Linux iptables includes that ability to mark individual network packets
with a "firewall mark".  Additionally there is a component called
"conntrack" which tries to string sequences of related packets together
into a "connection" (it even relates sequences of UDP and ICMP packets).
 There is a related mark for a connection called a "connection mark".
Marks can be copied freely between the firewall and connection marks

Using these two features it become possible to tag all related traffic
in arbitrary ways, eg authenticated users, traffic from a particular IP,
port, etc. Unfortunately any kind of "proxy" breaks this relationship
because network packets go in one side of the proxy and a completely new
connection comes out of the other side.  However, sometimes, we want to
maintain that relationship through the proxy and continue the connection
mark on packets upstream of our proxy

Dnsmasq includes such a feature enabled by the --conntrack
option. This allows, for example, using iptables to mark traffic from
a particular IP, and that mark to be persisted to requests made *by*
Dnsmasq. Such a feature could be useful for bandwidth accounting,
captive portals and the like. Note a similar feature has been 
implemented in Squid 2.2


As an example consider the following iptables rules:


1) iptables -t mangle -A PREROUTING -j CONNMARK --restore-mark
2) iptables -t mangle -A PREROUTING -m mark --mark 0 -s 192.168.111.137
-j MARK --set-mark 137
3) iptables -t mangle -A PREROUTING -j CONNMARK --save-mark

4) iptables -t mangle -A OUTPUT -m mark ! --mark 0 -j CONNMARK --save-mark

1-3) are all applied to the PREROUTING table and affect all packets
entering the firewall.

1) copies any existing connection mark into the firewall mark. 2) Checks
the packet not already marked and if not applies an arbitrary mark based
on IP address. 3) Saves the firewall mark back to the connection mark
(which will persist it across related packets)

4) is applied to the OUTPUT table, which is where we first see packets
generated locally. Dnsmasq will have already copied the firewall mark
from the request, across to the new packet, and so all that remains is
for iptables to copy it to the connection mark so it's persisted across
packets.

Note: iptables can be quite confusing to the beginner. The following
diagram is extremely helpful in understanding the flows
	http://linux-ip.net/nf/nfk-traversal.png
Additionally the following URL contains a useful "starting guide" on
linux connection tracking/marking
	http://home.regit.org/netfilter-en/netfilter-connmark/