A lot of things happened with my connectivity recently, so I thought
that I would sum them up here since:
- I was first pissed and then happily surprised RCN's service.
- it could be useful information to someone:
+ who wants cable modem in the Boston area.
+ who doesn't have a fixed IP and wants a static host name
(cool hack).

This is kind-of long, but the stuff at the bottom can be useful if you
want dynamic DNS with Linux.

About RCN[0]
------------

So it all started when I discovered that my hostname was not on the
authoritative name servers for rcn.com. To cut a very long story short,
I spent 2 weeks *trying* to report the problem and got answers such as:
- Your machine is pingable.
- We do not support Linux.
- It is resolved here (they were not using the authoritative servers).

At some point I was desperate and I smacked one of the head when I had
him on the phone when he wanted to come home and check my computer! The
effect was that he listened carefully to what I was saying, discovered
(oh surprise!) and acknowledged the problem (they haven't fixed it yet),
and ended up telling me that I should upgrade my cable modem and giving
me an appointment.

So I knew that it wasn't really the right fix, but since IP's where tied
to the cable modem serial numbers, I thought that it could give me
another IP address and a hostname which this time would be in the DNS.

The first time, they come the day before my appointment: "Sorry, we
missed you, please reschedule." So they accept to "reschedule" for the
same day, and here they are at home.

About cable modems and RCN's connectivity
-----------------------------------------

They replaced my Hybrid cable modem by a 3Com one[1]. The installation
was successful and quick, and I discovered that I was now connected to
another network.

Now I have to read about how it works to understand that; however the
specifications I found[2] are huge... They probably use virtual LANs
based on the frequency of the link between the cable modem and the
router.

Anyhow, my new LAN has interesting features compared to the one I was
using before.

The connectivity to MIT is much better. The number of hops between my
cable modem and www.mit.edu is 7, and ping statistics give: round-trip
min/avg/max = 12.2/13.5/16.8 ms. That is really cool because the latency
before was around 110ms and I was jealous of Eric's Mediaone connection.

My IP address changed, and I am back in the DNS (woohoo). Well, kind of,
because this time this is RCN's internal DNS servers who don't know
about me. Oh well...

However, IP addresses are not addressed based on the cable modem serial
number anymore, but apparently completely random. And their DHCP server
gives 1-hour leases! This is IMHO crazy since I guess that the average
use of a computer hooked up to cable modem is more than 1 hour. I tried
to get a longer one, but no luck.

Now I wonder when my IP is going to change, because my TCP connections
won't like that. Hopefully, my DHCP client should be able to get the
same one as long as I am online and I will get a different one only if I
unplug my cable modem for a little while. I need to read about DHCP[3]
too.

Dynamic DNS
-----------

I originally wanted to assign a hostname easy to type to my machine in
order to access it remotely, but since I don't have a fixed IP anymore,
it was tricky.

So I decided not to reinveint the wheel and look into free dynamic DNS
providers.

I don't really like dynamic DNS because it puts a heavy load on the name
servers. However, it's more or less the only was I can keep track of my
machine, and I don't plan to use it a lot (and don't expect others to
use it at all), so I feel OK about that.

The one I decided to use is justlinux's DynDNS[4] because it was easy to
set up, you can assign the IP address that you want (and not the one it
thinks you have - too bad if you are sitting behind a proxy) and they
were giving a Perl script for updates under Linux.

I wanted to automate that, so I looked into DHCP clients and got rid of
pump for dhclient[5] (dhcp-client Debian package) which is extremely
configurable.

I rewrote their Perl script using netcat[6] since Perl was kind-of heavy
for a simple HTTP GET request, and I wrote a hook for the client so that
when my IP address is first assigned or changes, I update my DNS entry.

Here is what my /etc/dhclient-exit-hooks looks like:

 #!/bin/sh

 setname() {
 URLPREFIX="/bin/controlpanel/dyndns/jlc.pl?direct=1"
 SERVER="www.justlinux.com"
 USER="@@@"
 PASS="@@@"
 HOST="@@@.penguinpowered.com"
 (echo "GET $URLPREFIX&username=$USER&password=$PASS&HOST=host&IP=$1 HTTP/1.0" \
  && echo && sleep 1) | /usr/bin/nc $SERVER 80
 
 }

 if [ x$reason = xBOUND ] || [ x$reason = xRENEW ] || \
    [ x$reason = xREBIND ]; then
 setname $new_ip_address;
 fi

 exit 0

This gives me a plug'n'play solution with a static hostname, which is
not too expensive (no cron job sending requests to justlinux every five
minutes).

Conclusion
----------

The conclusion of all that is that RCN's technical support is somewhat
clueless (well, I guess that the average tech support call isn't about a
problem with their DNS tables, but I couldn't talk to anybody who knew
what an authoritative answer was), but their connectivity is pretty good
and much better than what I had with my old cable modem.

 0. http://www.rcn.com/
 1. http://www.3com.com/products/cablemodem/external.html
 2. http://www.cablemodem.com/specifications.html
 3. http://www.faqs.org/rfcs/rfc2131.html
 4. http://www.justlinux.com/dynamic_dns.html
 5. http://www.isc.org/products/DHCP/
 6. http://www.l0pht.com/~weld/netcat/

--
Hugo Haas <[email protected]> - http://larve.net/people/hugo/
La libert� d'�crire et de parler impun�ment marque, soit l'extr�me
bont� du prince, soit le profond esclavage du peuple. On ne permet de
dire qu'� celui qui ne peut rien. -- Denis Diderot