Subject: Re: DNAT sporadically doesn't replace destination IP address

From: "Kris Op de Beeck" <[email-masked]>
Date: Thu, 22 May 2008 17:22:26 +0200

>>> On 22/05/2008 at 16:57, in message <[email-masked]>, Patrick
McHardy <[email-masked]> wrote:
> Kris Op de Beeck wrote:
>> * Problem:
>> The replacing of the destination IP address doesn't occur always.  I've got 
> a PC on which I run a program which retrieves web pages of 30 modems which 
> all have the same IP address.  In order to do this I created the 
> configuration as listed below.  For the program the IP addresses will be 
> 10.9.9.1 to 10.9.9.30.  Those will be send out on a specific VLAN and the IP 
> address will be replaced by 192.168.1.1.  A number of times I get a 'no route 
> to host' for those web pages, this happens sporadically.  If I start looking 
> at the ARP messages, I see that when it goes wrong an ARP message is send out 
> for an 10.9.9.x address which shouldn't be the case.
>> On an older setup I didn't  had this problem, so I downgraded the kernel 
> version from 2.6.22-14-generic to 2.6.17-12-generic on the setup where I had the 
> problem.  
>> With this downgraded kernel the problem didn't occur anymore. 
>> The distro is Ubuntu. An upgrade to Ubuntu 8.04 which has kernel 2.6.24 
> didn't solve the problem.
>> 
>> * Setup:
>> - On which I have the problem:
>> PC -> Vlan enabled ethernet switch -> 30 modems with the same IP address.
>> 
>> - Simplified:
>> PC -> PC
>> for 1 port use the configuration as mentioned below. For the other port use 
> 192.168.1.1 as IP address for each VLAN.
>> Then you can just ping 10.9.9.x to test.
>> I have not tried this.
>> 
>> * Perl script to generate configure script:
>> !/usr/bin/perl -w
>> 
>> my $vlan  = 100;
>> my $modem = 1;
>> my $subip = 2;
>> for (my $modem = 1;$modem <= 30;$modem++) {
>>     print "echo \"------------$vlan--------------------\"\n";
>>     print "vconfig add eth2 $vlan\n";
>>     print "ifconfig eth2.$vlan 192.168.1.$subip\n";
>>     print "route del -net 192.168.1.0/24\n";
>>     print "route add -host 10.9.9.$modem eth2.$vlan\n";
>>     print "iptables -t nat -A OUTPUT -o eth2.$vlan -j DNAT --to 192.168.1.1\n";
>> 
>>     print "iptables -t mangle -N VLAN$vlan\n";
>>     print "iptables -t mangle -F VLAN$vlan\n";
>>     print "iptables -t mangle -A VLAN$vlan -j MARK --set-mark $vlan\n";
>>     print "iptables -t mangle -A OUTPUT -o eth2.$vlan -j VLAN$vlan\n";
>>     print "ip ro add table $vlan default dev eth2.$vlan\n";
>>     print "ip ru add fwmark $vlan table $vlan\n";
>>     print "ip ro fl ca\n";
>>     $vlan++;
>>     $subip++;
>> };
>> 
> 
> Please try loading ipt_LOG and executing
> 
> "echo 255 >/proc/sys/net/netfilter/nf_conntrack_log_invalid"
> 
> and see if something shows up in the ringbuffer.

[  917.584000] nf_ct_tcp: invalid SYNIN= OUT= SRC=192.168.1.23 DST=10.9.9.22 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=3257 DF PROTO=TCP SPT=40018 DPT=80 SEQ=1136088214 ACK=0 WINDOW=5840 RES=0x00 SYN URGP=0 OPT (020405B40402080A00025B1C0000000001030307) UID=1000
[  918.800000] nf_ct_tcp: invalid SYNIN= OUT= SRC=192.168.1.22 DST=10.9.9.21 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=26411 DF PROTO=TCP SPT=47713 DPT=80 SEQ=1041054567 ACK=0 WINDOW=5840 RES=0x00 SYN URGP=0 OPT (020405B40402080A00025C4C0000000001030307) UID=1000
[  921.800000] nf_ct_tcp: invalid SYNIN= OUT= SRC=192.168.1.22 DST=10.9.9.21 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=26412 DF PROTO=TCP SPT=47713 DPT=80 SEQ=1041054567 ACK=0 WINDOW=5840 RES=0x00 SYN URGP=0 OPT (020405B40402080A00025F3A0000000001030307) UID=1000
[  924.204000] nf_ct_tcp: invalid SYNIN= OUT= SRC=192.168.1.30 DST=10.9.9.29 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=25744 DF PROTO=TCP SPT=52775 DPT=80 SEQ=2154890499 ACK=0 WINDOW=5840 RES=0x00 SYN URGP=0 OPT (020405B40402080A000261930000000001030307) UID=1000
[  927.204000] nf_ct_tcp: invalid SYNIN= OUT= SRC=192.168.1.30 DST=10.9.9.29 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=25745 DF PROTO=TCP SPT=52775 DPT=80 SEQ=2154890499 ACK=0 WINDOW=5840 RES=0x00 SYN URGP=0 OPT (020405B40402080A000264810000000001030307) UID=1000

For those DST ip addresses I've got failures

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