[tproxy] Fwd: Tproxy changes for performing dual NAT

Tóth László Attila panther at elte.hu
Tue Oct 30 11:49:55 CET 2007

On 2007.10.30., at 11:40, KOVACS Krisztian wrote:

> Hi,
> On k, okt 30, 2007 at 02:59:51 +0530, Arun S wrote:
>>> That seems ok and it works in the first scenario. This is what I
>>> tested with netcat patched to use IP_TRANSPARENT socket option.  
>>> But I
>>> didn't use SNAT because IP_TRANSPARENT lets the program bind to any
>>> IP address when it connects to any other server (this would be  the
>>> server-side connection of the squid if the binding to foreign  
>>> address
>>> is necessary).
>> That is fine. Since IP_TRANSPARENT lets the program to bind to any IP
>> address, the application can be made to use any IP address as the
>> source.
>> But let us assume the following scenario:
>> there are two outgoing WAN interfaces: eth0 and eth1.
>> 1. Outgoing Traffic from eth0 should not be SNAT-ted.
>> 2. Outgoing traffic from eth1 should be SNAT-ted.
>> All WWW traffic gets marked, hits TPROXY redirect rule, and goes to
>> TPROXY server.
>> Case 1 is fine for TPROXY traffic and other traffic.
>> But in Case 2, when SNAT happens, three-way handshake between TPROXY
>> server and Web server is not successful. This issue is only with the
>> Web traffic that is originated from TPROXY server (i.e., the server
>> with IP_TRANSPARENT option set).
>> Observation:
>> 1. TPROXY server sends SYN packet with foreign source IP to WWW  
>> server
>> 2. WWW server sends SYN-ACK to TPROXY server.
>> 3. TPROXY server is not sending ACK to WWW server that leads to a
>> half-open connection.
>> Please let me know if you require more information.
> This is probably a byproduct of the fact that PREROUTING/mangle is
> traversed before PREROUTING/nat.
> If you SNAT a TCP connection with non-local IP then the return  
> packet on
> mangle will have the modified destination IP and thus the socket match
> won't find the socket.

Ops, I forgot the routing table numbered 100. If the socket match  
doesn't find the socket, it won't be marked to 1 also the packet's  
destination won't be handled as a local address this is why it fails:
ip rule add fwmark 1 lookup 100
ip route add local dev lo table 100


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