Network Ports Used To Print (Network LPD Printing) I have been experimenting with an application that listens to network ports below 1024. 

I discovered that when that application is running, an attempt to print to a network printer (set up to print using LPD) will get stuck at "Printing - Connecting to printer". To be clear, the client is running the test application listening to sockets below 1024 and can't send print jobs to an LPD-enabled print server.

I've tried configuring the test application to not listen to ports 631 and 515, but still the print job gets stuck. Killing the test application allows printing to work fine.

My question is: when trying to print to a network print server using LPD, what ports below 1024 have a printing-related process connect to them? I want to "whitelist" them from the test application so printing works while the test application runs.

This is being done on OS X 10.10.3
You will want to try excluding ports TCP 721-731 as these tend to be the source ports for printing. TCP 515 is the listening port for LPD jobs.

Source (in the FAQ/questions section at the bottom)