man > ssh-keyscan(1)

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

NAME
       ssh-keyscan — gather SSH public keys from servers

SYNOPSIS
       ssh-keyscan   [-46cDHv]   [-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 docu‐
       mented 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 sec‐
       onds,  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 us‐
               able 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
       /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,dsa,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 AT lcs.edu>   wrote   the   initial   version,   and   Wayne  Davison
       <wayned AT users.net> added support for protocol version 2.

Debian                                    February 10, 2023                           SSH-KEYSCAN(1)