Archive for the ‘Packet Capture’ category
Port Numbers reused
Sharkfest 2015 is coming up fast (22 days, 12 hours to go when typing this), and so I spend the morning hours of my Saturday for preparation of materials for my three talks. Since that also involves adding features and fixing bugs in TraceWrangler (which I also need for the large demo part of my […]
Sanitizing IPv6 addresses
Tracewrangler was always supporting IPv6 from the start (even though without extension headers except fragmentation), but last weekend I realized that I could improve the sanitization feature due to something that is missing compared to IPv4: subnet masks. This may sound funny, but in fact the missing subnet masks help.
Deep Dive: Frame Timestamps
Last week Uwe, one of the instructors of the Wireshark class I created for FastLane, gave me a call in the evening. He was teaching a 5 day class in Hamburg at the time, and had had a student ask about a peculiar problem with frame/packet timestamps. I remembered that I had read something about […]
Zombie Packet Mayhem!
My previous post was about one of multiple false positives a network analyst needs to keep an eye out for to avoid writing down findings in a report that weren’t really there. So when I looked at my Sharkfest traces to see what other topic I could write this post about I realized that I […]
TCP Analysis and the Five-Tuple
The TCP expert of Wireshark is doing a pretty good job at pinpointing problems, helping analysts to find the packets where things go wrong. Unfortunately, there are some things that can throw the expert off pretty badly, which can fool inexperienced analysts in believing that there are big problems on the network. I did a […]
Advanced display filtering
Wireshark has a lot of display filters, and the filtering engine is really powerful. You can filter on almost anything in a packet, and ever since the filter box started suggesting possible filter expressions it got really easy to find the one you wanted.
Working with multi-point captures
Every now and then most analysts run into a troubleshooting situations where they need to capture the same packets at different locations in the network. Some reasons for such multi-point captures include having to determine if packets get delayed at some point in the network (this would be one of the few cases where “it […]
Determining network protocols
If you spent enough time using Wireshark or any other network analysis tool, you’ll sooner or later be able to even read bare hex dumps of packets, at least partially (it’s a little bit like Neo seeing the Matrix). So maybe you run across a text dump of a packet like this one: 0000 00 […]
Diagnosing intermittent “network” problems
There’s that one thing that customers usually ask, and that question is if I would be able to help diagnosing a problem on the network. My answer has two parts: If we can capture the problem situation in packets, I will find it When I find it, I’ll tell you if it’s a network problem […]
Determining frame forwarding latency
In some situations the question arises how much a frame was delayed by a device it has to pass through, e.g. firewalls, loadbalancers and sometimes even routers and switches. Usually, novice network analysts think that for that you need to synchronize the clocks of the capture PCs down to microseconds or even better, but that […]