The Code

Save this file as vtundconf, and run it each time you use a new wireless network to generate an appropriate vtund.conf for you on the fly:

#!/usr/bin/perl -w

# vtund wrapper in need of a better name.
#
# (c)2002 Schuyler Erle & Rob Flickenger
#
################ CONFIGURATION

# If TunnelName is blank, the wrapper will look at @ARGV or $0.
#
# Config is TunnelName, LocalIP, RemoteIP, TunnelHost, TunnelPort, Secret
#
my $TunnelName = ""; 
my $Config   = q{
  home    208.201.239.33 208.201.239.32 208.201.239.5  5000  sHHH
  tunnel2   10.0.1.100       10.0.1.1        192.168.1.4       6001  foobar
};

################ MAIN PROGRAM BEGINS HERE

use POSIX 'tmpnam';
use IO::File;
use File::Basename;
use strict;

# Where to find things...
#
$ENV{PATH}  = "/bin:/usr/bin:/usr/local/bin:/sbin:/usr/sbin:/usr/local/&return;sbin";
my $IP_Match = '((?:\d{1,3}\.){3}\d{1,3})';      # match xxx.xxx.xxx.xxx
my $Ifconfig = "ifconfig -a";
my $Netstat = "netstat -rn";
my $Vtund  = "/bin/echo";
my $Debug  = 1;

# Load the template from the data section.
#
my $template = join( "", );

# Open a temp file -- adapted from Perl Cookbook, 1st Ed., sec. 7.5.
#
my ( $file, $name ) = ("", "");
$name = tmpnam( )
  until $file = IO::File->new( $name, O_RDWR|O_CREAT|O_EXCL );
END { unlink( $name ) or warn "Can't remove temporary file $name!\n"; }

# If no TunnelName is specified, use the first thing on the command line,
# or if there isn't one, the basename of the script.
# This allows users to symlink different tunnel names to the same script.
#
$TunnelName ||= shift(@ARGV) || basename($0);
die "Can't determine tunnel config to use!\n" unless $TunnelName;

# Parse config.
#
my ($LocalIP, $RemoteIP, $TunnelHost, $TunnelPort, $Secret);
for (split(/\r*\n+/, $Config)) {
  my ($conf, @vars) = grep( $_ ne "", split( /\s+/ ));
  next if not $conf or $conf =~ /^\s*#/o; # skip blank lines, comments
  if ($conf eq $TunnelName) {
    ($LocalIP, $RemoteIP, $TunnelHost, $TunnelPort, $Secret) = @vars;
    last;
  }
}

die "Can't determine configuration for TunnelName '$TunnelName'!\n"
  unless $RemoteIP and $TunnelHost and $TunnelPort;

# Find the default gateway.
#
my ( $GatewayIP, $ExternalDevice );

for (qx{ $Netstat }) {
  # In both Linux and BSD, the gateway is the next thing on the line,
  # and the interface is the last.
  #
  if ( /^(?:0.0.0.0|default)\s+(\S+)\s+.*?(\S+)\s*$/o ) {
    $GatewayIP = $1;
    $ExternalDevice = $2;
    last;
  }
}

die "Can't determine default gateway!\n" unless $GatewayIP and $ExternalDevice;

# Figure out the LocalIP and LocalNetwork.
#
my ( $LocalNetwork );
my ( $iface, $addr, $up, $network, $mask ) = "";

sub compute_netmask {
  ($addr, $mask) = @_;
  # We have to mask $addr with $mask because linux /sbin/route
  # complains if the network address doesn't match the netmask.
  #
  my @ip = split( /\./, $addr );
  my @mask = split( /\./, $mask );
  $ip[$_] = ($ip[$_] + 0) & ($mask[$_] + 0) for (0..$#ip);
  $addr = join(".", @ip);
  return $addr;
}

for (qx{ $Ifconfig }) {
  last unless defined $_;

  # If we got a new device, stash the previous one (if any).
  if ( /^([^\s:]+)/o ) {
    if ( $iface eq $ExternalDevice and $network and $up ) {
      $LocalNetwork = $network;
      last;
    }
    $iface = $1;
    $up = 0;
  }

  # Get the network mask for the current interface.
  if ( /addr:$IP_Match.*?mask:$IP_Match/io ) {
    # Linux style ifconfig.
    compute_netmask($1, $2);
    $network = "$addr netmask $mask";
  } elsif ( /inet $IP_Match.*?mask 0x([a-f0-9]{8})/io ) {
    # BSD style ifconfig.
    ($addr, $mask) = ($1, $2);
    $mask = join(".", map( hex $_, $mask =~ /(..)/gs )); 
    compute_netmask($addr, $mask);
    $network = "$addr/$mask";
  }

  # Ignore interfaces that are loopback devices or aren't up.
  $iface = "" if /\bLOOPBACK\b/o;
  $up++    if /\bUP\b/o;
}

die "Can't determine local IP address!\n" unless $LocalIP and $LocalNetwork;

# Set OS dependent variables.
#
my ( $GW, $NET, $PTP );
if ( $^O eq "linux" ) {
  $GW = "gw"; $PTP = "pointopoint"; $NET = "-net";
} else {
  $GW = $PTP = $NET = "";
}

# Parse the config template.
#
$template =~ s/(\$\w+)/$1/gee;

# Write the temp file and execute vtund.
#
if ($Debug) {
  print $template;
} else {
  print $file $template;
  close $file;
  system("$Vtund $name");
}

__DATA_  _

options {
  port $TunnelPort;
  ifconfig /sbin/ifconfig;
  route /sbin/route;
}

default {
  compress no;
  speed 0;
}

# 'mytunnel' should really be `basename $0` or some such
# for automagic config selection
$TunnelName {   
  type tun;
  proto tcp;
  keepalive yes;

  pass $Secret;

  up {
   ifconfig "%% $LocalIP $PTP $RemoteIP arp";
   route "add $TunnelHost $GW $GatewayIP";
   route "delete default";
   route "add default $GW $RemoteIP";
   route "add $NET $LocalNetwork $GW $GatewayIP";
  };

  down {
   ifconfig "%% down";
   route "delete default";
   route "delete $TunnelHost $GW $GatewayIP";
   route "delete $NET $LocalNetwork";
   route "add default $GW $GatewayIP";
  };
}