I recently went on a mission to discover, which subnets are “local” to South African networks, whether by ISP peering arrangements, or direct connection. I wanted this information, so that I could setup my home linux router to use Telkom’s ADSL connection for local traffic, and to use Sentech’s MyWireless connection for international traffic. Reason: Sentech pings aren’t really good for local gaming, but international speeds are great.

BGP (Border Gateway Protocol) 

All route information is published, and synchronized between ISP peers, via the BGP protocol, which is a dynamic routing protocol.

Unfortunately, it’s not as simple as install something like Zebra (a routing daemon for linux that does BGP, set it up on a Linux machine, receiving a BGP feend and have it make clever routing decisions.

No ISP will let you connect to their routers’ BGP port. Easily, or without a fight, or without paying them money for transit. This kind of public routing information, is unfortunately only available to the end-user via a series of public route-servers, and there aren’t any that I know that will allow you to receive the feed via BGP either.

So, I looked at alternative methods. I went from writing scripts to dig through the ripe, arin, and radb databases, to turning to lists of IP ranges arrange by geographic location. All the time, using whois queries to resolve the AS (AutonomousSystem) numbers, and then querying them for their official public routes. The problem is, that these routing databases aren’t always up to date, and that it’s quite difficult to figure out which AS numbers are actually local ISPs.

In the end, Gregory Massel, of http://www.ispmap.org.za/ fame, helped me to get hold of directly accessible BGP route information, courtesy of telnet://route-server.is.co.za, a public service by Internet Solutions. SAIX also runs a route-server at telnet://tpr-route-server.saix.net/
I wrote a small script that would telnet to this router, and dump the BGP routing table. This table contains local subnets, which is exactly what I was after.

From here on, it’s pretty simple to modify the script to add routes on my linux machine for these subnets on a specific interface. The net result in my scenario: ADSL gets used for local traffic, and Sentech for international.

Example script:

#!/usr/bin/perl

use Net::Telnet;

$prompt = '/public-route-server>/';
$server="route-server.is.co.za";

print "Connecting to $servern";
my $session = Net::Telnet->new(Host => $server,Prompt => $prompt,Timeout=>30);
unlink("t.log");
$session->dump_log("t.log");

$session->waitfor($prompt);

#turn off paging
$session->cmd("terminal length 0");

#get list of local routes
print "Retrieving BGP routes\n";
my @output = $session->cmd("show ip bgp\n");
print @output;
print "Route list received\n";
$session->close;