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

NAME
     ssh-keyscan -- 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 possi-
     ble 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  ac-
     cess  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  net-
     work 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.

     -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  de-
	     fault is 5 seconds.

     -t type
	     Specify  the  type of the key to fetch from the scanned hosts.  The
	     possible values are "ecdsa", "ed25519",  "ecdsa-sk",  "ed25519-sk",
	     or "rsa".	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 verify-
     ing  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.

FreeBSD 15.1			  June 17, 2024 		  SSH-KEYSCAN(1)
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