Changing MAC address in linux

There are occasions when you may need to change your MAC (Media Access Control) address temporarily. This can be used:
1. While connected to a public wifi hot spot - it keeps your original MAC address anonymous.
2. To bypass a router/switch which has MAC address filtering.
3. To obtain a dhcp ip lease that is allocated based on MAC address.

And remember! MAC address is something that is soldered into your network adapter. You cant change it permanently; in other words, the change that you make is not persistent over reboots.

First let us find out the current MAC address of the Ethernet adapter in my machine

safeer@penguinpower:~$ ifconfig eth0|awk '/HWaddr/{print $1" "$5 }'
eth0 00:13:e4:5c:93:0a

Now I am going to change the MAC address of my Ethernet interface.

First, bring down the interface
safeer@penguinpower:~$ sudo ifconfig eth0 down

Then change the MAC addres to whatever you want
safeer@penguinpower:~$ sudo ifconfig eth0 hw ether 00:22:4d:b7:36:01

Bring up the interface
safeer@penguinpower:~$ sudo ifconfig eth0 up

Verify MAC address has changed.
safeer@penguinpower:~$ ifconfig eth0|awk '/HWaddr/{print $1" "$5 }'
eth0 00:22:4d:b7:36:01

The syntax of the command is "ifconfig [interface name] hw [encapsulation type] [MAC address] "

Every argument is self explanatory except the encapsulation type. This is "ether" (Ethernet) for both our Ethernet and wireless interfaces. For finding out encapsulation type for other interface types see ifconfig man pages. To see the encapsulation type of your network interfaces use the following command

safeer@penguinpower:~$ ifconfig -a|awk '/HWaddr/{split($3,enc,":"); print $1" "enc[2];}'
eth0 Ethernet
wlan0 Ethernet

GPRS connection in Linux using wvdial

If you have a mobile with GPRS enabled you can use it to connect your Linux Box to Internet. The program that we can use for this is wvdial. Most of the popular distributions will have it in its repository, if not, get it from here.

I am going to connect using:

Nokia E71
GSM based GPRS connection from Vodafone (India)
Kubuntu 9.04
WvDial 1.60

First connect your mobile phone with USB cable to your PC/Laptop. In the case of Nokia, four options will pop-up on the phone. PC Suite, Media Transfer, Mass Storage and Connect to PC to Web. Though I chose Connect PC to Web, it was not detected as a modem. So changed the mode to PC Suite on the phone, and then a modem was detected at /dev/ttyACM0. I got this message on dmesg - "ttyACM0: USB ACM device". Now we need to cerate a configuration file for wvdial. and save it to "/etc/wvdial-vodafone-gprs.conf" for the time being.

The file should start with:

[Dialer Defaults]

Add the basic settings for the modem.

Modem = /dev/ttyACM0
Modem Type = USB Modem
Baud = 460800

For any gprs connection the most important detail you need is the APN ( Access Point Name ) of the service provider, in the case of Vodafone, it is "portalnmms". This should go in the modem initialization commands as below:

Init1 = ATZ
Init2 = AT+CGDCONT=1,"IP","portalnmms"

Note that the APN is part of the Init2 command. Now we need to add the dialing phone number, username and password. This is same across all GSM providers (CDMA phones have different values).

Phone = *99#
Username = 0
Password = 0

Dirty Secret: GSM don't check the username and password for authentication, it just needs the entries to be present in the configuration - with some junk values.

Save the file and dial with wvdial command using /etc/wvdial-vodafone-gprs.conf as an argument:

safeer@penguinpower:~$ sudo wvdial -C /etc/wvdial-vodafone-gprs.conf

You will get a lot of output, look for the following lines ( the IP's shown may differ)


local IP address 10.16.25.127
remote IP address 10.6.6.6
primary DNS address 10.11.230.2
secondary DNS address 202.144.13.50

This indicate the modem is connected. At this point you will be able to contact your DNS server and resolve names. But you need to set proxy for your browser to start browsing. For Vodafone the proxy and the port are 10.10.1.100 and 9401. Set this on your browser and you can start browsing. When you are done, close the connection by pressing Control+C to terminate wvdial command.

If your connection is CDMA, the phone number will be #777 instead of *99#, and the username and password will be your mobile number ( without country code ). Also the Init2 string will be "ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0" (no APN name).

