What Makes a Good Firewalled Service? (Building Internet Firewalls, 2nd Edition)

13.4. What Makes a Good Firewalled Service?

The ideal service to run through a firewall is one that makes a single TCP connection in one direction for each session. It should make that connection from a randomly allocated port on the client to an assigned port on the server, the server port should be used only by this particular service, and the commands it sends over that connection should all be secure. The following sections look at these ideal situations and some that aren't so ideal.

13.4.1. TCP Versus Other Protocols

Because TCP is a connection-oriented protocol, it's easy to proxy; you go through the overhead of setting up the proxy only once, and then you continue to use that connection. UDP has no concept of connections; every packet is a separate transaction requiring a separate decision from the proxy server. TCP is therefore easier to proxy (although there are UDP proxies). Similarly, ICMP is difficult to proxy because each packet is a separate transaction. Once again, ICMP is harder to proxy than TCP but not impossible; some ICMP proxies do exist.

The situation is much the same for packet filters. It's relatively easy to allow TCP through a firewall and control what direction connections are made in; you can use filtering on the ACK bit to ensure that you allow internal clients only to initiate connections, while still letting in responses. With UDP or ICMP, there's no way to easily set things up this way. Using stateful packet filters, you can watch for packets that appear to be responses, but you can never be sure that a packet is genuinely a response to an earlier one, and you may be waiting for responses to packets that don't require one.

13.4.2. One Connection per Session

It's easy for a firewall to intercept the initial connection from a client to a server. It's harder for it to intercept a return connection. With a proxy, either both ends of the conversation have to be aware of the existence of the proxy server, or the server needs to be able to interpret and modify the protocol to make certain the return connection is made correctly and uniquely. With plain packet filtering, the inbound connection has to be permitted all the time, which often will allow attackers access to ports used by other protocols. Stateful packet filtering, like proxying, has to be able to interpret the protocol to figure out where the return connection is going to be and open a hole for it.

For example, in normal-mode FTP the client opens a control connection to the server. When data needs to be transferred:

The client chooses a random port above 1023 and prepares it to accept a connection.
The client sends a PORT command to the server containing the IP address of the machine and the port the client is listening on.
The server then opens a new connection to that port.

In order for a proxy server to work, the proxy server must:

Intercept the PORT command the client sends to the server.
Set up a new port to listen on.
Connect back to the client on the port the client specified.
Send a replacement PORT command (using the port number on the proxy) to the FTP server.
Accept the connection from the FTP server, and transfer data back and forth between it and the client.

It's not enough for the proxy server to simply read the PORT command on the way past because that port may already be in use. A packet filter must either allow all inbound connections to ports above 1023, or intercept the PORT command and create a temporary rule for that port. Similar problems are going to arise in any protocol requiring a return connection.

Anything more complex than an outbound connection and a return is even worse. The talk service is an example; see the discussion in Chapter 19, "Real-Time Conferencing Services", for an example of a service with a tangled web of connections that's almost impossible to pass through a firewall. (It doesn't help any that talk is partly UDP-based, but even if it were all TCP, it would still be a firewall designer's nightmare.)

13.4.3. One Session per Connection

It's almost as bad to have multiple sessions on the same connection as it is to have multiple connections for the same session. If a connection is used for only one purpose, the firewall can usually make security checks and logs at the beginning of the connection and then pay very little attention to the rest of the transaction. If a connection is used for multiple purposes, the firewall will need to continue to examine it to see if it's still being used for something that's acceptable.

13.4.4. Assigned Ports

For a firewall, the ideal thing is for each protocol to have its own port number. Obviously, this makes things easier for packet filters, which can then reliably identify the protocol by the port it's using, but it also simplifies life for proxies. The proxy has to get the connection somehow, and that's easier to manage if the protocol uses a fixed port number that can easily be redirected to the proxy. If the protocol uses a port number selected at configuration time, that port number will have to be configured into the proxy or packet filter as well. If the protocol uses a negotiated or dynamically assigned port, as RPC-based protocols do, the firewall has to be able to intercept and interpret the port negotiation or lookup. (See Chapter 14, "Intermediary Protocols", for more information about RPC.)

Furthermore, for security it's desirable for the protocol to have its very own assigned port. It's always tempting to layer things onto an existing protocol that the firewall already permits; that way, you don't have to worry about changing the configuration of the firewall. However, when you layer protocols that way, you change the security of the firewall, whether or not you change its configuration. There is no way to let a new protocol through without having the risks of that new protocol; hiding it in another protocol will not make it safer, just harder to inspect.

13.4.5. Protocol Security

Some services are technically easy to allow through a firewall but difficult to secure with a firewall. If a protocol is inherently unsafe, passing it through a firewall, even with a proxy, will not make it any safer, unless you also modify it. For example, X11 is mildly tricky to proxy, for reasons discussed at length in Chapter 18, "Remote Access to Hosts", but the real reason it's difficult to secure through firewalls has nothing to do with technical issues (proxy X servers are not uncommon as ways to extend X capabilities). The real reason is that X provides a number of highly insecure abilities to a client, and an X proxy system for a firewall needs to provide extra security.

The two primary ways to secure inherently unsafe protocols are authentication and protocol modification. Authentication allows you to be certain that you trust the source of the communication, even if you don't trust the protocol; this is part of the approach to X proxying taken by SSH. Protocol modification requires you to catch unsafe operations and at least offer the user the ability to prevent them. This is reasonably possible with X (and TIS FWTK provides a proxy called x-gw that does this), but it requires more application knowledge than would be necessary for a safer protocol.

If it's difficult to distinguish between safe and unsafe operations in a protocol, or impossible to use the service at all if unsafe operations are prevented, and you cannot restrict connections to trusted sources, a firewall may not be a viable solution. In that case, there may be no good solution, and you may be reduced to using a victim host, as discussed in Chapter 10, "Bastion Hosts". Some people consider HTTP to be such a protocol (because it may end up transferring programs that are executed transparently by the client).