ssh-keyscan - gather SSH public keys from servers


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


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:,

-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

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.




Print the RSA host key for machine hostname:

$ ssh-keyscan -t rsa hostname

Search a network range, printing all supported key types:

$ ssh-keyscan

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 -


ssh(1), sshd(8)

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


David Mazieres <> wrote the initial version, and Wayne
Davison <> added support for protocol version

illumos February 10, 2023 illumos