Getting the VPN to work

Update: Doesn't work

-+-

After connecting to the VPN server, can't access the sites. Need to add stuff to the routing table. Wrote the bash script for that:

Soon after getting connected on VPN, do ifconfig, get the IP address associated to the ppp0 (p-t-p), specify your local router gateway and the interface you are connected through (wireless or wired) and fire the script!

#!bin/sh
# Check for command line
if [ -z "$1" ]; then
  echo "usage: ./vpn.sh [p-t-p] [GW] [Interface]"
  exit
fi

if [ -z "$2" ]; then
  echo "usage: ./vpn.sh [p-t-p] [GW] [Interface]"
  exit
fi

if [ -z "$3" ]; then
  echo "usage: ./vpn.sh [p-t-p] [GW] [Interface]"
  exit
fi

VPN_REMOTE_IP=$1
ALLTARGET="198.82.0.0/16" # All class C addresses
GATEWAY=$2
INTERFACE=$3 #eth0 or eth1

# change!
route add -host $VPN_REMOTE_IP gw $GATEWAY dev $INTERFACE
route add -net $ALLTARGET dev ppp0

Run this script as sudo!

Using VNC over SSH

Okay, now I need to connect to this Linux box in the department which is running Matlab and access it on the wire.

Options:

1. SSH + X11 forwarding [slow]

% ssh -X username@hostname
% matlab &

In order to have SSH forward X11 by default change the following in /etc/ssh/ssh_config and also remove the #

ForwardAgent yes (default no)
ForwardX11 yes (default is no)
StrictHostKeyChecking no (default is ask)

With this you can now use

ssh username@hostname

2. VNC without SSH. [fast]
% vncviewer hostname:display

3. VNC tunneling over SSH [faster]
% ssh -C -L 5901/localhost/5901 username@hostname
% vncviewer localhost:5901

Ensure that vncserver is running on the remote node and also that you have set your vncpasswd. The "display" is the number that you get after starting the vncserver

NOTE:
If you are running compiz\beryl (any composite window manager), Matlab's pre-bundled java does not support it and hence it would not render it properly with option (1). You would have to point to the latest version of the JVM before starting matlab

% export MATLAB_JAVA=/usr/lib/java/blah-blah/jre
% matlab &

Update:
Got a local copy of Matlab and realized that it has the same problem and is solved by pointing to the latest JVM. Create a launcher that takes care of setting up the environment before the launch.

env MATLAB_JAVA="/usr/lib/jvm/java-6-sun/jre" xterm "/home/piyush/matlab-2007b/bin/matlab"

Using goto is not evil.

In fact I find it a much better way of handling exceptions in C as compared to that fucked up if-else nesting that I did for that protocol stack!

Read this.

Block copy and paste in emacs

1. Select the block that you need to cut or copy

Mark set with [control]+[spacebar]. Then move down the cursor to which you need to copy\cut. OR, [control]+[spacebar]+[spacebar] and page down to the desired block in one shot!

2. Copy using [alt]+[w], OR Cut using [control]+[w]

3. Then go to the line\buffer where you want to paste\yank using [control]+[y]

Viewing the initrd image

initrd is a RAM disk file image of the kernel used at the time of boot. On Ubuntu it is created using mkinitramfs or update-initramfs commands when a new kernel is installed. This image is a gunzip\cpio archive.

To see the contents do the following

1. Copy the initrd image which you want to open into a directory somewhere /home/node/temp. Let's say it is initrd-2.6.24-generic

2. Do the following now:

cat initrd-2.6.24-generic | gunzip | cpio -ivdm

OR,

bzcat initrd-2.6.24-generic | cpio -ivdm

your own system call in 5 easy steps

You *might* want to write your system call for various reasons

Assuming the path to your kernel source is "L". Create a new folder L/mysyscall. Inside the folder create the source file mysyscall.c and a Makefile

Step 1. Changing the System Table
L/arch/x86/kernel/syscall_table_32.S

Add your system call at the end of the file.

.long sys_new_system_call

Step 2. Changing the unistd.h
L/linux/include/asm-x86/unistd_32.h

Add your system call at the end of the existing list and append the next number

#define __NR_new_system_call XXX

Where XXX is the existing system call number plus 1. Also update the total system calls (as you just added another)
#define __NR_syscalls XXY
Where XXY is XXX+1

Step 3: Changing syscalls.h
L/include/linux/syscalls.h

Add the declaration of your system call at the end.
asmlinkage long new_system_call (whatever params you want to pass)

Step 4: Changing the kernel Makefile
Add the new folder to the kernel compile

core-y += /kernel /blah /blah /blah /mysyscall

Step 5: Write your system call

Write whatever crap you want to write inside the mysyscall.c file

asmlinkage long new_system_call (whatever params you want to pass)
{
  // whatever you want to do
}

Change the makefile as well and add the following line

obj-y := mysyscall.o

Compile your kernel and test the system call from a user level program. You can create a header file that the user space program can use.

/* header.h */
#include < linux/unistd.h >
#define __NR_new_system_call XXX

/* if you system call returns int and takes no parameter
* use this macro
*/
_syscall0(int,new_system_call)

/* Otherwise, depending on the number of parameters
* being passed use the _syscallN macro, N being the no
* of params, like
_syscall1(int, new_system_call, int)
*/

Last thing to do is to test the code:

/* test client */
#include "header.h"

int main (void)
{
  printf ("System call returned %d \n", new_system_call());
  return 1;
}

NOTE
Starting around kernel 2.6.18, the _syscallXX macros were removed from header files supplied to user space. Instead we need to use syscall() function.

printf ("System call returned %d \n", syscall (__NR_new_system_call, params_if_any));

or, make the following changes in the header.h

/* header.h */
#include < linux/unistd.h >
#include < sys/syscall.h >
#define __NR_new_system_call XXX

long new_system_call (params_if_any)
{
  return syscall (__NR_new_system_call, params_if_any);
}