Let me explain why the marking is done in POSTROUTING. The first packet of any connection get routed by the multipath routing entry. This happens AFTER PREROUTING, as you know. And this is what we want, letting the kernel decide based on the weights. (some people do think that we shouldn't let multipath decide routing, but thatz a different story). So where can this packet be marked? Obviously in POSTROUTING (so that local pkts also can be caught). We mark it and save it.(connmark).The mark is decoded by the chosen interface. (eg:-o WAN1 --set mark 1,-o WAN2 --set-mark 2) In PREROUTING, there is a restore-mark. You see iptables -t mangle -A PREROUTING -j CONNMARK --restore-mark. If this packet belong to a connection that has already sent a packet,this will restore the mark set in POSTROUTING. Then it will be routed by the corresponding routing table.(wan1 table lookup mark1 and wan2 table lookup mark2) If it is a new pkt, it will be routed by multipath routing statement,since no mark exists. -----Original Message----- From: lartc-bounces@xxxxxxxxxxxxxxx [mailto:lartc-bounces@xxxxxxxxxxxxxxx] On Behalf Of Peter Rabbitson Sent: Thursday, May 10, 2007 6:51 PM Cc: lartc@xxxxxxxxxxxxxxx Subject: Re: Load balancing using connmark Salim S I wrote: > Francis Brosnan Blazquez wrote: > >> Hi, > >> > >> I've been implementing a load balancing solution using CONNMARK, based > >> on solution described by Luciano Ruete at [1]. Gracias por el post y por > >> apuntar en la dirección correcta Luciano! > >> > >> Once implemented, I've found that due to some reason packets aren't > >> properly marked (or improperly remarked) and sent out using the wrong > >> interface. > >> > >> <snip> > >> > >> iptables -t mangle -A POSTROUTING -m mark --mark ! 0 -j ACCEPT > >> iptables -t mangle -A POSTROUTING -o eth1 -j MARK --set-mark 0x1 > >> iptables -t mangle -A POSTROUTING -o eth2 -j MARK --set-mark 0x2 > >> iptables -t mangle -A POSTROUTING -j CONNMARK --save-mark > > > > This is wrong. POSTROUTING is exactly what is is _POST_ routing. By the > > time you do your marks and stuff the kernel has _already_ assigned a > > packet to an interface, and you can not alter this anymore. > > > >> After a bit of testing with the second solution, it seems to behave > >> better, doing all marking job at the PREROUTING and OUTPUT. > > > > This is flawed too. OUTPUT suffers from the very same problem as > > POSTROUTING - by the time the packets hit the NF stack the process has > > already bound itself to an interface, which you can not change anymore. > > > > Peter > > > > Disagree with Peter. The marking in postrouting table is CONNMARK. This > is for marking the connection, which has already had a route decided for > it, so that all packets of the connection passes through this interface. > This marking is done for packets with NEW state, see the check for > mark==0 in the prev. line. The restore mark in PREROUTING will restore > the connmark and route the subsequent packets. > > This approach will work, but you need some sort of stateful-ness in > netfilter. > Connmark is exactly the statefullness you are talking about. The problem is that the marks by themselves do not mean anything. You mark packets and expect iproute to classify the packet in the correct routing table etc. CONNMARK is invisible to iproute - this is why you have only --save-mark and --restore-mark, and the rest of the rules deal with real MARKs. Further you (and the OP) seem to be confused by a mix of routing tasks. In the case of _forwarded_ traffic, you need to make sure that all packets within a connection leave to WAN over the same interface, and are SNATed to the same ip, so that they will come bak the same interface. The SNATting is trivial (as it can be done in POSTROUTING only), but you need to set all marks before the routing takes place (which is anywhere _but_ POSTROUTING). You might mark the connection with the proper CONNMARK. and subsequent packets might get routed correctly, but the _first_ packet (the one that you use to set the mark) is already assigned to an interface, and there is nothing you can do about it. In the case of _local_ traffic - it becomes even trickier. The problem is that when sockets are created they already have a source IP (the kernel determines that by looking at the default routing table, your marks do not exist yet). So since you can not alter the socket binding, the only way to make it leave on a different interface is by treating it as a forwarded connection and performing NAT on it. It is arguable if NATting locally originating connections is a good idea, but it can be done in OUTPUT just like it is done for forwarder connections in PREROUTING. I hope this clarifies things a bit, feel free to point out any inconsistencies you may find. Peter _______________________________________________ LARTC mailing list LARTC@xxxxxxxxxxxxxxx http://mailman.ds9a.nl/cgi-bin/mailman/listinfo/lartc _______________________________________________ LARTC mailing list LARTC@xxxxxxxxxxxxxxx http://mailman.ds9a.nl/cgi-bin/mailman/listinfo/lartc
