Linux as a Proxy Server

Proxy servers are software applications that run on your firewall machine in order to provide indirect Internet access to your network. The firewall can be either a “single-homed” host or a “dual-homed” host. A single-homed host is a machine with one network card. This configuration relies on the Internet router to block all packets to any machine except the firewall. A dual-homed host is a machine with two network cards that has routing capabilities disabled. Computers behind the firewall can talk to the dual-homed host, and computers on the Internet can communicate with the dual-homed host. However, since routing between the network cards is disabled, the computers behind the firewall cannot talk directly to the computers on the Internet.

The proxy server is used to allow Internet access from inside the protected network through either the single or dual-homed host firewall. The client applications speak directly to the proxy server and the proxy server in turn speaks directly to the Internet hosts on behalf of the client, thusacting as a proxy. This interaction allows Internet access to all clients on the internal network, but leaves only one machine, the firewall, directly vulnerable to attacks from the Internet.

The proxy server takes a packet from inside your network that is bound for the Internet and changes the “from” address to its own address. It then forwards the packet to the destination host. The beauty of the proxy server is that the destination host thinks it is talking only to the firewall. When the firewall receives the response from the destination host, the proxy server sends the packet back to the original requesting machine. The client has the illusion that it has been communicating directly with the host on the Internet. The host on the Internet has the illusion that it is only dealing with the firewall.

This method is a big advantage when you access FTP sites that do double-reverse lookups. These sites, as a security measure, want to ensure you are truly coming from the address you've given. The host name of the requesting IP address is looked up in the DNS records. The server then does a lookup of the host name it received. If the IP address it receives from this last lookup does not match the requesting one or if the DNS lookup failed to find any entries, the server denies access.

If you are denied access to one of these sites, there is most likely a problem with your DNS setup. When you have to manage several machines across your network, keeping all the entries up to date can be a daunting task. With a proxy server in place, your entire network appears to come from the IP address of the proxy server, thus reducing the total number of properly configured DNS entries you need.

Another advantage of using a proxy server is that since all outbound traffic must pass through the firewall, as an administrator, you can monitor which types of Internet activity are occurring. The proxy server has very robust logging capabilities which allow you to see who is accessing what on the Internet. Attempted access from the outside is also logged closely.

I will not go into the details of setting up a packet-filtering router, since that type of information is vendor specific. However, I will give you the basic information on setting up a dual-homed host firewall. Assuming you use a Linux machine for your host, you will need to have two network cards installed in your machine. Read the “Multiple-Ethernet” mini-HOWTO located at ftp://sunsite.unc.edu/. I used two 3Com509 cards.

Auto-sensing the modules to load is a common problem when using two identical cards, so I compiled the drivers into a monolithic kernel instead of a modular one. I also added the following line to my /etc/lilo.conf file:

append="ether=11,0x300,eth0 ether=10,0x270,eth1"

This ensures that the proper parameters are passed at boot time.

Configure your kernel to keep it from routing IP packets (see Listing 1). To further ensure protection and anonymity, use one of the “bogus” class addresses (see Table 1) as per RFC1918. These IP addresses are set aside by the INTERNIC for use behind a firewall. Any packet with one of these IP addresses is dropped by the Internet backbone routers. See Figure 1 for an example of a network topology with a dual-homed host firewall. The example configuration files in this article are based on this basic topology. Our protected network is assigned the “bogus” Class C address 192.168.50, and we assume that the valid IP address of the Internet side of the firewall is 111.222.333.1.

Listing 1

Table 1

Figure 1. Dual-homed Host Firewall