Wierd problem with irqs

["tanuki" <phake@sadomain.co.za>]

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Hi all


had an interresting problem the other day, just thought i'd share it with you all and see if 
any body else had a similar experience. 


I set up a small nat/firewall box for a client of ours. we had 5 interfaces and they were as follows 

eth0   192.168.10.1 netmask 255.255.255.255 pointopoint 192.168.10.2   <--- adsl modem  doing nat 
eth1   192.168.0.1 netmask 255.255.255.0 
eth2   192.168.1.1 netmask 255.255.255.0
eth3   192.168.2.1 netmask 255.255.255.0
eth4    192.168.3.1 netmask 255.255.255.255 pointopoint 192.168.3.2 <-- some upstairs router 

and the routing table looked like you would expect it to, with 

route add default gw 192.168.10.1 dev eth0 

also we had 

echo "1" > /proc/sys/net/ipv4/ip_forward 

for simplicity , iptables rules were as follows 

iptables -t nat --append POSTROUTING -o eth0 --jump MASQUERADE 

so, now all traffic using 192.168.0.1 , 1.1, 2.1 and 3.1 as a gateway should be able to reach the 
internet via the modem on 192.168.10.2 , right ? 

well, all icmp worked, perfectly 

but everything else , ie , udp, tcp didn't 

say for example http : packets get sent to tcp 80, tcp replies get recieved, but no data gets back to 
the user on 192.168.whatever

strange huh ? 

so i thought my mtu was befuqed, so i do 

iptables --append FORWARD --proto tcp --tcp-flags SYN,RST SYN --jump TCPMSS --clamp-mss-to-pmtu 

no luck though. 

tried a plethora of other stuff too , but didn't work, so i'll leave that there 

obviously the nat works, becuase all my icmp's are natted. 
mmm

so i go into the boxes bios set up and tell it to assign irq's to all pci devices automatically 

boot up into linux and do 

ifconfig eth4 up 

eth4: error fetching interface information: Device not found

eh ? wtf ? so i do ifconfig eth0 up 

and the device gets brought up 

then i do ifconfig eth4 up 

and it brings it up . Strange huh ? 

so now i see that all my cards are swopped arround. that which used to be eth0 is now eth4 and so 
on and so on. 

anyway, plug in the appropiate network cables to the relevant nics and run the script 

to a ping to google.com ... everything works fine. 
right, so far so good. right back were i started

now, do a HTTP-GET http://www.google.com and guess what 
i get a lovely html page. 

strange that changing the device irg assignment in the bios solved my problem ? 

any ideas what could have caused this ? 

thanks a lot for bearing with my idiotic ramblings so far 

----------------

tanuki 
























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