secsh-keyscan, ssh-keyscan -- gather ssh public keys

SYNOPSIS

secsh-keyscan [-cvDH46] [-p port] [-T timeout] [-t type] [-f file] [-O option] [host|addrlist namelist] [...]

ssh-keyscan [-cvDH46] [-p port] [-T timeout] [-t type] [-f file] [-O option] [host|addrlist namelist] [...]

secsh-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. secsh-keyscan provides a minimal interface suitable for use by shell and perl scripts.

secsh-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 ssh. For scanning, you do not need login access to the machines you are scanning, 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.

You can also call secsh-keyscan as ssh-keyscan.

Security

If a ssh_known_hosts file is constructed using secsh-keyscan without verifying the keys, users will be vulnerable to attacks. On the other hand, if the security model allows such a risk, ssh-keyscan can help you detect tampered keyfiles or man in the middle attacks which have begun after the ssh_known_hosts file was created.

Options

-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 secsh known_hosts file.

-f file

Read hosts or addrlist namelist pairs from file, one per line. If - is supplied instead of a filename, secsh-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

-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.

-H

Hashes all hostnames and addresses in the output. Hashed names may be used normally by secsh and secshd, but they do not reveal identifying information should the file's contents be disclosed.

-p port

Specifies the port to connect to on the remote host.

-T timeout

Sets 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, then the connection is closed and the host in question considered unavailable. 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, ed25519, ecdsa-sk, and ed25519-sk.

-v

(Verbose mode.) Print debugging messages about progress.

-4

Forces ssh-keyscan to use IPv4 addresses.

-6

Forces ssh-keyscan to use IPv6 addresses.

EXAMPLES

Print the RSA1 host key for machine hostname:

secsh-keyscan -t rsa hostname

Search a network range, printing all supported key types:

$ secsh-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:

secsh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \
	sort -u - ssh_known_hosts | diff ssh_known_hosts -

FILES

$ROOTDIR/etc/ssh_known_hosts

BUGS

It generates "Connection closed by remote host" messages on the consoles of all the machines it scans if the server is older than version 2.9. This is because it opens a connection to the ssh port, reads the public key, and drops the connection as soon as it gets the key.

AUTHOR

David Mazieres <dm@lcs.mit.edu> wrote the initial version, and Wayne Davison <wayned@users.sourceforge.net> added support for protocol version 2.

PORTABILITY

All UNIX systems. Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022. Windows Server 2025.

AVAILABILITY

PTC MKS Toolkit for System Administrators
PTC MKS Toolkit for Developers
PTC MKS Toolkit for Interoperability
PTC MKS Toolkit for Professional Developers
PTC MKS Toolkit for Professional Developers 64-Bit Edition
PTC MKS Toolkit for Enterprise Developers
PTC MKS Toolkit for Enterprise Developers 64-Bit Edition

secsh-keyscan, ssh-keyscan
gather ssh public keys

Command