Core dump file not found

Has it ever occurred to you that you ran a program, it aborted saying 'core dumped' , but then when you go around looking for the core dump file, you cant find it anywhere? 0-o

Well it happened to me today. I tried finding the 'core' file in each of the possible directories - the pwd, the executable directory, all the directories in the $PATH variable.

To my dismay, none of them had the core file in them.

After some google search, I found that there may be cases where the core file is not created to save space. (Core files are usually large ~1GB ).

I also found out that in my system (Ubuntu 12.04 LTS), the default size allocated to core files is 0.

This can be found using the command

ulimit -a


 If you see ulimit -c 0 , that means 0 blocks have been allocated to core dump file , and hence core dump file is not formed.

To enable the core dump, we can do the following :


ulimit -c unlimited

This increases the coredump size to unlimited. Now, try executing the code that caused Aborted (Core dump formed).

and check the pwd (present working directory) for 'core'  file. You will notice that core file is very large. And hence I feel that in Ubuntu, as  a protective measure, the core dump file has been allocated 0 blocks by default.

NExT up -> tips on how to use objdump for debugging through core file. :)

binary files diff and patch

I wanted to send this large sized data from a server to a client multiple times.
Though the change in data in every iteration was negligible, it was essential that the copy be made exactly similar on both client and server.

The time taken to transfer this data was huge. We had to decrease this. Hence it was required that only changes made in the file be diff 'ed  and this file be scp'ed over the client.

Also, at the client end, the file had to be patch'ed, so that the client had the exact same copy as the server.

Heres how we go about this:

1> Diff the file

diff old_file new_file > patch_file   

// old_file was created in previous iteration and new_file created in current iteration.

2> copy the patch file to the client

scp patch_file client@client_IP:patch_file

3> AT CLIENT END :: patch the file in the client

patch old_file patch_file > new_file

for more information please see man patch and man diff.

In order to do this for binary files, you need to install bsdiff.

for Ubuntu use

sudo apt-get install bsdiff


then do ::

bsdiff old_file new_file patch_file
scp patch_file client@clientip:
AT CLIENT END:
bspatch old_file patch_file > new_file.

NOTE :: Check whether the checksum of the newly created file is same as that of the previous file.
This can be done using cksum command.

If they are not the same, you can use jdiff (source code given here :: http://jojodiff.sourceforge.net/)

Also, note that jdiff and jpatch, the two binaries that can be used from the source code are compiled on 32 bit kernel. (use file command for checking the binary type)

For kernel version 3 and above, this will work perfectly fine. However if you plan to use jdiff and jpatch for machines with kernel level 2.6 or so, you will need to recompile it for 64 bit machine.

This regenerates jdiff and jpatch with 1-2 trivial error resolutions.

bsdiff  OR  jdiff, which one is better?

bsdiff takes more time but uses internal compression algorithm for creating small sized patch. It goes without saying the time taken to generate the patch is more.

jdiff on the other hand takes less time, generates larger patch, but gives you the gurantee that the final file after patch is exact replica of the original binary file (verified using cksum ). Also, it is meant for 32 bit OS. 

Analyse network traffic using tcpdump and tcpstat

The other day I was doing a set of system programming experiments which required me to generate graphs of network traffic per second between two hosts.

Though I knew wireshark was one reliable automated tool with a pretty GUI, I was in search of some better tool that could give me exact stats, rather exact distribution of the packets recieved per second by one host from the other host.

Searching over the net I tumbled upon '''tcpdump''', which has the capability to "sniff" packets over LAN or any localized network.

At first I tried running it by making it catch hold of all the packets that it could get a hand on. And that included network traffic of Office of the HOD of my college :P , and packets of some three or four other Professors. Though I didn't try and read the contents of the packets, i strongly believe tcpdump can be used to do that in verbose mode using vvv option. But thats something I'll try some other day.  

Well it was fun using it in amorous mode (yes thats the exact term given to listening to all the packets that are circulating in the network), but due to shortage of time and the perennial project advisor's pressure, I had to abstain myself from getting adventurous.

Comming back to tcpdump option - I used the following command to analyze the traffice between two hosts ::

tcpdump -f -n -S host $host1 and host $host2 -w file1.cap

This results in the generation of a binary file that can be read by tcpdump using the following command ::


tcpdump -r file1.cap

So far so good. But there was one problem. My intention was to give a comprehensive time varied traffic rate between two hosts. and tcpdump gave me a bunch of packets with their sizes. So creating a assorted list of packets and bandwidth usage per second would have required me to write a new script.

 This is when tcpstat came to the rescue. Using tcpstat, we can read the binary file (created above by the tcpdump command) and get a time sliced version of the network traffic using the following command

tcpstat -r file1.cap $timeslice

timeslice denotes the time scale on which packets are  analysed. for my purpose I set it to 5.

This blog in no means is comprehensive and I would strongly advise the reader to look at man page for tcpdump and tcpstat.

Notes::

How do we run tcpdump as a non-sudo user?

groupadd tcpdump

addgroup <username> tcpdump

chown root.tcpdump /usr/sbin/tcpdump

chmod 0750 tcpdump

setcap "CAP_NET_RAW+eip" /usr/sbin/tcpdump

==Don Jaffer==

UBUNTU ctrl key doesnt work on Vncviewer

its a gnome problem.

goto :  System -> Configuration tools ->  /desktop/gnome/peripherals/mouse/locate_pointer

unmark the checkbox. done.

Well for a strange reason sometimes this may not work.. So for that you need to install kde in Ubuntu.

This can be done using apt-get. No need to install kubuntu or anything.

Passwordless ssh into a server

You have a machine to which you have to login all the time. and that includes entering your password to authenticate yourself to the server. Taxing! isint it??

A simple quirk is to make yourself "known" to the server so that it doesnt ask you to authenticate yourself everytime you login to the server...

2 steps , thats all you need baby!

* ssh-keygen
* ssh-copy-id <servername>

Consider Host A and Host B. Host A is your local computer and Host B is your Server.

'''On Host A'''
 ssh-keygen
this will create two keys id_rsa and id_rsa.pub in your local machine.

'''On Host A'''
ssh-copy-id mcs112578@palasi.cse.iitd.ac.in
this will copy the id_rsa.pub file into the authenticate keys. Done!

Now try ssh ing into the Server... see the difference !! (No password required !!)

==Don Jaffer==

Linux Shortcuts : Open terminal - Maximize Terminal

<note>
I am mentioning the default keyboard shortcuts as available in Ubuntu, you can change it going into System > Preferences > Keyboard Shortcuts
</note>

Open the terminal using the shortcut
            Ctrl + Alt + T
Maximise by pressing the 3 keys in the following sequece (Note that timing and sequence is important here, you might take a little time to get a hang of it)

Press
             Alt + SpaceBar
one after the other in quick succession

Press
             'X'
after releasing Spacebar .

Vim Command to split the screen

To Split the screen in vim:

open the file in vim
   vim abc.txt

to split the screen

ESC
:sp

This is the default way to split the screen and results in the screen being split horizontally.

ESC
:vsp

This is used to split the screen vertically.
 
to switch Control between two screens

Ctrl + W +W (Hit W twice)