Saturday, April 18, 2009

How to ?

how to run a server as a deamon

Friday, April 17, 2009

Qualys Inc - Final Round

How to add a user in a linux machine?

How to check for the dependencies of a package?

SSH connection for the first time - Trusted Host ?
How to do it without prompting for a password
How SSH will store the keys at server side? How to add user ?
SSH as a root without password ?

md5 length and which algorithm it uses? how do u do it linux / windows

regex ! every time !!!!
How to fetch a name called any name containing patel from a text file
what is ^ and what is $
$ENV variable
CPAN and Object Oriented Perl

/etc/password content ?

where rpm packages are installed

how to add remove and force the older version of rpm package

nmap and to give range and os fingerprinting option?

Version of each OS ?

Tuesday, April 14, 2009

Facebook Second Round - Telephonic

rc.conf ?

boot-screen messages ?

3 phase wire vs 1 phase ?

Monday, April 13, 2009

Qualys Inc - QA Intern interview questions

linux / windows

how to check if rpm package is installed or not?
How to install and uninstall module?
how to check the bootscreen text?

how software gets installed in linux, windows..etc
how to edit the registry entry in windows using gui


Socket programming apis
TCP Connection establishment and close
Socket APIs

traceroute ? how it works and whatis the meaning of stars ???

path MTU ...what is the purpose ?

TCP header format , flags, every thing

Port numbers again and again

Sliding window algorithm n how to solve the problem if rx is dropping the packet (manually set the limit)

Security related

SQL injection - every single time.....

IP Governance
How to test Nessus


Sudo code of sniffer

Perl - regular expression
text substitution
fatching text and all that stuff...

Friday, April 3, 2009

Facebook - Pre screening round

1)How kill command works
2)Bootcode ?

Sunday, March 29, 2009

Twitter interview questions - Operation Engineer-Search

Twitter interview questions - Operation Engineer-Search

How to check the files been created within last hour

How to check the largest file in the directory

How to do recursive copy?

How to scp to multiple machine

What is a command for storing the output of tcpdump in ASCII format

Difference between du and df

ssh on different port

what is find ?

Perl : How to find the most repeated IP address from the Textfile

Friday, March 27, 2009

Difference between du and df

du - disk usage statistics
df - free disk space.

In some cases it is possible that there is a difference between the outcome of df and du.

In 99.9% of the cases this is caused by a lock on the file system. Meaning, if an application opens a file or files and an other application deletes these files while they are opened they will still be calculated by df.

The df command calculates this by looking at the inode table and the inode table has not yet released those files, meaning that, according to the inode table, the files are still using space on the disk.

Generally the solution is to stop/start the process that has a lock on those non existing files and the results returned by df and du will be the same again.

Tuesday, January 20, 2009

Windows Wireless Troubleshooting

Before you contact Microsoft PSS for wireless connectivity problems, follow these steps to collect the required information:

1. Click Start, right-click My Computer, and then click Properties. The Windows operating system, the build number, and the latest installed service pack are listed under System on the General tab. Write down this information.
2. Write down the vendor of your wireless network adaptor and the model number.
3. Right-click the wireless network connection in the Network Connections folder, and then click Properties.
4. On the General tab, click Configure, and then click the Driver tab. Write down the following information for the driver:
* Version
* Date
* Provider
5. See the documentation that was supplied with your wireless access point to obtain the following information:
* Wireless access point vendor
* Model number of the wireless modem
* Wireless access point firmware version
6. If you used a wireless configuration tool to configure your wireless settings instead of using Windows XP, write down the tool name and the tool version number.
7. Write down the type of wireless network to which you are trying to connect. For example, you may be connecting to a private network at work, to a home network, or to a public wireless network.
8. Write down the settings that are configured for your wireless connection and for your wireless access point. For example, write down authentication and encryption settings.

The information that you have collected will help the PSS support engineer to identify the cause of the problem.

Sometimes, PSS support engineers must view log files that record the activity of Windows components. To collect the log files, follow these steps:

1. On the Windows XP desktop, click Start, click Run, type cmd, and then click OK.
2. At the command prompt, type netsh ras set tra * ena, and then press ENTER. This command enables tracing.
3. Restart your computer.
4. Note the time, and then reproduce your wireless problem. Write down how long it took you to reproduce the problem.
5. Move to the %windir%\Tracing folder. This folder should contain the following files:


Have these log files ready to send to PSS for analysis together with the time that it took you to reproduce the problem.
6. Click Start, click Run, type cmd, and then click OK.
7. To disable tracing, type netsh ras set tra * dis, and then press ENTER.
8. Restart your computer.

