I'm using wireshark for the first time.
I run a client program that sends a command to server but the server response length is zero. I need to anlayse packets sent back from the server using wire shark in order to understand the problem
How can I see what is the size of data sent in a packet & what is the
data (human readable string) sent to destination using wireshark.
Please guide I'm new to networking and wireshark.
Thank you
I recommend this page for a guide on wireshark: Wireshark guide
I found the solution .Since, I'm using TCP .
Click the packet you want to analyse
See description- goto tcp
Under TCP click on data to see size of data and its value
Related
This question was posted on StackExchange - NetworkEngineering. People suggested me to post it here. Here is my original post.
I am trying to write a client that initiates a TCP connection and sends some data. On the server side, I am using nc that listens to a certain port. Now I am able to complete the 3-way handshaking. netstats shows that the connection is established. However, after my client starts sending data, it never gets an ACK.
The client is implemented on top of DPDK, and thus bypasses the kernel stack. The server binds to a different NIC. The two NICs are directly connected. The TCP part is handled by my own code. Due to the lack of knowledge, the implementation is greatly simplified in the sense that I set a lot of fields to some fixed numbers, such as the window size.
As a newbie in networking, I have no clue what could go wrong, and thus not sure what information I should provide to help you identify the problem. Here is a screenshot from wireshark. My client is at 192.168.0.10:12345 and my server listens at 192.168.0.42:3456. No ACK is sent from the server side after packet 6.
Also, the reason for the incorrect FCS is that I had to pad zeros to the SYN and the first ACK packets, so that they are at least 64 bytes, which is a requirement from the client NIC.
I did a comparison between packets from my client and packets from nc client. It seems that for the first data packet, the only real difference is that mine does not have any TCP options, while the nc one carries a timestamp. Could this be the problem?
Please let me know if you spot anything that may cause this no-ACK issue.
I am running a tcpdump on my server and capturing the data . I don't have access to the source server .
I can find the timestamp of the packet when it hits the NIC of the server where i am running tcpdump.
I would like to know if there a way to find out the timestamp when packets left the source server ?
I would like to know if there a way to find out the timestamp when packets left the source server ?
Yes - have somebody run tcpdump (or Wireshark or some other sniffer) on the source server. You can't determine that by capturing on some other machine.
Also bear in mind that sniffing on the source server will tell you when the packet reached the point in the networking stack when the packet gets handed to the packet capture mechanism; that's not necessarily the time when the packet is transmitted on the network.
I develop a network program that is used to transfer files , it works . But I just know it can works , and I don't know how to monitor and evaluate it . So I want to know what aspects a network program usually need to consider and monitor and how to monitor .
First make sure which protocol you have been used to send files (either TCP or UDP).
1.If you are using TCP at transport layer ,at the receiving end you can use TCPDUMP
packet analyzer to analyze all packets receiving on TCP port and its content.
2.If you want to analyze packets irrespective of protocols used at different layers, you can use wireshark packet analyzer to analyze all packets received on different networks like ethernet,PPP, loop back ,frame relay. you can use IP address of sender host as a reference to extract packets ( you need some reference to extract packets because wire shark will return all the packets received on the NIC interface). Once you extract the packets received from your sender host, you can analyze the packet payload to check whether files content has been received properly or not.
3.you can redirect data ( payload) of all received packets into some file. Once your program is done with receiving packets, you can check with that file to check data has been properly received or not. ( you can use this method only to test your client/server programs within a system)
I need to performance test an application [BRM remedy, ar user] that is using an TCP protocol. I don’t have much knowledge on TCP Protocol.
I tried with wireshark to analyze the requests, but it is showing TCP request with data checks [Assuming that my data is splitting into small chunks by TCP protocol implementation].
Can you please suggest me some good tool that can monitor TCP requests and responses [like HTTP fiddler for HTTP Protocol]?
I will be using Jmeter for the perf testing.
You might be seeing packets in the TCP format , but you can docode your packet in wire share too.
Open your capture in wireshark go to Analyze -> Decode As -> and all request will will traslated accodingly.
If you want analyse a specific TCP stream or want to to which tcp stream a packet belong to , right click on packet and click "Follow tcp..." , you will be finding allthe packets belong that particular tcp stream.
It mostly work for me with above two functionality , hope same for you.
I'm building my own webserver based on a tutorial.
I have found a simple way to initiate a TCP connection and send one segment of http data (the webserver will run on a microcontroller, so it will be very small)
Anyway, the following is the sequence I need to go through:
receive SYN
send SYN,ACK
receive ACK (the connection is now established)
receive ACK with HTTP GET command
send ACK
send FIN,ACK with HTTP data (e.g 200 OK)
receive FIN,ACK <- I don't recieve this packet!
send ACK
Everything works fine until I send my acknowledgement and HTTP 200 OK message.
The client won't send an acknowledgement to those two packages and thus
no webpage is being displayed.
I've added a pcap file of the sequence how I recorded it with wireshark.
Pcap file: http://cl.ly/5f5/httpdump2.pcap
All sequence and acknowledgement numbers are correct, checksum are ok. Flags are also right.
I have no idea what is going wrong.
I think that step 6. should be just FIN, without ACK. What packet from the client are you ACKing at that place? Also I don't see why 4. should be an ACK instead of just a normal data packet - the client ACKed the connection at 3.
This diagram on TCP states might help.
WireShark says (of the FIN packet):
Broken TCP: The acknowledge field is
nonzero while the ACK flag is not set
I don't know for sure that's what's causing your problem, but if WireShark doesn't like that packet, maybe the client doesn't either. So, it should be FIN+ACK, or you should set the acknowledge field to 0.
If that doesn't solve it, you might also try sending the data first, then a separate FIN packet. It's valid to include data with the FIN, but it's more common to send the FIN by itself (as seen in the other pcap trace you posted earlier).
Also, you should probably be setting the PUSH flag in the packet with the 200 OK
Finally, I don't see any retransmission attempts for the FIN packet - is that because you stopped the capture right away?
The IP length field was consequently counting 8 bits too much. I made a mistake in my calculations. Everythings works like a charm now!