How do you run a script after a network interface comes up?

I haven't tried this solution my self - yet, but found this thread: https://superuser.com/questions/201478/run-script-in-os-x-10-6-on-network-connection-like-etc-network-if-up-d/261004#261004

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.

The following works on Mac OS X 10.11.3 (El Capitan)

1. Create a file: networkchange.plist

<?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>networkchange</string>
  <key>LowPriorityIO</key>
  <true/>
  <key>ProgramArguments</key>
  <array>
    <string>/Users/Shared/bin/networkchange.sh</string>
  </array>
  <key>WatchPaths</key>
  <array>
    <string>/etc/resolv.conf</string>
    <string>/var/run/resolv.conf</string>
    <string>/private/var/run/resolv.conf</string>
  </array>
  <key>RunAtLoad</key>
  <true/>
</dict>
</plist>

2. Then, register by running the following commands in Terminal:

launchctl load networkchange.plist
launchctl start networkchange

Note that the script is run by the user registering the plist.