Aseem Rastogi wrote:
in continuation:
i am actually trying to understand how nat and default filter table work
together. my understanding is this:
when a packet is encountered it is either:
1. a new connection creation request packet.
2. a packet associated with a connection that has been mangled by NAT
earlier.
3. a packet assiciated with a connection that has not been mangled by
NAT earlier.
Not really. Each packet is one of:
-> NEW packet with SYN set
-> NEW packet without SYN set
-> Part of a connection which has seen at least the SYN packet
Basically, a NEW packet means, that there is no entry in the conntrack
table.
Each case goes like this:
Case 1: NAT table is considerd. Packet passes through PREROUTING chain,
routing decision and then POSTROUTING chain.
No. Then filter/INPUT or filter/FORWARD - always.
Case 1a -- If either of them modified the packet, this packet and all
subsequent packets of this connection DO NOT PASS THROUGH FILTER TABLE
CHAINS.
Case 1b -- None of NAT tables modifies packet. It passes through FILTER
table chains as usual.
No. See above.
Case 2: This packet follows the fate of its earlier packets. (PREROUTING
AND POSTROUTING NAT table chains BUT NO FILTER table chains)
Case 3. Passes through FILTER TABLE chains.
is this correct??
No. It is almost vice-versa. Only NEW packet pass nat table, but all
packets pass filter table.
If you apply NAT to the first packet, these subsequent packet will be
NATed automagically, so - in your words - they follow the fate of their
earlier packet concerning NAT.
HTH and have a nice time,
Joerg
PS: May I recommend Oskar Andreasson's excellent iptables tutorial at
http://iptables-tutorial.frozentux.net/chunkyhtml/index.html ?
Aseem Rastogi wrote:
Hi,
I have a small query.
I have read that whenever a packet requesting a connection is
encountered, NAT table is used. My question is : Does it mean that for
new connection request packets ONLY NAT table is considered and not
default FILTER table?
