Re: [net-next PATCH 16/16] Add documentation

[Philip Craig <philipc@snapgear.com>]

Jan Engelhardt wrote:
>> +2. Redirecting traffic
>> +======================
>> +
>> +Transparent proxying often involves "intercepting" traffic on a router. This is
>> +usually done with the iptables REDIRECT target, however, there are serious
>> +limitations of that method. One of the major issues is that it actually
>> +modifies the packets to change the destination address -- which might not be
>> +acceptable in certain situations. (Think of proxying UDP for example: you won't
>> +be able to find out the original destination address. Even in case of TCP
>> +getting the original destination address is racy.)
> 
> IIRC, you _can_ find out, though I agree it's rather a hack (with 
> tproxy, you can just use the address as received via recvmsg):
> 
> 	getsockopt(fd, SOL_IP, SO_ORIGINAL_DST, &sockaddr, &sizeptr);

Yes, but the problem is that SO_ORIGINAL_DST is only implemented for TCP.
And I guess that the race for TCP is that the conntrack may not exist when you
call getsockopt() (not sure that is something you'll hit in practice though).