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man pages section 1: User Commands

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Updated: July 2017



ssh-keyscan - gather public ssh host keys of a number of hosts


ssh-keyscan [-v46] [-p port] [-T timeout] [-t type] 
     [-f file] [-] [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. ssh-keyscan provides a minimal interface suitable for use by shell and perl scripts. The output of ssh-keyscan is directed to standard output.

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 ssh. For scanning, one does not need login access to the machines that are being scanned, nor does the scanning process involve any encryption.

File Format

Input format:,

Output format for rsa1 keys:

host-or-namelist bits exponent modulus

Output format for rsa and dsa keys, where keytype is either ssh-rsa or `ssh-dsa:

host-or-namelist keytype base64-encoded-key


The following options are supported:

–f filename

Read hosts or addrlist namelist pairs from this file, one per line. If you specify - instead of a filename, ssh-keyscan reads hosts or addrlist namelist pairs from the standard input.

–p port

Port to connect to 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 is considered unavailable. The default is for timeout is 5 seconds.

–t type

Specify the type of the key to fetch from the scanned hosts. The possible values for type are rsa1 for protocol version 1 and rsa or dsa for protocol version 2. Specify multiple values by separating them with commas. The default is rsa1.


Specify verbose mode. Print debugging messages about progress.


Force to use IPv4 addresses only.


Forces to use IPv6 addresses only.


If a ssh_known_hosts file is constructed using ssh-keyscan without verifying the keys, users are vulnerable to man-in-the-middle attacks. 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.


Example 1 Printing the rsa1 Host Key

The following example prints the rsa1 host key for machine hostname:

$ ssh-keyscan hostname

Example 2 Finding All Hosts

The following commands finds 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 -f ssh_hosts | \
     sort -u - ssh_known_hosts | diff ssh_known_hosts -



Contains list of public ssh host keys.

Exit Status

The following exit values are returned:


No usage errors. ssh-keyscan might or might not have succeeded or failed to scan one, more or all of the given hosts.


Usage error.


See attributes(5) for descriptions of the following attributes:

Interface Stability

See Also

ssh(1), sshd(1M), attributes(5)


David Mazieres wrote the initial version, and Wayne Davison added support for protocol version 2.


ssh—keyscan generates the following messages on the consoles of all machines it scans if the server is older than version 2.9:

Connection closed by remote host

This is because ssh-keyscan opens a connection to the ssh port, reads the public key, and drops the connection as soon as it gets the key.