ssh-keyscan(1) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | FILES | EXAMPLES | SEE ALSO | AUTHORS | COLOPHON

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

       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.

       -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

       /etc/ssh/ssh_known_hosts

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 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,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 <[email protected]> wrote the initial version, and
       Wayne Davison <[email protected]> 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
       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-9.7p1.tar.gz
       fetched from
       ⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
       2024-06-14.  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                         February 10, 2023             SSH-KEYSCAN(1)