Pulling back from the archives this is a repost of a previous blog post. This time ripped from a guest spot at The Security Stack Exchange Community Blog.

This question on Security.SE made me think in a rather devious way. At first, I found it to be rather poorly worded, imprecise, and potentially not worth salvaging. After a couple of days I started to realize exactly how many times I've really been asked this question by well intentioned, and often, knowledgeable people. The real question should be, "Is there a recommended set of firewall rules that can be used as a standard config?" Or more plainly, I have a bunch of systems, so what rules should they all have no matter what services they provide. Now that is a question that can reasonably be answered, and what I've typically given is this:

-A INPUT -i lo -j ACCEPT
-A INPUT -p icmp --icmp-type any -j ACCEPT
# Force SYN checks
-A INPUT -p tcp ! --syn -m state --state NEW -j DROP
# Drop all fragments
-A INPUT -f -j DROP
# Drop XMAS packets
-A INPUT -p tcp --tcp-flags ALL ALL -j DROP
# Drop NULL packets
-A INPUT -p tcp --tcp-flags ALL NONE -j DROP
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

The first and last lines should be pretty obvious so I won't go into too much detail. Enough services use loopback that, except in very restrictive environments, attempting to firewall them will almost definitely be more work than is useful. Similarly, the ESTABLISHED keyword allows return traffic for outgoing connections. The RELATED keyword takes care of things like FTP that use multiple ports and may trigger multiple flows that are not part of the same connection, but are none-the-less dependent on each other. In a perfect world these would be at the top for performance reasons. Since rules are processed in order we really want the fewest number of rules processed as possible, however in order to get the full benefit from the above rule set, we want to run as many packets as possible by them.

The allow all on ICMP is probably the most controversial part of this set. While there are reconnaissance concerns with ICMP, the network infrastructure is designed with the assumption that ICMP is passed. You're better off allowing it (at least within your organization's network space), or grokking all of the ICMP messages and determining your own balance. Look at this question for some worthwhile discussion on the matter.

Now, down to the brass tacks. Let's look at each of the wonky rules in order.

Forcing SYN Checks

-A INPUT -p tcp ! --syn -m state --state NEW -j DROP

This rule performs two checks:

1. Is the SYN bit NOT set, and
2. Is this packet NOT part of a connection in the state table

If both conditions match, then the packet gets dropped. This is a bit of a low-hanging fruit kind of rule. If these conditions match, then we're looking at a packet that we just downright shouldn't be interested in. This could indicate a situation where the connection has been pruned from the conntrack state table, or perhaps a malicious replay event. Either way, there isn't any typical benefit to allowing this traffic, so let's explicitly block it.

Fragments Be Damned

-A INPUT -f -j DROP

This is an easy one. Drop any packet with the fragment bit set. I fully realize this sounds pretty severe. Networks were designed with the notion of fragmentation, in fact the the IPv4 header specifically contains a flag that indicates whether or not that packet should or should not be fragmented. Considering that this is a core feature of IPv4, fragmentation is still a bit of a touchy subject. If the DF bit is set, then MTU path discovery should just work. However, not all devices respond back with the correct ICMP message. The above rule is one of them, however that's because ICMP type 3 code 4 (wikipedia) isn't a reject option in iptables. As a result of this one can't really know whether or not your packets will get fragmented along the network. Nowadays, on internal networks at least, this usually isn't a problem. You may run into problems, however, when dealing with VPNs and similar where your 1500 byte ethernet segment suddenly needs to make space for an extra header.

So now that we've talked about all the reasons to not drop fragments, here's the reason to. By default, standard iptables rules are only applied to the packet marked as the first fragment. Meaning, any packet marked as a fragment with an offset of 2 or greater is passed through, the assumption being that if we receive an invalid packet then reassembly will fail and the packets will get dropped anyway. In my experience, fragmentation is a small enough problem that I don't want to deal with risk and block it anyway. This should only get better with IPv6 as path MTU discovery is placed more firmly on the client and is considered less "Magic."

Christmas in July

-A INPUT -p tcp --tcp-flags ALL ALL -j DROP

Network reconnaissance is a big deal. It allows us to get a good feel for what's out there so that when doing our work we have some indications of what might exist instead of just blindly stabbing in the dark. So called ‘Christmas Tree Packets' are one of those reconnaissance techniques used by most network scanners. The idea is that for whatever protocol we use, whether TCP/UDP/ICMP/etc, every flag is set and every option is enabled. The notion is that just like a old style indicator board the packet is "lit up like a Christmas tree". By using a packet like this we can look at the behavior of the responses and make some guesses about what operating system and version the remote host is running. For a White Hat, we can use that information to build out a distribution graph of what types of systems we have, what versions they're running, where we might want to focus our protections, or who we might need to visit for an upgrade/remediation. For a Black Hat, this information can be used to find areas of the network to focus their attacks on or particularly vulnerable looking systems that they can attempt to exploit. By design some flags are incompatible with each-other and as a result any Christmas Tree Packet is at best a protocol anomaly, and at worst a precursor to malicious activity. In either case, there is normally no compelling reason to accept such packets, so we should drop them just to be safe.

I have read instances of Christmas Tree Packets resulting in Denial of Service situations, particularly with networking gear. The idea being that since so many flags and options are set, the processing complexity, and thus time, is increased. Flood a network with these and watch the router stop processing normal packets. In truth, I do not have experience with this failure scenario.

Nothing to See Here

-A INPUT -p tcp --tcp-flags ALL NONE -j DROP

When we see a packet where none of the flags or options are set we use the term Null Packet. Just like with the above Christmas Tree Packet, one should not see this on a normal, well behaved network. Also, just as above, they can be used for reconnaissance purposes to try and determine the OS of the remote host.