dyn-nsupdate: Self-made DynDNS

Introduction

Welcome to dyn-nsupdate, a collection of tools using BIND, CGI and Python to provide DynDNS services with your own nameserver. Both IPv4 and IPv6 are fully supported.

dyn-nsupdate consists of two pieces: The server part provides a way to update IP addresses in Bind’s DNS zones via CGI, in a safe manner. The client part uses CGI to update some domain to the current address(es) of the machine it is running on. Alternatively, some routers can be configured to do this themselves. The FritzBox is known to be supported.

Server Setup

In the following, replace dyn.example.com by whatever domain will be managed through DynDNS. I assume that BIND has already been set up for dyn.example.com as a dynamic zone that can be updated through nsupdate -l. This can be achieved by setting update-policy local; in the zone configuration. Furthermore, I assume the directory /var/lib/bind/ exists.

There are two pieces that have to be installed: A setuid wrapper which checks the passwords, and applies the updates; and some CGI scripts offered through a webserver. Please read this guide carefully and make sure you understand the security implications of what you are doing. setuid wrappers are not toys!

Let’s first set up the setuid wrapper. To compile it, you will need cmake and boost, including the regex and program_options boost packages. Starting in the source directory, run::

cd nsupd-wrapper
mkdir -p build
cd build
DIR=/var/lib/bind
cmake .. -DCMAKE_BUILD_TYPE=Release -DDYNNSUPDATE_CONFIG_FILE=$DIR/dyn-nsupdate.conf
make

This should compile the binary dyn-nsupdate. Notice that the path to the configuration file will be hard-coded into the binary. If it were run-time configurable, then a user could call the script with her own configuration file, gaining access to all domains BIND lets you configure. If you want to put the files in another directory, change the configuration file name accordingly. Make sure the file (and all of the directories it is in) can not be written by non-root. The setuid wrapper trusts that file. You can now install it and the sample configuration file, and set their permissions::

sudo install dyn-nsupdate $DIR/dyn-nsupdate -o bind -g bind -m +rx,u+ws
sudo install ../../dyn-nsupdate.conf.dist $DIR/dyn-nsupdate.conf -o bind -g bind -m u+rw

Finally, edit the config file. The format should be pretty self-explanatory. In particular, change the password!

Now, let’s go on with the CGI scripts. They are using Python 2, so make sure you have that installed. There are two scripts: One is used for clients to detect their current external IP address, and one is used to do the actual update of the domain. The first script is used by the “web” IP detection method (see client configuration below). It should be available on a domain that is available only through a single protocol, i.e., IPv4 only or IPv6 only. This is required to reliably detect the current address of the given protocol. If you want to support both IPv4 and IPv6, I suggest you have three domains ipv4.ns.example.com, ipv6.ns.example.com and ns.example.com where only the latter is available via both protocols (this is something you have to configure in your example.com DNS zone). All can serve the same scripts (e.g. via a ServerAlias in the apache configuration). I also strongly suggest you make these domains HTTPS-only, as the client script will send a password!

Choose some directory (e.g., /srv/ns.example.com) for the new domain, and copy the content of server-scripts there. Now configure your webserver appropriately for CGI scripts to be executed there. You can find a sample configuration for apache in apache-ns.example.com.conf. If you used a non-default location for the dyn-nsupdate wrapper, you have to change the path in the update CGI script accordingly.

That’s it! Your server is now configured. You can use curl to test your setup::

DOMAIN=test.dyn.example.com
PW=some_secure_password
curl 'https://ns.example.com/update?domain=$DOMAIN&password=$PW&ip=127.0.0.1'

Client setup (using the script)

You can find the client script at client-scripts/dyn-ns-client. It requires Python 3 and the dnspython package. Copy that script to the machine that should be available under the dynamic domain. Also copy the sample configuration file dyn-ns-client.conf.dist to $HOME/.config/dyn-nsupdate/dyn-ns-client.conf. You can choose another name, but then you will have to tell the script about it. Call dyn-ns-client --help for this and other options the script accepts. An important aspect of configuration is how to detect the current addresses of the machine the script is running on. For IPv4, this can only be “web”, which can deal with NAT. For IPv6, the script can alternatively attempt to detect the correct local address to use. The sample file contains comments that should explain everything.

Note that the script can update a list of domain names, in case you need the machine to have several names. It is preferable to use a CNAME instead, this will reduce the number of updates performed in the zone.

To run the script regularly, simply set up a cronjob. You can do so by running crontab -e, and add a line as follows::

*/15 * * * * /home/user/dyn-ns-client

This sets the update interval to 15min. If your IP address changes daily, you may want to reduce this to 5min to have a smaller timeframe during which your server is not available.

If you want to be emailed about changes in your IP address, pass -v as argument. The script will then only produce output if it has to update the DNS record.

Client setup (using a router)

Some routers are able to perform the update of the domain names themselves. The FritzBox is known to be supported. To configure it to tell your server about the current IP address, go to the DynDNS configuration section of the FritzBox and choose the “custom” DynDNS provider. Then enter the following settings:

Note that the user name is ignored.

Source, License

You can find the sources in the git repository (also available on GitHub). They are provided under a 2-clause BSD license. See the file LICENSE-BSD for more details.

Contact

If you found a bug, or want to leave a comment, please send me a mail. All sorts of feedback are welcome :)