Prev | Contents | Next

# 36 Firewalls

A favorite component of every bad hacker movie ever is the firewall. It’s clear from those bad movies that it has something to do with security, but the exact role is ambiguous; it’s only apparent that a bad guy getting through the firewall is a bad thing.

But back to reality: a firewall is a computer (often a router) that restricts certain types of traffic between one interface and another. So instead of forwarding every packet of every type from every port, it might decide to drop those packets instead.

The upshot is that if someone in trying to connect to a port on the far side of a firewall, the firewall might prevent that connection depending on how it is configured.

In this chapter, we’ll be speaking conceptually about firewalls, but won’t get into any specifics about practical configuration. There are many implementations of firewalls out there, and they all tend to have different methods of configuration.

## 36.1 Firewall Operation

If you think about a packet arriving at a router, that router has the capability to inspect that packet all it wants. It can look at the source and destination IPs, or the ports, or even the application layer data.

So that gives it the opportunity to make a decision about whether or not to keep forwarding that packet based on any of that data.

For example, the firewall might be configured to allow any port 80 (HTTP) traffic to come from the outside of the firewall to the inside, but block incoming traffic on all other destination ports.

Or maybe it’ll be configured to only allow certain IP addresses to connect.

So the firewall, on receipt of a packet, looks at its configured rules and decides whether or not to drop the packet (if it’s being filtered out), or forward the packet normally.

When it decides to drop the packet, it has a more options. It can either silently drop it, or it can reply with a ICMP “destination unreachable” message.

That basic filtering gives some a control, but there are still some things you might want to do that it can’t manage.

For example, if someone behind the firewall establishes a TCP connection over the Internet from a random port, we want to be able to get traffic back to that host from that random port.

With plain filtering, we’d just have to allow traffic on all those ports.

But with stateful filtering, the firewall tracks the state of TCP connections that have been initiated from behind the firewall. The firewall sees the TCP three-way handshake and knows then that there is a valid connection. It can then safely allow traffic back that’s destined for the inside computer’s random port number.

Stateful filtering is also used with UDP traffic, even though it’s connectionless. The firewall sees a UDP packet go out, and it’ll allow incoming UDP packets to that port. (For a while. Since it’s connectionless, there’s no way to tell when the server and client and done. So the firewall will timeout UDP rules after they haven’t been used for a while.)

> In order to keep the firewall from timing-out on any of its rules, the programs can send keepalive messages. TCP actually has a specific message type for this, and the OS will periodically send empty `ACK` packets out to keep anyone from timing out. (If programming with sockets, you will likely need to set the `SO_KEEPALIVE` option.)

> The keepalive can also be effectively implemented by a program using UDP, as well. It could be configured to send a custom keepalive packet to the receiver (who would reply with some kind of ACK) every so often.

> Keepalive is only an issue for programs that have long periods of no network traffic.

Finally, filtering could also be done at the application layer. If the firewall digs deeply enough into the packet, it might see that it’s HTTP or FTP data, for example. With that knowledge, it could allow or deny all HTTP traffic, even if it’s arriving on a non-standard port.

## 36.2 Firewalls and NAT

In a home network, it’s really common for the firewall to also be the router and the switch and do NAT.

It’s all rolled into one.

But there’s nothing stopping us from having a separate firewall computer on the network that only decides whether or not to allow traffic.

And there’s no rule that one side of the firewall must be a private network and the other side a public network. Both sides could be private networks or public networks, or one private and one public.

The role of the firewall at its core is not to separate public and private networks–that’s NAT’s role–but rather to control which traffic is allowed to pass through the firewall.

## 36.3 Local Firewalls

You can also set up a firewall just on your computer (and this is commonly done). Typically this is set up to allow all connections from the computer going out, but to restrict connections from other computers coming in.

In MacOS and Windows, the firewall is something that can simply be turned on and then it will start blocking traffic based on some common rules.

If you need some specific ports unblocked, you’ll have to manually add those.

Linux has firewall support through a mechanism called iptables. This isn’t the most straightforward thing to configure, but it’s powerful enough to build a NAT router/firewall from scratch.

## 36.4 Reflect

- What’s the difference/relationship between a firewall and a router?

- What’s the difference/relationship between a firewall and NAT?

- What’s the funniest or most painful thing in this NCIS clip?

Prev | Contents | Next