Linux CLI reference

Command Description
apropos whatis Show commands pertinent to string.
man -t man | ps2pdf – > man.pdf make a pdf of a manual page
which command Show full path name of command
time command See how long a command takes
time cat Start stopwatch. Ctrl-d to stop.
nice info Run a low priority command (The “info” reader in this case)
renice 19 -p $ Make shell (script) low priority. Use for non interactive tasks
dir navigation
cd – Go to previous directory
cd Go to $HOME directory
(cd dir && command) Go to dir, execute command and return to current dir
pushd . Put current dir on stack so you can popd back to it
file searching
alias l=’ls -l –color=auto’ quick dir listing
ls -lrt List files by date.
ls /usr/bin | pr -T9 -W$COLUMNS Print in 9 columns to width of terminal
find -name ‘*.[ch]‘ | xargs grep -E ‘expr’ Search ‘expr’ in this dir and below.
find -type f -print0 | xargs -r0 grep -F ‘example’ Search all regular files for ‘example’ in this dir and below
find -maxdepth 1 -type f | xargs grep -F ‘example’ Search all regular files for ‘example’ in this dir
find -maxdepth 1 -type d | while read dir; do echo $dir; echo cmd2; done Process each item with multiple commands (in while loop)
find -type f ! -perm -444 Find files not readable by all (useful for web site)
find -type d ! -perm -111 Find dirs not accessible by all (useful for web site)
locate -r ‘file[^/]*.txt’ Search cached index for names. This re is like glob *file*.txt
look reference Quickly search (sorted) dictionary for prefix
grep –color reference /usr/share/dict/words Highlight occurances of regular expression in dictionary
archives and compression
gpg -c file Encrypt file
gpg file.gpg Decrypt file
tar -c dir/ | bzip2 > dir.tar.bz2 Make compressed archive of dir/
bzip2 -dc dir.tar.bz2 | tar -x Extract archive (use gzip instead of bzip2 for tar.gz files)
tar -c dir/ | gzip | gpg -c | ssh user@remote ‘dd of=dir.tar.gz.gpg’ Make encrypted archive of dir/ on remote machine
find dir/ -name ‘*.txt’ | tar -c –files-from=- | bzip2 > dir_txt.tar.bz2 Make archive of subset of dir/ and below
find dir/ -name ‘*.txt’ | xargs cp -a –target-directory=dir_txt/ –parents Make copy of subset of dir/ and below
( tar -c /dir/to/copy ) | ( cd /where/to/ && tar -x -p ) Copy (with permissions) copy/ dir to /where/to/ dir
( cd /dir/to/copy && tar -c . ) | ( cd /where/to/ && tar -x -p ) Copy (with permissions) contents of copy/ dir to /where/to/
( tar -c /dir/to/copy ) | ssh -C user@remote ‘cd /where/to/ && tar -x -p’ Copy (with permissions) copy/ dir to remote:/where/to/ dir
dd bs=1M if=/dev/sda | gzip | ssh user@remote ‘dd of=sda.gz’ Backup harddisk to remote machine
rsync (Network efficient file copier: Use the –dry-run option for testing)
rsync -P rsync:// file Only get diffs. Do multiple times for troublesome downloads
rsync –bwlimit=1000 fromfile tofile Locally copy with rate limit. It’s like nice for I/O
rsync -az -e ssh –delete ~/public_html/’~/public_html’ Mirror web site (using compression and encryption)
rsync -auz -e ssh remote:/dir/ . && rsync -auz -e ssh . remote:/dir/ Synchronize current directory with remote one
ssh (Secure SHell)
ssh $USER@$HOST command Run command on $HOST as $USER (default command=shell)
ssh -f -Y $USER@$HOSTNAME xeyes Run GUI command on $HOSTNAME as $USER
scp -p -r $USER@$HOST: file dir/ Copy with permissions to $USER’s home directory on $HOST
ssh -g -L 8080:localhost:80 root@$HOST Forward connections to $HOSTNAME:8080 out to $HOST:80
ssh -R 1434:imap:143 root@$HOST Forward connections from $HOST:1434 in to imap:143
wget (multi purpose download tool)
(cd dir/ && wget -nd -pHEKk Store local browsable version of a page to the current dir
wget -c Continue downloading a partially downloaded file
wget -r -nd -np -l1 -A ‘*.jpg’ Download a set of files to the current directory
wget ftp://remote/file[1-9].iso/ FTP supports globbing directly
wget -q -O- | grep ‘a href’ | head Process output directly
echo ‘wget url’ | at 01:00 Download url at 1AM to current dir
wget –limit-rate=20k url Do a low priority download (limit to 20KB/s in this case)
wget -nv –spider –force-html -i bookmarks.html Check links in a file
wget –mirror Efficiently update a local copy of a site (handy from cron)
networking (Note ifconfig, route, mii-tool, nslookup commands are obsolete)
ethtool eth0 Show status of ethernet interface eth0
ethtool –change eth0 autoneg off speed 100 duplex full Manually set ethernet interface speed
iwconfig eth1 Show status of wireless i
nterface eth1
iwconfig eth1 rate 1Mb/s fixed Manually set wireless interface speed
iwlist scan List wireless networks in range
ip link show List network interfaces
ip link set dev eth0 name wan Rename interface eth0 to wan
ip link set dev eth0 up Bring interface eth0 up (or down)
ip addr show List addresses for interfaces
ip addr add brd + dev eth0 Add (or del) ip and mask (
ip route show List routing table
ip route add default via Set default gateway to
tc qdisc add dev lo root handle 1:0 netem delay 20msec Add 20ms latency to loopback device (for testing)
tc qdisc del dev lo root Remove latency added above
host Lookup DNS ip address for name or vice versa
hostname -i Lookup local ip address (equivalent to host `hostname`)
whois Lookup whois info for hostname or ip address
netstat -tupl List internet services on a system
netstat -tup List active connections to/from system
windows networking (Note samba is the package that provides all this windows specific networking support)
smbtree Find windows machines. See also findsmb
nmblookup -A Find the windows (netbios) name associated with ip address
smbclient -L windows_box List shares on windows machine or samba server
mount -t smbfs -o fmask=666,guest //windows_box/share /mnt/share Mount a windows share
echo ‘message’ | smbclient -M windows_box Send popup to windows machine (off by default in XP sp2)
text manipulation (Note sed uses stdin and stdout. Newer versions support inplace editing with the -i option)
sed ‘s/string1/string2/g’ Replace string1 with string2
sed ‘s/(.*)1/12/g’ Modify anystring1 to anystring2
sed ‘/ *#/d; /^ *$/d’ Remove comments and blank lines
sed ‘:a; /\$/N; s/\n//; ta’ Concatenate lines with trailing
sed ‘s/[ t]*$//’ Remove trailing spaces from lines
sed ‘s/([`”$])/\1/g’ Escape shell metacharacters active within double quotes
seq 10 | sed “s/^/      /; s/ *(.{7,})/1/” Right align numbers
sed -n ’1000{p;q}’ Print 1000th line
sed -n ’10,20p;20q Print lines 10 to 20
sed -n ‘s/.*<title>(.*)</title>.*/1/ip;T;q Extract title from HTML web page
sed -i 42d ~/.ssh/known_hosts Delete a particular line
sort -t. -k1,1n -k2,2n -k3,3n -k4,4n Sort IPV4 ip addresses
echo ‘Test’ | tr ‘[:lower:]‘ ‘[:upper:]‘ Case conversion
tr -dc ‘[:print:]‘ < /dev/urandom Filter non printable characters
history | wc -l Count lines
set operations (Note you can export LANG=C for speed. Also these assume no duplicate lines within a file)
sort file1 file2 | uniq Union of unsorted files
sort file1 file2 | uniq -d Intersection of unsorted files
sort file1 file1 file2 | uniq -u Difference of unsorted files
sort file1 file2 | uniq -u Symmetric Difference of unsorted files
join -t’′ -a1 -a2 file1 file2 Union of sorted files
join -t’′ file1 file2 Intersection of sorted files
join -t’′ -v2 file1 file2 Difference of sorted files
join -t’′ -v1 -v2 file1 file2 Symmetric Difference of sorted files
echo ‘(1 + sqrt(5))/2′ | bc -l Quick math (Calculate ?).
echo ‘pad=20; min=64; (100*10^6)/((pad+min)*8)’ | bc More complex (int) e.g. This shows max FastE packet rate
echo ‘pad=20; min=64; print (100E6)/((pad+min)*8)’ | python Python handles scientific notation
echo ‘pad=20; plot [64:1518] (100*10**6)/((pad+x)*8)’ | gnuplot -persist Plot FastE packet rate vs packet size
echo ‘obase=16; ibase=10; 64206′ | bc Base conversion (decimal to hexadecimal)
echo $((0x2dec)) Base conversion (hex to dec) ((shell arithmetic expansion))
units -t ’100m/9.58s’ ‘miles/hour’ Unit conversion (metric to imperial)
units -t ’500GB’ ‘GiB’ Unit conversion (SI to IEC prefixes)
units -t ’1 googol’ Definition lookup
seq 100 | (tr ‘n’ +; echo 0) | bc Add a column of numbers.
cal -3 Display a calendar
cal 9 1752 Display a calendar for a particular month year
date -d fri What date is it this friday.
[ $(date -d “tomorrow” +%d) = “01” ] || exit exit a script unless it’s the last day of the month
date –date=’25 Dec’ +%A What day does xmas fall on, this year
date –date=’@2147483647′ Convert seconds since the epoch (1970-01-01 UTC) to date
TZ=’America/Los_Angeles’ date What time is it on west coast of US (use tzselect to find TZ)
date –date=’TZ=”America/Los_Angeles” 09:00 next Fri’ What’s the l
ocal time for 9AM next Friday on west coast US
echo “mail -s ‘get the train’ < /dev/null” | at 17:45 Email reminder
echo “DISPLAY=$DISPLAY xmessage cooker” | at “NOW + 30 minutes” Popup reminder
printf “%’dn” 1234 Print number with thousands grouping appropriate to locale
BLOCK_SIZE=’1 ls -l get ls to do thousands grouping appropriate to locale
echo “I live in `locale territory`” Extract info from locale database
LANG=en_IE.utf8 locale int_prefix Lookup locale info for specific country.
locale | cut -d= -f1 | xargs locale -kc | less List fields available in locale database
recode (Obsoletes iconv, dos2unix, unix2dos)
recode -l | less Show available conversions (aliases on each line)
recode windows-1252.. file_to_change.txt Windows “ansi” to local charset (auto does CRLF conversion)
recode utf-8/CRLF.. file_to_change.txt Windows utf8 to local charset
recode iso-8859-15..utf8 file_to_change.txt Latin9 (western europe) to utf8
recode ../b64 < file.txt > file.b64 Base64 encode
recode /qp.. < file.qp > file.txt Quoted printable decode
recode ..HTML < file.txt > file.html Text to HTML
recode -lf windows-1252 | grep euro Lookup table of characters
echo -n 0×80 | recode latin-9/x1..dump Show what a code represents in latin-9 charmap
echo -n 0x20AC | recode ucs-2/x2..latin-9/x Show latin-9 encoding
echo -n 0x20AC | recode ucs-2/x2..utf-8/x Show utf-8 encoding
gzip < /dev/cdrom > cdrom.iso.gz Save copy of data cdrom
mkisofs -V LABEL -r dir | gzip > cdrom.iso.gz Create cdrom image from contents of dir
mount -o loop cdrom.iso /mnt/dir Mount the cdrom image at /mnt/dir (read only)
cdrecord -v dev=/dev/cdrom blank=fast Clear a CDRW
gzip -dc cdrom.iso.gz | cdrecord -v dev=/dev/cdrom – Burn cdrom image (use dev=ATAPI -scanbus to confirm dev)
cdparanoia -B Rip audio tracks from CD to wav files in current dir
cdrecord -v dev=/dev/cdrom -audio -pad *.wav Make audio CD from all wavs in current dir (see also cdrdao)
oggenc –tracknum=’track’ track.cdda.wav -o ‘track.ogg’ Make ogg file from wav file
disk space
ls -lSr Show files by size, biggest last
du -s * | sort -k1,1rn | head Show top disk users in current dir.
df -h Show free space on mounted filesystems
df -i Show free inodes on mounted filesystems
fdisk -l Show disks partitions sizes and types (run as root)
rpm -q -a –qf ‘%10{SIZE}t%{NAME}n’ | sort -k1,1n List all packages by installed size (Bytes) on rpm distros
dpkg-query -W -f=’${Installed-Size;10}t${Package}n’ | sort -k1,1n List all packages by installed size (KBytes) on deb distros
dd bs=1 seek=2TB if=/dev/null of=ext3.test Create a large test file (taking no space).
> file truncate data of file or create an empty file
tail -f /var/log/messages Monitor messages in a log file
strace -c ls >/dev/null Summarise/profile system calls made by command
strace -f -e open ls >/dev/null List system calls made by command
ltrace -f -e getenv ls >/dev/null List library calls made by command
lsof -p $ List paths that process id has open
lsof ~ List processes that have specified path open
tcpdump not port 22 Show network traffic except ssh.
ps -e -o pid,args –forest List processes in a hierarchy
ps -e -o pcpu,cpu,nice,state,cputime,args –sort pcpu | sed ‘/^ 0.0 /d’ List processes by % cpu usage
ps -e -orss=,args= | sort -b -k1,1n | pr -TW$COLUMNS List processes by mem (KB) usage.
ps -C firefox-bin -L -o pid,tid,pcpu,state List all threads for a particular process
ps -p 1,2 List info for particular process IDs
last reboot Show system reboot history
free -m Show amount of (remaining) RAM (-m displays in MB)
watch -n.1 ‘cat /proc/interrupts’ Watch changeable data continuously
system information (‘#’ means root access is required)
uname -a Show kernel version and system architecture
head -n1 /etc/issue Show name and version of distribution
cat /proc/partitions Show all partitions registered on the system
grep MemTotal /proc/meminfo Show RAM total seen by the system
grep “model name” /proc/cpuinfo Show CPU(s) info
lspci -tv Show PCI info
lsusb -tv Show USB info
mount | column -t List mounted filesystems on the system (and align output)
grep -F capacity: /proc/acpi/battery/BAT0/info Show state of cells in laptop battery
# dmidecode -q | less Display SMBIOS/DMI information
# smartctl -A /dev/sda | grep Power_On_Hours How long has this disk (system) been powered on in total
# hdparm -i /dev/sda Show info about disk sda
# hdparm -tT /dev/sda Do a read speed test on disk sda
# badblocks -s /dev/sda Test for unreadable blocks on disk sda
readline Line editor used by bash, python, bc, gnuplot, …

screen Virtual terminals with detach capability, …
mc Powerful file manager that can browse rpm, tar, ftp, ssh, …
gnuplot Interactive/scriptable graphing
links Web browser
xdg-open . open a file or url with the registered desktop application
alias hd=’od -Ax -tx1z -v’ Handy hexdump. (usage e.g.: • hd /proc/self/cmdline | less)
alias realpath=’readlink -f’ Canonicalize path. (usage e.g.: • realpath ~/../$USER)
set | grep $USER Search current environment
touch -c -t 0304050607 file Set file timestamp (YYMMDDhhmm)
python -m SimpleHTTPServer Serve current directory tree at http://$HOSTNAME:8000/

This is just a repost from

RawCap sniffer for Windows released (via

We are today proude to announce the release of RawCap, which is a free raw sockets sniffer for Windows.

Here are some highlights of why RawCap is a great tool to have in your toolset:

  • Can sniff any interface that has got an IP address, including (localhost/loopback)
  • RawCap.exe is just 17 kB
  • No external libraries or DLL’s needed
  • No installation required, just download RawCap.exe and sniff
  • Can sniff most interface types, including WiFi and PPP interfaces
  • Minimal memory and CPU load
  • Reliable and simple to use


RawCap takes two arguments; the first argument is the IP address or interface number to sniff from, the second is the path/file to write the captured packets to.

C:Tools>RawCap.exe dumpfile.pcap

You can also start RawCap without any arguments, which will leave you with an interactive dialog where you can select NIC and filename:

Network interfaces:
0.    Local Area Connection
1.    Wireless Network Connection
2.   3G UMTS Internet
3.   VMware Network Adapter VMnet1
4.   VMware Network Adapter VMnet2
5.       Loopback Pseudo-Interface
Select network interface to sniff [default ‘0’]: 1
Output path or filename [default ‘dumpfile.pcap’]:
Sniffing IP :
File        : dumpfile.pcap
Packets     : 1337

For Incident Responders

RawCap comes in very handy for incident responders who want to be able to sniff network traffic locally at the clients of the corporate network. Here are a few examples of how RawCap can be used for incident response:

  1. A company laptop somewhere on the corporate network is believed to exfiltrate sensitive coporate information to a foreign server on the Internet by using a UMTS 3G connection on a USB dongle. After finding the internal IP address on the corporate network the Incident Response Team (IRT) use the Sysinternals tool PsExec to inject RawCap.exe onto the laptop and sniff the packets being exfiltrated through the 3G connection. The generated pcap file can be used to determine what the external 3G connection was used for.
  2. A computer is suspected to be infected with malware that uses an SSL tunnelling proxy (stunnel) to encrypt all Command-and-Control (C&C) communication. The data that is to be sent into the tunnel is first sent unencrypted to localhost ( aka loopback interface) before it enters the encrypted tunnel. Incident responders can use RawCap to sniff the traffic to/from localhost on the Windows OS, which is something other sniffing tools cannot do.
  3. A corporate laptop connected to the companies WPA2 encrypted WiFi is found to have suspicious TCP sessions opened to other computers on the same WiFi network. Incident responders can run RawCap locally on any of those machines in order to capture the WiFi network traffic to/from that machine in unencrypted form.

For Penetration Testers

RawCap was not designed for pen-testers, but I realize that there are some situations where the tool can come in hany when doing a penetration test. Here are some examples:

  1. After getting remote access and admin privileges on a Windows XP machine the pen-tester wanna sniff the network traffic of the machine in order to get hold of additional credentials. Sniffing tools like dumpcap, WinDump and NMCap can unfortunately not be used since no WinPcap or NDIS driver is installed. RawCap does, however, not need any special driver installed since it makes use of the Raw Sockets functionality built into Windows. Pen-testers can therefore run RawCap.exe to sniff traffic without installing any drivers.
  2. After getting admin on a box the pen-tester wanna sniff the network traffic, but box uses a WiFi network so traditional sniffing tools won’t work. This is when RawCap comes in handy, since it can sniff the WiFi traffic of the owned machine just as easily as if it had been an Ethernet NIC.

Download RawCap

RawCap is provided for free and can be downloaded from here:

Original post:

Lazy Linux: 10 essential tricks for admins (by Vallard Benincosa via developerWorks)

The best systems administrators are set apart by their efficiency. And if an efficient systems administrator can do a task in 10 minutes that would take another mortal two hours to complete, then the efficient systems administrator should be rewarded (paid more) because the company is saving time, and time is money, right?

The trick is to prove your efficiency to management. While I won’t attempt to cover that trick in this article, I will give you 10 essential gems from the lazy admin’s bag of tricks. These tips will save you time—and even if you don’t get paid more money to be more efficient, you’ll at least have more time to play Halo.

Trick 1: Unmounting the unresponsive DVD drive

The newbie states that when he pushes the Eject button on the DVD drive of a server running a certain Redmond-based operating system, it will eject immediately. He then complains that, in most enterprise Linux servers, if a process is running in that directory, then the ejection won’t happen. For too long as a Linux administrator, I would reboot the machine and get my disk on the bounce if I couldn’t figure out what was running and why it wouldn’t release the DVD drive. But this is ineffective.

Here’s how you find the process that holds your DVD drive and eject it to your heart’s content: First, simulate it. Stick a disk in your DVD drive, open up a terminal, and mount the DVD drive:

# mount /media/cdrom
# cd /media/cdrom
# while [ 1 ]; do echo "All your drives are belong to us!"; sleep 30; done

Now open up a second terminal and try to eject the DVD drive:

# eject

You’ll get a message like:

umount: /media/cdrom: device is busy

Before you free it, let’s find out who is using it.

# fuser /media/cdrom

You see the process was running and, indeed, it is our fault we can not eject the disk.

