Ssh HostbasedAuthentication

From Brandonhutchinson.com

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Overview

The following notes on HostbasedAuthentication apply to OpenSSH.

HostbasedAuthentication files

These files are checked in the following order on the remote system if HostbasedAuthentication is enabled:

1. /etc/hosts.equiv
2. /etc/ssh/shosts.equiv
3. ~/.shosts
4. ~/.rhosts

Relevant sshd_config directives

    HostbasedAuthentication
            Specifies whether rhosts or /etc/hosts.equiv authentication
            together with successful public key client host authentication is
            allowed (host-based authentication).  This option is similar to
            RhostsRSAAuthentication and applies to protocol version 2 only.
            The default is “no”.

    HostbasedUsesNameFromPacketOnly
            Specifies whether or not the server will attempt to perform a
            reverse name lookup when matching the name in the ~/.shosts,
            ~/.rhosts, and /etc/hosts.equiv files during
            HostbasedAuthentication.  A setting of “yes” means that sshd(8)
            uses the name supplied by the client rather than attempting to
            resolve the name from the TCP connection itself.  The default is
            “no”.

    IgnoreRhosts
            Specifies that .rhosts and .shosts files will not be used in
            RhostsRSAAuthentication or HostbasedAuthentication.

            /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used.  The
            default is “yes”.

    IgnoreUserKnownHosts
            Specifies whether sshd(8) should ignore the user’s
            ~/.ssh/known_hosts during RhostsRSAAuthentication or
            HostbasedAuthentication.  The default is “no”.

    Protocol
            Specifies the protocol versions sshd(8) supports.  The possible
            values are ‘1’ and ‘2’.  Multiple versions must be comma-sepa‐
            rated.  The default is “2,1”.  Note that the order of the proto‐
            col list does not indicate preference, because the client selects
            among multiple protocol versions offered by the server.  Specify‐
            ing “2,1” is identical to “1,2”.

    RhostsRSAAuthentication
            Specifies whether rhosts or /etc/hosts.equiv authentication
            together with successful RSA host authentication is allowed.  The
            default is “no”.  This option applies to protocol version 1 only.

HostbasedAuthentication with the root user

If you want to use HostbasedAuthentication with the root user, you must:

• Place the fully-qualified domain name of the trusted system in ~root/.shosts or ~root/.rhosts on the remote system.
• Set IgnoreRhosts no in sshd_config on the remote system.

The system-wide /etc/rhosts.equiv or /etc/ssh/shosts.equiv files will not work with HostbasedAuthentication of the root user.

Allowing HostbasedAuthentication on the client system

Even if HostbasedAuthentication is properly enabled on the server, it may denied as a form of authentication on the client system.

For example, on RHEL 5, HostbasedAuthentication is implicitly denied on clients, as the default is "no".

To enable, modify the site-wide or user-specific ssh configuration file as appropriate.

Reverse DNS

By default (see HostbasedUsesNameFromPacketOnly above), OpenSSH will not trust the client's self-identifying hostname in the hostbased authentication request, and will perform a reverse DNS lookup of the IP address.

e.g., /var/log/auth.log

Jul 14 16:44:24 ssh_server sshd[10794]: userauth_hostbased mismatch: client sends host.example.com, but we resolve 192.216.212.193 to 192.216.212.193

In this case, I needed to add 193.212.216.192.in-addr.arpa to DNS.

• How do I get trusted-host (SSH-2 "hostbased", SSH-1 "RhostsRSA") authentication working?

Miscellaneous notes

• If any of the four files used in HostbasedAuthentication contain a + in either the hostname(s) or user name(s), the line is ignored.

debug1: Remote: Ignoring wild host/user names in /etc/hosts.equiv.
debug1: Remote: Ignoring wild host/user names in /etc/ssh/shosts.equiv.
debug1: Remote: Ignoring wild host/user names in $HOME/.shosts.
debug1: Remote: Ignoring wild host/user names in $HOME/.rhosts.