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

NAME
     ssh-keyscan M-bM-^@M-^S gather SSH public keys from servers

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

DESCRIPTION
     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 M-bM-^@M-^\addrlist namelistM-bM-^@M-^] pairs from file, one per line.
             If M-bM-^@M-^X-M-bM-^@M-^Y 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 M-bM-^@M-^\sha1M-bM-^@M-^]
                     and M-bM-^@M-^\sha256M-bM-^@M-^].  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 M-bM-^@M-^\ecdsaM-bM-^@M-^], M-bM-^@M-^\ed25519M-bM-^@M-^], M-bM-^@M-^\ecdsa-skM-bM-^@M-^], M-bM-^@M-^\ed25519-skM-bM-^@M-^],
             or M-bM-^@M-^\rsaM-bM-^@M-^].  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.

FILES
     /etc/ssh/ssh_known_hosts

EXAMPLES
     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 -

SEE ALSO
     ssh(1), sshd(8)

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

AUTHORS
     David Mazieres <dm@lcs.mit.edu> wrote the initial version, and Wayne
     Davison <wayned@users.sourceforge.net> added support for protocol version
     2.

OpenBSD 7.6                      June 17, 2024                     OpenBSD 7.6
