Flush the DNS Cache on Yosemite (using a nice quick alias)

Problem: I’ve updated some Name Server records on a domain and I want to check they’ve propagated using Dig. When I dig mydomain.com ns I keep getting the old results.

Solution: The local DNS cache on the machine needs to be cleared. As far as the OS is concerned, it’s already looked up that domain name and doesn’t need to do a fresh lookup until the domain record’s TTL (time to live) has expired.

On OS X Yosemite there are two name resolution caches that need emptying, Multicast DNS and Unicast DNS. They can be cleared / emptied / flushed using two commands in Terminal:

sudo discoveryutil mdnsflushcache
sudo discoveryutil udnsflushcaches

A Nice Quick Alias: But really… who’s going to remember that? So I’ve added an alias to make it easier. Aliases are added (amongst other places) to your user’s .bash_profile file. To edit your bash profile file, open it with the following command in terminal:

nano ~/.bash_profile

Then once nano (the text editor) is open, add the following line to the file:

alias flushdns='sudo sh -c "discoveryutil mdnsflushcache; discoveryutil udnsflushcaches"'

To save the file, press Control (ctrl) + x and follow the prompts.
Once this is saved, load the changes you’ve made into your current terminal session with the command:

source ~/.bash_profile

Now when you type flushdns in terminal, your caches will be cleared! woo!

Tips:
1. The quickest way to open Terminal (or anything actually) is to use Spotlight. Press the Command (cmd) + Space keys, and type ‘Terminal’ in the resulting input, then press Return / Enter.
2. If your bash profile doesn’t exist, it might be empty when you type the nano command above. This is ok, when you Control (ctrl) + x after adding the line, the new file will be created.
3. When opening the bash profile file, I used ~/ before the file name. This is to ensure that it opens the profile file of the user you’re currently logged in as.

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