Re: Does Redirect/NAT change the destination port of reverse tuple ?

[Pascal Hambourg <pascal.mail@plouf.fr.eu.org>]

Hello,

Nishit Shah a écrit :
> 	I have following iptable rule in system.
> 
> 	iptables -I PREROUTING -t nat -p tcp --dport 443 -j REDIRECT
> --to-ports 3128
> 	kernel - 2.6.16.13
> 
> 	I am using following scenario for load testing
> 
> 	192.168.206.200 -----> 192.168.121.125 	-----> 	72.14.223.83
> 	Client			iptables/proxy server		server
> 
> 	and I am getting following entry through conntrack binay as well as
> in /proc/net/ip_conntrack
> 
> 	[NEW] tcp 6 120 NONE src=192.168.206.200 dst=72.14.223.83
> sport=63423 dport=443 packets=1 bytes=48 [UNREPLIED] src=192.168.121.125
> dst=192.168.206.200 sport=3128 dport=46873 packets=0 bytes=0 id=28187887
> 
> Now here original and reverse tuples are --> 
> 	Original tuple 192.168.206.200:63423->72.14.223.83:443
> 	Reply tuple    192.168.121.125:3128->192.168.206.200:46873
> 
> So, here destination port of reverse tuple is 46873. Is it correct ?

Yes. NAT may implicitly change the original source port in order to 
avoid a clash with an existing connection. However the original port 
will be restored in reply packets before they leave the box, so the 
client won't see anything. Remember that the tuples in ip_conntrack 
contain the addresses and ports when packets enter the PREROUTING or 
OUTPUT chains, not when they leave the POSTROUTING or INPUT chains.
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