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I run a custom python script to update various dynamic DNS servers through launchctl and it runs every 15 minutes. This is both overkill and underkill.

It would be nice if the script would execute only when reachability changes, and then as a fallback maybe every 30 minutes. I could easily enough update the Python script to check if the external address has changed before calling the update, but if my connection goes down and comes back up with a different IP address, I wouldn't want to have to wait 15-30 minutes.

Note this is different than the keep alive parameter. Odds are my computer will never lose its network connection. It's on a LAN and everything has backup power. So, only Internet reachability matters.

stormj
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  • How often are you wanting to check if your network connection is up/down? – l'L'l May 10 '15 at 22:50
  • I don't want to check; I want the system to run it when network availability comes back. – stormj May 12 '15 at 04:50
  • In order for something to tell it to run there has to be another thing that tells it if it's up or down - that's what i'm asking. – l'L'l May 12 '15 at 23:39
  • The system performs a number of tasks depending on network status, including reachability. I'm sure there are ways of extending it, just a question of whether it's doable with documented APIs. – stormj May 14 '15 at 03:59

1 Answers1

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I don't think launchd has any built-in trigger for the network coming up, but you can monitor file paths for changes. So if we can find a file that is updated when the network connects, we can implement the desired functionality.

One such file is /var/run/resolv.conf - this file is written whenever the computer renews a DHCP lease. It will be updated when you first connect, and periodically as the DHCP lease is renewed (the timing is set by the DHCP server).

Here is an example launchd plist file. 

<?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>org.example.on-dhcp-renew</string>
    <key>ProgramArguments</key>
    <array>
        <string>/Users/darrin/bin/on-dhcp-renew.sh</string>
    </array>
    <key>WatchPaths</key>
    <array>
        <string>/var/run/resolv.conf</string>
    </array>
</dict>
</plist>

This will only work if you use DHCP addressing for your LAN - as the DHCP client is the program that updates your resolv.conf file.

Darrin
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