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The default ASP.NET Core port configured in .NET container images has been updated from port 80 to 8080.

We also added the new ASPNETCORE_HTTP_PORTS environment variable as a simpler alternative to ASPNETCORE_URLS . The new variable expects a semicolon delimited list of ports numbers, while the older variable expects a more complicated syntax.

Apps built using the older WebHost.CreateDefaultBuilder() API won't respect the new ASPNETCORE_HTTP_PORTS environment variable. And now that ASPNETCORE_URLS is no longer set automatically, they'll switch to use a default URL of http://localhost:5000 , rather than http://*:80 , as formerly.

Previous behavior

Prior to .NET 8, you could run a container that expected port 80 to be the default port and be able to access the running app.

For example, you could run the following command and then access the app locally at port 8000, which is mapped to port 80 in the container:

docker run --rm -it -p 8000:80 <my-app>

New behavior

Starting with .NET 8, if you map to port 80 in the container without explicitly setting the ASP.NET Core port used in the container to 80, any attempt to connect to that mapped port will fail.

For example, if you run the following command, you'd be unable to connect to the application locally using port 8000.

docker run --rm -it -p 8000:80 <my-app>

Instead, change the command to use port 8080 within the container:

docker run --rm -it -p 8000:8080 <my-app>

You can see the difference in behavior in the following examples.

Mapping port 80 (failure case):

$ docker run --rm -d -p 8000:80 mcr.microsoft.com/dotnet/samples:aspnetapp
ba88b746bd7097e503f8ab6e5320c595640e242f6de4f734412944a0e2836acc
$ curl http://localhost:8000/Environment
curl: (56) Recv failure: Connection reset by peer
$ docker kill ba88b746bd7097e503f8ab6e5320c595640e242f6de4f734412944a0e2836acc
ba88b746bd7097e503f8ab6e5320c595640e242f6de4f734412944a0e2836acc
Mapping port 8080:
$ docker run --rm -d -p 8000:8080 mcr.microsoft.com/dotnet/samples:aspnetapp
74d866bdaa8a5a09e4a347bba17ced321d77a2524a0853294a123640bcc7f21d
$ curl http://localhost:8000/Environment
{"runtimeVersion":".NET 8.0.0-rc.1.23419.4","osVersion":"Alpine Linux v3.18","osArchitecture":"Arm64","user":"root","processorCount":4,"totalAvailableMemoryBytes":4123820032,"memoryLimit":0,"memoryUsage":30081024,"hostName":"74d866bdaa8a"}
$ docker kill 74d866bdaa8a5a09e4a347bba17ced321d77a2524a0853294a123640bcc7f21d
74d866bdaa8a5a09e4a347bba17ced321d77a2524a0853294a123640bcc7f21d
Mapping port 80 with ASPNETCORE_HTTP_PORTS set to port 80:
$ docker run --rm -d -p 8000:80 -e ASPNETCORE_HTTP_PORTS=80 mcr.microsoft.com/dotnet/samples:aspnetapp
3cc86b4b3ea1a7303d83171c132b0645d4adf61d80131152936b01661ae82a09
$ curl http://localhost:8000/Environment
{"runtimeVersion":".NET 8.0.0-rc.1.23419.4","osVersion":"Alpine Linux v3.18","osArchitecture":"Arm64","user":"root","processorCount":4,"totalAvailableMemoryBytes":4123820032,"memoryLimit":0,"memoryUsage":95383552,"hostName":"3cc86b4b3ea1"}
$ docker kill 3cc86b4b3ea1a7303d83171c132b0645d4adf61d80131152936b01661ae82a09
3cc86b4b3ea1a7303d83171c132b0645d4adf61d80131152936b01661ae82a09
Version introduced
.NET 8 Preview 1
Type of change
This change is a behavioral change.
Reason for change
The change to the port number was made because of the need to provide a good usability experience when switching to a non-root user. Running as a non-root user requires the use of a non-privileged port in some environments. Since port 80, the previous default port, is a privileged port, the default was updated to port 8080, which is a non-privileged port.
Recommended action
There are two ways to respond to this breaking change:
(Recommended action) Explicitly set the ASPNETCORE_HTTP_PORTS, ASPNETCORE_HTTPS_PORTS, and ASPNETCORE_URLS environment variables to the desired port. Example: docker run --rm -it -p 8000:80 -e ASPNETCORE_HTTP_PORTS=80 <my-app>
Update existing commands and configuration that rely on the expected default port of port 80 to reference port 8080 instead. Example: docker run --rm -it -p 8000:8080 <my-app>
If your app was built using the older WebHost.CreateDefaultBuilder() method, set ASPNETCORE_URLS (not ASPNETCORE_HTTP_PORTS). Example: docker run --rm -it -p 8000:80 -e ASPNETCORE_URLS=http://*:80 <my-app>.
If you're using Kubernetes or Docker Compose, you need to change the port per those schemas. For examples, see Using .NET with Kubernetes.
If you use Azure container services to host your ASP.NET Core container app, you need to update your container environment to set the desired port:
For Azure App Service, set WEBSITES_PORT using the CLI or in the portal.
For Azure Container Apps, you can change the port as part of resource creation.
For Azure Container Instances, you can change the port as part of resource creation.
Affected APIs
None.
See also
New non-root 'app' user in Linux images
Containerize a .NET app
Blog: Secure your .NET cloud apps with rootless Linux containers
Blog: Running non-root .NET containers with Kubernetes