[tproxy] tproxy can't work with ip_conntrack

[KOVACS Krisztian]

Hi,

On sze, okt 22, 2008 at 04:11:09 +0800, Dong Wei wrote:
> >> I think that we can add some new fields to identify the two HTTP
> >> connections in ip_conntrack structure(one is Client->TPROXY Server,
> >> and the other is TPROXY Server->HTTP Server).
> >> I hope we can get the right conntrack when we just know the tuple
> >> containing src_ip,src_port,dst_ip,dst_port. Because there is lots of
> >> kernel code like this:
> >>
> >> 1. get the tuple 4 fields(src_ip,src_port,dst_ip,dst_port) from skb
> >> 2. call: nf_conntrack_find_get(tuple) to find the corresponding ip_conntrack.
> >> So in this case we can only know the 4 fields, we can't get any more
> >> message from skb.
> >>
> >> Assuming there are 2 ip_conntrack:
> >> [1]: 192.168.2.2:12345->192.168.1.2:80(client->TPROXY Server)
> >> [2]: 192.168.2.2:12345->192.168.1.2:80(TPROXY Server->HTTP Server)
> >> When we process TCP packets between client and TPROXY Server, we find
> >> the conntrack for 192.168.2.2:123456->192.168.1.2:80, the result
> >> should
> >> be [1], and if the TCP packets belong to TPROXY Server and HTTP Server
> >> connection, the result should be [2]
> >>
> >> Does anyone have good idea about the requirement mentioned above?
> >
> > I think most users work around the problem: you usually don't really
> > need the source port to be preserved exactly and by choosing a different
> > source port the problem goes away.
> >
> > (If you don't care about the source port then bind the socket to port 0
> > and the kernel will choose an unused port.)
> 
> Thanks for your reply.  But, I think maybe this solution still have problems.
> 
> Here is a sample:
> Client -> TPROXY Server (192.168.2.2:12345 ->192.168.1.2:80)
> TPROXY Server -> HTTP Server(192.168.2.2:54321->192.168.1.2:80)
> Now the two connections have different ip_conntrack. TPROXY Server use
> port 54321 instead of 12345 as src port
> 
> Next time, the client connect to the HTTP Server
> Client -> HTTP Server(192.168.2.2:54321 -> 192.168.1.2:80).
> Now Client use the port 54321 connecting to 192.168.1.2:80(This is
> common for a NAT server connecting to the same HTTP Server using
> different src port)
> When  TPROXY receive this packet, and find ip_conntrack. It will match
> the old one - TPROXY Server -> HTTP Server.
> 
> This still exist the ip_conntrack conflict.

Yes, and unfortunately you can't do much about that if you're using
connection tracking. (Except for implementing some method of
differentiating between the two conntracks in ip_conntrack/nf_conntrack,
of course.)

-- 
KOVACS Krisztian