Quick Premier on migration to Curl for a socket programmer

I have been thinking of putting this out for quite some time. In future this will find its home on stackoverflow (libcurl documentation) which still is in proposal state. This is very high level and only provides the basic guideline. There are many options and cases that are better documented on libcurl website.

Check https://curl.haxx.se/libcurl/c/libcurl-tutorial.html which provides many examples. Documentation for libcurl also provides small code snippets to use for each API. For some they even provide high level steps just like I mentioned below. Continue reading

Net-SNMP configuration for non-root AgentX application

It is imperative to run the application when developing it. You may be testing it, debugging it or troubleshooting something. By default Net-SNMP uses named socket for AgentX communication which does not allow a non-root user to connect making troubleshooting difficult. There are security reasons for not allowing this kind of widely open access so do not set this up in your production environment. There are other ways to control the access which I will narrate in future posts.

To enable AgentX and allow non-root applications/Agents to connect to snmpd you can setup TCP socket as follows. TCP socket provides a cleaner access and allows easier troubleshooting e.g you could capture network traffic between snmpd and the AgentX application. Update /etc/snmp/snmpd.conf and ensure that following directives are set for TCP based AgentX communication.

rocommunity public default # or whatever community string/access control you want
master agentx # Run as an AgentX master agent
agentXSocket tcp:localhost:705 # Listen for localhost network connections instead of /var/agentx/master

Restart snmpd (/etc/init.d/snmpd restart)

Alternate is to set correct permissions for /var/agentx/master named socket or whatever you have configured.

Capture local network traffic for multi-homed host

If both end-points of a socket are on local system, network traffic will be seen on loopback interface even if applications are using non-loopback interface (e.g. eth0, wlan0…). Capturing data over loopback is quite obvious. But here I am discussing that applications are using one of the external interfaces (e.g. eth0, eth1, wlan0 …..).

Since both end-points are on  local system, kernel will shunt the traffic and not send it to the wire. The data will be delivered internally by queuing it to the read queue of other end-point. So we cannot capture the traffic on that particular interface, but this traffic is visible on loopback interface. Lets see an example.

I use netcat for setting up our test client and server program. nc -kl 9090 will run server on all interfaces on port 9090. And nc 10.1.1.100 9090 will setup a client. Here 10.1.1.100 is the external IP of my system(wlan0). Now instead of using the interface name associated with that IP (in my case wlan0), we have to use loopback interface lo to capture the traffic as below.

tcpdump -i lo tcp port 9090

Now anything that is typed on the client terminal when sent will be seen by tcpdump. Problem solved.

bash : grep for pattern from certain location in the file

Syntax for grep to search for a pattern in a file is very well-known. But there are times when one has to grep for the pattern from a certain location or after a certain offset in the file. For example if we are searching for a pattern in a log file which could appear multiple times. Each time we grep, it will provide us all the matching lines from top to bottom of the file and then we have to find which lines were new since our last run. Using dd, the file can be sliced and then grep can be applied for the pattern on that slice. Lets see an example. Continue reading

bash : search multiple file patterns using single find command

One reader asked how find can be used to find various file patterns. For example in a directory which could be littered with various logs and other files, how do I use a single find command to find all shell scripts, perl scripts and say php scripts. Simple answer is to use multiple -name arguments combined with -o (or ORed) and if needed with -a (or ANDed). Other find conditions (like -mtime, -type etc) can be combined as well. Continue reading

VirtualBox : Use Raw Disk to load Windows under Linux

Here I explain how to use a Physical disk partition for a guest OS under VirtualBox. This is also called Raw Disk partition use for VirtualBox. My use case was to run WindowsXP as guest OS from a physical installation under Linux and still be able to boot up the system in same Windows installation when needed. My system is running Ubuntu 11.04 on core2 duo, 3GB memory, two hard disks (one with Ubuntu and other with WindowsXP installation), with VirtualBox 4.0.4. Process is simple but took quite a while to get all information/steps collected and tested. In nutshell, first we need to know the partition that we will use, then the user who is going to use it needs to have access to it. After that an mbr has to be created and finally a vmdk file is created to use the Raw Disk. Keep reading for the full process. Continue reading

Howto rollover a file when size exceeds using unix find

Here is a one liner to rollover a file to file.old when it exceeds the size using find command. Lets say we have a script in cron that runs and prints messages in a log file. Overtime the log file will grow and we would want to rollover the log file to log.old. Many solutions exist by finding the size and comparing it. Here is one elegant solution in one liner. Thanks to my colleague Vlad who gave the idea for using find and exec, and I added the automatic substitution or brace expansion from my knowledge-base. Happy sharing of knowledge.

find /var/log/ -name myapp.log -size +1M -exec mv {}{,.old} ;

Continue reading

Unix : find affected by current working directory

On may Unix variants, find first looks for current working directory before proceeding with what it was asked to find. Ubuntu 10.10 and Debian Squezze not affected and I did not check older versions, but debian 5.0.6 or Lenny is affected and list includes Solaris 10 and Solaris 11 Express. It is very easy to fall in this pitfall if you have some automated package installation which may invoke some scripts for starting applications at the end of installation while cleaning up the temporary directory the package was running from. I wasted couple of hours in going over all my scripts to understand what was going on. The ls command was working but find was not able to get me the list of files to process from unrelated directories. So I ended up redirecting find’s error out to standard out and viola, solution presents itself. That redirection should have been on top of my list. It tells you that “find : cannot get the current working directory”. Why it needs that? I don’t know. Linux has this fixed for some time now, but for some reason SunOS is still using the old find variant including Solaris 11 Express which is the latest version out. Maybe some historical reasons. If anyone know, please share.

So the solution to the problem was that before invoking the command that will continue to run and may need to call find, start it in a directory that will persist after package installation is complete, e.g. / or /tmp.
Continue reading