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Annotation of embedaddon/dnsmasq/contrib/conntrack/README, revision 1.1.1.2

1.1       misho       1: Linux iptables includes that ability to mark individual network packets
                      2: with a "firewall mark".  Additionally there is a component called
                      3: "conntrack" which tries to string sequences of related packets together
                      4: into a "connection" (it even relates sequences of UDP and ICMP packets).
                      5:  There is a related mark for a connection called a "connection mark".
                      6: Marks can be copied freely between the firewall and connection marks
                      7: 
                      8: Using these two features it become possible to tag all related traffic
                      9: in arbitrary ways, eg authenticated users, traffic from a particular IP,
                     10: port, etc. Unfortunately any kind of "proxy" breaks this relationship
                     11: because network packets go in one side of the proxy and a completely new
                     12: connection comes out of the other side.  However, sometimes, we want to
                     13: maintain that relationship through the proxy and continue the connection
                     14: mark on packets upstream of our proxy
                     15: 
1.1.1.2 ! misho      16: Dnsmasq includes such a feature enabled by the --conntrack
1.1       misho      17: option. This allows, for example, using iptables to mark traffic from
                     18: a particular IP, and that mark to be persisted to requests made *by*
1.1.1.2 ! misho      19: Dnsmasq. Such a feature could be useful for bandwidth accounting,
1.1       misho      20: captive portals and the like. Note a similar feature has been 
                     21: implemented in Squid 2.2
                     22: 
                     23: 
                     24: As an example consider the following iptables rules:
                     25: 
                     26: 
                     27: 1) iptables -t mangle -A PREROUTING -j CONNMARK --restore-mark
                     28: 2) iptables -t mangle -A PREROUTING -m mark --mark 0 -s 192.168.111.137
                     29: -j MARK --set-mark 137
                     30: 3) iptables -t mangle -A PREROUTING -j CONNMARK --save-mark
                     31: 
                     32: 4) iptables -t mangle -A OUTPUT -m mark ! --mark 0 -j CONNMARK --save-mark
                     33: 
                     34: 1-3) are all applied to the PREROUTING table and affect all packets
                     35: entering the firewall.
                     36: 
                     37: 1) copies any existing connection mark into the firewall mark. 2) Checks
                     38: the packet not already marked and if not applies an arbitrary mark based
                     39: on IP address. 3) Saves the firewall mark back to the connection mark
                     40: (which will persist it across related packets)
                     41: 
                     42: 4) is applied to the OUTPUT table, which is where we first see packets
1.1.1.2 ! misho      43: generated locally. Dnsmasq will have already copied the firewall mark
1.1       misho      44: from the request, across to the new packet, and so all that remains is
                     45: for iptables to copy it to the connection mark so it's persisted across
                     46: packets.
                     47: 
                     48: Note: iptables can be quite confusing to the beginner. The following
                     49: diagram is extremely helpful in understanding the flows
                     50:        http://linux-ip.net/nf/nfk-traversal.png
                     51: Additionally the following URL contains a useful "starting guide" on
                     52: linux connection tracking/marking
                     53:        http://home.regit.org/netfilter-en/netfilter-connmark/
                     54: