From 3ba53df00517ff5e01d4369e535b9776f16f62b6 Mon Sep 17 00:00:00 2001
From: Mathias Brodala <mbrodala@pagemachine.de>
Date: Mon, 29 Jul 2024 10:22:04 +0200
Subject: [PATCH] Document Docker port selection on multiple exposed ports

---
 docs/content/providers/docker.md         | 10 ++++++----
 docs/content/routing/providers/docker.md |  5 +++--
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/docs/content/providers/docker.md b/docs/content/providers/docker.md
index d47e9d78f..5202dfe32 100644
--- a/docs/content/providers/docker.md
+++ b/docs/content/providers/docker.md
@@ -117,12 +117,14 @@ When using Docker Compose, labels are specified by the directive
 
 Traefik retrieves the private IP and port of containers from the Docker API.
 
-Port detection works as follows:
+Port detection for private communication works as follows:
 
 - If a container [exposes](https://docs.docker.com/engine/reference/builder/#expose) a single port,
-  then Traefik uses this port for private communication.
+  then Traefik uses this port.
 - If a container [exposes](https://docs.docker.com/engine/reference/builder/#expose) multiple ports,
-  or does not expose any port, then you must manually specify which port Traefik should use for communication
+  then Traefik uses the lowest port.  E.g. if `80` and `8080` are exposed, Traefik will use `80`.
+- If a container does not expose any port, or the selection from multiple ports does not fit,
+  then you must manually specify which port Traefik should use for communication
   by using the label `traefik.http.services.<service_name>.loadbalancer.server.port`
   (Read more on this label in the dedicated section in [routing](../routing/providers/docker.md#port)).
 
@@ -738,7 +740,7 @@ providers:
 _Optional, Default=false_
 
 If the parameter is set to `true`,
-any [servers load balancer](../routing/services/index.md#servers-load-balancer) defined for Docker containers is created 
+any [servers load balancer](../routing/services/index.md#servers-load-balancer) defined for Docker containers is created
 regardless of the [healthiness](https://docs.docker.com/engine/reference/builder/#healthcheck) of the corresponding containers.
 It also then stays alive and responsive even at times when it becomes empty,
 i.e. when all its children containers become unhealthy.
diff --git a/docs/content/routing/providers/docker.md b/docs/content/routing/providers/docker.md
index b120abbb7..1c30f7824 100644
--- a/docs/content/routing/providers/docker.md
+++ b/docs/content/routing/providers/docker.md
@@ -66,10 +66,11 @@ With Docker, Traefik can leverage labels attached to a container to generate rou
     ```
 
     !!! important "Traefik Connecting to the Wrong Port: `HTTP/502 Gateway Error`"
-        By default, Traefik uses the first exposed port of a container.
+        By default, Traefik uses the lowest exposed port of a container as detailed in
+        [Port Detection](../providers/docker.md#port-detection) of the Docker provider.
 
         Setting the label `traefik.http.services.xxx.loadbalancer.server.port`
-        overrides that behavior.
+        overrides this behavior.
 
 ??? example "Specifying more than one router and service per container"