diff --git a/README.md b/README.md
index 6a263f5..6b7f1dc 100644
--- a/README.md
+++ b/README.md
@@ -26,9 +26,6 @@ location /saml/ {
     proxy_set_header Host $http_host;
     proxy_set_header X-Forwarded-Proto $scheme;
     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-    proxy_set_header X-Saml-Entity-Id https://samldemo.iamdev.s.uw.edu/saml;
-    # acs - post-back url registered with the IdP.
-    proxy_set_header X-Saml-Acs /saml/login;
     proxy_pass http://saml:5000/;
 }
 
@@ -50,3 +47,50 @@ randomly-generated string. Otherwise, we generate one on the fly, which only
 works as long as the app is running, and won't work in a distributed environment.
 SECRET_KEY is used to sign cookies, so setting a new key effectively
 invalidates all existing sessions.
+
+
+## Service Provider (SP) Entity ID and ACS URL
+
+There are two ways to declare your SP entity-id and acs-url. With both of
+these, the `X-Forwarded-` headers listed are crucial.
+
+### By inference
+
+With this the saml proxy will make assumptions that the request URL and proxy
+path are registered as an SP.
+
+If host https://example.com has the following proxy config...
+
+```
+location /saml/ {
+    proxy_set_header Host $http_host;
+    proxy_set_header X-Forwarded-Proto https;
+    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+    proxy_set_header X-Forwarded-Prefix /saml/;
+    proxy_pass http://saml:5000/;
+  }
+```
+
+Then the SP entity-id to register is `https://example.com/saml` and the ACS
+endpoint to register is `https://example.com/saml/login`.
+
+### Explicitly
+
+You can also declare these items explicitly by passing them in as headers...
+
+```
+location /saml/ {
+    proxy_set_header Host $http_host;
+    proxy_set_header X-Forwarded-Proto https;
+    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+    proxy_set_header X-Forwarded-Prefix /saml/;
+    proxy_set_header X-Saml-Entity-Id https://samldemo.iamdev.s.uw.edu/saml;
+    proxy_set_header X-Saml-Acs /saml/login;
+    proxy_pass http://saml:5000/;
+}
+```
+
+You typically won't need to explicitly declare `X-Saml-Acs`. `X-Saml-Entity-Id`
+may need to be declared if for any reason the entity-id doesn't match with how
+we infer it, such as with an existing entity-id, or when multiple hosts
+share a single Entity ID.
diff --git a/app.py b/app.py
index bbff115..0e28ecd 100644
--- a/app.py
+++ b/app.py
@@ -1,13 +1,14 @@
 from flask import Flask, Response, request, session, abort, redirect
 import flask
-from werkzeug.contrib.fixers import ProxyFix
+from werkzeug.middleware.proxy_fix import ProxyFix
 import uw_saml2
 from urllib.parse import urljoin
 from datetime import timedelta
 import os
 import secrets
 app = Flask(__name__)
-app.wsgi_app = ProxyFix(app.wsgi_app, x_proto=1)
+app.wsgi_app = ProxyFix(app.wsgi_app, x_proto=1, x_prefix=1)
+POSTBACK_ROUTE = '/login'
 if os.environ.get('SECRET_KEY'):
     app.secret_key = os.environ['SECRET_KEY']
 else:
@@ -56,10 +57,13 @@ def status(group=None):
 
 def _saml_args():
     """Get entity_id and acs_url from request.headers."""
-    return {
-        'entity_id': request.headers['X-Saml-Entity-Id'],
-        'acs_url': urljoin(request.url_root, request.headers['X-Saml-Acs'])
-    }
+    entity_id = request.url_root[:-1]  # remove trailing slash
+    acs_url = urljoin(request.url_root, POSTBACK_ROUTE[1:])
+    if 'X-Saml-Entity-Id' in request.headers:
+        entity_id = request.headers['X-Saml-Entity-Id']
+    if 'X-Saml-Acs' in request.headers:
+        acs_url = urljoin(request.url_root, request.headers['X-Saml-Acs'])
+    return dict(entity_id=entity_id, acs_url=acs_url)
 
 
 @app.route('/login/')
@@ -75,6 +79,7 @@ def login_redirect(return_to=''):
     return_to - the path to redirect back to after authentication. This and
         the request.query_string are set on the SAML RelayState.
     """
+    app.logger.error(f'URL ROOT {request.url_root}')
     query_string = '?' + request.query_string.decode()
     if query_string == '?':
         query_string = ''
@@ -85,7 +90,7 @@ def login_redirect(return_to=''):
     return redirect(uw_saml2.login_redirect(return_to=return_to, **args))
 
 
-@app.route('/login', methods=['GET', 'POST'])
+@app.route(POSTBACK_ROUTE, methods=['GET', 'POST'])
 def login():
     """
     Process a SAML Response, and set the uwnetid and groups on the session.
diff --git a/test/nginx/server.conf b/test/nginx/server.conf
index 6b381c8..09c7ed2 100644
--- a/test/nginx/server.conf
+++ b/test/nginx/server.conf
@@ -28,10 +28,9 @@ server {
 
   location /saml/ {
     proxy_set_header Host $http_host;
-    proxy_set_header X-Forwarded-Proto $scheme;
+    proxy_set_header X-Forwarded-Proto https;
     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-    proxy_set_header X-Saml-Entity-Id https://samldemo.iamdev.s.uw.edu/saml;
-    proxy_set_header X-Saml-Acs /saml/login;
+    proxy_set_header X-Forwarded-Prefix /saml/;
     proxy_pass http://saml:5000/;
   }