An Introduction to Using Linux as a Multipurpose Firewall

We currently have a Linux PC, connected in the middle of two networks. It can see both, and both can see it. The PC is also wide open with all the default ports turned on. We want to restrict this as much as possible. People are always looking for new ways of breaking into systems. The more we lock down this firewall to the outside, the less vulnerable we are to attacks. Nothing is perfect, and the only true way to be sure people are kept out is to unplug your Ethernet connection when you are not there. Since that's undesirable for most of us, this is the next best thing.

What needs to be done now is disabling all services we don't need. If you are making this a true firewall, you can disable almost everything except TELNET and FTP, and these two will be limited to ports from only inside your LAN and trusted outside IP addresses.

Listing 3

The file /etc/inetd.conf, as shown in Listing 3, is where these ports are configured. This file affects traffic terminating at the firewall, not passing through it. Disabling something like POP3 or IMAP is acceptable, since when you go to get your mail from a PC inside your network, this traffic will pass through the firewall (but not stop) on its way to your ISP's POP3 or IMAP mail server.

Remember, the more ports and addresses you choose to leave open, the more closely you will need to watch your firewall for break-in attempts. We have left TELNET and FTP open, so we'll want to restrict the originating IP addresses on both networks to those we want to let in.

Setting Services

This is done by editing the files /etc/hosts.deny and /etc/hosts.allow. By editing these files, you can deny access to everyone except a few specific addresses or range of addresses, or you can allow everyone in by default and disable problem IP addresses down the road when you discover unwanted access from those points. If this is the case, be sure to watch your system logs closely. See the “Setting Services” sidebar for more details. In one sense, we could have left inetd.conf alone and restricted people from those ports via the /etc/hosts.deny table; however, it is always best to lock down ports in multiple ways.

By default, most UNIX systems do not allow root to log in from anywhere but the console. If your system is not set up that way, it should be. You will at least want to slow down someone who might want in your system. If they can't log in directly as root, this is an additional security benefit. Check the file /etc/securetty. In Red Hat 6.0, look for pty1, pty2, etc. entries in the table. In Slackware, look for ttyp0, ttyp1, etc. entries in the table. If these entries are in place, root login is allowed on those TELNET ttys; therefore, remove the entries. The other remaining entries in the table cover your various consoles and serial ports.

Since you can't log in remotely as root and you do not have a console with a monitor and keyboard, it would be best to add a second user to the firewall to ensure you can “su to root” to do work on the firewall.

useradd -g 100 -d /home/USER -s /bin/tcsh -c\
 'YOURNAME' USER passwd USER

The -g controls which group this user will belong to. In this example, 100 was used, as this is the user's group in Red Hat 6.0. If this does not work for you, check out /etc/group to find a suitable group. YOURNAME is whatever you want to put in the Name field of the user account, and USER is the ID chosen for the user, i.e., I may choose to use jeff as my ID.

In a small system, the only processes we want running are ones that pertain to the operation of the firewall. This means disabling processes: all but one or two consoles, Sendmail and anything else you don't need. You can see what is running right now by typing:

ps -xa

To keep Sendmail from starting next time, you will need to move or edit the file where it starts. Linux usually starts up in runlevel 3. In Red Hat 6.0, you can check that by looking at /etc/inittab and looking for the line that reads id:3:initdefault:. The 3 indicates runlevel 3. Therefore, in /etc/rc.d/rc3.d, there is a file called S80sendmail. Move this file to 80sendmail, as follows:

mv /etc/rc.d/rc3.d/S80sendmail\
/etc/rc.d/rc3.d/80sendmail

Some programs like elm require that sendmail be running to operate properly. This opens up a potential hole to to the outside world since it also means port 25 will be open to possible attacks and possibly even mail relaying—allowing others on the Internet to use your firewall to send out spam mail. Turning off port 25 access is the easiest way to prevent this problem. Other solutions can be found at http://www.sendmail.org/.

In Slackware, edit /etc/rc.d/rc.M and change the line:

/usr/sbin/sendmail -bd -q15m

to:

/usr/sbin/sendmail -bm -q15m

In Red Hat 6.0, edit /etc/rc.d/rc3.d/S80sendmail and change the line:

daemon /usr/sbin/sendmail $([ "$DAEMON" = yes ] && echo -bd) \

to:

daemon /usr/sbin/sendmail $([ "$DAEMON" = yes ] && echo -bm) \