UNTRACKED is a rather special keyword when it comes to connection tracking in Linux. Basically, it is used to match packets that has been marked in the raw table not to be tracked.
The raw table was created specifically for this reason. In this table, you set a NOTRACK mark on packets that you do not wish to track in netfilter.
Notice how I say packets, not connection, since the mark is actually set for each and every packet that enters. Otherwise, we would still have to do some kind of tracking of the connection to know that it should not be tracked.
As we have already stated in this chapter, conntrack and the state machine is rather resource hungry. For this reason, it might sometimes be a good idea to turn off connection tracking and the state machine.
One example would be if you have a heavily trafficked router that you want to firewall the incoming and outgoing traffic on, but not the routed traffic. You could then set the NOTRACK mark on all packets not destined for the firewall itself by ACCEPT'ing all packets with destination your host in the raw table, and then set the NOTRACK for all other traffic. This would then allow you to have stateful matching on incoming traffic for the router itself, but at the same time save processing power from not handling all the crossing traffic.
Another example when NOTRACK can be used is if you have a highly trafficked webserver and want to do stateful tracking, but don't want to waste processing power on tracking the web traffic. You could then set up a rule that turns of tracking for port 80 on all the locally owned IP addresses, or the ones that are actually serving web traffic. You could then enjoy statefull tracking on all other services, except for webtraffic which might save some processing power on an already overloaded system.
There is however some problems with NOTRACK that you must take into consideration. If a whole connection is set with NOTRACK, then you will not be able to track related connections either, conntrack and nat helpers will simply not work for untracked connections, nor will related ICMP errors do. You will have to open up for these manually in other words. When it comes to complex protocols such as FTP and SCTP et cetera, this can be very hard to manage. As long as you are aware of this, you should be able to handle this however.