Perl TCP Listener

As a note to self, comes in handy whenever you're missing netcat:

#!/usr/bin/perl -w
use IO::Socket; 
use Net::hostent;              
$PORT = 6379;

$server = IO::Socket::INET->new( Proto     => 'tcp',
                                 LocalPort => $PORT,
                                 Listen    => 5,
                                 Reuse     => 1) or die "can't setup server" unless $server;

print "SERVER Waiting for client connection on port $PORT\n";

 while ($client = $server->accept()) {
  $client->autoflush(1);
  while ( <$client> ) {
    if (/quit|exit/i) { exit; }                           
    else { print $_;}
  }
  close $client; 
}

How to enable IP Forwarding in Mac OS X

Another note to self: how to enable IP forwarding in Mac OS X:

sudo sysctl -w net.inet.ip.forwarding=1

(credit: Stack Exchange)

How to disable ASLR in Linux

Quick note to self on how to disable ASLR (address space layout randomization) in Ubuntu Linux:

$ sudo sh -c "echo 0 > /proc/sys/kernel/randomize_va_space"

How to get a List of Installed JVMs on a Mac

I got this tip from a fellow worker:

$ /usr/libexec/java_home -V

This command will get you the list of installed Java Virtual Machines installed on your Mac OS X system.

Inspecting the Process Environment Variables with GDB

While trying to solve the 4th level of the vortex wargame, I found it was necessary to learn how to inspect the location and content of the environment variables within the process memory.

GDB has built-in commands to inspect the process environment, see the GDB manual. You can either list all environment variables or a specific one (e.g. FOOBAR) using the following commands, which will output their values:

(gdb) show environment
(gdb) show environment FOOBAR

In order to locate the environment variables within the process memory, you can query the variable char** environ (see the libc reference and this entry on stack overflow):

(gdb) x/s *((char **)environ)

This will print the location of the first environment variable and its representation as string. To print the next variables, simply add an offset to the variable:

(gdb) x/s *((char **)environ + 1)

I also found these links to be useful:

• GDB macros by IBM developers resources
• Some more GDB tips & tricks

how to (quickly) perform a dns lookup without nslookup, dig, etc

Use perl for example:

$ perl -e 'use Socket; print inet_ntoa(inet_aton("example.com"));'