Now, if you are root, you can exercise your godlike powers and kill processes:

# fuser -k /media/cdrom

Boom! Just like that, freedom. Now solemnly unmount the drive:

# eject

fuser is good.


Trick 2: Getting your screen back when it’s hosed

Try this:

# cat /bin/cat

Behold! Your terminal looks like garbage. Everything you type looks like you’re looking into the Matrix. What do you do?

You type reset. But wait you say, typing reset is too close to typing reboot or shutdown. Your palms start to sweat—especially if you are doing this on a production machine.

Rest assured: You can do it with the confidence that no machine will be rebooted. Go ahead, do it:

# reset

Now your screen is back to normal. This is much better than closing the window and then logging in again, especially if you just went through five machines to SSH to this machine.


Trick 3: Collaboration with screen

David, the high-maintenance user from product engineering, calls: “I need you to help me understand why I can’t compile supercode.c on these new machines you deployed.”

“Fine,” you say. “What machine are you on?”

David responds: ” Posh.” (Yes, this fictional company has named its five production servers in honor of the Spice Girls.) OK, you say. You exercise your godlike root powers and on another machine become David:

# su - david

Then you go over to posh:

# ssh posh

Once you are there, you run:

# screen -S foo

Then you holler at David:

“Hey David, run the following command on your terminal: # screen -x foo.”

This will cause your and David’s sessions to be joined together in the holy Linux shell. You can type or he can type, but you’ll both see what the other is doing. This saves you from walking to the other floor and lets you both have equal control. The benefit is that David can watch your troubleshooting skills and see exactly how you solve problems.

At last you both see what the problem is: David’s compile script hard-coded an old directory that does not exist on this new server. You mount it, recompile, solve the problem, and David goes back to work. You then go back to whatever lazy activity you were doing before.

The one caveat to this trick is that you both need to be logged in as the same user. Other cool things you can do with the screen command include having multiple windows and split screens. Read the man pages for more on that.

But I’ll give you one last tip while you’re in your screen session. To detach from it and leave it open, type: Ctrl-A D . (I mean, hold down the Ctrl key and strike the A key. Then push the D key.)

You can then reattach by running the screen -x foo command again.


Trick 4: Getting back the root password

You forgot your root password. Nice work. Now you’ll just have to reinstall the entire machine. Sadly enough, I’ve seen more than a few people do this. But it’s surprisingly easy to get on the machine and change the password. This doesn’t work in all cases (like if you made a GRUB password and forgot that too), but here’s how you do it in a normal case with a Cent OS Linux example.

First reboot the system. When it reboots you’ll come to the GRUB screen as shown in Figure 1. Move the arrow key so that you stay on this screen instead of proceeding all the way to a normal boot.

Figure 1. GRUB screen after reboot
GRUB screen after reboot

Next, select the kernel that will boot with the arrow keys, and type E to edit the kernel line. You’ll then see something like Figure 2:

Figure 2. Ready to edit the kernel line
Ready to edit the kernel line

Use the arrow key again to highlight the line that begins with kernel, and press E to edit the kernel parameters. When you get to the screen shown in Figure 3, simply append the number 1 to the arguments as shown in Figure 3:

Figure 3. Append the arg
ument with the number 1

Append the argument with the number 1

Then press Enter, B, and the kernel will boot up to single-user mode. Once here you can run the passwd command, changing password for user root:

sh-3.00# passwd
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully

Now you can reboot, and the machine will boot up with your new password.


Trick 5: SSH back door

Many times I’ll be at a site where I need remote support from someone who is blocked on the outside by a company firewall. Few people realize that if you can get out to the world through a firewall, then it is relatively easy to open a hole so that the world can come into you.

In its crudest form, this is called “poking a hole in the firewall.” I’ll call it an SSH back door. To use it, you’ll need a machine on the Internet that you can use as an intermediary.

In our example, we’ll call our machine The machine behind the company firewall is called ginger. Finally, the machine that technical support is on will be called tech. Figure 4 explains how this is set up.

Figure 4. Poking a hole in the firewall
Poking a hole in the firewall

Here’s how to proceed:

  1. Check that what you’re doing is allowed, but make sure you ask the right people. Most people will cringe that you’re opening the firewall, but what they don’t understand is that it is completely encrypted. Furthermore, someone would need to hack your outside machine before getting into your company. Instead, you may belong to the school of “ask-for-forgiveness-instead-of-permission.” Either way, use your judgment and don’t blame me if this doesn’t go your way.

  2. SSH from ginger to with the -R flag. I’ll assume that you’re the root user on ginger and that tech will need the root user ID to help you with the system. With the -R flag, you’ll forward instructions of port 2222 on blackbox to port 22 on ginger. This is how you set up an SSH tunnel. Note that only SSH traffic can come into ginger: You’re not putting ginger out on the Internet naked.

    You can do this with the following syntax:

    ~# ssh -R 2222:localhost:22

    Once you are into blackbox, you just need to stay logged in. I usually enter a command like:

    thedude@blackbox:~$ while [ 1 ]; do date; sleep 300; done

    to keep the machine busy. And minimize the window.

