#!/usr/bin/perl -W
#
# Maximilian Wilhelm <max@rfc2324.org>
#  --  Sat 11 Feb 2017 10:29:29 PM CET
#

use strict;

# Search for interface entry for an external interface with an IPv4 default
# route configured, like this:
#
#auto eth0
#iface eth0
#	address 5.196.106.54/32
#	gateway 5.196.106.48
#	mtu 1500
#	pointopoint 5.196.106.48
my $gateway = undef;
open (ENI, "< /etc/network/interfaces")
	or die "Failed to open '/etc/network/interfaces': $!\n";
while (my $line = <ENI>) {
	chomp $line;

	# New interface stanza
	if ($line =~ /^iface (.*)/) {
		$gateway = undef;
	}

	# gateway set?
	elsif ($line =~ m/gateway\s+([0-9.]+)$/) {
		$gateway = $1;
	}

	# Interface part of vrf_external
	elsif ($line =~ m/vrf vrf_external/) {
		last;
	}
}
close (ENI);

# If there's no gateway configured for vrf_external, nothing to do
if (not defined $gateway) {
	exit (0);
}


# Check for current default route in vrf_external
my $default_route_active = undef;
open (ROUTE, "ip -4 route show table 1023 | grep ^default |")
	or die "Failed to read default route from table 1023: $!\n";
while (my $line = <ROUTE>) {
	if ($line =~ m/^default via ([0-9.]+)/) {
		$default_route_active = $1;
	}
}
close (ROUTE);


# If we didn't find an active default route, re-add it.
if (not defined $default_route_active) {
	system ("ip route add default via $gateway table 1023");
}