Re: [mpls-linux-general] iptables problem?

SourceForge Headquarters 225 Broadway Suite 1600 San Diego, CA 92101 +1 (858) 422-6466

Tom,

I think I understand the issue you're seeing.  I'm going to try 
to duplicate your setup and see if I can figure out a work around.

On Wed, Apr 22, 2009 at 02:08:48PM +0200, Tom Kleiberg wrote:
> Hello James,
>
> Thanks for your reply. Flushing the cache indeed "solves" the problem.  
> It may be my perception, but I find this behavior somewhat erratic. It  
> basically means that once there are several streams of packets towards a 
> specific destination, albeit with different portnumbers and/or protocol, 
> and there are one or more rules in the mangle table, all the packets to 
> the destination are mangled and mapped onto the LSP. Different streams 
> that match with different rules, are indeed treated differently, which is 
> correct. But packets that do not match any of the rules are also mangled, 
> this I find awkward...or am I mistaken? Is there a way to circumvent this 
> problem?
>
> Thanks for your efforts!!
>
> Tom
>
> Btw, I recently find that the mailinglists are no longer searchable/ 
> browsable on the web.
>
>
>
>
> On Apr 21, 2009, at 6:09 PM, James R. Leu wrote:
>
>> Hello Tom,
>>
>> I'm going to top-post since my response is more generalized:
>>
>> I think you are observing the effects of the Linux route cache.
>> All traffic that goes through the IPv4 stack has two 'stages' to
>> its forwarding.  The first couple of packets go through the 'slow'
>> path in which a full iptables/route lookup is done.  The results
>> of that may be summarized into a route cache entry.  From that point
>> on all traffic that matches the src and dst of the route cache use
>> the 'fast' path and are forwarded according to the contents of the
>> route cache.
>>
>> I believe the route cache entries timeout after ~300 seconds.
>> You can look at the contents of the route cache by doing:
>>
>>    ip route show cache
>>
>> And you can flush it via:
>>
>>    ip route flush cache
>>
>> Does this explain what you are seeing?
>>
>> On Tue, Apr 21, 2009 at 04:21:24PM +0200, Tom Kleiberg wrote:
>>> Hello all,
>>>
>>> I have a question regarding iptables and mpls. I use four nodes (as  
>>> in
>>> the MPLS for Linux example on the SF.net website), mpls version  
>>> 1.962,
>>> FC8. The mpls packages are installed from the repository (so no  
>>> custom
>>> patching etc). I try to filter the packets using iptables and mangle
>>> them so that they are directed onto an LSP, e.g.
>>>
>>> sudo /sbin/mpls nhlfe add key 0 instructions push gen 1001 nexthop  
>>> eth1
>>> ipv4 10.0.0.2
>>> (returns 0x2)
>>> sudo /sbin/iptables -t mangle -A POSTROUTING -s 10.0.0.1 -d  
>>> 10.0.0.10 -p
>>> tcp --source-port 4000 --destination-port 4001 -j mpls --nhlfe 0x2
>>>
>>>
>>> At the downstream nodes, the label is swapped and finally removed at 
>>> the
>>> last hop before the destination.
>>> Now when I generate traffic (using d-itg), I see the following  
>>> tcpdump
>>> at the first downstream node (10.0.0.2):
>>>
>>> 16:01:25.713116 MPLS (label 1001, exp 0, [S], ttl 64)
>>> 	IP (tos 0x0, ttl 64, id 22294, offset 0, flags [DF], proto TCP (6),
>>> length 564) 10.0.0.1.4000 > 10.0.0.10.4001: P 958977:959489(512) ack 
>>> 1
>>> win 46 <nop,nop,timestamp 3001565 3070458>
>>> 16:01:25.713342 IP (tos 0x0, ttl 62, id 4498, offset 0, flags [DF],
>>> proto TCP (6), length 52) 10.0.0.10.4001 > 10.0.0.1.4000: ., cksum
>>> 0xa1e4 (correct), ack 959489 win 5024 <nop,nop,timestamp 3070459
>>> 3001565>
>>> 16:01:25.714115 MPLS (label 1001, exp 0, [S], ttl 64)
>>> 	IP (tos 0x0, ttl 64, id 22295, offset 0, flags [DF], proto TCP (6),
>>> length 564) 10.0.0.1.4000 > 10.0.0.10.4001: P 959489:960001(512) ack 
>>> 1
>>> win 46 <nop,nop,timestamp 3001566 3070459>
