Hello,
longraider a écrit :
linux-2.6.14.2 with imq patch
eth0 - iface where two inet connections are attached
eth1 - server
eth2 - LAN
There is SNAT involved on one net connection. The other conn is for
servers, and there is proxy-arp active (at eth0 and eth1).
I type:
iptables -t nat -A PREROUTING -i eth0 -j LOG
And after that, dmesg shows something like that:
17:08:53 IN=eth0 OUT= SRC=some_remote_IP DST=IP_of_the_linux_box
Shouldn't be there DST=10.0.0.5 for example (ie. de-SNATed)?
This packet probably does not belong to a SNAT-ed or MASQ-ed connection.
Actually, with this rule you won't see the return packets belonging to
your SNAT-ed connection. In short, the 'nat' table chains only see the
first packet of a "connection", and only if it has the state NEW (not
RELATED). All the subsequent valid packets belonging or related to that
connection (state NEW, ESTABLISHED, or RELATED) don't go through theses
chains. The action taken by these packets is automatically determined by
the NAT operation applied to the first packet and the direction of the
packet.
For instance, with this rule :
iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 1.2.3.4
The first 'direct' packet of an outgoing connection on eth0 goes through
the nat POSTROUTING chains and matches this rule, so the SNAT operation
is applied. Instead of going through the POSTROUTING chain, the
subsequent direct packets (in the same direction) of the connection will
automatically be applied the same SNAT operation. The return packets (in
the opposite direction) of the connection will automatically be applied
the de-SNAT operation instead of going through the nat PREROUTING chain.
By the way, the subsequent packets of the connection don't need to go in
or out eth0 (funny, huh ?) to be properly NATed.
And all that I want to do is ingress queuing using IMQ. I want to fwmark
packets according to their de-SNATed destination adress (and some other
things also), and then put them into the IMQ ingress queue.
I could use the packet matching available in the ingress queue itself
(by ip tool), but I don't know if the packets that go into IMQ are
de-SNATed or not.
So, where the de-SNAT actually takes place?
De-SNAT takes place in the NF_IP_PRE_ROUTING Netfilter hook at the same
place as the nat PREROUTING chain, after the mangle PREROUTING chain and
before the input routing decision.
For DNAT which occured in the PREROUTING chain, de-DNAT takes place in
the NF_IP_POST_ROUTING Netfilter hook, at the same place as the nat
POSTROUTING chain, after the mangle POSTROUTING chain.
For DNAT which occured in the OUTPUT chain, I observed that de-DNAT
takes place in the NF_IP_LOCAL_IN Netfilter hook, after the mangle and
filter INPUT chains.
BTW is this diagram correct?
http://www.docum.org/docum.org/kptd/
I think so, at least for the pure Netfilter part which matches my own
diagram http://www.plouf.fr.eu.org/bazar/netfilter/schema_netfilter.txt.
I don't know about the IMQ and QoS parts.
I think not, since traversing the magle PREROUTING can't occur
simulatenously with de-MASQ.
Incoming packets traverse the mangle PREROUTING chain just before being
de-MASQ-ed if needed.
And is this de-MASQUERADE a de-SNAT also?
Yes. Actually MASQUERADE and SNAT are similar, the only difference being
in the choice of the new source address.
De-MASQ and de-SNAT both are destination address rewrite operations, so
it is consistent that they take place in the same place as the nat
PREROUTING chain which performs DNAT. But keep in mind that they take
place *instead* of trversing the nat PREROUTING chain, so you will never
see packets being de-MASQ-ed or de-SNAT-ed in any nat chain.