  3. Now instruct your friends at tech to SSH as thedude into blackbox without using any special SSH flags. You’ll have to give them your password:

    root@tech:~# ssh .

  4. Once tech is on the blackbox, they can SSH to ginger using the following command:

    thedude@blackbox:~$: ssh -p 2222 root@localhost

  5. Tech will then be prompted for a password. They should enter the root password of ginger.

  6. Now you and support from tech can work together and solve the problem. You may even want to use screen together! (See Trick 4.)


Trick 6: Remote VNC session through an SSH tunnel

VNC or virtual network computing has been around a long time. I typically find myself needing to use it when the remote server has some type of graphical program that is only available on that server.

For example, suppose in Trick 5, ginger is a storage server. Many storage devices come with a GUI program to manage the storage controllers. Often these GUI management tools need a direct connection to the storage through a network that is at times kept in a private subnet. Therefore, the only way to access this GUI is to do it from ginger.

You can try SSH’ing to ginger with the -X option and launch it that way, but many times the bandwidth required is too much and you’ll get frustrated waiting. VNC is a much more network-friendly tool and is readily available for nearly all operating systems.

Let’s assume that the setup is the same as in Trick 5, but you want tech to be able to get VNC access instead of SSH. In this case, you’ll do something similar but forward VNC ports instead. Here’s what you do:

  1. Start a VNC server session on ginger. This is done by running something like:

    root@ginger:~# vncserver -geometry 1024x768 -depth 24 :99

    The options tell the VNC server to start up with a resolution of 1024×768 and a pixel depth of 24 bits per pixel. If you are using a really slow connection setting, 8 may be a better option. Using :99 specifies the port the VNC server will be accessible from. The VNC protocol starts at 5900 so specifying :99 means the server is accessible from port 5999.

    When you start the session, you’ll be asked to specify a password. The user ID will be the same user that you launched the VNC server from. (In our case, this is root.)

  2. SSH from ginger to forwarding the port 5999 on blackbox to ginger. This is done from ginger by running the command:

    root@ginger:~# ssh -R 5999:localhost:5999

    Once you run this command, you’ll need to keep this SSH session open in order to keep the port forwarded to ginger. At this point if you were on blackbox, you could now access the VNC session on ginger by just running:

    thedude@blackbox:~$ vncviewer localhost:99

    That would forward the port through SSH to ginger. But we’re interested in letting tech get VNC access to ginger. To accomplish this, you’ll need another tunnel.

  3. From tech, you open a tunnel via SSH to forward your port 5999 to port 5999 on blackbox. This would be done by running:

    root@tech:~# ssh -L 5999:localhost:5999

    This time the SSH flag we used was -L, which instead of pushing 5999 to blackbox, pulled from it. Once you are in on blackbox, you’ll need to leave this session open. Now you’re ready to VNC from tech!

  4. From tech, VNC to ginger by running the command:

    root@tech:~# vncviewer localhost:99 .

    Tech will now have a VNC session directly to ginger.

While the effort might seem like a bit much to set up, it beats flying across the country to fix the storage arrays. Also, if you practice this a few times, it becomes quite easy.

Let me add a trick to this trick: If tech was running the Windows® operating system and didn’t have a command-line SSH client, then tech can run Putty. Putty can be set to forward SSH ports by looking in the options in the sidebar. If the port were 5902 instead of our example of 5999, then you would enter something like in Figure 5.

Figure 5. Putty can forward SSH ports for tunneling
Putty can forward SSH ports for tunneling

If this were set up, then tech could VNC to localhost:2 just as if tech were running the Linux operating system.


Trick 7: Checking your bandwidth

Imagine this: Company A has a storage server named ginger and it is being NFS-mounted by a client node named beckham. Company A has decided they really want to get more bandwidth out of ginger because they have lots of nodes they want to have NFS mount ginger’s shared filesystem.

The most common and cheapest way to do this is to bond two Gigabit ethernet NICs together. This is cheapest because usually you have an extra on-board NIC and an extra port on your switch somewhere.

So they do this. But now the question is: How much bandwidth do they really have?

Gigabit Ethernet has a theoretical limit of 128MBps. Where does that number come from? Well,

1Gb = 1024Mb; 1024Mb/8 = 128MB; “b” = “bits,” “B” = “bytes”

But what is it that we actually see, and what is a good way to measure it? One tool I suggest is iperf. You can grab iperf like this:

# wget

You’ll need to install it on a shared filesystem that both ginger and beckham can see. or compile and install on both nodes. I’ll compile it in the home directory of the bob user that is viewable on both nodes:

tar zxvf iperf*gz
cd iperf-2.0.2
./configure -prefix=/home/bob/perf
make install

On ginger, run:

# /home/bob/perf/bin/iperf -s -f M

This machine will act as the server and print out performance speeds in MBps.

On the beckham node, run:

# /home/bob/perf/bin/iperf -c ginger -P 4 -f M -w 256k -t 60

You’ll see output in both screens telling you what the speed is. On a normal server with a Gigabit Ethernet adapter, you will probably see about 112MBps. This is normal as bandwidth is lost in the TCP stack and physical cables. By connecting two servers back-to-back, each with two bonded Ethernet cards, I got about 220MBps.

