The script runs whenever a network connection is made or broken by using the
launchd feature to check a path for changes. In this case, I used resolv.conf.The scripts determines the default gateway IP address and then keep trying to find the corresponding MAC address in the ARP table. Then it sets up the static ARP entry.
Two files are needed: one launchd configuration file and a shell script file. You need to give execute rights on the shell script with
chmod +x. Copy the files in place and rename/edit the filenames. You need to restart to make the configuration active.The contents of
/Users/darkfader/static-arp.sh:#!/bin/bash
# if the resolv.conf file was deleted, create an empty one to enable file watch again
touch /var/run/resolv.conf
while true; do
IP=$(netstat -rn | grep -m 1 default | tr -s ' ' | cut -d' ' -f 2)
if [ "$IP" == "" ]; then
exit 0
fi
MAC=$(arp -an | grep -m 1 $IP | tr -s ' ' | cut -d' ' -f 4)
if [ "$MAC" == "" ]; then
sleep 1
continue
fi
arp -S $IP $MAC
exit 0
done
The contents of
/Library/LaunchDaemons/net.darkfader.static-arp.plist:<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>net.darkfader.static-arp</string>
<key>ProgramArguments</key>
<array>
<string>/Users/darkfader/static-arp.sh</string>
</array>
<key>QueueDirectories</key>
<array/>
<key>WatchPaths</key>
<array>
<string>/var/run/resolv.conf</string>
</array>
</dict>
</plist>
