Run script in OS X 10.6 on network connection. (like /etc/network/if-up.d/)

Is there any way to run a script when a network interface comes up in Mac OS X?

I've tried putting a script named ip-up in /etc/ppp but it doesn't seem to fire when I disconnect and reconnect to a wireless network.

I'd prefer not to have to run a cron job to constantly check whether the network status has changed. In some linux distros, you can accomplish this by putting a script in /etc/network/if-up.d/ . Is there anything similar for Mac OS X?

You could try MarcoPolo or one of the similar utilities listed on its website. Location Changer looks promising if you're a minimalist.

A launchd agent watching /etc/resolv.conf, and two network related .plist files under /Library/Preferences/SystemConfiguration/ seems to work for me (in Mac OS X 10.8.4):

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" \
  "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>Label</key>
  <string>ifup.ddns</string>

  <key>LowPriorityIO</key>
  <true/>

  <key>ProgramArguments</key>
  <array>
    <string>/Users/Shared/bin/ddns-update.sh</string>
  </array>

  <key>WatchPaths</key>
  <array>
    <string>/etc/resolv.conf</string>
    <string>/Library/Preferences/SystemConfiguration/NetworkInterfaces.plist</string>
    <string>/Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist</string>
  </array>

  <key>RunAtLoad</key>
  <true/>
</dict>
</plist>

I had previously only used /etc/resolv.conf, but there were cases where that wasn't enough.

You should consider using crankd, which precisely allows you to run scripts in response to many system events such as network changes, filesystem activity, application launching, etc.

As I couldn't find any sensible documentation, I also wrote a small blog post on getting started using crankd.