In reality, what you see with NFS on bonded networks is around 150-160MBps. Still, this gives you a good indication that your bandwidth is going to be about what you’d expect. If you see something much less, then you should check for a problem.

I recently ran into a case in which the bonding driver was used to bond two NICs that used different drivers. The performance was extremely poor, leading to about 20MBps in bandwidth, less than they would have gotten had they not bonded the Ethernet cards together!


Trick 8: Command-line scripting and utilities

A Linux systems administrator becomes more efficient by using command-line scripting with authority. This includes crafting loops and knowing how to parse data using utilities like awk, grep, and sed. There are many cases where doing so takes fewer keystrokes and lessens the likelihood of user errors.

For example, suppose you need to generate a new /etc/hosts file for a Linux cluster that you are about to install. The long way would be to add IP addresses in vi or your favorite text editor. However, it can be done by taking the already existing /etc/hosts file and appending the following to it by running this on the command line:

# P=1; for i in $(seq -w 200); do echo "192.168.99.$P n$i"; P=$(expr $P + 1);
done >>/etc/hosts

Two hundred host names, n001 through n200, will then be created with IP addresses through Populating a file like this by hand runs the risk of inadvertently creating duplicate IP addresses or host names, so this is a good example of using the built-in command line to eliminate user errors. Please note that this is done in the bash shell, the default in most Linux distributions.

As another example, let’s suppose you want to check that the memory size is the same in each of the compute nodes in the Linux cluster. In most cases of this sort, having a distributed or parallel shell would be the best practice, but for the sake of illustration, here’s a way to do this using SSH.

Assume the SSH is set up to authenticate without a password. Then run:

# for num in $(seq -w 200); do ssh n$num free -tm | grep Mem | awk '{print $2}';
done | sort | uniq

A command line like this looks pretty terse. (It can be worse if you put regular expressions in it.) Let’s pick it apart and uncover the mystery.

First you’re doing a loop through 001-200. This padding with 0s in the front is done with the -w option to the seq command. Then you substitute the num variable to create the host you’re going to SSH to. Once you have the target host, give the command to it. In this case, it’s:

free -m | grep Mem | awk '{print $2}'

That command says to:

  • Use the free command to get the memory size in megabytes.
  • Take the output of that command and use grep to get the line that has the string Mem in it.
  • Take that line and use awk to print the second field, which is the total memory in the node.

This operation is performed on every node.

Once you have performed the command on every node, the entire output of all 200 nodes is piped (|d) to the sort command so that all the memory values are sorted.

Finally, you eliminate duplicates with the uniq command. This command will result in one of the following cases:

  • If all the nodes, n001-n200, have the same memory size, then only one number will be displayed. This is the size of memory as seen by each operating system.
  • If node memory size is different, you will s
    ee several memory size values.
  • Finally, if the SSH failed on a certain node, then you may see some error messages.

This command isn’t perfect. If you find that a value of memory is different than what you expect, you won’t know on which node it was or how many nodes there were. Another command may need to be issued for that.

What this trick does give you, though, is a fast way to check for something and quickly learn if something is wrong. This is it’s real value: Speed to do a quick-and-dirty check.


Trick 9: Spying on the console

Some software prints error messages to the console that may not necessarily show up on your SSH session. Using the vcs devices can let you examine these. From within an SSH session, run the following command on a remote server: # cat /dev/vcs1. This will show you what is on the first console. You can also look at the other virtual terminals using 2, 3, etc. If a user is typing on the remote system, you’ll be able to see what he typed.

In most data farms, using a remote terminal server, KVM, or even Serial Over LAN is the best way to view this information; it also provides the additional benefit of out-of-band viewing capabilities. Using the vcs device provides a fast in-band method that may be able to save you some time from going to the machine room and looking at the console.


Trick 10: Random system information collection

In Trick 8, you saw an example of using the command line to get information about the total memory in the system. In this trick, I’ll offer up a few other methods to collect important information from the system you may need to verify, troubleshoot, or give to remote support.

First, let’s gather information about the processor. This is easily done as follows:

# cat /proc/cpuinfo .

This command gives you information on the processor speed, quantity, and model. Using grep in many cases can give you the desired value.

A check that I do quite often is to ascertain the quantity of processors on the system. So, if I have purchased a dual processor quad-core server, I can run:

# cat /proc/cpuinfo | grep processor | wc -l .

I would then expect to see 8 as the value. If I don’t, I call up the vendor and tell them to send me another processor.

Another piece of information I may require is disk information. This can be gotten with the df command. I usually add the -h flag so that I can see the output in gigabytes or megabytes. # df -h also shows how the disk was partitioned.

And to end the list, here’s a way to look at the firmware of your system—a method to get the BIOS level and the firmware on the NIC.

To check the BIOS version, you can run the dmidecode command. Unfortunately, you can’t easily grep for the information, so piping it is a less efficient way to do this. On my Lenovo T61 laptop, the output looks like this:

#dmidecode | less
BIOS Information
Vendor: LENOVO
Version: 7LET52WW (1.22 )
Release Date: 08/27/2007

This is much more efficient than rebooting your machine and looking at the POST output.

