TCPtrace - An introduction

What is TCPtrace ?

Wireshark wins over TCPtrace on GUI

 

Its a tool designed to analyze the output logs from TCPdump. Previously, in my introduction to TCPdump I highlighted that the output logs created by TCPdump were not plain text and only special programs could interpret them, TCPdump is one of these program, as is Wireshark and TCPtrace.

So TCPtrace takes the output file from TCPdump as an input and it then outputs useful information and graphs.

How do I get TCPtrace ?

I downloaded it from the Ubuntu repositories using the typical 'sudo apt-get install tcptrace'. If this is not possible you can download it from here.

How do I input a TCPdump file to TCPtrace ?

You can call TCPtrace with a TCPdump file using  'tcptrace <my-file>' where my-file is the name of the file outputted by TCPdump. For example you could do something like:
$ sudo tcpdump -v -i wlan0 -w my_tcpdump_output -c 100
$ tcptrace my_tcpdump_output

The above will run TCPdump and create the output file called "my_tcpdump_output", this file is then passed as a argument to the TCPtrace tool

How do i interpret the basic TCPtrace output ?

The structure of the output is (in order from the top) :

• 1st line returns the name of the TCPdump output file that TCPtrace is currently analyzing
• Then it printed the version of TCPtrace and then this version was last compiled
• The next line states the number of packets seen and the number of those which were TCP
• The following line gives the elapsed wallclock time, this is the time TCPtrace took to process the file and it then gives the average number of packets processed by TCPtrace per second
• The following line gives the trace file elasped time, this is the time between the capture of the first packet and the last packet in the file
• The sequential lines contain information on each TCP connection
• First it gives the address and ports of the two machines involved in the connection
• Then is the label given to this connection by TCPtrace is printed in parenthesis
• The number proceeding '>' is the number of packets seen in the first host to second host direction
• The number proceeding '<' is the number of packets seen in the second host to the first host direction
• Then the connection is labelled with '(complete)' or '(reset)', with the connection being labelled as complete if SYN and FIN packets were seen

This output is TCPtrace's brief output. Just like TCPdump, you can stop the translation of IP address to domain names using the '-n' opinion.

When using TCPdump, you can see more detailed output using the '-v' option but with TCPtrace you can see more detailed output using the '-l' option.

When adding options to TCPtrace, you need to ensure the you place the extra options before the name of the input file and after the tool name.

When viewing the output from the long mode (when -l is the option) then all information is labelled. I'm now going to explain each label given in long output (warning .. this might take a while):

Packets and ACKS

• total packets - number of packets sent in that specific direction
• ack pkts sent - number of ACKs sent in that direction
• pure acks sent - number of ACK sent without data and the SYN,FIN&RST not set
• sack pkts sent - number of selective ACKs sent in that direction
• dsack pkts sent - number of duplicate selective ACKs sent in that direction
• max sack blks/ack - maximum number of selective ACK blocks seen in any selective ACK packet

Retransmissions

• unique bytes sent - total number of bytes sent excluding retransmittions
• actual data pkts - total number of packet with at least 1 byte of TCP payload
• actual data bytes - total number of bytes seen including retransmittion
• rexmt data pkts - total number of packets that where retransmittions
• rexmt data bytes - total number of bytes of data that where retransmittions

Window scaling / Probing

• zxnd probe pkts - total number of window probe packets seen
• zxnd probe bytes - total number of bytes sent in window probe packets
• outoforder pkts - number of packets that arrived out of order
• pushed data pkts - number of packets with the PUSH bit set, this means that the buffered data should be sent to the receiving application
• SYN/FIN pkts sent - number of packets with SYN or FIN bits set

etc... (sorry I hate leaving things half done, but I really wanted to move on, its in my to-do list)

How do I get RTT (Round-Trip-Time) from TCPtrace ?

TCPtrace will generate statistics on RRT when using with the opinions '-r' and '-l'. This will give data on RRT including the number of RTT samples found, RTT minimum,RTT maximum, RTT average, RTT standard deviation, RTT from TCP's hand shake. The same data is then available again for full-sized RTT samples only.

TCPdump - An introduction

As per usual, if you find any mistakes, concepts that could be explained better or have something to add, then please comment below and I'll correct it.

What is TCPdump ?

TCPdump is a command line tool that is used to look at packets on the network. TCPdump does not generate the packets itself but instead it analyzes the packets generated by other applications and from this, it can determine network behaviour and performance.

Despite being called TCPdump, you can choose from a range of protocols including IP,ARP, TCP and UDP.

How do I get TCPdump ?

From Ubuntu, I simply got it via 'sudo apt-get install tcpdump'. Otherwise the public repo is located here, along side some useful information and the manual pages.

You can also check the dependencies of TCPdump using 'apt-cache depends tcpdump'. The output that this returns for me is:

  Depends: libc6

  Depends: libpcap0.8

  Depends: libssl1.0.0

  Suggests: apparmor

  Conflicts: tcpdump:i386

 

If you get the package for the public repo, then you need will extract the content of the .tar.gz file using 'tar -zxf tcpdump-4.3.0.tar.gz' after using cd to move to the directory that you downloaded the file to, then install the program.