A few more things to note, depending on your phone, the modem device location may change, it can be /dev/ttyUSB0, /dev/ttySL0 etc... always check dmesg to see how it is detected - use command dmesg|tail after connecting your usb cable.

For good measure, you can add the following settings as well to the wvdial configuration:

Dial Command = ATDT
Stupid Mode = 1
ISDN = 0
New PPPD = yes

To find out your provider's GPRS settings, you can visit Manoj Tivari's blog . I got the Vodafone proxy details from this web site.

Truncating a file

There are situations when you want to adjust the size of a file rather than deleting it. Especially when the file is being used by a process and you want to reduce its size without interrupting the process. Removing a file and creating another file by the same name will not work in such cases as the inode number of the file will change. This happens mainly with log files, where you want to reduce the size of the log - to save space -but don't want to stop the process that is writing to the log.

BSD has an excellent tool for this - the "truncate" command. This command can be used either to reduce/increase the size of a file. Let us consider a live example.

safeer->vps1.safeer.in ~ $ du -hs mysql-slowquery.log
476M mysql-slowquery.log

To reduce the size to 300MB

safeer->vps1.safeer.in ~ $ truncate -s 300M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ du -hs mysql-slowquery.log
300M mysql-slowquery.log

Instead of specifying the exact size, if you just want to free up a 100Mb of disk space, you can do it this way:

safeer->vps1.safeer.in ~ $ truncate -s -100M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ du -hs mysql-slowquery.log
200M mysql-slowquery.log

When you truncate a file by this method, the data is lost from the end of the file.

Truncate also has an option to extend a file's size, but it do not allocate any space for that. So it is not so useful.

safeer->vps1.safeer.in ~ $ truncate -s +400M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ du -hs mysql-slowquery.log
200M mysql-slowquery.log

Another way of specifying size for truncation is to give a reference file. Truncate will calculate the size of the referenced file and truncate the given file to its length.

safeer->vps1.safeer.in ~ $ du -hs httpd.log mysql-slowquery.log
153M httpd.log
200M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ truncate -r httpd.log mysql-slowquery.log
safeer->vps1.safeer.in ~ $ du -hs httpd.log mysql-slowquery.log
153M httpd.log
153M mysql-slowquery.log

truncate do not change the inode of the file.

safeer->vps1.safeer.in ~ $ ls -i mysql-slowquery.log
48980316 mysql-slowquery.log
safeer->vps1.safeer.in ~ $ truncate -s -132M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ ls -i mysql-slowquery.log
48980316 mysql-slowquery.log

As you can see, this is a very useful command, but unfortunately we don't have a similar tool in Linux. So we can use any of the following method to emulate some truncate features.

To empty a file (truncate -s 0 )

safeer@penguinpower:~$ du -hs mysql-slowquery.log
1.1G mysql-slowquery.log
safeer@penguinpower:~$ >mysql-slowquery.log
safeer@penguinpower:~$ du -hs mysql-slowquery.log
0 mysql-slowquery.log

For resizing to an exact size you can use the dd command.

safeer@penguinpower:~$ du -hs mysql-slowquery.log
747M mysql-slowquery.log
safeer@penguinpower:~$ dd if=/dev/null seek=705 bs=1M of=mysql-slowquery.log
0+0 records in
0+0 records out
0 bytes (0 B) copied, 5.1892e-05 s, 0.0 kB/s
safeer@penguinpower:~$ du -hs mysql-slowquery.log
706M mysql-slowquery.log

Here dd argument bs specifies the unit of file size where as seek will specify the number of units to which you want to resize the file. Technically, copying /dev/null to a file will erase the content in the destination file. But with the "seek" option we specify that "seek value" X "block size" data from the beginning of the file should be preserved and the rest of the data should be erased. So in the above example, file was reduced to ~705MB (705 * 1M).