>>> 16:01:25.714337 IP (tos 0x0, ttl 62, id 4499, offset 0, flags [DF],
>>> proto TCP (6), length 52) 10.0.0.10.4001 > 10.0.0.1.4000: ., cksum
>>> 0x9fe2 (correct), ack 960001 win 5024 <nop,nop,timestamp 3070460
>>> 3001566>
>>> 16:01:25.715116 MPLS (label 1001, exp 0, [S], ttl 64)
>>> 	IP (tos 0x0, ttl 64, id 22296, offset 0, flags [DF], proto TCP (6),
>>> length 564) 10.0.0.1.4000 > 10.0.0.10.4001: P 960001:960513(512) ack 
>>> 1
>>> win 46 <nop,nop,timestamp 3001567 3070460>
>>> 16:01:25.715339 IP (tos 0x0, ttl 62, id 4500, offset 0, flags [DF],
>>> proto TCP (6), length 52) 10.0.0.10.4001 > 10.0.0.1.4000: ., cksum
>>> 0x9de0 (correct), ack 960513 win 5024 <nop,nop,timestamp 3070461
>>> 3001567>
>>>
>>> However, when I change the source or destination port used by the
>>> traffic generator (or the protocol), then that traffic (which does  
>>> not
>>> match the iptables rule), is still mapped onto the LSP (in this case 
>>> the
>>> destination port is changed to 4002, but any other traffic to  
>>> 10.0.0.10
>>> is in fact mapped onto the LSP).
>>>
>>> 16:11:31.590680 MPLS (label 1001, exp 0, [S], ttl 64)
>>> 	IP (tos 0x0, ttl 64, id 30521, offset 0, flags [DF], proto TCP (6),
>>> length 564) 10.0.0.1.4000 > 10.0.0.10.4002: P 1021953:1022465(512)  
>>> ack 1
>>> win 46 <nop,nop,timestamp 3607443 3676339>
>>> 16:11:31.591011 IP (tos 0x0, ttl 62, id 46089, offset 0, flags [DF],
>>> proto TCP (6), length 52) 10.0.0.10.4002 > 10.0.0.1.4000: ., cksum
>>> 0xeeb5 (correct), ack 1022465 win 5876 <nop,nop,timestamp 3676340
>>> 3607443>
>>> 16:11:31.591681 MPLS (label 1001, exp 0, [S], ttl 64)
>>> 	IP (tos 0x0, ttl 64, id 30522, offset 0, flags [DF], proto TCP (6),
>>> length 564) 10.0.0.1.4000 > 10.0.0.10.4002: P 1022465:1022977(512)  
>>> ack 1
>>> win 46 <nop,nop,timestamp 3607444 3676340>
>>> 16:11:31.592009 IP (tos 0x0, ttl 62, id 46090, offset 0, flags [DF],
>>> proto TCP (6), length 52) 10.0.0.10.4002 > 10.0.0.1.4000: ., cksum
>>> 0xeca3 (correct), ack 1022977 win 5892 <nop,nop,timestamp 3676341
>>> 3607444>
>>> 16:11:31.592681 MPLS (label 1001, exp 0, [S], ttl 64)
>>> 	IP (tos 0x0, ttl 64, id 30523, offset 0, flags [DF], proto TCP (6),
>>> length 564) 10.0.0.1.4000 > 10.0.0.10.4002: P 1022977:1023489(512)  
>>> ack 1
>>> win 46 <nop,nop,timestamp 3607445 3676341>
>>> 16:11:31.593007 IP (tos 0x0, ttl 62, id 46091, offset 0, flags [DF],
>>> proto TCP (6), length 52) 10.0.0.10.4002 > 10.0.0.1.4000: ., cksum
>>> 0xea90 (correct), ack 1023489 win 5909 <nop,nop,timestamp 3676342
>>> 3607445>
>>> 16:11:31.593681 MPLS (label 1001, exp 0, [S], ttl 64)
>>> 	IP (tos 0x0, ttl 64, id 30524, offset 0, flags [DF], proto TCP (6),
>>> length 564) 10.0.0.1.4000 > 10.0.0.10.4002: P 1023489:1024001(512)  
>>> ack 1
>>> win 46 <nop,nop,timestamp 3607446 3676342>
>>> 16:11:31.594005 IP (tos 0x0, ttl 62, id 46092, offset 0, flags [DF],
>>> proto TCP (6), length 52) 10.0.0.10.4002 > 10.0.0.1.4000: ., cksum
>>> 0xe87e (correct), ack 1024001 win 5926 <nop,nop,timestamp 3676342
>>> 3607446>
>>>
>>> When I query the number of packets that match an iptables rule  
>>> (iptables
>>> -t mangle -vnL), the query does not show any increment between after 
>>> the
>>> second batch of traffic is generated (so according to the iptables  
>>> output
>>> the packets that do not match the filter are not mapped onto the LSP,
>>> although in fact it DOES happen).
>>>
>>> More interestingly, the incorrect behavior disappears after some
>>> time...so when I do not send packets from the source to the  
>>> destination,
>>> the filtering recovers itself (until the packets are being send that 
>>> do
>>> match the filter)...
>>>
>>> I hope my question makes sense and someone can help me figure out the
>>> problem.
>>>
>>> thanks
>>> tom
>>>
>>>
>>>
>>
>> -- 
>> James R. Leu
>> jl...@mi...
>

-- 
James R. Leu
jl...@mi...