ssh-keyscan(1) — Linux manual page


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

NAME         top

       ssh-keyscan — gather SSH public keys from servers

SYNOPSIS         top

       ssh-keyscan [-46cDHv] [-f file] [-O option] [-p port] [-T
       timeout] [-t type] [host | addrlist namelist]

DESCRIPTION         top

       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).  provides a minimal interface suitable for use by shell
       and perl scripts.

       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 to use IPv4 addresses only.

       -6      Force to use IPv6 addresses only.

       -c      Request certificates from target hosts instead of plain

       -D      Print keys found as SSHFP DNS records.  The default is to
               print keys in a format usable as a ssh(1) known_hosts

       -f file
               Read hosts or “addrlist namelist” pairs from file, one
               per line.  If ‘-’ is supplied instead of a filename, 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:


       -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:

                       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.

       -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 “dsa”, “ecdsa”,
               “ed25519”, “ecdsa-sk”, “ed25519-sk”, or “rsa”.  Multiple
               values may be specified by separating them with commas.
               The default is to fetch “rsa”, “ecdsa”, “ed25519”,
               “ecdsa-sk”, and “ed25519-sk” keys.

       -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 man in the middle attacks.
       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

FILES         top


EXAMPLES         top

       Print the RSA host key for machine hostname:

             $ ssh-keyscan -t rsa hostname

       Search a network range, printing all supported key types:

             $ ssh-keyscan

       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 -

SEE ALSO         top

       ssh(1), sshd(8) Using DNS to Securely Publish Secure Shell (SSH)
       Key Fingerprints, RFC 4255, 2006.

AUTHORS         top

       David Mazieres <> wrote the initial version, and
       Wayne Davison <> added support for
       protocol version 2.

COLOPHON         top

       This page is part of the openssh (Portable OpenSSH) project.
       Information about the project can be found at  If you have a bug report
       for this manual page, see ⟨⟩.
       This page was obtained from the tarball openssh-9.6p1.tar.gz
       fetched from
       ⟨⟩ on
       2023-12-22.  If you discover any rendering problems in this HTML
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       part of the original manual page), send a mail to

GNU                         February 10, 2023             SSH-KEYSCAN(1)