If your problem relates to the user interface, such as to a dialog box, obtain a screen shot of the user interface item. For example, if something looks incorrect in the View Available Networks dialog box, capture a screen shot of the dialog box that you can send to PSS. To do this, follow these steps:

1. Switch to the user interface that is incorrect, and then press ALT+PRINT SCREEN. This command copies the active window to the Clipboard.
2. Paste the screen shot into an e-mail message. You can also paste the screen shot into Paint, save the image as a file, and attach the file to an e-mail message.

PSS may also require status information for the Wireless Zero Configuration component or for the Wireless Configuration services. To obtain this status, follow these steps:

1. On the Windows desktop, click Start, click Run, type cmd, and then click OK.
2. At the command prompt, type sc query wzcsvc, and then press ENTER.
3. Capture a screen shot of the command output.

Sunday, January 18, 2009

Network Troubleshooting script

# Network testing script v 1.0
# (c) 2005 Javier Fernandez-Sanguino
# This script will test your system's network configuration using basic
# tests and providing both information (INFO messages), warnings (WARN)
# and possible errors (ERR messages) by checking:
# - Interface status
# - Availability of configured routers, including the default route
# - Proper host resolution, including DNS checks
# - Proper network connectivity (the remote host can be configured, see
# below)
# The script does not need special privileges to run as it does not
# do any system change. It also will not fix the errors by itself.
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# GNU General Public License for more details.
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
# You can also find a copy of the GNU General Public License at
# - Works only on Linux, can this be generalised for other UNIX systems
# (probably not unless rewritten in C)
# - Does not check for errors properly, use -e and test intensively
# so that expected errors are trapped
# (specially for tools that are not available, like netcat)
# - If the tools are localised to languages != english the script might
# break
# - Ask 'host' maintainer to implement error codes as done with
# dlint
# - Should be able to check if DNS server is in the same network, if
# it doesn't answer to pings, check ARP in that case.
# - DHCP checks?
# - Other internal services tests? (LDAP if using pam...)
# - Generate summary of errors in the end (pretty report?)
# - Check if packets are being dropped by local firewall? (use dmesg
# and look for our tests)
# - Support wireless interfaces? (use iwconfig)
# - Check other TODOs inline in the code

# BEGIN configuration
# Configure to your needs, these values will be used when
# checking DNS and Internet connectivity
# DNS name to resolve
# Web server to check for
# END configuration

# Extract the interface of our default route

defaultif=`netstat -nr |grep ^ | awk '{print $8}' | head -1`
defaultroutes=`netstat -nr |grep ^ | wc -l`
if [ -z "$defaultif" ] ; then
echo "WARN: This system does not have a default route"
elif [ "$defaultroutes" -gt 1 ] ; then
echo "WARN: This system has more than one default route"
echo "INFO: This system has exactly one default route"

# Check loopback
check_local () {
# Is there a loopback interface?
if [ -n "`ip link show lo`" ] ; then
# OK, can we ping localhost
if ! check_host localhost 1; then
# Check instead (not everybody uses this IP address however,
# although its the one commonly used)
if ! check_host 1; then
echo "ERR: Cannot ping localhost (, loopback is broken in this system"
echo "ERR: Localhost is not answering but, check /etc/hosts and verify localhost points to"
echo "INFO: Loopback interface is working properly"

echo "ERR: There is no loopback interface in this system"
return $status

# Check network interfaces
check_if () {
[ -z "$ifname" ] && return 1
# Find IP addresses for $ifname
inetaddr=`ip addr show $ifname | grep inet | awk '{print $2}'`
if [ -z "$inetaddr" ] ; then
echo "WARN: The $ifname interface does not have an IP address assigned"
# TODO: WARN if more than 2 IP addresses?
echo $inetaddr | while read ipaddr; do
echo "INFO: The $ifname interface has IP address $ipaddr assigned"

