Testing your connectivity

May 21, 2009 Technical, General

Recently I blogged about our new IPv4 address allocation. While we don’t need to start using it for a while as we have been conserving IP addresses quite well, and gave ourselves plenty of time before we actually need to use the new allocation, it is a good idea to check that it is accessible to the Internet at large.

Our new allocation is from the block which was only allocated to the Asia-Pacific regional registry APNIC last November. Prior to it being allocated to APNIC, it would have been in a state affectionately known as “bogon” to network administrators. Bogons are network ranges that aren’t in use, and therefore can be safely ignored by all live networks on the Internet. There have been cases where spammers or other parties looking to conduct illegal activity on the Internet have attempted to use unallocated network ranges for various reasons, so most knowledgable network administrators will block all bogon networks. There are several projects such as the Team Cymru Bogon Reference which put together lists of current bogons to aid network administrators in this task.

The problem comes at the time of removing these bogons from the list. There are currently over 30000 active ASs (Autonomous Systems) on the IPv4 Internet, and effectively each of these must update their own bogon list (if they are not peering with an automatic service such as what Team Cymru provides). Not all network administrators are up to date on the IANA allocations so this process can take months. We are lucky enough to have an allocation from a brand new APNIC range – others are not so lucky and often will have been allocated a range that was previously used by spammers.

Faced with this situation, we’ve decided to try to find out exactly how reachable our new allocation is. I consulted the members of the NANOG mailing list, and pondered their suggestions. I’ve documented below the success of the various methods they suggested and a method which we thought up and decided to try as well.

Do Nothing

One member of the mailing list said:

IMHO, if a network doesn't either update filters based on IANA
notifications or follow Cymru BOGON, then they don't deserve to receive
traffic from your network ;)

The BOFH within me likes this response very much, but sadly I don’t think that response would be accepted by the boss… I’d also like to take more of an active role in determining our connectivity.

RIPE Debogon Prefix Reachability


The RIPE regional registry has this page which is effectively just a rudimentary looking glass allowing you to ping or traceroute to your new IP address space. Unfortunately I found dubious results when pinging from some of the routers listed, on all of our address ranges. I suspect not all routers are available or the script behind the page needs updating. If you only have a single address range it would be hard to figure out if results are correct.

RIPE also performs their own testing of de-bogoned address space and graphs the output of their reachability tests. This only really helps you if your allocation has come from RIPE though.

Looking Glasses

Similar to the above method, this involves advertising a small segment of IP space for testing and then using as many public looking glasses on the web as you can find to test connectivity. It is quite thorough, although very time consuming.

Notify network operator groups

One suggestion from the list was simply to post a message on NOG mailing lists and ask the participants to check their filters and optionally attempt to ping the address space in question. This requires participation from the network administrators on the lists, but my main reservation with this method is that if they are knowledgeable enough to subscribe to NANOG or a similar mailing list, they probably take care of their BGP filters anyway so this method probably won’t reveal too many misconfigurations.

Active testing from BGP data

I’m always interested in using BGP data for new and exciting things, so this was a good challenge. From our border routers we can assemble a list of endpoint ASs (as we’re not that interested in strictly transit ASs unless we spot a problem we can pin down to one of them) and pick at least one subnet advertised by each AS. Then we attempt to ping or in some other way communicate with one IP address on each of those subnets. We do this from a working IP on our existing IP allocation range.

We then take the results of that testing and perform the tests again from an IP address on our new allocation range. In theory if the range has been correctly debogoned we will see identical results (even if not all IPs are reachable). If there are discrepancies we can determine which ASs may have bogon-related issues and attempt to contact them.

