Re: conntrack, NAT and icmp echo reply

On Fri, 12 Oct 2012, Denys Fedoryshchenko wrote:

On 2012-10-12 09:13, Vigneswaran R wrote:

quoted

On Thursday 11 October 2012 03:32 PM, Denys Fedoryshchenko wrote:

quoted

On 2012-10-11 12:57, Eric Dumazet wrote:

quoted

On Thu, 2012-10-11 at 12:41 +0300, Denys Fedoryshchenko wrote:

quoted

I have NAT box, with very simple rule
iptables -t nat -I POSTROUTING -s 10.0.0.0/8 -j MASQUERADE
It can be SNAT also, and it works fine, as NAT.

When i generate icmp _reply_ packet, to some host
hping -I ppp0 -1 --icmptype 0 8.8.8.8

It will pass the box, and will exit it without NAT, e.g. with original
IP 10.x.x.x
on outgoing interface, which is not expected behavior IMHO.
Is it a bug or feature?

It depends, -s 10.0.0.0/8 wont match the rule if the source address
should be 198.23.44.55 I guess ?

I would try the more obvious

iptables -t nat -I POSTROUTING -o device -j MASQUERADE

Source is correct, it is 10.0.0.0/8 range. I tested also ICMP code 3, it
wont be NATed also.
But ICMP echo passing OK.
Also TCP RST generated same way, (i guess that don't have any match in
conntrack table), won't be NATed too.
hping -I ppp0 -R 8.8.8.8
13:01:07.074134 IP 10.0.0.142.2106 > 8.8.8.8.0: Flags [R], seq 510333079,
win 512, length 0
13:01:08.074239 IP 10.0.0.142.2107 > 8.8.8.8.0: Flags [R], seq 1169580528,
win 512, length 0
13:01:09.074253 IP 10.0.0.142.2108 > 8.8.8.8.0: Flags [R], seq 186548661,
win 512, length 0
13:01:10.074376 IP 10.0.0.142.2109 > 8.8.8.8.0: Flags [R], seq 2135508128,
win 512, length 0
13:01:11.074553 IP 10.0.0.142.2110 > 8.8.8.8.0: Flags [R], seq 1507433100,
win 512, length 0

And ICMP here you can see correct behavior with icmp echo request:

12:58:22.917458 IP 10.0.0.142 > 8.8.8.8: ICMP echo reply, id 62548, seq 0,
length 8
12:58:23.917543 IP 10.0.0.142 > 8.8.8.8: ICMP echo reply, id 62548, seq
256, length 8
12:58:24.917657 IP 10.0.0.142 > 8.8.8.8: ICMP echo reply, id 62548, seq
512, length 8
12:58:31.047475 IP 10.0.0.142 > 8.8.8.8: ICMP net 5.6.7.8 unreachable,
length 36
12:58:32.047562 IP 10.0.0.142 > 8.8.8.8: ICMP net 5.6.7.8 unreachable,
length 36
12:58:33.047734 IP 10.0.0.142 > 8.8.8.8: ICMP net 5.6.7.8 unreachable,
length 36
12:58:54.014601 IP X.146.153.X > 8.8.8.8: ICMP echo request, id 10462, seq
0, length 8
12:58:54.081897 IP 8.8.8.8 > X.146.153.X: ICMP echo reply, id 10462, seq
0, length 8

I think, the following may be the reason for the behaviour you
observed. (I may be wrong, I am not an expert in iptables.)

"nat" table only consulted for "NEW" connections. ref:
<http://inai.de/images/nf-packet-flow.svg>

The ICMP echo _reply_ may not be considered as part of a "NEW"
connection, as it must be a _reply_ to some already received
_request_. So _request_ is new and _reply_ is not.

Yes, they are not related to existing and it is not new connection, but 
there is similar issues related to TCP, and it is handled differently. 
For example if nf_conntrack_tcp_loose is set to 0 - it will not pickup 
already established connections, and just they will have INVALID state. 
If set to 1 (and it is default value) - it will pickup the connection, 
even it is established state. I was expecting ICMP and especially RST 
can be consistent with that.

TCP is a connection-oriented procol while ICMP isn't. There is no point
in creating a connection entry from a bogus-looking ICMP reply packet.

The packets you generated neither belong nor related to an existing 
connection, thus are marked as INVALID. INVALID packets are not NAT-ed and 
if not dropped explicitely, will be sent out without natted.

Best regards,
Jozsef
-
E-mail  : kadlec@blackhole.kfki.hu, kadlecsik.jozsef@wigner.mta.hu
PGP key : http://www.kfki.hu/~kadlec/pgp_public_key.txt
Address : Wigner Research Centre for Physics, Hungarian Academy of Sciences
          H-1525 Budapest 114, POB. 49, Hungary