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Wednesday, April 27th, 2016

DNS 1: Dynamic DNS

Categories: [ IT ]

Now that I have my own server, I can finally have my own DNS server and my own domain name for my home computer that has a (single) dynamic IP address.

The following notes assumes there is already a running instance of Bind 9 on a Debian Jessie system for an imaginary domain example.com, served by a name server named ns.example.com and you want to dynamically update the DNS records for home.example.com. This is largely based on the Debian tutorial on the subject, solving the problem that bind cannot modify files in /etc/bind.

On the server

Create a shared key that will allow to remotely update the dynamic zone:
dnssec-keygen -a HMAC-MD5 -b 128 -r /dev/urandom -n USER DDNS_UPDATE
This creates a pair of files (.key and .private) with names starting with Kddns_update.+157+. Look for the value of Key: entry in the .private file and put that value in a file named /etc/bind/ddns.key with the following content (surrounding it with double quotes):
key DDNS_UPDATE {
        algorithm HMAC-MD5.SIG-ALG.REG.INT;
        secret "THIS IS WHERE YOU PUT THE KEY";
};

You can then delete the two Kddns_update.+157+ files. Ensure that /etc/bind/ddns.key belongs to "root" and to the "bind" group, and is not readable by other users.

Then in named.conf.local, include the key file and declare a new zone:

include "/etc/bind/ddns.key";

zone "home.example.com" { type master; file "/var/cache/bind/db.home.example.com"; allow-update { key DDNS_UPDATE; }; journal "/var/cache/bind/db.home.example.com.jnl"; };

In /var/cache/bind create the file db.home.example.com by copying /etc/bind/db.empty and adapting it to your needs. For convinience, create a db.home.example.com symbolic link in /etc/bind pointing to /var/cache/bind/db.home.example.com.

In db.example.com (that is, the parent zone), add a NS entry to delegate the name home.example.com to the DNS server of the parent zone:
home.example.com NS ns.example.com

You can now reload the bind service to apply the configuration changes.

I also found examples of how to test the dynamic zone with nsupdate.

On the home computer

I decided to use ddclient 3.8.3 because it supports dynamic dns updates using the nsupdate tool. I backported that version of ddclient manually from a Debian Testing package; it's written in Perl and the backporting is trivial.

Copy /etc/bind/ddns.key from the server to /etc/ddns.key on the home computer (the one running ddclient), ensuring only root can read it. Then add the following to /etc/ddclient.conf (be careful with the commas, there is no comma at the end of the second last line):
protocol=nsupdate, \
zone=home.example.com, \
ttl=600, \
server=THE_IP_ADDRESS_OF_THE_DNS_SERVER, \
password=/etc/ddns.key \
home.example.com

You can then try out the new setup.

[ Posted on April 27th, 2016 at 18:15 | 1 comment | ]

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