The following commands are equivalent.

truncate -s 0 mysql-slowquery.log
>mysql-slowquery.log
cat /dev/null > mysql-slowquery.log
dd if=/dev/null of=mysql-slowquery.log

ssh-agent and passwordless ssh

In addition to password authentication, ssh also provides public key authentication. Setup of public key authentication is described here . The summary of authentication is as follows:

* Generate a private key /public key pair:

ssh-keygen -t rsa

This will create a private key named id_rsa and public key named id_rsa.pub under ~/.ssh directory of the user. Here we are using RSA version 2 as the encryption protocol. Alternatively you can specify DSA ( -t dsa ) or RSA version 1 ( -t rsa1 ). If you want to create the key pair with an alternate name/location, specify it with -f option. The above command will prompt you for a passphrase for the private key. You can either give a passphrase or just press enter to have a blank passphrase (alternatively use -P '' option). The problem with having a blank passphrase is that anybody who has access to your private key file will be able to view and use it to impersonate your login. A passphrase for the private key prevents this from happening.

* Copy your public key to destination server and append it to ~/.ssh authorized_keys.

Once this is setup you can login to the destination server without using your account password. If you have provided a blank passphrase while generating public/private key you will be directly logged in. But if you have give a passphrase for you private key, you wil be prompted for that. While this gives a more secure way of logging in, for the end user it is just like replacing one password with another one. To get around this without compromising on security, we can use ssh-agent which will cache your password for repeated use.

ssh-agent is a key management program that can manage all our keys. We just need to unlock/decrypt our private keys - with the passphrase - once and add them into the agent, and for the rest of that session ssh-agent will automatically supply your key credentials whenever a new ssh connection is initiated.

Let us see the usage now.

* Start the agent.

[safeer@fcds ~]$ ssh-agent
SSH_AUTH_SOCK=/tmp/ssh-EQayeR4473/agent.4473; export SSH_AUTH_SOCK;
SSH_AGENT_PID=4474; export SSH_AGENT_PID;
echo Agent pid 4474;

Now that the agent is running you need to add your keys to the agent.

[safeer@fcds ~]$ ssh-add
Enter passphrase for /home/safeer/.ssh/id_rsa:
Identity added: /home/safeer/.ssh/id_rsa (/home/safeer/.ssh/id_rsa)
Identity added: /home/safeer/.ssh/id_dsa (/home/safeer/.ssh/id_dsa)

here you will prompted to enter the pasphrace for your keys. if all your private keys have the same passphrase, you will be prompted only once.

To see the keys added to ssh:

[safeer@fcds ~]$ ssh-add -l
2048 20:83:de:3d:7c:5f:a4:32:c2:75:c9:12:f6:7f:f2:be /home/safeer/.ssh/id_rsa (RSA)
1024 7a:29:5a:73:6b:56:d0:0d:73:4e:70:67:39:99:80:c8 /home/safeer/.ssh/id_dsa (DSA)

Since the keys are added to ssh-agent you can use private keys without being prompted for passphrase throughout this session.

To remove a specific key from the agent

[safeer@fcds ~]$ ssh-add -d /home/safeer/.ssh/id_rsa
Identity removed: /home/safeer/.ssh/id_rsa (/home/safeer/.ssh/id_rsa.pub)

If you want to remove all the keys, just use "ssh-add -d"

Recovering your kjots books

For those who don't know about kjots, it is a note taking utility that comes with the KDE desktop environment. It allows you to organize your notes into Books. Each book will have pages where you can actually save your notes. It has formating options like font selection,color,alignment etc. It allows you to export your books and/or pages to text/html formats. There are many other handy function like bookmarking searching etc.. It is a very handy and lightweight application.

I was using this for quite some time and had two books full of notes. One fine morning my KDE crashed leaving my system in hung state. When it came up after the reboot, I found that all the notes in Kjots were gone leaving two blank books in it. So I was sitting there scratching my head, suddenly it came to my mind that Kjots has an automatic backup facility and decided to try my luck.

All kde applications store their per user configuration and user data inside the local kde directory under the respective user's home directory. In most of the cases it will be .kde directory under the users home directory. But in KDE4 this wil change to .kde4. So to determine the kde directory for your user account you need to run the following command.

