dns-update

• dns-update - small dynamic DNS suite using SSH
• Copyright (C) 2014,2019 Christian Garbs mitch@cgarbs.de
• Licensed under GNU GPL v3 (or later)
• Homepage: https://github.com/mmitch/dns-update

ABOUT

This is a small suite to enable dynamic DNS updates from remote hosts. It's my personal replacement for the not-free-any-more DynDNS service. Clients use SSH to update their IP. While this won't work with J. Random Router that has a DynDNS client built in, it works on all of my systems (they have the capabilites for scripts and SSH connections), it's very easy to build and use and it provides the full SSH possibilities of encryption and authentication - for free.

HOW TO INSTALL ON THE SERVER

1. Install a nameserver of your choice and serve a zone dedicated to your dynamical IPs. You probably need a delegation for that zone from somewhere else. All this is NOT handled in this howto.

   My setup works with nsd as nameserver but apart from "please reload the zonefile" there is no interaction with the nameserver, so it should be easily portable.

   The example configuration creates the zone dynip.example.com in the file ZONEFILE=/home/dns-update/zones/dynip.example.com.zone. In /etc/nsd/nsd.conf, the zone paragraph would look like this:

   zone:
           name: dynip.example.com
           zonefile: /home/dns-update/zones/dynip.example.com.zone
   

2. Create a new user for the sole purpose of handling the dynamic DNS updates. This user needs a normal shell (/bin/sh should be enough - with /bin/false, the SSH forced commands won't run…), a home directory and SSH access (be sure to include him in AllowUsers/AllowGroups if you use them to restrict access via SSH). The password of the user can and should be disabled (either after configuration is finished or you'll have to use su(1) while configuring stuff).

   This repository is configured for a user dns-update with a home directory of /home/dns-update

3. Allow this new user to reload your nameserver configuration when a zonefile changes.

   My setup does this via sudo(8). I created the file /etc/sudoers.d/local-dns-update which just contains the line dns-update ALL =NOPASSWD: /bin/systemctl reload nsd. YMMV.

4. Clone this git repository RIGHT INTO the home directory of the dedicated user. Log in as or su(1) to the user and do:

   $ git clone https://github.com/mmitch/dns-update.git /home/dns-update
   

5. Create a local branch (git checkout -b local). Do your local configuration:

   • edit .ssh/authorized_keys (see notes in file)
   • edit the parameters on the top of update-client e.g. zone file name
   • edit config/zonefile.input to your needs (§SERIAL§ will be replaced automatically)
   • edit the bottom part of update-client if you used a different sudo configuration than in this example

   Check in your changes (git commit -a).

   Fix the file permissions on .ssh and .ssh/authorized_keys?! TODO: watch what git is doing there and what can be done against it.

   (In case you're wondering why a non-functional authorized_keys file is part of this repository: That way it's automatically tracked in your local branch and you can't forget to git-add it. Also: inline help on first edit.)

6. Basically, you're ready to go now!

HOW TO UPDATE THE SERVER

1. Log in/su(1) to user dns-update.

2. Switch git branch back to master (be sure to have no pending local changes):

   $ git checkout master
   

3. Get updates:

   $ get pull
   

4. Switch back to your local branch:

   $ git checkout local
   

5. Merge the updates. It's best to do a rebase to keep a clean history (of course, using merge instead of rebase is also possible). You might have to solve merge conflicts, be ready for that:

   $ git rebase master
   

LOGGING ON THE SERVER

Your SSH server probably already logs to /var/log/auth.log. There you can find successful client logins as well as diagnostic error messages when something goes wrong.

HOW TO INSTALL ON A CLIENT

• create local SSH key

• add pubkey to .ssh/authorized_keys on server

  • copy existing line
  • change hostname in command="" string
  • change public key

• to update, call ssh -i /path/to/new/identity.pub dns-update@ns.example.com update-client <new.ip.add.ress>

  • or use auto instead of the ip address to use the ssl source ip address automatically (useful if you're behind a router and don't know your outside IP, but want to send exactly this IP to the server)

• ppp-update-script not finished yet (but at least on Debian based distributions, you can simply write the ssh commandline (preferably with auto) into a new file under /etc/ppp/ip-up.d and you're set)

IPV4 AND IPV6

A simple heuristic tries to tell apart IPv4 and IPv6 addresses and generate A or AAAA records automatically. This allows a single client to have two addresses (one of each).

If a client updates using the auto keyword, you can call the SSH client (at least the OpenSSH client) with the additional parameters -4 or -6 to use the respective protocol. To update both address types, two ssh invocations are needed:

1. ssh -4 -i /path/to/new/identity.pub dns-update@ns.example.com update-client auto
2. ssh -6 -i /path/to/new/identity.pub dns-update@ns.example.com update-client auto