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.
Configuring and Using the Proxy Services

This section focuses on the configuration and use of the service proxies for telnet, ftp, http, and SMTP mail. Common methods for configuring these services are discussed, but each of them has many options and are very flexible. The man pages for each of the services should be reviewed to determine if other configurations might suit your installation better. The proxies for rlogin and X-Windows are configured similarly to the telnet proxy. The Generic TCP plug-through proxy can be used for NNTP news transfers, talk sessions, or any other TCP service a site wishes to pass through the firewall.

The telnet proxy is called tn-gw. The tn-gw has many options, including the permit-hosts and deny-hosts lines as seen on other services, and a number of message options. The fwtk configuration manual discusses how to use netacl to allow both telnet service to the bastion host and the telnet proxy to coexist on the bastion host, although this is just one of many possible configurations.

First, use netacl to switch the service based on the origin, as shown in Listing 7, which shows an /etc/inetd.conf entry, and Listing 8, which shows the additions to the netperm-table.

When a telnet connection is started, inetd calls netacl. Netacl looks at the source IP address, and if it is not from the bastion host, it calls the tn-gw proxy. The tn-gw proxy prints a denial message and closes, if the source address is unknown, and will allow a non-authenticated connection to only from Connections from the internal net have no restrictions (the default) on their destinations, and users are allowed to change their bastion host passwords if they are coming from the internal network. Additionally, connections from the internal net to the bastion host itself are allowed. Finally, all other hosts are allowed to go to any destination other than the bastion host itself, after they authenticate with the specified authserver. You might not want to use this setup (allowing unauthenticated access from any external site is not a good idea) but it presents many of the options the toolkit offers.

If a user is on the internal network and wants to have telnet access to the external network, they telnet and then type c to connect to the external host.

If a user is on the external network and wants to connect to an internal host they will have to telnet, enter their userid and authentication as required by specific authentication type, and then c

Finally, if an administrator wants to connect to the bastion host from the internal network, they telnet bastion host, then c, and netacl will start the real telnet service on the bastion host.

Next, we will configure the ftp proxy system. We will assume your site does not want to provide anonymous ftp service from the bastion host to the external network. The TIS configuration guide discusses in more detail how to configure a site that supports anonymous ftp and the ftp proxy on the same host. Our example will only have the ftp proxy on the bastion host.

The inetd.conf file will need to be modified to call the ftp-gw proxy. Netacl is not used since we are not switching the service being provided, as we were in the telnet example. The inetd.conf line is shown in Listing 9. Then establish the permissions in the netperm-table, as shown in Listing 10.

These lines will print the ftp-deny.txt file and close the connection if the reverse name lookup fails; internal nodes will be allowed to ftp through the gateway without authentication, but RETR and STOR transactions are logged. Additionally, it will allow external nodes to connect to the internal ftp server after authentication with the authserver, logging RETR and STOR transactions.

Users on the internal network will ftp to the bastion host and enter their destination at the username prompt. To be forwarded to the site big.archive as user bob, they need to enter bob@big.archive at the bastion host's username prompt, and it will forward the connection. Users on the external network will have to authenticate themselves first and then enter their destination. This is discussed in more detail in the ftp-gw man page.

Now, configure the proxy for http. This is similar to the other proxies and can have a relatively simple configuration. First, add the line in Listing 11 to the /etc/inetd.conf file, and then set up the netperm-table configuration entries shown in Listing 12.

This configuration will allow internal hosts to go out to wherever they want without authentication and allow external hosts to connect to the host without authentication. Check the http-gw man page for options involving specific authentication of particular http actions.

For users on the internal network, we can configure their proxy-aware web browser to transparently pass through the firewall http proxy. We will use the Netscape browser as our example. Find the Network Preferences menu under the Options menu. Under that, there is a section on proxy configuration. Select manual proxy configuration and enter the IP address of your bastion host for each of the proxied services and their respective ports. Now you can again use normal http addresses, and the browser will do all necessary requests automatically through the bastion host.

Our final service to be proxied is SMTP mail. The fwtk comes with two programs—smap and smapd—which serve to reduce problems with sendmail and insulate it from some attacks. They do use sendmail, however, so this section will assume that the sendmail configuration for the bastion host has already been setup and debugged. Sendmail configuration is well outside the scope of this article. The Linux Electronic Mail HOWTO can be consulted as necessary.

Smap is a minimal SMTP client that is invoked by inetd, accepts SMTP mail messages, and writes them to a special spool directory. Smapd is a daemon that replaces sendmail in rc.M (or whichever boot script starts sendmail in your distribution). Smapd will look at the spool directory and deliver messages using sendmail periodically. This time, three changes need to be made. Listing 13 shows an /etc/inetd.conf entry to smap, Listing 14 shows how to start smapd from a boot script, and Listing 15 shows the netperm-table entries.

Next, create the /var/spool/inspool directory and make it owned by uucp. Run mkdir /var/spool/inspool; chown uccp /var/spool/inspool. Finally, run sendmail from a cron job so that it can process any entries that could not be delivered. A line like:

0,30 * * * * /usr/lib/sendmail -q >/dev/null 2>&1

should be added to root's crontab.