weird network problem

Hi,

I have linux box two NIC cards. It is Intel 815E motherbard PC with
PIII processor and two rtl 8139 PCI cards. One is connected to
internet throogh Motorola SURFBoard TV cable modem using cat5 ethernet
cable, and second is connected to 5-port switch.

Both cards runs at 100Mbps ful-duplex.

I have firewall and NAT set up on that box, and I do some port
forwarding all using iptables.

I run dhcpcd on eth1 to obtain ip and default gw from TV company, and
dhcpd on eth0 for my local network.

OS is Slackware linux 9.0 with kernel 2.4.20.

This configuration worked for me from about one year and now I run
into problems that I can't solve.

The problem is that NIC connected to internet stops working after some
peroid of time (5 minutes to couple of hours - depending on network
activity).

When it happens, I can't connect to any host on network and cant ping
my default gateway any more. There is nothing in logs, and when I run
tcpdump on that card I notice that there _is_ some network activity -
I see lot of arp queries from routers and my icmp echo request to
router without any reply.

I can ping eth1 but nothing else.

When I reset modem everythig goes back to normal for some time and
then eth1 stops working again.

Eth0 card connected to local network works properly all the time.

I tried to solve problem by:
- replacing rtl8139 with other 8139 card
- replacing rtl8139 with build-in eepro100 NIC
- downgrading kernel to 2.4.17
- upgarding 8139too driver to 0.9.22a
- making cable TV company to replace my cable modem with new one
- disabling all iptables rules
- disconnecting local network and removing NIC card.
- switched networks on eth0 and eth1
- replacing cable between eth1 and cable modem.
- reseting dhcpcd after hang
- reloading rtl8139 module after hang.

All above actions give me no results.

I connected another computer to cable modem (notebook with old 3Com
EtherlinkIII PCMCIA card running slackware 8.1 with kernel 2.4.7) and
it works without any problems so I assume problem is in my linux box.

When eth1 hangs, and i unplug it from cable modem and plug it into my
switch I can ping other computer connected to that swich, so I think
NIC is working. This is very strange. I get no errors and no dropped
packets in ifconfig stats for eth1.

mii-tool does not work at all - it shows no link and wrong network
speed. I can't switch to 10Mbps with this tool.

If someone has any idea what can cause this or what else could I check
to locate the problem I would be very grateful.

Regards,
£ukasz

Possibly a DHCP problem, when your modem tries to update the IP if you are
using certain iptables rules you might have inadvertently prevented DHCP
updates. Preventing ICMP requests can also cause this problem. Flush
iptables rules and monitor the problem , understandably you will only be
able to do this from the box connected to the modem but it will rule out the
firewall as the cause.

--
Nathan D Higgins

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