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
 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 an 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
 default 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
 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

 /opt/local/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.
macOS 15.7 October 4, 2025 macOS 15.7