# Lookup TX and RX statistics
# TODO: This is done using ifconfig but could use /proc/net/dev for
# more readibility or, better, 'netstat -i'
txpkts=`ifconfig $ifname | awk '/RX packets/ { print $2 }' |sed 's/.*://'`
rxpkts=`ifconfig $ifname | awk '/RX packets/ { print $2 }' |sed 's/.*://'`
txerrors=`ifconfig $ifname | awk '/TX packets/ { print $3 }' |sed 's/.*://'`
rxerrors=`ifconfig $ifname | awk '/RX packets/ { print $3 }' |sed 's/.*://'`
# TODO: Check also frames and collisions, to detect faulty cables
# or network devices (cheap hubs)
if [ "$txpkts" -eq 0 ] && [ "$rxpkts" -eq 0 ] ; then
echo "ERR: The $ifname interface has not tx or rx any packets. Link down?"
elif [ "$txpkts" -eq 0 ]; then
echo "WARN: The $ifname interface has not transmitted any packets."
elif [ "$rxpkts" -eq 0 ] ; then
echo "WARN: The $ifname interface has not received any packets."
echo "INFO: The $ifname interface has tx and rx packets."
# TODO: It should be best if there was a comparison with tx/rx packets.
# a few errors are not uncommon if the card has been running for a long
# time. It would be better if a relative comparison was done (i.e.
# less than 1% ok, more than 20% warning, over 80% major issue, etc.)
if [ "$txerrors" -ne 0 ]; then
echo "WARN: The $ifname interface has tx errors."
if [ "$rxerrors" -ne 0 ]; then
echo "WARN: The $ifname interface has rx errors."
return $status

check_netif () {
ip link show | egrep '^[[:digit:]]' |
while read ifnumber ifname status extra; do
ifname=`echo $ifname |sed -e 's/:$//'`
if [ -z "`echo $status | grep UP\>`" ] ; then
if [ "$ifname" = "$defaultif" ] ; then
echo "ERR: The $ifname interface that is associated with your defualt route is down!"
elif [ "$ifname" = "lo" ] ; then
echo "ERR: Your lo inteface is down, this might cause issues with local applications (but not necessarily with network connectivity)"
echo "WARN: The $ifname interface is down"
# Check network routes associated with this interface
echo "INFO: The $ifname interface is up"
check_if $ifname
check_netroute $ifname
return $status

check_netroute () {
[ -z "$ifname" ] && return 1
netstat -nr | grep "${ifname}$" |
while read network gw netmask flags mss window irtt iface; do
# For each gw that is not the default one, ping it
if [ "$gw" != "" ] ; then
if ! check_router $gw ; then
echo "ERR: The default route is not available since the default router is unreachable"

check_router () {
# Checks if a router is up
[ -z "$router" ] && return 1
# First ping the router, if it does not answer then check arp tables and
# see if we have an arp. We use 5 packets since it is in our local network.
ping -q -c 5 "$router" >/dev/null 2>&1
if [ "$?" -ne 0 ]; then
echo "WARN: Router $router does not answer to ICMP pings"
# Router does not answer, check arp
routerarp=`arp -n | grep "^$router" | grep -v incomplete`
if [ -z "$routerarp" ] ; then
echo "ERR: We cannot retrieve a MAC address for router $router"
if [ "$status" -eq 0 ] ; then
echo "INFO: The router $router is reachable"
return $status

check_host () {
# Check if a host is reachable
# - if the host is in our local network (no route needs to be used) then
# check ARP availability
# - if the host is not on our local network then check if we have a route
# for it
[ -z "$host" ] && return 1
# Use 10 packets as we expect this to be outside of our network
[ -n "$2" ] && COUNT=$2
ping -q -c $COUNT "$host" >/dev/null 2>&1
if [ "$?" -ne 0 ]; then
echo "WARN: Host $host does not answer to ICMP pings"
echo "INFO: Host $host answers to ICMP pings"
return $status

