Creating A Linux Firewall Using the TIS Firewall Toolkit

If you have a valuable or fragile network to protect, you may want to protect it with a very strong, well-proven firewall. In this article, Benjamin Ewy explains very thoroughly how to build your own 'bastion host' firewall with Linux.
Providing a Network Authentication Service

Authsrv is the authentication server for the firewall toolkit. The authentication server is optional but allows multiple types of authentication to be managed in a consistent manner. Support for the authsrv daemon's authentication by is built into all of the proxies in the firewall toolkit, and can be selectively enabled in the netperm-table on a per-proxy or even per-permiti-hosts basis.

Authsrv has support for many different types of authentication, including internal plain-text passwords, and several forms of strong authentication using one time passwords compatible with Bellcore's S/Key, Security Dynamic's SecurID, Enigma Logics' Silver Card, and Digital Pathways' SNK004 Secure Net Key. In our example, we have compiled S/Key and its support into authsrv, but the other mechanisms are similar and their details can be found by looking in the /usr/src/fwtk/auth directory. S/Key is a challenge-response one-time password system that will present you with a sequence number and a key at login time. You must give the sequence number, key, and your own private pass-phrase to an S/Key calculator, and it will return a 6 word password. That password will be valid only for that particular sequence number, and it has the property that it was created using a non-reversible algorithm, so it is not possible to easily calculate the next password even if the current one is known. This type of strong authentication is one of the best features of using the fwtk.

To configure the authsrv it must be added to the inetd.conf so that inetd will start it, as shown in Listing 5.

Since authsrv is not a well known service, an unused port must be selected and added to the /etc/services file. The fwtk configuration manual suggests port 7777. The corresponding /etc/services entry would then be:

# Example services entry
authsrv 7777/tcp

As before, when you change the /etc/inetd.conf file, you must send a -HUP signal to inetd to cause it to read in the changes.

Configure authsrv itself by setting up its options in the netperm-table. Authsrv recognizes the database, permit-hosts, nobugus, userid, and badsleep options. The database option tells authsrv where to find its database, and the permit-hosts can be used to restrict which hosts can query the authsrv. It is recommended that authsrv be run on the bastion host so that the database is protected from misuse. An example config might include the entries shown in Listing 6 in the netperm-table.

In our example, the bastion host is running the authsrv daemon, and the bastion host is the only host with proxies requiring authentication by our server. We restrict the authsrv requests to come only from the bastion host to prevent unauthorized probing of the database.

Next, we set up an S/Key-based admin account on our bastion host. First, the auth database needs to be initialized. The best way to do this is to run authsrv as root. Then add an admin user and enable logins for that user by entering the following at the authsrv prompt:

authsrv# adduser admin
authsrv# enable admin

Set the protocol to skey, and give the the user wizard privileges:

authsrv# proto admin skey
authsrv# superwiz admin

Then you need to set the admin password. S/Key allows your password to be several words long. Enter your phrase between quotes:

authsrv# password admin "my neat password phrase"
ID admin s/key is 664 wa56038
authsrv# exit

The output returned by authsrv after your pass-phrase is the next sequence number it will use for a challenge and its key. You can use them to generate one-time passwords, as needed, using the key program (read its man page, found in /usr/src/skey-2.2/key). For example, if you are challenged with:

S/Key Challenge: s/key 663 wa56038

run key 663 wa56038 and enter in your pass-phrase. It will respond with a six-word phrase that you can enter to authenticate yourself. If you are going to be traveling, there are Macintosh and DOS versions of the S/Key calculator, or you can have key print out a list of your next passwords by running key -n number 663 wa56038, and it will print out your next number passwords and their appropriate sequence numbers.

There is another program called authmgr which can be used to remotely administer the authentication database. There are many features in addition to those shown here, such as groups and group permissions for users, and the ability to specify authentication on a per-user, per-time basis. These additional features can be found in the authd man pages. Finally, there are two utilities called authdump and authload which allow you to take snapshots of the current database, for archival or administrative purposes, and then reload the database.