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Old 06-12-2002, 10:37 AM   #1
shchuka
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Registered: Jun 2002
Location: Virginia, USA
Distribution: Mandrake 8.1
Posts: 2

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Post Strange network problem


Hello All,

I'm having a strange problem with my linux network connection. The box has a 3COM 3c509 card and is sitting on a cable-modem DHCP network. Everything used to work just fine until about three weeks ago, when my ISP made some changes in there network.

Now, when I bring the interface up, it gets and IP address fine - with the gateway, DNS, broadcast, etc. After that, I cannot connect to anything - I cannot even ping the gateway. The ping says "x packets sent, 0 packets received, packet loss 100%" with x anywhere up to several thousand.

The most interesting thing is that I can connect to the box from outside! I can connect to the http server running there, I can log in using SSH, and so on. I don't have any problems connecting using SSH and doing whatever I need to do.

Note that if I connect a windows 2000 machine to the same cable modem, everything works fine.

The ISP is less than helpful - their tech support people basically hang up on me as soon as I say that I use linux, they become very defensive and rude, announce that they only support windows machines, that I'm out of luck and hang up on me. I don't have other alternatives for an ISP, since I live in a rural area where no DSL is available - even from Verizon, who's my telephone carrier.

Please help me resolve the problem!

Thanks in advance,
-Aleks
 
Old 06-12-2002, 10:55 AM   #2
Griffon26
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Registered: Sep 2001
Location: The Netherlands
Distribution: Gentoo, Debian, Mandrake, LFS
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I can try to help you with it in about an hour or 2 maybe 3. (For some 'debugging' I mean.)
Will you be available then?
 
Old 06-12-2002, 02:06 PM   #3
shchuka
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Registered: Jun 2002
Location: Virginia, USA
Distribution: Mandrake 8.1
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Solution to the strange problem

The greatest thanks to Griffon26 for helping me resolve the problem.

Here's what was wrong. In my firewall/masquarade script, in the beginning, I had the rule

    iptables -F

assuming that it would flush all rules in all tables. I was mistaken. Apparently, it did not flush the 'nat' table. Therefore the 'nat' table still had my old IP address stored in it and try to present the packets from my linux box with my old IP address, effectively spoofing the address. The solution was extreamly simply, just like everything great. I replaced the line above with these three lines:

    iptables -F -t filter
    iptables -F -t nat
    iptables -F -t mangle

and restarted my firewall/masquerade script - and now everything works fine.

Thank you again, Griffon26!

-Aleks
 
Old 06-13-2002, 03:06 AM   #4
Noerr
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Registered: May 2002
Location: Dalec, HU
Distribution: Redhat 7.3
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also check your routing
route -n
make sure you have default gw, network or you ISP, and local network entry
 
Old 06-13-2002, 05:16 AM   #5
Bert
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Registered: Jul 2001
Location: 406292E 290755N
Distribution: GNU/Linux Slackware 8.1, Redhat 8.0, LFS 4.0
Posts: 1,004

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Assuming you're using ethernet - packet loss or corrupted frames can be due to physical connections - akways worth just checking.

Bert

Last edited by Bert; 06-13-2002 at 05:18 AM.
 
Old 06-13-2002, 05:26 AM   #6
Noerr
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Registered: May 2002
Location: Dalec, HU
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I guess you could check your cable + NIC, but it probably routing or iptables setup with old parameters
 
  


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