safeer@cms-netbook:~$ kde-config --localprefix
/home/safeer/.kde/

The kjots data is located in the path share/apps/kjots under ~/.kde. Lets examine the contents of this directory:

safeer@cms-netbook:~/.kde/share/apps/kjots$ ls
Oj6443.book Oj6443.book.2 Oj6443.book.4 Oj6443.book.6~ xA4048.book xA4048.book.10~ xA4048.book.3~ xA4048.book.6~ xA4048.book.8~
Oj6443.book.1 Oj6443.book.2~ Oj6443.book.4~ Oj6443.book.7~ xA4048.book.1 xA4048.book.2 xA4048.book.4~ xA4048.book.7 xA4048.book.9
Oj6443.book.1~ Oj6443.book.3 Oj6443.book.5 Oj6443.book.8~ xA4048.book.1~ xA4048.book.2~ xA4048.book.5~ xA4048.book.7~ xA4048.book.9~
Oj6443.book.10~ Oj6443.book.3~ Oj6443.book.5~ Oj6443.book.9~ xA4048.book.10 xA4048.book.3 xA4048.book.6 xA4048.book.8

Kjots by default assigns a random filename to each of your kjots books. This will not be the same as the name you have assigned to the book. Kjots stores its books in xml format. If you want to know which file corresponds to which book, use this command. Here the name I gave to this kjots book is "TrainingNotes".

safeer@cms-netbook:~/.kde/share/apps/kjots$ grep -A 1 -m 1 KJotsBook Oj6443.book.6~|grep Title|cut -d '>' -f2|cut -d '<' -f1
TrainingNotes

Kjots takes backup of your books every time you close it. In addition to the original book, kjots will keep 10 most recent backups of each book. The backup and book naming scheme is as follows.

Book name convention: .book
Eg: Oj6443.book

Backup book name: .book.~ with .book.1~ being the most recent book.
Eg: Oj6443.book.3~
Most recent backup for the above book will have name Oj6443.book.1~

Now that you know about the kjots backup scheme, let us see how I recovered my kjots. First find out the good backup of each books.

safeer@cms-netbook:~/.kde/share/apps/kjots$ ls -l
total 576
-rw------- 1 safeer safeer 0 2009-09-10 16:14 Oj6443.book
-rw------- 1 safeer safeer 0 2009-09-24 18:39 Oj6443.book.1~
-rw------- 1 safeer safeer 19774 2009-09-10 16:52 Oj6443.book.10~
-rw------- 1 safeer safeer 0 2009-09-19 16:37 Oj6443.book.2~
-rw------- 1 safeer safeer 0 2009-09-18 18:16 Oj6443.book.3~
-rw------- 1 safeer safeer 48278 2009-09-17 21:45 Oj6443.book.4~
-rw------- 1 safeer safeer 48278 2009-09-17 21:21 Oj6443.book.5~
-rw------- 1 safeer safeer 48278 2009-09-17 21:21 Oj6443.book.6~
-rw------- 1 safeer safeer 48278 2009-09-16 17:15 Oj6443.book.7~
-rw------- 1 safeer safeer 48278 2009-09-16 17:02 Oj6443.book.8~
-rw------- 1 safeer safeer 19774 2009-09-16 16:59 Oj6443.book.9~
-rw------- 1 safeer safeer 0 2009-09-24 18:39 xA4048.book
-rw------- 1 safeer safeer 0 2009-09-24 18:39 xA4048.book.1~
-rw------- 1 safeer safeer 39850 2009-09-10 16:52 xA4048.book.10~
-rw------- 1 safeer safeer 0 2009-09-19 16:37 xA4048.book.2~
-rw------- 1 safeer safeer 0 2009-09-18 18:16 xA4048.book.3~
-rw------- 1 safeer safeer 42450 2009-09-17 21:45 xA4048.book.4~
-rw------- 1 safeer safeer 42450 2009-09-17 21:21 xA4048.book.5~
-rw------- 1 safeer safeer 42450 2009-09-17 21:21 xA4048.book.6~
-rw------- 1 safeer safeer 42450 2009-09-16 17:15 xA4048.book.7~
-rw------- 1 safeer safeer 39850 2009-09-16 17:02 xA4048.book.8~
-rw------- 1 safeer safeer 39850 2009-09-16 16:59 xA4048.book.9~

As you can see, the most recent good copy for both the books were backed up on 2009-09-17 21:45 (this should be the time when I closed the Kjots on the previous day). All files after this has zero size. So copying this backup files to the corresponding books will get my books back. Make sure to close Kjots before doing this.

safeer@cms-netbook:~/.kde/share/apps/kjots$ cp Oj6443.book.4~ Oj6443.book
safeer@cms-netbook:~/.kde/share/apps/kjots$ cp xA4048.book.4~ xA4048.book

Relaunched Kjots and my books were back with all the contents!