Reusing most of the BGP dump manipulation script I wrote for my BGP Data Visualisation article, I was able to pull out a list of unique endpoint AS numbers and a subnet for each from our live BGP data within a few seconds. The wc utility tells me that there are 31056 unique AS numbers, which sounds about right based on the recent AS reports. Here is the perl script I used to generate the list of ASs and a subnet for each. You simply pipe the output of the “show ip bgp” command from your router into it and it will print one AS and one target subnet per line:


my %aslist;

while (<STDIN>) {
    # Skip the first 5 lines of header data
    if ($. < 6) { next; }

    # Skip any blank lines
    if ( m/^$/ ) { next; }

    # Skip lines without an AS path or subnet
    if ( m/ 0 (i|e|?)$/ ) { next; }
    if ( m/^... / ) { next; }
    unless (m/ 0 / ) { next; }

    # Skip the last summary line
    if ( m/^Total number of prefixes.*$/ ) { next; }

    # Grab the AS path and subnet
    s/^...([0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}(/[0-9]{1,2})?).* 0 (.*) (i|e|?)$/1 3/;
    s/,/ /g;

    # Turn the AS path string into an array
    my @path = split(' ', $_);

    # Add classful subnet designations
    $path[0] =~ s/^([0-9]{1,3}).0.0.0$/;
    $path[0] =~ s/^([0-9]{1,3}.[0-9]{1,3}).0.0$/1.0.0/16/;
    $path[0] =~ s/^([0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}).0$/1.0/24/;

    # Add last AS to our global list of ASs

while (($key, $value) = each(%aslist)){
    print "$key $valuen";

Since this was very much just a proof of concept I didn’t have much motivation to ensure absolute correctness or make the process as efficient as possible. Ideally I’d have the entire thing within the one script/program which intelligently pings multiple hosts in separate threads with some sort of limiting involved. Instead, I hacked up a couple of quick shell scripts; the first takes the list of ASs and subnets and passes them to the second script which is forked off for each pair. Forking off indiscriminately would lead to the process scheduler having a fit and the machine becoming unresponsive pretty quick so there is a quick check to make sure there aren’t more than 250 concurrent pings running before forking off another instance.

cat AS | while read as subnet; do
        while [ `ps h -C ping | wc -l` -gt 250 ]; do
                sleep 60
        /data/pingloop $as $subnet &

“AS” is the file with AS/subnet pairs.

for ip in `/usr/bin/ipcalc $subnet | grep Hostroute | awk '{print $2}'`; do
        ping -c 5 -i 0.2 -w 5 $ip >/dev/null 2>/dev/null
        if [ $? -eq 0 ]; then
                ping -I X.X.X.X -c 5 -i 0.2 -w 5 $ip >/dev/null 2>/dev/null
                if [ $? -eq 0 ]; then
                        echo "$as $ip reachable" >> output
                        echo "$as $ip bogoned" >> output
                exit 0

In the above “pingloop” script, we hamfistedly generate a sequence of IPs on the target subnet and attempt to find one reachable IP address then ping it from our new allocation immediately after to see if it is reachable from both subnets.

The results came back in about 10 hours, which isn’t bad for some fairly non-aggressive ICMP reachability testing of effectively the entire IPv4 Internet. Out of 25446 ASs we were able to reach initially, 1716 couldn’t be reached from our new address space which works out to be around 6.7%. Not terrible, but not great either. From here, we’ll look at the ASs that couldn’t be reached and see if there are any patterns that suggest common upstreams need to update their filters.

One disadvantage to this method is raising the ire of network administrators. The amounts of ICMP traffic the scripts generate is pretty minimal but some networks have overly sensitive network monitoring that will trigger if you perform a sequential ICMP “scan” of their network. Of course, it wasn’t performed with malicious intent to really they have no cause to complain.

On DNS and GeoIP

While network-based bogon lists are the prime concern, you should also consider DNS resolver ACLs and GeoIP data. Many DNS administrators will maintain bogon lists in their configurations and these are probably updated even less frequently than BGP bogon lists. If you run into issues with nameservers on your new IP allocation range, you will know that someone out there hasn’t updated their BIND configuration. Similarly, a lot of web services utilise GeoIP to determine the location of a remote IP. By virtue of the allocation to APNIC, our new range is displayed as being in Australia, but it does not show a city or geographical coordinates. Sending an email to GeoIP with your details can rectify this problem.