Synology/Mac Custom Domain updater for Dynamic IP addresses

This is a handy little bash script to dynamically update DNS entries for your custom domains you want to point to your Synology NAS when you do not have a static IP.

This is for you, if

• You own 1..n domains and
• Want to host the contents behind them on top level on you Synology NAS
• Your Synology NAS sits behind a router that has a dynamic IP which your ISP changes (usually once / 24h)

Example: You own the domain example.com and want to host your wordpress site on you Synology NAS, making it available at https://example.com

Remark: If you only want to use subdomains (e.g. photos.example.com) you can do this easier with CNAME records that point to a DynDNS domain. A-records for the domain itself are not allowed to be CNAMES. Thus, the dynamic update of the A-records is required.

Prerequisites

DNS Service that allows for dynamic updating

If you are at a "typical" hoster, like [domainfactory.eu], they will not offer the chance to dynamically update your URL's A or AAAA records with an API. However, DESEC ([desec.io]) allows this. All you need to do is create an entry in your hoster's nameserver settings that determines the DESEC-nameservers to be used whenever your domain is being resolved.

At Domainfactory, the menu is Nameserver Settngs and the nameservers to be configured are the following:

So steps you need to do are

1. Configure your hoster's nameserver settings so that the DESEC nameservers are used
2. Configure your URL in your DESEC account

Preparations on your Synology NAS

Remark: Tested with Synology DSM 7.2.1

• Enable SSH access in Control Center -> Terminal & SNMP
• Install the DNS Server package

Deploy the Script

Copy the 2 script files to your Synology NAS. You can do this via drag & drop from the Browser UI.

Adapt the Config-Parameters in the config.sh file.

Somewhere in your user's home directory is fine, e.g. ~/url_update/.

Log in to your Synology NAS via SSH.

Make sure, the DynDNS_Update.sh is executable. If it is not, make it executable with: chmod +x DynDNS_Update.sh

Edit your Synology's Crontab

To make sure, the script checks for a change in your external IPs on a regular basis, you need to schedule the execution in your Synology's crontab.

Connect to your Synology NAS via SSH.

Edit the /etc/crontab file using a text editor like nano or vi (you need administrator privileges): sudo vi /etc/crontab

What the script does

The script contains of 2 parts: The DynDNS_Update.sh contains the script logic and the config.sh contains the custom paramaters to be configured.

Config Parameters

The config.sh files has to be located in the same directory as the script itself. It contains your DESEC-token which you need to generate here [https://desec.io/tokens] which serves as your authentication and authorization against the DESEC update endpoint.

TOKEN="yourDESEC_TokenValue"
DOMAINS=(domain1 domain2 ...)
OLD_IPv4="0.0.0.0"
OLD_IPv6="::::::"

The last two variables store the IP addresses from the last script execution to be compared to the external addresses in the next execution.

Determining your external IP addresses

After reading the config.sh file the script has a check on which operation system it runs, as the commands to determine your external IP address differ depending on the platform you are on.

If you are on a Mac (Darwin-Kernel operating system) you have the dig command available natively. On the Synology NAS you can also use dig, but only if you have the DNS server package installed, as it comes as a utility function with this package.

The following block of code determines, which OS you are on and then uses the Google request DNS resolver to determine your external IP.

if [[ $(uname) == "Darwin" ]]; then
    # Code specific to macOS --> dig is available
    ipv4=$(echo "$(dig -4 TXT +short o-o.myaddr.l.google.com @ns1.google.com)" | sed 's/^"//; s/"$//')
    ipv6=$(echo "$(dig -6 TXT +short o-o.myaddr.l.google.com @ns1.google.com)" | sed 's/^"//; s/"$//')
    echo "This code runs on macOS."
elif [[ $(uname) == "Linux" ]]; then
    # Code specific to Syno-DSM --> dig binary taken from the DNS Server package
    ipv4=$(echo "$(/var/packages/DNSServer/target/bin/dig -4 TXT +short o-o.myaddr.l.google.com @ns1.google.com)" | sed 's/^"//; s/"$//')
    # Remember: Enable IPv6 on Synology in the Network --> Network Interface --> Manage section first
    ipv6=$(echo "$(/var/packages/DNSServer/target/bin/dig -6 TXT +short o-o.myaddr.l.google.com @ns1.google.com)" | sed 's/^"//; s/"$//')
    echo "This code runs on SynoDSM"
else
    # Code for other operating systems (optional)
    echo "This code runs on an unknown operating system."
    exit 1
fi

Sanity Checks

If the IP addresses have not changed compared to the values stored in the config.sh, the script will exit.

If the external IP addresses could not be properly determined and are empty, the script will exit.

Execute the Update via API

With the collected parameters the proper URL for the DESEC update API is constructed for every domain given in the $DOMAINS-array in the config.sh. The update is performed for every URL given.

Store values in Config-file

Lastly, the IP addresses in the config.sh are being updated.

Create Cron-Task

Connect to your Synology NAS via SSH.

Edit the /etc/crontab file using a text editor like nano or vi: sudo vi /etc/crontab

Note: Using sudo is important as the crontab file requires root privileges for modification.

Add a new line to the file with the following format:

*/5 * * * * your_username cd ~/url_update/ ./DynDNS_Update.sh

This line

• runs every 5 minutes
• under the user identity given in your_username
• sets the working directory to the directory where you copied the script files (e.g. ~/url_update/)
• defines the script to be executed

After your edited the crontab, restart the cron service: sudo systemctl restart crond