diff --git a/docs/oidc.md b/docs/oidc.md
index 7f85b397..60ca2896 100644
--- a/docs/oidc.md
+++ b/docs/oidc.md
@@ -12,7 +12,7 @@ With a subpath (`URL_BASE=/shelfmark/`):
 https://<your-shelfmark-domain>/shelfmark/api/auth/oidc/callback
 ```
 
-The callback URL is constructed from the incoming request, so your reverse proxy must forward `X-Forwarded-Proto` and `X-Forwarded-Host` correctly. PKCE (S256) is used automatically.
+The callback URL is constructed from the incoming request, so your reverse proxy must forward `X-Forwarded-Proto` and `X-Forwarded-Host` correctly, including the external port when it is not the protocol default. PKCE (S256) is used automatically.
 
 ## Settings
 
diff --git a/docs/reverse-proxy.md b/docs/reverse-proxy.md
index 294fa2b5..5282bbc9 100644
--- a/docs/reverse-proxy.md
+++ b/docs/reverse-proxy.md
@@ -23,10 +23,11 @@ server {
     location / {
         proxy_pass http://shelfmark:8084;
         proxy_http_version 1.1;
-        proxy_set_header Host $host;
+        proxy_set_header Host $http_host;
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
         proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_set_header X-Forwarded-Host $http_host;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection $connection_upgrade;
     }
@@ -56,11 +57,11 @@ All Shelfmark paths (UI, API, assets, Socket.IO) are served under the base path.
 location /shelfmark/ {
     proxy_pass http://shelfmark:8084/shelfmark/;
     proxy_http_version 1.1;
-    proxy_set_header Host $host;
+    proxy_set_header Host $http_host;
     proxy_set_header X-Real-IP $remote_addr;
     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
     proxy_set_header X-Forwarded-Proto $scheme;
-    proxy_set_header X-Forwarded-Host $host;
+    proxy_set_header X-Forwarded-Host $http_host;
     proxy_set_header Upgrade $http_upgrade;
     proxy_set_header Connection $connection_upgrade;
     proxy_read_timeout 86400;
@@ -136,11 +137,11 @@ location /shelfmark/ {
 
     proxy_pass http://shelfmark:8084/shelfmark/;
     proxy_http_version 1.1;
-    proxy_set_header Host $host;
+    proxy_set_header Host $http_host;
     proxy_set_header X-Real-IP $remote_addr;
     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
     proxy_set_header X-Forwarded-Proto $scheme;
-    proxy_set_header X-Forwarded-Host $host;
+    proxy_set_header X-Forwarded-Host $http_host;
     proxy_set_header Upgrade $http_upgrade;
     proxy_set_header Connection $connection_upgrade;
     proxy_read_timeout 86400;
@@ -158,6 +159,7 @@ If login, settings saves, or downloads appear to fail in the browser but the act
 - Do not force `Connection: upgrade` on every request. That can break normal `POST` and `PUT` responses while the backend still processes them.
 - If your proxy UI does not support conditional websocket headers, remove the forced websocket headers entirely and let Shelfmark fall back to polling.
 - Keep the standard forwarded headers: `Host`, `X-Forwarded-For`, `X-Forwarded-Proto`, and `X-Forwarded-Host` when using a subpath or OIDC.
+- Preserve the original port in `Host` and `X-Forwarded-Host` by using `$http_host` rather than `$host` when Shelfmark is exposed on a custom port.
 
 This is especially relevant for Nginx Proxy Manager or custom advanced config snippets that add websocket headers globally.
 
diff --git a/shelfmark/main.py b/shelfmark/main.py
index 46fdbff9..0cb94d11 100644
--- a/shelfmark/main.py
+++ b/shelfmark/main.py
@@ -118,7 +118,7 @@ BASE_PATH = normalize_base_path(normalize_optional_text(app_config.get("URL_BASE
 app = Flask(__name__)
 app.config["SEND_FILE_MAX_AGE_DEFAULT"] = 0  # Disable caching
 app.config["APPLICATION_ROOT"] = BASE_PATH or "/"
-wsgi_app = cast(Any, ProxyFix(app.wsgi_app))
+wsgi_app = cast(Any, ProxyFix(app.wsgi_app, x_host=1, x_port=1))
 if BASE_PATH:
     wsgi_app = cast(Any, PrefixMiddleware(wsgi_app, BASE_PATH, bypass_paths={"/api/health"}))
 app.wsgi_app = wsgi_app
diff --git a/tests/core/test_proxy_headers.py b/tests/core/test_proxy_headers.py
new file mode 100644
index 00000000..629b9cc6
--- /dev/null
+++ b/tests/core/test_proxy_headers.py
@@ -0,0 +1,46 @@
+import importlib
+from unittest.mock import Mock, patch
+
+import pytest
+
+
+@pytest.fixture(scope="module")
+def main_module():
+    with patch("shelfmark.download.orchestrator.start"):
+        import shelfmark.main as main
+
+        importlib.reload(main)
+        return main
+
+
+@pytest.mark.parametrize(
+    "headers",
+    [
+        {
+            "X-Forwarded-Proto": "https",
+            "X-Forwarded-Host": "library.example.com:12345",
+        },
+        {
+            "X-Forwarded-Proto": "https",
+            "X-Forwarded-Host": "library.example.com",
+            "X-Forwarded-Port": "12345",
+        },
+    ],
+)
+def test_oidc_redirect_uses_forwarded_external_port(main_module, headers):
+    oidc_client = Mock()
+    oidc_client.authorize_redirect.return_value = ("", 302)
+
+    with patch(
+        "shelfmark.core.oidc_routes._get_oidc_client",
+        return_value=(oidc_client, {}),
+    ):
+        response = main_module.app.test_client().get(
+            "/api/auth/oidc/login",
+            headers=headers,
+        )
+
+    assert response.status_code == 302
+    oidc_client.authorize_redirect.assert_called_once_with(
+        "https://library.example.com:12345/api/auth/oidc/callback"
+    )