I have a custom wireshark dissector, but it takes a very long time to load/dissect large captures.
Is it possible to only dissect packets when they are displayed/on scroll?
No, it's not. The Wireshark core has no idea whether a given dissector constructs, for some or all packets, state needed for dissecting later packets, so it has to dissect every packet when it reads the file in.
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I'm using Wireshark to monitor network traffinc to test a new software installed on a router. The router itself lets other networks (4g, mobile devices through usb etc) connect to it and enhance the speed on that router.
What I'm trying to do is to disconnect the connected devices and discover if there are any packet losses while doing this. I know I can simply use a filter stating "tcp.analysis.lost_segment" to track down lost packets, but how can I eventually isolate the specific device that causes the packet loss? Or even know if the reason was because of a disconnected device when there is a loss?
Also, what is the most stable method to test this with? To download a big file? To stream a video? Etc etc
All input is greatly appreciated
You can't detect lost packets solely with Wireshark or any other packet capture*.
Wireshark basically "records" what was seen on the line.
If a packet is lost, then by definition you will not see it on the line.
The * means I lied. Sort of. You can't detect them as such, but you can extremely strongly indicate them by taking simultaneous captures at/near both devices in the data exchange... then compare the two captures.
COMPUTER1<-->CAPTURE-MACHINE<-->NETWORK<-->CAPTURE-MACHINE<-->COMPUTER2
If you see the data leaving COMPUTER1, but never see it in the capture at COMPUTER2, there's your loss. (You could then move the capture machines one device closer on the network until you find the exact box/line losing your packets... or just analyze the devices in the network for eg configs, errors, etc.)
Alternately if you know exactly when the packet was sent, you could not prove but INDICATE its absence with a capture covering a minute or two before and after the packet was sent which does NOT have that packet. Such an indicator may even stand on its own as sufficient to find the problem.
I want to store large number of packets in a pcap file (say around 200000) and then send it using tcpreplay . The problem is the loop option in tcpreplay sends at a very low speed .
Now I am capturing packets using wireshark but wireshark does not respond after sending a lot of packets . How can i increase the length of the pcap file by multiplying the number of packets already stored in it ? How can I achieve good throughput using tcpreplay?
If you'd like to multiple a single pcap, consider the mergecap command, shipped with wireshark.
Regarding the packet pumping speed of tcpreplay take a look at its FAQs, and in particular consider the -T option to pick a timer mechanism that works well. I've found rdtsc to work very well. Also consider using a short trace that fits into memory, and iterating playback of that, to avoid disk I/O. For this, consider the -K option.
when does Wireshark timestamp the packets? After fully receiving a frame? Or already at receiving the first bytes of a frame? I read the following description about Wireshark timestamps but this text only states: "While packets are captured, each packet is time stamped as it comes in".
Consider the following scenario and an accurate OS time:
sender ----> Wireshark ----> receiver
The sender starts the transmission of the frame at time x. The frame is fully received at the receiver at time y (y = transmission start x + frame length / link speed). Will the captured frame appear in the Wireshark with a timestamp close to x or will it be y?
Best regards,
Jonas
Well, Wireshark doesn't time stamp the packets itself; it relies on libpcap to do them and, on almost all operating systems, libpcap doesn't time stamp them itself, either, the OS's packet capture mechanism, as used by libpcap does. The main exception is Windows, where WinPcap has to provide its own capture mechanism in the kernel, atop NDIS, but the mechanism behaves very similarly to the mechanisms inside most UN*Xes, and will give similar behavior. (The other exception is HP-UX, where the OS's capture mechanism doesn't time-stamp packets at all, so libpcap does so; that gives answers that are somewhat similar to other OSes, but with a potentially even longer delay before the packets are time-stamped.)
If Wireshark (or any other packet sniffer!) is run on the sender, the packets will be "wrapped around" within the OS and handed to the capture mechanism; the time stamp could be applied before the sender starts transmitting the packet, but the time stamp will be closer to x than to y.
If Wireshark (or any other packet sniffer) is run on the receiver, the time stamp is applied at some time after the entire packet has been received; that could involve delays due to packet being queued up, interrupts being "batched", some amount of network-stack processing being done before the packet is time stamped, etc.. The time stamp will be closer to y than to x.
If Wireshark (or any other packet sniffer) is being run on some third machine, passively sniffing a network, the time stamp will probably be closer to y than to x, but there's a difference due to the receiver and the sniffer being separate machines that might see the packet at different times, have different code paths for receiving, etc..
I have a ton of wireshark traces containing varying amount of ISCSI packets. I need to parse out the command being sent by the initiator (in bytes) and write it to a file for each packet. I was originally going to do this manually, as it is easily viewable inside the wireshark application (see SS below), but some of these traces are huge (1-2 Gb), and it would take forever to do by hand.
I've been looking into tshark and rawshark documentation, but I'm not sure either is able to get me what I need. A friend suggested using libpcap to parse the traces myself, but from what I can tell I'd need to find some way to identify the bytes I need to pull out of each packet. Ideally I'd like to use something that recognizes it for me (ie wireshark's ISCSI dissector).
Can anyone point me in the right direction? I need some way to parse out these commands from each ISCSI packet without looking through the raw packet data and trying to identify which bytes I need. As a note - It's not always the last 16 bytes in the packet as shown above, so I can't just go through and take the last 16 bytes.
If you export the packets to PDML/XML (File->Export...->File->Save As Type PDML) you will get a nice XML file with all the protocol fields. You may be able to use this for your requirements, or use it as an index to locate the raw bytes in each packet.
I'm trying to write a c application that logs ethernet frame arrival times to a database. I been doing some analysis with wireshark and can see that it displays the arrival time of the frame. I'm going to use libcap which analyses the pcap file and I gather wireshark also uses the pcap file. My question is how does wireshark calculate the arrival time of frames?
From https://www.wireshark.org/docs/wsug_html_chunked/ChAdvTimestamps.html:
While capturing, Wireshark gets the time stamps from the libpcap (WinPcap) library, which in turn gets them from the operating system kernel.