Linux networking/ip notes

Networking can be a pain, and in linux there are several config files that I must remember where they are and what they do. Note Ubuntu is a bit different.

/etc/sysconfig/network
For various configuration like hostname, gateway, and/or the type of protocol to use.
The below is a summary of possible field and their values, thanks to this faq

NETWORKING=answer, where answer is yes or no -Configure networking or not to configure networking.
FORWARD_IPV4=answer, where answer is yes or no -Perform IP forwarding or not to perform IP forwarding.
HOSTNAME=hostname, where hostname is the hostname of your server.
GATEWAY=gwip, where gwip is the IP address of the remote network gateway -if available.
GATEWAYDEV=gwdev, where gwdev is the device name eth# you use to access the remote gateway.
an example network file can be

NETWORKING=yes
NETWORKING_IPV6=no
HOSTNAME=jack-machine
GATEWAY=10.11.0.1


/etc/sysconfig/network-scripts/ifcfg-eth0
This file contains the configuration for a specific network device (in our case it is ifcfg-eth0)

DEVICE=devicename, where devicename is the name of the physical network device.
IPADDR=ipaddr, where ipaddr is the IP address.
NETMASK=netmask, where netmask is the netmask IP value.
NETWORK=network, where network is the network IP address.
BROADCAST=broadcast, where broadcast is the broadcast IP address.
ONBOOT=answer, where answer is yes or no. Do the interface need to be active or inactive at boot time.
BOOTPROTO=proto, where proto is one of the following :
none - No boot-time protocol should be used.
bootp - The bootp now pump protocol should be used.
dhcp - The dhcp protocol should be used.
USERCTL=answer, where answer is one of the following:
yes - Non-root users are allowed to control this device.
no - Only the super-user root is allowed to control this device.

an example network-script file can be

DEVICE=eth0
IPADDR=208.164.186.1
NETMASK=255.255.255.0
NETWORK=208.164.186.0
BROADCAST=208.164.186.255
ONBOOT=yes
BOOTPROTO=none
USERCTL=no

/etc/resolv.conf
Contains information on the dns that this machine is using
An example file would be

search example.com
;the name server that the resolver should query.
nameserver 10.11.0.5

A special note on the search parameter; it is the list of domains that we will search by default. for example if we nslookup on svn, it will actually lookup for svn.example.com. if we didn’t have the search rule below, it would just search for svn and give a not found response.

Ubuntu 10.10 Checking battery state…

A few days ago I had to reinstall by ubuntu on my T-400. After the re installation, it required me to upgrade my video driver using a proprietary software. After I restarted my computer, the machine gets stuck at the boot stage, displaying the message “Checking battery state….”

I attempted at searching on Google for ways to remedy this my problem, but no luck. I was frustrated and figured that I might be able to figure something out if I had a terminal. I went to tty1 by pressing ctrl + alt + F1, I then removed my graphics configuration by issuing the following command
sudo rm /etc/X11/xorg.conf
I then issued a reboot
sudo reboot
and everything was fine and I could login into my machine normally.

Using awk to remove python2.7

Today I wanted to remove python 2.7, and I didn’t really want to do it manually by doing sudo rm on individual python2.7 file/folder. So instead I used whereis, xargs, and grep to help me. I used the following command 

whereis python | awk 'BEGIN{FS=" "}{for(i=1;i<NF;i++) print $i}' | grep python2.7 |xargs sudo rm -fr

Lets walk this through step by step.

>whereis python

Finds all python files that are in the standard directory, for example in my machine it produced
python2: /usr/bin/python2.7 /usr/bin/python2.6-config /usr/bin/python2.6 /etc/python2.7 /etc/python2.6 /usr/lib/python2.7 /usr/lib/python2.6 /usr/local/bin/python2.7 /usr/local/bin/python2.7-config /usr/local/lib/python2.7

We want to split this line by white spaces; this is where the awk expression comes along.
awk 'BEGIN{FS=" "}{for(i=1;i<NF;i++) print $i}

The above uses awk to split white space delimited strings into seperate lines, the FS is a special awk’s built-in variable that determines the delimited string, in our case it is a space. The NF stands for the total number of elements.
I got the following on my machine.

/usr/bin/python2.7
/usr/bin/python2.6-config
/usr/bin/python2.6
/etc/python2.7
/etc/python2.6
/usr/lib/python2.7
/usr/lib/python2.6
/usr/local/bin/python2.7
/usr/local/bin/python2.7-config
/usr/local/lib/python2.7
/usr/local/lib/python2.6

then with some help of grep and xargs I was able to successfully remove python2.7 from my system with precision.

grep python2.7 |xargs sudo rm -fr

This is only a little taste of awk’s power, it fully supports regular expression and makes command line scripting a breeze.
For example if you wanted to kill all processes that contains the word java, then you can just do

ps ax | grep java| awk '/java/{print $2;}' |xargs kill -9

especially useful for killing multiple java processes that hangs.
I for one have just begin to use awk, and I see it as a essential tool for all scripters in linux environment.

vim regex

Ever wonder just how useful vim REALLY is? Well, the substitution command in VIM makes it really useful, it supports all sorts of regex, and makes editing very easy and fast assuming that the user knows what he/she is doing. Vim is one of those tools that takes more thinking than doing at the beginning. For example it took me 30 minutes to surround lines from 24 to 35 with =>” and “.  To give you a better picture of my problem look at the below example.

line containing contents foo bar will become =>”foo bar” for arbitrary number of lines.

There are several ways of doing this. One of the ways is by using the Visual block mode to select lines 24 to 35 and press I to insert =>” at the beginning of the lines. Then again select the lines and press A to append “ to the end of the lines. But I didn’t think this was a satisfactory solution to my problem. What if there were 50 lines that needs to be encapsulated instead of 11 lines? (visual block selection can be a pain sometimes)

So we can use subsitution command :

:24,35s/^.*$/=>"&"/

to solve our problem.

What the command says is that from lines 24 to 35 we are going to substitute the from beginning of the line to the end of the line (denote this as X), with the X but with =>” before it and “ after it. The & symbol represents the contents which we just matched (in this case it is X).

Here is another useful substitution command you might use

:23s/[a-z]*_*[a-z]*\s*/=>"&",\r/g

The above command takes a line (i.e line 23) of multiple words for example:

23: foo bar lulz cat

to

=>"foo",

=>"bar",

=>"lulz",

=>"cats",

this is specifically useful if you want to construct a large php array of multiple keys and values. What I do is I first type out the words that will be the values then use the above command, then type in the corresponding key for each value. e.g.

array(

1=>"foo",

2=>"bar",

3=>"lulz",

4=>"cats"

);

I hope the above will help you constructing large php arrays as it did for me.