To examine the driver and firmware versions of your Ethernet adapter, run ethtool:

# ethtool -i eth0
driver: e1000
version: 7.3.20-k2-NAPI
firmware-version: 0.3-0


There are thousands of tricks you can learn from someone’s who’s an expert at the command line. The best ways to learn are to:

  • Work with others. Share screen sessions and watch how others work—you’ll see new approaches to doing things. You may need to swallow your pride and let other people drive, but often you can learn a lot.
  • Read the man pages. Seriously; reading man pages, even on commands you know like the back of your hand, can provide amazing insights. For example, did you know you can do network programming with awk?
  • Solve problems. As the system administrator, you are always solving problems whether they are created by you or by others. This is called experience, and experience makes you better and more efficient.

I hope at least one of these tricks helped you learn something you didn’t know. Essential tricks like these make you more efficient and add to your experience, but most importantly, tricks give you more free time to do more interesting things, like playing video games. And the best administrators are lazy because they don’t like to work. They find the fastest way to do a task and finish it quickly so they can continue in their lazy pursuits.




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About the author

Vallard Benincosa is a lazy Linux Certified IT professional working for the IBM Linux Clusters team. He lives in Portland, OR, with his wife and two kids.

Original article:

Access Any Website Or Forum Without Registering (by Nakodari via

Visit any forum or website to find something useful and they will ask you to register. Every time a forum asks me to register, I simply close the site. You would probably do the same. But this time, lets face it.

Before I begin, you should know how things work. All websites and forums will block unregistered users, but they won’t block Google Bot. What we will do is to switch our User Agent to that of Google Bot and freely browse any website or forum without registering.

First grab the add-on for Firefox called ‘user agent’ here and install it. Now go to Tools > User Agent Switcher > Options and then again to Options.

user agent switcher options


Select User Agent from the left sidebar and click Add. Now in the description field type:

and in user agent field type:

Googlebot/2.1 (+

as shown in the screenshot below.

adding a new user agent

Select Google Bot as your User Script by going to Tools > User Agent Switcher.

selecting google bot as user agent

Now browse any website or forum without registering. Alternatively you can also check out BugMeNot, it is a free online service where people share login information of thousands of websites and forums. Enjoy!

Original post:

Java Hangs When Converting 2.2250738585072012e-308 (by Rick Regan via

Konstantin Preisser made an interesting discovery, after reading my article “PHP Hangs On Numeric Value 2.2250738585072011e-308”: Java — both its runtime and compiler — go into an infinite loop when converting the decimal number 2.2250738585072012e-308 to double-precision binary floating-point. This number is supposed to convert to 0x1p-1022, which is DBL_MIN; instead, Java gets stuck on 0x0.fffffffffffffp-1022, the largest subnormal double-precision floating-point number.


Read the whole post:

Java Cisco Group Password Decrypter (by Patrick Toomey via Neohapsis blog)

For whatever reason I have found myself needing to “decrypt” Cisco VPN client group passwords throughout the years.  I say “decrypt” , as the value is technically encrypted using 3DES, but the mechanism used to decrypt the value is largely obfuscative (the cryptographic key is included in the ciphertext).  As such, the encryption used is largely incidental, and any simplistic substitution cipher would have protected the group password equally well (think newspaper cryptogram).

I can’t pinpoint all the root reasons, but it seems as though every couple months I’m needing a group password decrypted.  Sometimes I am simply moving a Cisco VPN profile from Windows to Linux.  Other times I’ve been on a  client application assessments/penetrations test where I’ve needed to gain access to the plaintext group password.  Regardless, I inevitably find myself googling for “cisco group password decrypter”.  A few different results are returned.  The top result is always a link here.  This site has a simple web app that will decrypt the group password for you, though it occurs server-side.  Being paranoid by trade, I am always apprehensive sending information to a third party if that information is considered sensitive (whether by me, our IT department, or a client).  I have no reason to think the referenced site is malicious, but it would not be in my best interest professionally not to be paranoid security conscious, particularly with client information.  The referenced site has a link to the original source code, and one can feel free to download, audit, compile, and use the provided tool to perform all of the decryption client-side.  That said, the linked file depends on libgcrypt.  That is fine if I am sitting in Linux, but not as great if I am on some new Windows box (ok, maybe I’m one of the few who doesn’t keep libgcrypt at the ready on my fresh Windows installs).  I’ve googled around to see if anything exists that is more portable.  I found a few things, including a link to a Java applet, but the developer seems to have lost the source code…..  So, laziness won, and I decided it would be easier to spend 30 minutes to write my own cross-platform Java version than spend any more time on Google.

The source code can be found  here and an executable jar can be found here.  I am not a huge fan of Java GUI development, and thus leveraged the incredible GUI toolkit built in to NetBeans.  The referenced source code should compile cleanly in NetBeans if you want the GUI.  If you simply want to decrypt group passwords with no dependencies you can run a command line version by compiling the “GroupPasswordDecrypter” class file.  This file has zero dependencies on third-party libraries and should be sufficient for anyone that doesn’t feel compelled to use a GUI (me included).

As a quick example, I borrowed a sample encrypted group password from another server-side implementation .  The encrypted group password we would like to decrypt is


Or, if you prefer the command line version

Hope it comes in handy!

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