What are the the big drawbacks to relying exclusively on MAC filtering for securing a small, personal network?
Update: 2 people have mentioned MAC spoofing. I'm not sure I by that at face-value: A MAC address has a key space of 16 ^ 12, whereas some wireless security protocols limit you to 8 alphanumeric characters, a key space of 36 ^ 8 (smaller than the MAC address).
Unless I'm missing something? Perhaps MAC addresses, like credit cards, can be narrowed down to a much smaller range of values?
When adding a new wireless device, you try for about an hour to connect it with the correct key and wondering why it isn't working.... then you get a eureka moment and realise you had it turned on... (Happened to me a while ago at a client!).
In all seriousness, when it comes to security, as long as you remember the settings, every little helps - unless you are the sort of person who lets friends on the wireless all the time, there is little reason not to use it.
MAC addresses can always be forged by an attacker, but at the end of the day, every little helps when it comes to security and it is another hurdle that an attacker will have to jump over.
Edit, Just read the question again... Relying just on MAC filtering is a bad idea, if you mean using no key at all. This would mean that any rogue device will be able to listen in on pretty much anything going on over your network.
Update to your edited question -...MAC addresses are sent in the packet header to the router, if you are using no encryption, it will be EASY to read in plain text with the correct tools... If you use a key in combination to MAC security, it will still be guessable, but it will take significant time (encryption level depending) for someone to crack it.
You can easily spoof a mac address, and means that you need to allow anyone who comes over to your house access to the network.
@Wil is right. The nature of ethernet means that MAC addresses are sent with every packet, and so it's not so much a matter of your attacker having to bruteforce the MAC address keyspace as just monitoring a few minutes worth of traffic with NetStumbler, Kismet, or similar and then picking a MAC address that he's seen. Once he assigns that to his system's network interface he is part of your network, for all intents and purposes.
So relying only on MAC address filtering is bad. Relying on it as part of another strategy, where it's one more thing an attacker has to deal with is better. More of the 'I don't have to be faster than the bear, I just have to be faster than you' school of thought. If that makes your network just enough more difficult to compromise, an attacker who's solely interested in getting 'net access is going to move on to easier meat.
A friend of mine told me that they rate safes in minutes. That didn't make sense to me, at first, but he clarified: no safe can be crack-proof. But safes with various characteristics will hold up for n minutes against a skilled attacker with normal (non-stationary) tools. Wireless networks are much the same. No network is uncrackable, but anything you can add to make it more frustrating to crack increases the possibility of your attacker moving on. This, of course, does not take into account a personally motivated attacker like the black hat whose wife you might have stolen, but that's beyond the scope of this discussion.
