Cannot connect to port 53 (DNS) on external servers

Cannot connect to port 53 (DNS) on external servers

Cannot connect to port 53 (DNS) on external servers
Cannot connect to port 53 (DNS) on external servers
2024-09-30 01:27:28 - last edited 2024-09-30 01:40:53
Model: Deco X60  
Hardware Version: V3
Firmware Version: 1.2.9 Build 20240722 Rel. 43212

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.

 

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Re:Cannot connect to port 53 (DNS) on external servers
2024-09-30 11:47:29

  @pjam 

Hi,

 

Deco will not block your connection to internet servers on port 53.

 

 

You cannot use telnet commend to check port 53 on the local network.

Since port 53 is the standard port for DNS, please use nslookup commend.

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Re:Cannot connect to port 53 (DNS) on external servers
2024-10-01 03:12:52 - last edited 2024-10-01 03:23:34

Sunshine wrote

  @pjam 

Hi,

 

Deco will not block your connection to internet servers on port 53.

 

 

You cannot use telnet commend to check port 53 on the local network.

Since port 53 is the standard port for DNS, please use nslookup commend.

 

Please read my message again - I showed what works and what doesn't work. Did you try it yourself? I know the difference between telnet and nslookup, I've been using the internet since 1992.

 

I cannot make connections to external computers on port 53. If you don't want to try it with telnet, try it with netcat (or nc on macOS). it doesn't connect when using my home network:

 

nc -z -n -v 8.8.8.8 53
[hangs here]^C

 

Whereas when connected to a 4G network:

 

nc -z -n -v 8.8.8.8 53
Connection to 8.8.8.8 port 53 [tcp/*] succeeded!

 

What is causing this behaviour if not the Deco system? It happens on multiple computers that also operate 100% correct when not on the Deco network.

 

I can make a UDP connection to port 53, so it seems like it's just TCP that doesn't work:

 

nc -z -n -v -u 8.8.8.8 53
Connection to 8.8.8.8 port 53 [udp/*] succeeded!

 

 

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