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Building Internet Firewalls

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Previous: 6.3 What Does a Packet Look Like?Chapter 6
Packet Filtering
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6.4 What Does the Router Do with Packets?

Once a packet filtering router has finished examining a specific packet, what can it do with that packet? There are two choices:

6.4.1 Logging Actions

Regardless of whether the packet is forwarded or dropped ("permitted" or "denied" in some packet filtering implementations), you might want the router to log the action that has been taken. This is especially true if you drop the packet because it runs afoul of your packet filtering rules. In this case, you'd like to know what's being tried that isn't allowed.

You probably aren't going to log every packet that is allowed, but you might want to log some of these packets. For example, you might want to log start-of-connection TCP packets, so that you can keep track of incoming and outgoing TCP connections. Not all packet filters will log allowed packets.

Different packet filtering implementations support different forms of logging. Some will log only specific information about a packet, and others will forward or log an entire dropped packet. Generally, your packet filter will need to be configured to log to a host somewhere via the syslog service. You don't want the only copy of the logs to be on the packet filter if it is compromised. Most packet filtering also occurs on dedicated routers, which rarely have large amounts of disk space to dedicate to logging. See the discussion of setting up logging in Chapter 5, Bastion Hosts and Chapter 12, Maintaining Firewalls.

6.4.2 Returning ICMP Error Codes

If a packet is to be dropped, the router may or may not send back an ICMP error code indicating what happened. Sending back an ICMP error code has the effect of warning the sending machine not to retry sending the packet; thereby saving some network traffic and some time for the user on the remote side. (If you send back an ICMP error code, the user's connection attempt will fail immediately; otherwise it will time out, which may take several minutes.)

There are two sets of relevant ICMP codes to choose from:

ICMP's designers intended the first pair of ICMP error codes that the router might return, "host unreachable" or "network unreachable", to indicate serious network problems: the destination host is down or something in the only path to the host is down. These error codes predate firewalls and packet filtering. The problem with returning one of these error codes is that some hosts (particularly if they're running older versions of UNIX) take them quite literally. If these machines get back a "host unreachable" for a given host, they will assume that the host is totally unreachable and will close all currently open connections to it, even if the other connections were permitted by the packet filtering.

The second set of ICMP error codes the router might return, "host administratively unreachable" and "network administratively unreachable", were added to the official list of ICMP message types a few years ago, specifically to give packet filtering systems something to return when they dropped a packet. Many systems do not yet recognize these codes, although that should not cause your system problems. Systems are supposed to simply ignore ICMP error codes they don't understand, so this should be equivalent to returning no error code to such systems. On the other hand, many devices comply poorly with standards, and this is the kind of boundary condition some of them may not handle gracefully. The fact that the standard requires a system to ignore unknown error codes does not ensure that a system will not, in fact, react fatally to such an error code. It simply ensures that you will be able to argue persuasively that it is not your fault if they do so!

There are several issues to consider when you are deciding whether or not your packet filtering system should return ICMP error codes.

Which set of error codes makes sense for your site?

Returning the old "host unreachable" and "network unreachable" codes is technically incorrect (remember that the host may or may not be unreachable, according to the packet filtering policy, depending on what host is attempting to access what service). Also, these error codes can cause many systems to react excessively (shutting down all connections to that host or network).

Returning the new "host administratively unreachable" or "network administratively unreachable" codes advertises the fact that there is a packet filtering system at your site, which you may or may not want to do. These codes may also cause excessive reactions in faulty IP implementations.

There is another consideration as well. Generating and returning ICMP error codes takes a certain small amount of effort on the part of the packet filtering router. An attacker could conceivably mount a denial of service attack by flooding the router with packets the router would reject, and for which it would try to generate ICMP error packets. The issue isn't network bandwidth; it's CPU load on the router. (While it's busy generating ICMP packets, it's not able to do other things as quickly, like make filtering decisions.) On the other hand, not returning ICMP error codes will cause a small amount of excess network traffic, as the sending system tries and retries to send the packet being dropped. This traffic shouldn't amount to much, because the number of packets blocked by a packet filtering system should be a fraction of the total number of packets processed. (If it's not a small fraction, you've got more serious problems, because people are apparently trying lots of things that "aren't allowed.")

If your router returns an ICMP error code for every packet that violates your filtering policy, you're also giving an attacker a way to probe your filtering system. By observing which packets evoke an ICMP error response, attackers can discover what types of packets do and don't violate your policy (and thus what types of packets are and are not allowed into your network). You should not give this information away, because it greatly simplifies the attacker's job. The attacker knows that packets that don't get the ICMP error are going somewhere, and can concentrate on those protocols, where you actually have vulnerabilities. You'd rather that the attacker spent plenty of time sending you packets that you happily throw away. Returning ICMP error codes speeds up attack programs; if they get back an ICMP error for something they try, they don't have to wait for a timeout.

All in all, the safest thing to do seems to be to drop packets without returning any ICMP error codes. If your router offers enough flexibility, it might make sense to configure it to return ICMP error codes to internal systems (which would like to know immediately that something is going to fail, rather than wait for a timeout), but not to external systems (where the information would give an attacker a means to probe the filtering configuration of the firewall). Even if your router doesn't seem to offer such flexibility, you may be able to accomplish the same result by specifying packet filtering rules to allow the relevant inbound ICMP packets and disallow the relevant outbound ICMP packets.

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