Dynamic local dns changer.

You probably know of ddns services that allow the dynamic change of a (sub) hostname. to point a webaddress to a changing web ip address. This is kinda the same but then for local system. But there is DNS/DHCP (and a lot of other local systems) i hear you say that always point to the right ip. Well your not wrong but there are two use cases that are not capable with dns/dhcp and other local systems

1st ip of a list of servers

For my home assistant setup i have local whisper setup for when the internet is down. I dont have home assistant on a fast system. My fastest system is my laptop blue-mage but that one is not up 24/7 so i need a fail over. By making a list of ip addresses in priority of system speed and then ping then untill you get a respond you have the ip for a hostname for your fastest whisper server.

vpn on/off hostname

My laptops have a vpn system to keep connected to my homelab. By selecting between the local ip or the vpn ip i have a hostname that always connects to my laptop where ever it is. (vpn needs to be enabled when outside local lan)

Code and stuff

This script uses pihole or dnsmasq as dns system with a additional host file. that is fed by scripts in a crontab.

pihole/dnsmasq config.

Pi hole is under everlasting change. the setup just changed i hope it stays this for a while set etc_dnsmasq_d = true in misc in /etc/pihole/pihole.toml

create a file /etc/dnsmasq.d/99-dyndns.conf with the content addn-hosts=/etc/dyn-hosts.conf

create a empty file with atleast one row end (or else dnsmasq wont start) /etc/dyn-hosts.conf

ddns script

here is the overall script to manage the /etc/dns-hosts.conf file. if you use dnsmasq you need to edit the systemctl restart dont forget making it execuable /usr/local/bin/dyndnsmasq.sh

#!/bin/bash

# Script to add or update a hostname's IP address in $dyn_file

# Check for required arguments
if [ $# -ne 2 ]; then
  echo "Usage: $0 <ip_address> <hostname>"
  exit 1
fi

ip_address="$1"
hostname="$2"
dyn_file="/etc/dyn-hosts.conf"

# Check if IP address is valid (basic check)
if [[ ! "$ip_address" =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
  echo "Invalid IP address: $ip_address"
  exit 1
fi

# Check if hostname is valid (basic check - can be more strict)
if [[ ! "$hostname" =~ ^[a-zA-Z0-9.-]+$ ]]; then
  echo "Invalid hostname: $hostname"
  exit 1
fi

# Create a temporary file to store the modified $dyn_file content
temp_file=$(mktemp)

# Check if the hostname already exists in $dyn_file and update or add
if grep -q "$hostname" "$dyn_file"; then
  # Hostname exists, update the IP address.  Capture the changes.
  if sed "s/^\([0-9.]*\)\s\+${hostname}\$/${ip_address} ${hostname}/" "$dyn_file" > "$temp_file"; then
      echo "Updated IP address for $hostname to $ip_address"
  else
      echo "Error updating IP address for $hostname"
      rm "$temp_file"
      exit 1
  fi

else
  # Hostname does not exist, add a new entry
  echo "$ip_address $hostname" >> "$temp_file"
  echo "Added $hostname with IP address $ip_address"
  # Copy the original content of $dyn_file to the temp file, then append the new line.
  cat "$dyn_file" >> "$temp_file"
fi


# Compare the original $dyn_file with the new content in the temp file
if ! cmp -s "$dyn_file" "$temp_file"; then  # -s for silent comparison
  cat "$temp_file" > "$dyn_file" 
  # Files are different, overwrite $dyn_file and restart dnsmas
systemctl restart pihole-FTL # or dnsmasq if you use dnsmasq

else
  echo "No changes needed to $dyn_file."
  rm "$temp_file" # Clean up the temporary file
fi

exit 0

script for a hostname

Here is one example script edit and duplicate i keep them in /root/bin/

#!/bin/bash

# List of IP addresses to ping
IP_STRING="192.168.5.80 10.209.212.9 192.168.5.6"
hostname="whisper.hlab"

# Function to check if IP is reachable
check_ip_reachable() {
    ip="$1"
    ping -c 1 -W 1 "$ip" >/dev/null 2>&1
}

# Main script
for ip in $IP_STRING; do
    if check_ip_reachable "$ip"; then
        echo "Found reachable IP: $ip"
        /usr/local/bin/dyndnsmasq.sh "$ip" "$hostname"
        exit 0
    fi
done

echo "No reachable IP found in the list."

Final result.

if you loaded it in a crontab or executed the script. you have whisper.hlab pointing to the 1st ip found. No online services are used so the system works when your home is offline.

• Dns
• Pihole