-update-host-aliases.md

update-host-aliases

This Gist provides a script that updates host aliases in the /etc/hosts file based on comments following the pattern # alias ${sourcehostname}=${targethostname}. It ensures that entries for the specified source hostnames are updated with the correct IP addresses for their corresponding target hostnames.

Features

• Automatically updates /etc/hosts based on alias comments.
• Resolves IP addresses using DNS if not found in /etc/hosts.
• Supports a dry-run option to preview changes before applying them.

Installation Steps

To install the script and the associated systemd service and timer, follow these steps:

1. Download the Script and Systemd Files

   Use curl or wget to download the script and the systemd service/timer files from the latest release of this Gist to a temporary location. Run the following commands:

   curl -L -o /tmp/update-host-aliases.sh https://gist.githubusercontent.com/aslafy-z/527ecafc0e77188448b61749a9ad4b87/raw/update-host-aliases.sh
   sudo mv /tmp/update-host-aliases.sh /usr/local/bin/
   sudo chmod +x /usr/local/bin/update-host-aliases.sh
   
   curl -L -o /tmp/update-host-aliases.service https://gist.githubusercontent.com/aslafy-z/527ecafc0e77188448b61749a9ad4b87/raw/update-host-aliases.service
   sudo mv /tmp/update-host-aliases.service /etc/systemd/system/
   
   curl -L -o /tmp/update-host-aliases.timer https://gist.githubusercontent.com/aslafy-z/527ecafc0e77188448b61749a9ad4b87/raw/update-host-aliases.timer
   sudo mv /tmp/update-host-aliases.timer /etc/systemd/system/
   
   sudo systemctl daemon-reload
   sudo systemctl enable update-host-aliases.timer
   sudo systemctl start update-host-aliases.timer

Usage

Running the Script

You can run the script directly or allow the timer to handle execution. To run the script manually, use:

sudo /usr/local/bin/update-host-aliases.sh

Dry Run Option

To preview changes without applying them, use the --dry-run option:

sudo /usr/local/bin/update-host-aliases.sh --dry-run

This will display the differences between the current /etc/hosts file and the updated version, allowing you to review what changes would be made.

Contributing

If you find any issues or want to suggest improvements, feel free to open a comment.

update-host-aliases.service

[Unit]
Description=Update host aliases based on alias comments in /etc/hosts
After=network-online.target
Wants=network-online.target

[Service]
Type=oneshot
ExecStart=/usr/local/bin/update-host-aliases.sh

update-host-aliases.sh

#!/bin/bash

HOSTFILE="/etc/hosts"
TEMPFILE=$(mktemp)
DIG_TIMEOUT=1

# Create a new temp file to build updated host file
cp $HOSTFILE $TEMPFILE

# Loop through each line of the host file
while read -r line; do
    # Check if the line is an alias comment
    if [[ $line =~ \#\ alias\ ([^=]+)=([^[:space:]]+) ]]; then
        source_hostname="${BASH_REMATCH[1]}"
        target_hostname="${BASH_REMATCH[2]}"

        # Find the IP of the target hostname
        target_ip=$(grep -Ew "[0-9]+.[0-9]+.[0-9]+.[0-9]+ $target_hostname" "$TEMPFILE" | head -n 1 | awk '{print $1}')
        
        if [ -z "$target_ip" ]; then
            # If target_hostname is not in /etc/hosts, resolve it using DNS
            target_ip=$(dig +short "$target_hostname" -4 +time=$DIG_TIMEOUT | head -n 1)
        fi

        if [ -n "$target_ip" ]; then
            sed -i "/# alias $source_hostname=/d" $TEMPFILE
            sed -i "/$source_hostname # alias/d" $TEMPFILE
            echo "# alias $source_hostname=$target_hostname" >> $TEMPFILE
            echo "$target_ip $source_hostname # alias" >> $TEMPFILE
        fi
    fi
done < "$HOSTFILE"

if [ -n "$1" ]; then
  # Dry run
  diff -u "$HOSTFILE" "$TEMPFILE"
  rm $TEMPFILE
  exit 0
fi
 

if ! cmp -s $HOSTFILE $TEMPFILE; then
    # Replace the old host file with the updated one
    mv $TEMPFILE $HOSTFILE
    chmod 644 $HOSTFILE
else
    # No changes detected, remove the temp file
    rm $TEMPFILE
fi

update-host-aliases.timer

[Unit]
Description=Run update-host-aliases script every 10 seconds

[Timer]
OnBootSec=10sec
OnUnitActiveSec=10sec
Persistent=true

[Install]
WantedBy=timers.target