Slow Network and loss of gateway connection

Posted on 2004-10-19
Last Modified: 2012-06-27
My network for some reason is grinding to halting speed and all computers seem to lose connectivity with the gateway - this effect is affecting vpn's coming in from the outside as well.  My hubs are all showing constant network traffic.

I've been running ethereal but i'm not sure what to look for.

Any help here would be greatly appreciated.

thx in advance
Question by:loupis
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Expert Comment

ID: 12353104
what sort of network? domain based?

if u can post/host a text file of the ethereal traffic somewhere we can all have a look.

Expert Comment

ID: 12354260
Also while you are at it can you do an internal portscan of the subnet and post the ports open? We need to find out for one as well wether this traffic is coming from the inside or the outside. Next I would scan your outside network to see whats open. My first guess is filesharing. Look for ports like 6889 and up and LOTS of packets coming form them. One or two machines on a small network using BitTorrenet can bring things to a screeching halt. What do you have for a firewall?

LVL 79

Expert Comment

ID: 12360659
Sure looks like you have a worm loose on your network...

Try shutting down PC's and servers one at a time until they are all offline and see if you can catch the culprit...
Then bring them all back up one at a time. Someone else might take over and bring the network back down..

We saw this all over the place with MSBlast, Welchia/Nachi worms earlier this year.

If you can, block ICMP at the router ingress port (stop all ICMP from going out the router/firewall)
Can you be more specific in the network equipment you have?
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Author Comment

ID: 12364706
the domain is a 2k sbs and the router is a cisco1720 with an adsl card

icmp should be blocked out but not in

i'll attach an etherial clip - how many lines of text can I post here- or would you need to see?

I'm in the process of a powerdown on all computers now.


LVL 79

Expert Comment

ID: 12365080
If you have a 1720, try this to help:

access-list 110 deny icmp any any echo
access-list 110 permit ip any any

  interface fast 0
   ip access-group 110 in
   ip accounting output-packets
   ip route-cache flow

Then use:
  "sho access-list 110"  to see the hit-count increase on the deny entry - yes | no = clue
  "sho ip accounting output-packets"  highest number of packets to inside IP = clue
  "sho ip cache flow" shows source/destination pairs and ports (in hex)

Sort your ethereal capture by port number. look for any specific port that has a lot of activity that is outside the "normal' ports on your network..


Expert Comment

ID: 12373792
I agree with lrmore,

You will find many ports open but the majority of which will have nothing traffic wise of the magnatude or the storm you are getting. The offending port(s) will be spewing out packets. If you have your machines on a managed switch like a cisco 2924 you could type from enable mode "sho int accounting" and if need be clear the interface counters and watch the ports. "clear counters" and then just keep re-running the "sho int accounting" command.


Author Comment

ID: 12374397
I think I found a solution here -  Though maybe i'm running down the wrong highway.

I examined the majority of the packets and they were dns.  So i dug in a little bit deeper and found out that the primary dns server at my isp had been changed without letting me know.  I plugged in the new numbers into my dns forewarders and in my IP NAME-SERVER on my c1720 and it seems to be running smoothly right now.  (i can at least get here to communicate with you gurus.)  

Does this make sense/


Accepted Solution

browolf earned 125 total points
ID: 12375144
if the problems' gone away then that was most likely the solution.
you probably had dns packets going round in circles.  

Author Comment

ID: 12375771
hmm - dns wasn't the answer -

I didn't get much from the cisco - when the problems hapenning i can't access the router

catch 22

Assisted Solution

ecrit earned 125 total points
ID: 12376525
With little information this is gonna be hard ut here is a guess.
goto command prompt and type net stat.
if you see alot of traffic on port 445 I would bet you have a worm.
Yes port 445 is used for netbios but recent worms have been using this port also.

If you are using ethereal what ips and ports are repiticious?

Assisted Solution

pakitloss earned 125 total points
ID: 12376572

What you might want to do is get a laptop and plug into the hub and start sniffing packets with a clean machine while the problem is happening. Then post some of the results up here where everyone can see them.  One thing you can do is clear the counters in your router and after the next time this happens post the "sho ip accounting output-packets", "sho access-list 110" and "sho ip cache flow" from lrmoore's suggestion and paste them up here.  I'd still like to see a port scan as well myself if you could. As far as your question about the etherreal post, do what lrmoore said and sort by port and post @ 5 or so lines of the ports with the heaviest usage.

LVL 79

Assisted Solution

lrmoore earned 125 total points
ID: 12403501
Any progress? Are you still working on this?
Seems the GaoBot worm has been doing this to networks lately..

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