Question

How do I interpret Wireshark's ouput?

Asked by: priceep

I have a small network I am responsible for, with 4 switches and 8 servers. I've recently been trying to isolate a problem on that network that causes clients to lose their connection to one or more servers. The drop in the connection is only momentary, but still causes issues. The protocol is TCP. Since the switches were old I took the opportunity to converge 3 switches into 1, and replace network cables to the servers in question, yet the problem remains. I've been told that Wireshark can help me determine the issue, but as of yet I cant make heads or tails of what the output is really telling me, and what I need to be looking at.

Im working my way through the manual from Wireshark.org, but can someone give me a top 10 things to look for when Im scanning through these captured filters? Right now, it feels like a wild goose chase, and I dont even know what a goose looks like!

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Asked On
2008-02-25 at 06:10:10ID23190193
Tags

Wireshark

Topics

Network Analysis Software

,

TCP/IP

,

Network Management

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Answers

 

by: warrenbucklesPosted on 2008-02-25 at 06:43:38ID: 20975703

The manual is the place to start, but a familiarity with TCP/IP is also important.

Wireshark has two modes - live capture and trace analysis.  

Both modes have filter capabilities but the filter scripts are different (not great design here).

You can filter on practically anything - IP address, protocol, sub-protocol, host name, etc.

In your case you might want to build a filter that captures a specific IP combination (source and destination) and protocol (SMB? - I'm not sure what you mean by 'losing connection')  and look for retransmitted packets, retransmit requests, bad packets (crc or other errors)).  Wireshark has a nice coloring scheme for packets that makes it easy to scan a dataset.

If you can repeat the problem on the network you have a better chance of catching it - build a set of snapshots that cover the time period of the errors and try to pull out what they have in common.

I don't have a copy of wireshark running on this box (I'm away from home for another week) and can't give you specific examples - maybe other people on this site can help here.

Microsoft also has a network monitoring package (http://www.microsoft.com/downloads/details.aspx?FamilyID=18b1d59d-f4d8-4213-8d17-2f6dde7d7aac&displaylang=en) but I don't have as much experience with it as with wireshark - others may favor this approach.  Any comments out there?

wb

 

by: Bamit99Posted on 2008-02-25 at 23:19:24ID: 20982226

'warrenbuckles' is right here. For me, Wireshark or for that matter any Network Analysis tool can be a big help provided you know what you are looking for. It is similar to Internet where the information is immense but is only useful, when you know what you are looking for. Now, you have not specified what kind of connections are you trying to monitor. TCP is a very wide term. You need to be more specific like SMB is TCP 445, RDP is TCP 3389 and so on. If you are using Custom Application then it will have some specific ports as well. Now, to filter TCP session. First, we need to narrow down the traffic i.e. Wireshark will capture anything and everything which is on the wire which means a lot of garbage packet as well (Broadcasts and all). If you know that traffic is getting disrupted...let's say between two machines then begin with narrowing down the traffic between those two.

I have given some Example Commands for WireShark but like I said, it all depends on what you are trying to look for...or may be, if you can tell me what is the exact issue...I might be able to give you more pointers...

Also, note that the brackets and the Signs (&&, ||, !) play a very important role in the trace.

TC..:)

1. ip.addr==192.168.1.10 && ip.addr==192.168.1.20
 
This rule under Wireshark will show only the traffic which is between IP 192.168.1.10 and 192.168.1.20.
 
 
2. tcp.port==80 || tcp.port==3389
 
This rule under Wireshark will show traffic from any machine but it will only be TCP Port 80 (HTTP) or TCP Port 3389 (RDP)
 
 
3. !(ip.addr==192.168.1.10 && ip.addr==192.168.1.20)
 
This rule will show all the traffic except the traffic between these two machines. Note the Exclamation (!) mark outside the bracket.
 
 
4. (ip.addr==192.168.1.10 && ip.addr==192.168.1.20) && tcp.port==445
 
This rule will show traffic between these two machines and that too, SMB only (SMB is the traffic that you normally see when copying file from One System to the other). In Windows 95\98\ME it used to NetBios over TCP\IP...or TCP port 139..but in XP, 2000, 2003...It can be a combination these two.
 
 
5. (ip.addr==192.168.1.10 && ip.addr==192.168.1.20) && (udp.port==67 || udp.port==68)
 
This rule will show all the DHCP based traffic between these two machines. Just an example..:)

                                              
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