I can no longer make connections from any of my local devices to internet servers on port 53. The problem seemed to start some time in August 2024, I do not remember making any networking change or Deco changes around then. It's like Deco is blocking the connection.

I noticed because there are a number of iOS/Android app development libraries that contain code to detect if the device has a working internet connection. They do this by opening a connection to port 53 at a list of servers - 1.1.1.1, 8.8.8.8 etc. The app I work on fell back to offline mode seemingly overnight. I traced it to this issue.

Simply running telnet 8.8.8.8 53 will show the problem. This is what should happen:

telnet 8.8.8.8 53
Trying 8.8.8.8...
Connected to dns.google.
Escape character is '^]'.
Connection closed by foreign host.

But what actually happens is:

telnet 8.8.8.8 53
Trying 8.8.8.8...

I have confirmed this on multiple devices (Macbook Pro, Raspberry Pi, iPhone). Switching to a network connection that doesn't use the Deco - eg. hotspot to my iPhone's Optus 5G network, or my work connection - works fine.

This is going to cause confusion to people who use apps that test port 53. I have one misbehaving app that now doesn't work and I'm pretty sure it's the same issue.