MAKE-SSH-KNOWN-HOSTS(1) SSH TOOLS MAKE-SSH-KNOWN-HOSTS(1)NAME
make-ssh-known-hosts - make ssh_known_hosts file from DNS data
domain_name [take_regexp [remove_regexp]]"
make-ssh-known-hosts is a perl5 script that helps create the
/etc/ssh_known_hosts file, which is used by ssh to contain the host
keys of all publicly known hosts. Ssh does not normally permit login
using rhosts or /etc/hosts.equiv authentication unless the server knows
the client's host key. In addition, the host keys are used to prevent
In addition to /etc/ssh_known_hosts, ssh also uses the
$HOME/.ssh/known_hosts file. This file, however, is intended to con‐
tain only those hosts that the particular user needs but are not in the
global file. It is intended that the /etc/ssh_known_hosts file be
maintained by the system administration, and periodically updated to
contain the host keys for any new hosts.
The make-ssh-known-hosts program finds all the hosts in a domain by
making a DNS query to the master domain name server of the domain. The
master domain name server is located by searching for the SOA record of
the domain from the initial domain name server (which can be specified
with the --initialdns option). The master domain name server can also
be given directly with the --server option.
After getting the hostname list make-ssh-known-hosts tries to get the
public key from every host in the domain. It first tries to connect ssh
port to check check if the host is alive, and if so, it tries to run
the command cat /etc/ssh_host_key.pub on the remote machine using ssh.
If the command succeeds, it knows the remote machine has ssh installed
properly, and it then extracts the public key from the output, and
prints the /etc/ssh_known_hosts entry for it to STDOUT. Because make-
ssh-known-hosts is usually run before remote machines have
/etc/ssh_known_hosts file you may have to use RSA-authentication to
allow access to hosts.
If the command fails for some reason, it checks if the ssh client still
got the public key from the remote host in the initial dialog, and if
so, it will print a proper entry, and if --notrustdaemon option is
given comment it out.
Domain_name is the domain name for which the file is to be generated.
By default make-ssh-known-hosts extracts also all subdomains of domain.
Many sites will want to include several domains in their
/etc/ssh_known_hosts file. The entries for each domain should be
extracted separately by running make-ssh-known-hosts once for each
domain. The results should then be combined to create the final file.
Take_regexp is a perl regular expression that matches the hosts to be
taken from the domain. The data matched contains all the DNS records in
the form "fieldname=value". The fields are separated with newline, and
the perl match is made in multiline mode and it is case insensetive.
The multiline mode means that you can use a regexp like "^wks=.*tel‐
net.*$" to match all hosts that have WKS (well known services) field
that contains value "telnet".
Remove_regexp is similar but those hosts that match the regexp are not
added (it can be used for example to filter out PCs and Macs using the
hinfo field: "^hinfo=.*(mac|pc)").
Set the initial domain name server used to query the SOA record
of the domain.
Set the master domain name server of the domain. This host is
used to query the DNS list of the domain.
Comma separated list of subdomains that are added to hostnames.
For example, if subdomainlist is ",foo, foo.bar, foo.bar.zappa,
foo.bar.zappa.hut.fi" then when host foobar is added to
/etc/ssh_known_hosts file it has aliases "foobar, foobar.foo,
foobar.foo.bar, foobar.foo.bar.zappa, foo‐
bar.foo.bar.zappa.hut.fi". The default action is to take all
subparts of the host but the second last on a host by host
basis. (The last element is usually the country code, and some‐
thing like foobar.foo.bar.zappa.hut would not make sense.)
Set the debug level. Default is 5, bigger values give more out‐
put. Using a big value (like 999) will print lots of debugging
Timeout when executing ssh command. The default is 60 seconds.
Timeout when trying to ping the ssh port. The default is 3 sec‐
Timeout when asking password for ssh command. Default is that no
passwords are queried. Use value 0 to have no timeout for pass‐
-notr If the ssh command fails, use the public key stored in the local
known hosts file and trust it is the correct key for the host.
If this option is not given such entries are commented out in
the generated /etc/ssh_known_hosts file.
-nor Tell make-ssh-known-hosts that it should only extract keys for
the given domain, and not to be recursive.
-do Split the domainname to get the list of subdomains. Use this
option if you don't want hostname to splitted to pieces automat‐
ically. Default splitting is done host by host basis. If the
domain is zappa.hut.fi, and the host name is foo.bar then
default action adds entries "foo, foo.bar, foo.bar.zappa,
foo.bar.zappa.hut.fi" and this options adds entries "foo.bar,
-si Be silent.
-k Output list of all hosts in format "ipaddr1,ipaddr2,...ipaddrn
name_entries". The output of this can be feeded to ssh-keyscan
to fetch keys.
Path to the nslookup program.
Path to the ssh program, including all options.
The following command:
example# make-ssh-known-hosts cs.hut.fi > /etc/ssh_known_hosts
finds all public keys of the hosts in cs.hut.fi domain and put them to
/etc/ssh_known_hosts file splitting domain names on a per host basis.
example% make-ssh-known-hosts hut.fi '^wks=.*ssh' > hut-hosts
finds all hosts in hut.fi domain, and its subdomains having own name
server (cs.hut.fi, tf.hut.fi, tky.hut.fi) that have ssh service and
puts their public key to hut-hosts file. This would require that the
domain name server of hut.fi would define all hosts running ssh to have
entry ssh in their WKS record. Because nobody yet adds ssh to WKS, it
would be better to use command
example% make-ssh-known-hosts hut.fi '^wks=.*telnet' > hut-hosts
that would take those host having telnet service. This uses default
example% make-ssh-known-hosts hut.fi 'dipoli.hut.fi'
'^hinfo=.*(mac|pc)' > dipoli-hosts
finds all hosts in hut.fi domain that are in dipoli.hut.fi subdomain
(note dipoli.hut.fi does not have own name server so its entries are in
hut.fi-server) and that are not Mac or PC.
/etc/ssh_known_hosts Global host public key list
SEE ALSOssh(1), sshd(8), ssh-keygen(1), ping(8), nslookup(8), perl(1),
Tero Kivinen <email@example.com>
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a per‐
mission notice identical to this one.
Permission is granted to copy and distribute translations of this man‐
ual into another language, under the above conditions for modified ver‐
sions, except that this permission notice may be included in transla‐
tions approved by the the author instead of in the original English.
SSH TOOLS November 8, 1995 MAKE-SSH-KNOWN-HOSTS(1)