ssh-keyscan(1) — Linux manual page


SSH-KEYSCAN(1)         BSD 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

     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

     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

     -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:


     -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 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.

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
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BSD                         February 10, 2023                        BSD