I have a big PCAP file downloaded from this website. (You can download the original pcap file : 368 MBs)
You can also download a short version that contains only some of the buggy packets.
There is something strange with some packets inside this file. There are 1113 packets labeled with sFlow inside it that it doesn't matters which wireshark-filter you apply to the packets, you always will see them (or part of them) in the window:
To be more clear let see some screen shots:
No filter applied:
Filter to see tcp packets only:
Filter to see udp packets only:
Filter to see packets with ip.addr == 68.64.21.64
What's wrong with these packets?
These packets are of sFlow type. They are used for network sampling, so they contain samples of other network packets within. Display filter seems to be applied not only to sFlow packet itself, but to every inner packet as well. So "tcp" display filter leaves those sFlow packets (they are obviously udp), which contain tcp sample within. Same for address filtering.
You can inspect inner packets as shown on the picture
Not sure if filter behavior is correct, I was amused by output as well. I think, it'd be great to open ticket at Wireshark bug database to hear developers' opinion.
Related
I am having an issue with wireshark capture. The wireshark doesn't show SNMP protocol but as UDP and complaints as malformed packet.1: https://i.stack.imgur.com/EXcfN.jpg May I know the reason and solution for this issue.
Thanks and Regards,
Alexander.
Looks like your packet is cut out in the middle.
Notice that both the IP and the SNMP length field are indicating longer payload than there actually is (they are even marked red for that reason).
Somewhere in the way from the sender to wireshark the packet was cut short. could be because of a malfunctioning router, BER, or just corrupted pcap file.
Wireshark has a really hard time trying to dissect packets (remember it doesn't know the configurations on the end components), but it does it's best with heuristics defined by the community. A malformed packet not being dissected right is not surprising.
If you decide to change WS's heuristics on your PC you may as well do that using LUA plugins.
There is a “Don't fragment” flag in IP header.
Could applications set this flag ?
When to set this flag and why?
If the 'DF' bit is set on packets, a router which normally would fragment a packet larger than MTU (and potentially deliver it out of order), instead will drop the packet. The router is expected to send "ICMP Fragmentation Needed" packet, allowing the sending host to account for the lower MTU on the path to the destination host. The sending side will then reduce its estimate of the connection's Path MTU (Maximum Transmission Unit) and re-send in smaller segments.This process is called PMTU-D ("Path MTU Discovery").
A fragmentation causes extra overhead on CPU processing to re-assemble packets at the other end (and to handle missing fragments).
Typically, the 'DF' bit is a configurable parameter for the IP stack. I know of ping utility with an option to set DF.
It is often useful to avoid fragmentation, since apart from CPU utilization for fragmentation and re-assembly, it may affect throughput (if lost fragments need re-transmission). For this reason, it is often desirable to know the maximum transmission unit. So the 'Path MTU discovery' is used to find this size, by simply setting the DF bit (say for a ping)
Further down in RFC 791 it says:
If the Don't Fragment flag (DF) bit is set, then internet
fragmentation of this datagram is NOT permitted, although it may be
discarded. This can be used to prohibit fragmentation in cases
where the receiving host does not have sufficient resources to
reassemble internet fragments.
So it appears what they had in mind originally was small embedded devices with the simplest possible implementation of IP, and little memory. Today, you might think of an IoT device like a smart light bulb or smoke alarm. They might not have the code or memory to reassemble fragments, and so the software communicating with them would set DF.
The only situations I can think of where you would possibly want to set this flag is:
If you are building something like a client-server application where
you don't want the other side having to deal with a fragmented
packet but rather prefer a packet loss to that.
Or if you are on a
network with a very specific set of restrictions, possibly caused by
bandwidth issues or a specific firewall behaviour.
Except for such specific circumstances you would likely never touch it.
From RFC 791:
Fragmentation of an internet datagram is necessary when it
originates in a local net that allows a large packet size and must
traverse a local net that limits packets to a smaller size to reach
its destination.
An internet datagram can be marked "don't fragment." Any internet
datagram so marked is not to be internet fragmented under any
circumstances. If internet datagram marked don't fragment cannot be
delivered to its destination without fragmenting it, it is to be
discarded instead.
Could applications set this flag ?
Yes if you write low-level enough code that you are dealing with the IP header. This part of the question is a bit broad for giving a more specific answer, you should probably figure out if you want to set it before worrying about the how.
I'm pretty new to Wireshark and stuck with a filter task.
I have network traffic and error messages from a certain system. I need to trace the SYN packet of one of my error messages.
For Wireshark, that means I need to filter for one specific IP-port combination x.x.x.x:xxxx among the SYN packets.
With tcp.flags.syn == 1 as a display filter I have been able to narrow down Wireshark's output to only SYN packets, but it's still far too many to find the one packet belonging to the port where we see the error and that we would like to follow.
Can you help me with that?
Looking only at SYN packets is not very helpful if you need to find a conversation that has problems - it's usually better to gather as much information about the IPs involved in the problem and filter on them. E.g. if you know that the computer with the IP 192.168.1.1 has a problem, and your capture has tons of conversations, you can filter on that IP by using the following filter:
ip.addr==192.168.1.1
If you also know the layer 4 protocol and port (e.g. TCP on port 1025) you can filter on both IP and port, like this:
ip.addr==192.168.1.1 and tcp.port==1025.
If you have a plain text protocol and know the text of the error message (if it is actually visible in a packet, and not just some coded thing), you could use the "find" option and search for the string (don't forget to set the search type to "string", because the default is "display filter").
I wanted to simulate a situation where when I send some packets say ping x.x.x.x, some of my packets outgoing from my Linux box should be dropped out and number of dropped packets should be then captured through Tx dropped packets in ifconfig tool.
I know some of option/rule that we can provide in iptables but not sure about them.
Can anybody let how to simulate packet drop and dropped packet capture in Linux?
netem provides functionality to simulate various network related situation like emulate wide area network, packet loss/drop/delay.
Please refer this excerpt for more information on packet drop as your have provided less requirements.
Also see SO answer
How does wireshark interpret physical packets?
As far as I know, all packets look to be the same, so how does it decode them to pass to next higher protocol?
When it's used to capture live traffic it knows the type of the interface and therefore the L2 encapsulation of packets, and when it reads a pcap file, the file has a field in the header indicating network type.
There are probably a number of different mechanisms. You can download the dissectors and study the source to find out the various methods.
I wrote a dissector for a network sniffer and ported it to Ethereal and then Wireshark (or maybe someone else ported it; I don't remember). But the basic logic is that the dissector gets added to the list of possible dissectors. Wireshark calls a dissector and it decodes the packet if it can. If not, it calls the next one in the chain.
In the code I wrote, I simply analyzed the packet (UDP in my situation) to determine if it fit the profile of the desired packet using checksums and known data in the packet. If it decided it was the packet I was interested in I just extracted the various pieces of interesting data from the packet. The function tvb_get_ptr returns a pointer to the start of the data.