Using Firewall Builder, Part II
Once your bastion host's loopback needs have been attended to, turn your attention to its global policy. This requires a little thought. You want Netfilter to provide a meaningful amount of protection but not at the expense of desired functionality.
Our example host, Trillian, is a web server, so we want to allow other hosts to access it with HTTP and HTTPS. We also want to allow Trillian to perform DNS lookups for coherent logging. In addition, some sort of administrative connectivity should be allowed. The tool of choice for this purpose is SSH, so we'll also allow inbound SSH connections but only from our internal network. Figure 2 shows such a policy as defined for Trillian.
I'll spare you a blow-by-blow description of how I created every single rule, but several things are worth noting. First, in the object hierarchy on the left-hand side of the window, you can see that I had selected the global Policy object in the hierarchical level directly below Trillian, rather than either of the interface-specific policy objects.
I also referenced not only the Trillian object, but also a network object named Net_Internal, which is the example network object from last month's column. This object refers to an entire network's worth of IP addresses, 192.168.111.0 to be exact. Whereas rule 02 uses a single IP address (Trillian's) as its source IP address, rule 03 matches packets whose source IP address is any of the entire range, 192.168.111.1-192.168.111.254.
Another important tip for rule building is to get at Firewall Builder's handy prebuilt service objects, click the Standard tab on the left-hand side of the window. Be careful, though; if you do anything besides drag a service object (for example, dns_tcp) into your rules, the rules display on the right-hand part of the window is replaced with information about whatever you've selected.
In other words, if you're working on a policy, you can click on the Standard tab, click on the + (expand) and - (collapse) icons in the hierarchy window and click and drag service objects from it, all without changing the mode of the right-hand part of the window. But if you simply select a service object or category in the Standard hierarchy (by clicking on it once without dragging), that object's properties are displayed on the right. You have to go back to the User tab and reselect your firewall's global policy to display your rules again. You do not lose any data, but this can be inconvenient and unsettling if you aren't expecting it.
A more substantial observation is that in all of these rules, I left stateful inspection turned on. I skipped step 7 from our loopback-rules procedure. Normally, we want the kernel to keep state information on network transactions; this is why we can describe most transactions with a single bidirectional rule rather than with two unidirectional rules. For example, thanks to stateful inspection, whenever a transaction matches rule 02 from Figure 2, which allows inbound SSH traffic from hosts on the internal network, Trillian's kernel matches not only those inbound SSH packets, but also the SSH packets that Trillian sends back out in reply. Had I turned off stateful inspection for rule 02, I'd need another rule allowing all packets originating from TCP port 22 on Trillian to accommodate those replies.
Finally, all rules but the last one have logging turned off, as described in step 6 of our loopback-rules procedure. Most people don't find it a useful or justifiable use of disk space or I/O overhead to log every packet their firewall rules process. Personally, I tend to focus on dropped packets and forego logging on allow rules. Thus, the sample rules in Figure 2 end with a cleanup rule at the bottom that explicitly drops any packet not matching the other rules or the rules in any interface-specific policies such as the loopback policy.
This rule's sole purpose in life is for logging. Firewall Builder automatically sets the default policy for all my iptables chains to DROP, but these dropped-by-default packets aren't logged unless you tell Netfilter to do so.
An experimental dropped-table patch is available for Netfilter that allows automatic logging of all dropped packets, but I recommend you wait for this code to stabilize before going out of your way to compile it into your kernel. If you can't wait for some reason, you can access this feature from Firewall Builder by selecting your firewall object, clicking its Firewall Properties tab and checking the box next to Log all dropped packets. For more information on the dropped-table patch, see www.netfilter.org/documentation/pomlist/pom-summary.html.
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