ssh-keyscan to use IPv6 addresses only.
Print keys found as DNS Resource Records (CERT or SSHFP). The default is to print keys in a format usable as a ssh(1) known_hosts file.
Print keys found as DNS Resource Records (CERT or SSHFP) in generic format.
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. Input is expected in the format:,,name,,n,,
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.
Connect to port 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, the connection is closed and the host in question considered unavailable. The default is 5 seconds.
Specify the host-key algorithms as pattern-list to fetch from the scanned hosts. See PATTERNS in ssh_config(5) for more information on patterns.

The supported host-key algorithms are “x509v3-ecdsa-sha2-nistp256”, “x509v3-ecdsa-sha2-nistp384”, “x509v3-ecdsa-sha2-nistp521”, “x509v3-rsa2048-sha256”, “x509v3-ssh-rsa”, “x509v3-sign-rsa”, “x509v3-ssh-dss”, “x509v3-sign-dss”, “ssh-ed25519”, “ssh-rsa”, “rsa-sha2-256”, “rsa-sha2-512”, “ssh-dss”, “ecdsa-sha2-nistp256”, “ecdsa-sha2-nistp384”, or “ecdsa-sha2-nistp521”. The default is “*” , i.e. all supported algorithms.

Note that host-key algorithms “rsa-sha2-256”, “rsa-sha2-512”, and “ssh-rsa” share one and the same format of host-key.

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.


Print the all RSA host-keys for machine hostname:

$ ssh-keyscan -t '*rsa*' 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 -f ssh_hosts | \
	sort -u - ssh_known_hosts | diff ssh_known_hosts -

ssh(1), sshd(8)

  1. D. Eastlake and O. Gudmundsson, Storing Certificates in the Domain Name System (DNS), RFC 2538, March 1999.
  2. R. Arends, R. Austein, M. Larson, D. Massey, and S. Rose, Resource Records for the DNS Security Extensions, RFC 4034, March 2005.
  3. J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, January 2006.

David Mazieres <> wrote the initial version, Wayne Davison <> added support for protocol version 2, and Roumen Petrov contributed support for X.509 certificates.
8 March 2021 PKIX-SSH


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