check_dns () {
# Check the nameservers defined in /etc/resolv.conf
tempfile=`mktemp tmptestnet.XXXXXX` || { echo "ERR: Cannot create temporary file! Aborting! " >&2 ; exit 1; }
trap " [ -f \"$tempfile\" ] && /bin/rm -f -- \"$tempfile\"" 0 1 2 3 13 15
cat /etc/resolv.conf |grep nameserver |
awk '/nameserver/ { for (i=2;i<=NF;i++) { print $i ; } }' >$tempfile
for nameserver in `cat $tempfile`; do
nsfound=$(( $nsfound + 1 ))
echo "INFO: This system is configured to use nameserver $nameserver"
check_host $nameserver 5
if check_ns $nameserver ; then
nsok=$(( $nsok +1 ))
#Could also do:
#nsfound=`wc -l $tempfile | awk '{print $1}'`
/bin/rm -f -- "$tempfile"
trap 0 1 2 3 13 15
if [ "$nsfound" -eq 0 ] ; then
echo "ERR: The system does not have any nameserver configured"
if [ "$status" -ne 0 ] ; then
if [ "$nsfound" -eq 1 ] ; then
echo -e "ERR: There is one nameserver configured for this system but it does not work properly"
echo "ERR: There are $nsfound nameservers configured for this system and none of them works properly"
if [ "$nsfound" -eq 1 ] ; then
echo "INFO: The nameserver configured for this system works properly"
echo "INFO: There are $nsfound nameservers is configured for this system and $nsok are working properly"
return $status

check_ns () {
# Check the nameserver using host
# TODO: use nslookup?
# nslookup $CHECK_HOST -$nameserver
[ -z "$nameserver" ] && return 1
# Using dnscheck:
dnscheck=`host -t A $CHECK_HOST $nameserver 2>&1 | tail -1`
if [ -n "`echo $dnscheck |grep NXDOMAIN`" ] ; then
echo "ERR: Dns server $nameserver does not resolv properly"
elif [ -n "`echo $dnscheck | grep \"timed out\"`" ] ; then
echo "ERR: Dns server $nameserver is not available"
elif [ -z "`echo $dnscheck | egrep \"$CHECK_RESULT\"`" ] ; then
echo "WARN: Dns server $nameserver did not return the expected result for $CHECK_HOST"
echo "INFO: Dns server $nameserver resolved correctly $CHECK_HOST"

# Using dlint
# dlint $CHECK_HOST @$nameserver >/dev/null 2>&1
# if [ $? -eq 2 ] ; then
# echo "ERR: Dns server $nameserver does not resolv properly"
# elif [ $? -ne 0 ]; then
# echo "ERR: Unexpected error when testing $nameserver"
# else
# echo "INFO: Dns server $nameserver resolved correctly $CHECK_HOST"
# status=0
# fi

return $status

check_conn () {
# Checks network connectivity
if ! check_host $CHECK_WEB_HOST >/dev/null ; then
echo "WARN: System does not seem to reach Internet host $CHECK_WEB_HOST through ICMP"
echo "INFO: System can reach Internet host $CHECK_WEB_HOST"
# Check web access, using nc
echo -e "HEAD / HTTP/1.0\n\n" |nc $CHECK_WEB_HOST $CHECK_WEB_PORT >/dev/null 2>&1
if [ $? -ne 0 ] ; then
echo "WARN: Cannot access web server at Internet host $CHECK_WEB_HOST"
echo "INFO: System can access web server at Internet host $CHECK_WEB_HOST"

# TODO: checks could be conditioned, i.e. if there is no proper
# interface setup don't bother with DNS and don't do some Inet checks
# if DNS is not setup properly

exit 0

Wireless Troubleshooting guide can also be scripted

Wireless Troubleshooting

Hardware Verification

The first critical step is to ensure that your wireless device is recognized by your system. There are a variety of methods to verify that your system did this successfully. Here are some methods:

The “dmesg” command can quite often contain detailed messages indicating that the wirelss devices was properly detected.
If the card is an ISA card, you are usually out of luck.
If the card is a PCI card (miniPCI/miniPCI Express/PCI Express), you need to use the command “lspci” to display the card identification strings.
If the hardware is a USB dongle, you need to use the command “lsusb” to display the dongle identification strings. In some case, “lsusb” doesn't work (for example if usbfs is not mounted), and you can get the identification strings from the kernel log using “dmesg” (or in /var/log/messages).
If the card is a Cardbus card (32 bits Pcmcia), and if you are using kernel 2.6.X or kernel 2.4.X with the kernel Pcmcia subsystem, you need to use the command “lspci” to display the card identification strings. If the card is a Cardbus card (32 bits Pcmcia), and if you are using an older kernel with the standalone Pcmcia subsystem, you need to use the command “cardctl ident” display the card identification strings. Try both and see what comes out.
If the card is a true Pcmcia card (16 bits), and if you are using kernel 2.6.14 or later, you need to use the command “pccardctl ident” to display the card identification strings. If the card is a true Pcmcia card (16 bits), and if you are using an older kernel, you need to use the command “cardctl ident” display the card identification strings. Note that cardmgr will also write some identification strings in the message logs (/var/log/daemon.log) that may be different from the real card identification strings.

