Container Apps - Binding to ports under 1024

This post will discuss binding to ports under 1024 for your application to listen on

Overview

This post will discuss binding to ports under 1024 for your application to listen on.

There is a fairly well known notion on Linux that binding to ports under 1024 require root access, certain “capabilities” (eg., CAP_NET_BIND_SERVICE), or something like setting net.ipv4.ip_unprivileged_port_start = 0 on a host machine. Since Container Apps inheritly only supports Linux-based containers, this same concept applies here.

However, the above cannot be done since Container Apps being a PaaS - does not grant that kind of priviledged access. If you encounter this issue with port binding, it’s typically because of a mix of the following:

• Running as a non-root USER in your container. Which is set in your Dockerfile or container image.
• Attempting to bind to a port under 1024 while running as this non-root user.

If this happens, the application will exit - since it can’t listen on the specified address:port. This will also manifest as an HTTP 502 or 503 to clients as the container in the pod will fail to start.

In application logs, you can see the full error. If you’re using Log Analytics, use the ContainerAppConsoleLogs_CL table - for Azure Monitor, use ContainerAppConsoleLogs, you can use Log Stream or the Azure CLI as well.

Using the above tables or log viewing measures, you’d see an error like the below, which depends on your application stack:

• error: listen EACCES: permission denied 0.0.0.0:80 (Node.js-based)
• nginx: [emerg] bind() to 0.0.0.0:80 failed (13: permission denied) (from NGINX)
• Caused by: java.lang.Exception: Socket bind failed: [13] [Permission denied] (Java)
• failed to listen: listen tcp4 :80: bind: permission denied (Go)
• [ERROR] Can't connect to ('0.0.0.0', 80) (Gunicorn)
• etc.

The common theme being permission denied when trying to listen on the specified port.

NOTE: Another port binding failure is address in use, which is for a different reason. See Address or port in use

Resolution

To resolve this, you can do either of the two options:

1. If wanting to still use a non-root USER - simply listen to a port higher than 1024. Ensure to update your application listening port and the Ingress option on Container Apps for your application (if using Ingress)
2. Or, if wanting to bind to a port under 1024 - change the user to root - . In most cases, not having a USER instruction would implicitly set this to root. However, this depends on the image used - you may need to explicitly set and/or add a root USER

As a best practice, ensure to test these user changes locally.