ASP.NET Core: 'Failed to determine the https port for redirect'

warn: Microsoft.AspNetCore.HttpsPolicy.HttpsRedirectionMiddleware[3] Failed to determine the https port for redirect. means the app calls app.UseHttpsRedirection() but listens on HTTP only, so the middleware has no HTTPS address to send the browser to. In the test it then served the request on plain HTTP. Give it an HTTPS port and the redirect works.

Two fixes were tested: setting ASPNETCORE_HTTPS_PORT, and adding an https:// address to --urls. Both turned the 200 into a 307 Temporary Redirect to the HTTPS port and the warning was gone.

The error

info: Microsoft.Hosting.Lifetime[14]
      Now listening on: http://127.0.0.1:15709
...
warn: Microsoft.AspNetCore.HttpsPolicy.HttpsRedirectionMiddleware[3]
      Failed to determine the https port for redirect.

Why it happens

The app was the empty web template with one extra line:

var app = builder.Build();
app.UseHttpsRedirection();
app.MapGet("/", () => "Hello World!");

It was started with --urls http://127.0.0.1:15709, an HTTP address and nothing else. The warning appeared in the log after the startup lines, once a request had been made, and that request was not redirected: curl.exe -s -D - http://127.0.0.1:15709/ got HTTP/1.1 200 OK and Hello World!. So the app kept working, but the redirection you asked for silently did not happen. This is a warning, not an exception, which makes it easy to miss in a busy log.

The fix

Fix A, tell the middleware the port with an environment variable (same code):

$env:ASPNETCORE_HTTPS_PORT='15716'; & ./run.ps1 https 15715; $env:ASPNETCORE_HTTPS_PORT=$null

curl.exe -s -D - http://127.0.0.1:15715/
HTTP/1.1 307 Temporary Redirect
Content-Length: 0
Date: Fri, 06 Nov 2026 07:55:11 GMT
Server: Kestrel
Location: https://127.0.0.1:15716/

Fix B, let the app listen on HTTPS too, with no port setting (this PC has the ASP.NET Core development certificate installed):

$env:FIXMODE='https'; $p2 = Start-Process -FilePath "$d/FixLab/bin/Debug/net10.0/FixLab.exe" -ArgumentList '--urls','"http://127.0.0.1:15717;https://127.0.0.1:15718"' -WorkingDirectory "$d/FixLab" -WindowStyle Hidden -RedirectStandardOutput "$d/logs/https2-out.txt" -RedirectStandardError "$d/logs/https2-err.txt" -PassThru; Start-Sleep 3

curl.exe -s -D - http://127.0.0.1:15717/
HTTP/1.1 307 Temporary Redirect
Content-Length: 0
Date: Fri, 06 Nov 2026 07:55:14 GMT
Server: Kestrel
Location: https://127.0.0.1:15718/

In both runs the log had no warning line after startup. run.ps1 is the test's own script; its start line is $p = Start-Process -FilePath $exe -ArgumentList "--urls", "${Url}:$Port" -WorkingDirectory (Join-Path $root "FixLab") -WindowStyle Hidden -RedirectStandardOutput "$out\$Mode-out.txt" -RedirectStandardError "$out\$Mode-err.txt" -PassThru, after $env:FIXMODE = $Mode, with $Url defaulting to http://127.0.0.1. In fix B, $d is the test folder. Note that fix A only sets where the redirect points: nothing in that run listened on 15716, so a browser following that redirect would have reached a closed port. Fix A fits when something else in front of the app serves HTTPS on that port; with fix B the redirect pointed at an address the app itself was listening on, as the second Now listening on: https://127.0.0.1:15718 line showed.

How it was reproduced

A dotnet new web project (net10.0) with the three lines above, built once and started directly in the Production environment, called with curl 8.21.0. .NET SDK 10.0.403, Microsoft.AspNetCore.App 10.0.14, Windows 11.

Frequently asked

Does 'Failed to determine the https port for redirect' break my app?

Not in the test. The request was served over plain HTTP with 200 OK; only the redirect to HTTPS did not happen, and a warning was logged.

How do I set the HTTPS port for UseHttpsRedirection?

Setting the environment variable ASPNETCORE_HTTPS_PORT=15716 made the app answer 307 with Location: https://127.0.0.1:15716/. Listening on an https:// URL as well also worked.

Does ASPNETCORE_HTTPS_PORT make the app listen on HTTPS?

No. In the test it only changed the Location of the redirect to https://127.0.0.1:15716/. Nothing listened on 15716; to serve HTTPS from the app itself, add an https:// URL.

More decoded errors in the Fixes category. For where the app's addresses come from, see Kestrel ports and URLs.