Needless to say, if your wireless device is not detected by your system, you will have to investigate and correct the problem.

Start by running “modprobe ”.
View iwconfig output

Run the “iwconfig” command and look for wireless devices. Based on the driver, look for an appropriately named interface such as ath0, rausb0, etc. The presence indicates that at least the driver is loaded. The absence likely means it did not. This at least gives you a starting point on the problem solving.

A common problem is that your system has both ieee80211 and mac80211 versions of the drivers. Having wmaster0 typically indicates you are using the new mac80211 drivers. Having wifi0 or eth0 typically means you are using the older (legacy) ieee80211 drivers. Having both wmaster0 and wifi0/eth0 (as well as weird interface names like wlan0_rename) might indicate a udev problem. Based on what which ones you really want, you may have to blacklist or move one or more drivers.
View dmesg output

Run the “dmesg” command and look for errors relating to your wireless device. At a minimum there should be some messages relating to your device loading and the module initializing it. If there are no messages or errors, you will have to investigate and correct the problem.

See the next entry of a problem commonly seen: “unknown symbol”.
"unknown symbol" error

When loading the driver kernel module you get a “unknown symbol” error message for one more field names. Sometimes you will see this in the dmesg output as well. This is caused by module you are loading not being matching the kernel version you are running.

First, determine which kernel you are running with “uname -r”. Then use your package manager to determine if you have kernels, kernel headers or kernel development packages that are older.

If you use the RPM package manager then “rpm -qa | grep kernel”. So if you get something like:


In the example above, there are kernel headers and a kernel development package that match the kernel we are running. If you are missing them, the use yum or equivalent on your distribution to install them such as:

yum -y install kernel-headers
yum -y install kernel-devel

Lets assume that “uname -r” returned “” then all the ones are old and need to be removed. So you remove all the old ones:

rpm -e
rpm -e kernel-
rpm -e kernel-devel-

Also change to ”/lib/modules” and do a directory listing and remove any directory referring to old kernel versions.

Once you are finished, you can do ”“rpm -qa | grep kernel” and confirm everything looks good. At this point, recompile your wireless drivers and reboot the system.
View lsmod output

Run the “lsmod” command can be used to see the loaded modules. Confirm that the kernel module for your wireless device is actually loaded. If it is not loaded, you will have to investigate and correct the problem.

Sometimes other modules conflict with the one you are trying to run. See blacklisting below. Additionally, conflicting modules can be moved out of the module tree. If you do this, run “depmod -ae” afterwards.
View modinfo output

Run “modinfo ”. This will confirm the module is actually in the modules tree. As well, confirm it is the correct version. Do a “ls -l ” and confirm the date matches when you compiled it. It is not uncommon that you are not running the correct module version.

A common problem on newer kernels is that the new mac80211 version of the driver gets loaded instead of the older legacy driver, or vice versa. If that is the case, then you need to blacklist the wrong modules by editing /etc/modprobe.d/blacklist. First, determine the broken module names and add them to the blacklist file as “blacklist ”.

Specifically for madwifi-ng, do a locate or find for ath5k.ko. If ath5k.ko exists then add “blacklist ath5k” to /etc/modprobe.d/blacklist and reboot. Same for the other way around: if you want to load ath5k, but madwifi-ng gets loaded instead, add “blacklist ath_pci” to /etc/modprobe.d/blacklist.
Reload Driver

Although it is not very “scientific”, sometimes simply unloading then reloading the driver will get it working. This is done with the rmmod and modprobe commands.

For b43 and b43legacy, it might also be necessary to reload the underlying SSB module. Similarly, rt2x00 and p54 might need reloading of the common modules (p54common, rt2x00lib, rt2x00usb, rt2x00pci). Sometimes (especially with mac80211 drivers), reloading the stack (for example, modules “cfg80211” and “mac80211”) might do the trick.