DNAT sporadically doesn't replace destination IP address

["Kris Op de Beeck" <kris.op.de.beeck@newtec.eu>]

* 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++;
};

* HW related info as far it is relevant:
10:04.0 Ethernet controller: Intel Corporation 82546GB Gigabit Ethernet Controller (rev 03)
10:04.1 Ethernet controller: Intel Corporation 82546GB Gigabit Ethernet Controller (rev 03)
3f:00.0 Ethernet controller: Broadcom Corporation NetXtreme BCM5755 Gigabit Ethernet PCI Express (rev 02)

Kind regards,
Kris

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