So I've got it.. but how do I use it ?

To start with use 'sudo tcpdump' to output to the terminal, this gives the standard output of TCPdump, using the default arguments since you have not passed TCPdump any arguments yet. To the output will be printed to the terminal, until you stop the program using Ctrl-C.

Don't be scared ... we will now begin the break this output down into understandable section (if you not scared then try 'sudo tcpdump -v' or 'sudo tcpdump -vv'  for a verbose output).

Changing network interface

When I'm using tcpdump, it listened on eth0 by default, but you can change this. Entering 'sudo tcpdump -D' will return a list of network interfaces that you can choose between. For me this returns:
        1.eth0
        2.wlan0
        3.usbmon1 (USB bus number 1)
        4.usbmon2 (USB bus number 2)
        5.any (Pseudo-device that captures on all interfaces)
        6.lo
The first item in the list is the Ethernet port, the 2nd is the Wireless card and the 3,4,5th is self explanatory. The 6th is the loopback virtual interface (more information here on wikipedia)

You can then change the interface, by calling TCPdump using 'sudo tcpdump -i <name-of-interface> '. For most of my work, I use the local Wi-Fi so can change the interface from Ethernet to wireless using 'sudo tcpdump -i wlan0'. 

Creating a file to store the tcpdump arguments

So far, we have only added one argument when we call TCPdump, but there will be a lot more to come. We can save these arguments, which filter the output of TCPdump to a file and then use the file when we call TCPdump.

Create the file using your favourite text editors (mine in gedit at the moment) and then pass the file to TCP dump using 'sudo tcpdump -F <my-file>'. The file dose not need any special file extension or layout.

You may need to change the file permissions so that TCPdump can extract the file. You can view the file premissions using 'ls -l <my-file>'. The 'ls' part is command line tool to list the files in a directory and the argument -l means use long format so the file permissions will be included. The file 10 characters are the file permissions, they are decoded as follows:

1. 'd' means this is a directory and '-' means this is a file
2. 'r' means that the file owner can read the file, '-' means they can't
3. 'w' means that the file owner can write to the file,  '-' means they can't
4. 'x'  means that the file owner can execute the file,  '-' means they can't. If this is a directory then 'x' means that the owner can list the files in directory, '-' means they can't.
5. 'r' means that the file group can read the file, '-' means they can't
6. 'w' means that the file group can write to the file,  '-' means they can't
7. 'x'  means that the file group can execute the file,  '-' means they can't. If this is a directory then 'x' means that the owner can list the files in directory, '-' means they can't.
8. 'r' means that everyone else can read the file, '-' means they can't
9. 'w' means that everyone else can write to the file,  '-' means they can't
10. 'x'  means that everyone else can execute the file,  '-' means they can't. If this is a directory then 'x' means that the owner can list the files in directory, '-' means they can't.

For me, the default file permissions are '-rw-rw-r--'. This therefore means that the file owner doesn’t have permission to execute the file. This can be changed using 'chmod u+x <my-file>'. The command line tool is chmod, this is used for changing file permissions, 'u' means we are considering the users permissions, '+' means we are granting a permission and 'x' means we can considering the execute permission. Now, if I redo 'ls -l <my-file>' the new result is -rwxrw-r--.

  

Sending the output to file

You can send the output of tcpdump to a file instead of printing it to the terminal using 'sudo tcpdump -w <my-file>'. This output is not saved as a plain text so you can't just read it in a text editor instead you need to use a special program to read the file and extract information from it. You can do this using tcpdump by 'tcpdump -r <my-file>'. Alternatively you can open it using wireshark, launch wireshark and using File>Open.

 

Filtering information by protocol

To filter packets by protocol, add the name of protocol to the arguments of tcpdump so use something like 'sudo tcpdump <name-of-protocol>'. The main protocols that I'm likely are use are IP, ARP, TCP and UDP but others are available, see the man pages for a full list

Filtering information by direction to different hosts

To filter packets by a direction and host, add the direction and then the host name. Possible direction options are src, dst, src or dst, src and dst. You specify the host using its IP address. You can use the logical operators not, or and and. For example, you can look up the local IP address of your machine using 'ifconfig <name-of-interface>'. If you don't specify the name of the interface, then all interfaces will be listed. Now if you only want to view incoming traffic you can use 'sudo udpdump dst <ip-address>'.

Change the level of detail 

Compared to the level of detail provided by the standard query, The detail can be reduced using '-q' for quiet output or increased using '-v' for verbose, '-vv' for more verbose and '-vvv' for even more verbose.

The opinion '-t' means do not print timestamp on each line and the option '-a' allows you to display each packet in ASCII or '-x' to display in hex or '-X' to display in hex and ASCII

View IP address instead of domain names

You can stop the translation of IP address to domain names using the '-n' opinion and you can stop the translation of port number too using the '-nn' opinion

Sources of Info

Wikipedia article, which contains very little information

Official public repo, including the man pages and FAQ 

Useful online tutorial at openmaniak, this site also has good tutorials on networking tools that I've previously covered including wireshark, OpenVPN, Iperf and ping.