8.12. Putting It All TogetherThis section works through a few more examples to show how many of the concepts we've talked about in this chapter come together in the real world. For detailed discussions of the packet filtering characteristics of particular protocols, see the chapters in Part III, "Internet Services".
This section is designed to demonstrate the process of developing a filter set; filters are elaborated as we go on, rather than being produced in final form. We aren't attempting to show a complete filter set for any site. Every site is different, and you can get burned by packet filtering if you don't understand all the details and implications of its use in your particular environment. We want people to carefully consider and understand what they're doing -- not blindly copy something out of a book (even ours!) without a careful consideration of how relevant and appropriate it is for their own situation. In any case, a full solution for a site requires considering packet filtering, proxying, and configuration issues. That process is illustrated in Chapter 24, "Two Sample Firewalls".
TIP: We assume in this example that, for each packet, your filtering system looks at the rules in order. It starts at the top until it finds a rule that matches the packet, and then it takes the action specified.Now, let's consider some sample packets to see what happens. Let's say that your host has IP address 172.16.1.1, and that someone is trying to send you mail from the remote host with IP address 192.168.3.4. Further, let's say the sender's SMTP client uses port 1234 to talk to your SMTP server, which is on port 25. (SMTP servers are always assumed to be on port 25; see the discussion of SMTP in Chapter 16, "Electronic Mail and News").
Figure 8-8 shows this case.
Figure 8-8. Packet filtering: inbound SMTP (sample packets 1 and 2)In this case, the packet filtering rules permit your incoming email:
Figure 8-9 shows this case.
Figure 8-9. Packet filtering: outbound SMTP (sample packets 3 and 4)Again, in this case, the packet filtering rules permit your outgoing email:
Figure 8-10 shows this case.
Figure 8-10. Packet filtering: inbound SMTP (sample packets 5 and 6)The preceding rule set allows this connection to take place! In fact, this rule set allows any connection to take place as long as both ends of the connection are using ports above 1023. Why?
What can you do about this? Well, what if you also looked at the source port in making your filtering decisions? Here are those same five basic rules with the source port added as a criterion.
And here are those same six sample packets, filtered by the new rules.
As you can see, when the source port is also considered as a criterion, the problem packets (numbers 5 and 6, representing an attack on your web proxy server) no longer meet any of the rules for packets to be permitted (rules A through D). The problem packets end up being denied by the default rule.
OK, now what if you're dealing with a slightly smarter attacker? What if the attacker uses port 25 as the client port on his end (he might do this by killing off the SMTP server on a machine he controls and using its port, or by carrying out the attack from a machine that never had an SMTP server in the first place, like a PC) and then attempts to open a connection to your web proxy server? Here are the packets you'd see.
Figure 8-11 shows this case.
As you can see, the packets would be permitted, and the attacker would be able to make connections through your proxy server (as we discuss in Chapter 15, "The World Wide Web", this would certainly be annoying and could be disastrous).
So what can you do? The solution is to also consider the ACK bit as a filtering criterion. Again, here are those same five rules with the ACK bit also added as a criterion.
Figure 8-11. Packet filtering: inbound SMTP (sample packets 7 and 8)Now, packet 7 (the attacker attempting to open a connection to your web proxy server) will fail, as follows.
The only differences in this rule set are in rules B and D. Of these, rule D is the most important because it controls incoming connections to your site. Rule B applies to connections outgoing from your site, and sites are generally more interested in controlling incoming connections than outgoing connections.
Rule D now says to accept incoming packets from things that are supposedly SMTP servers (because the packets are coming from port 25) only if the packets have the ACK bit set; that is, only if the packets are part of a connection started from the inside (from your client to his server).
If someone attempts to open a TCP connection from the outside, the very first packet that he or she sends will not have the ACK bit set; that's what's involved in "opening a TCP connection". (See the discussion of the ACK bit in Section 4.3.1, "TCP" of the Section 4.3, "Protocols Above IP" discussion in Chapter 4, "Packets and Protocols ".) If you block that very first packet (packet 7 in the preceding example), you block the whole TCP connection. Without certain information in the headers of the first packet -- in particular, the TCP sequence numbers -- the connection can't be established.
Why can't an attacker get around this by simply setting the ACK bit on the first packet? Because the packet will get past the filters, but the destination will believe the packet belongs to an existing connection (instead of the one with which the packet is trying to establish a new connection). When the destination tries to match the packet with the supposed existing connection, it will fail because there isn't one, and the packet will be rejected.
TIP: As a basic rule of thumb, any filtering rule that permits incoming TCP packets for outgoing connections (that is, connections initiated by internal clients) should require that the ACK bit be set.At this point, you now have a simple rule set that allows the traffic you set out to allow, and only that traffic. It's not a full rule set (it doesn't include the default rules we discussed earlier), and it's not a very interesting rule set (you almost certainly want to allow more protocols than just SMTP). But it's a functioning rule set that you understand precisely, and from here you can build a configuration that actually meets your needs, using the information in the rest of this book.
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