Custom Error Pages (#1675)

* custom error pages

docs/basics.md

@@ -396,6 +396,39 @@ Here is an example of backends and servers definition:
 - `backend2` will forward the traffic to two servers: `http://172.17.0.4:80"` with weight `1` and `http://172.17.0.5:80` with weight `2` using `drr` load-balancing strategy.
 - a circuit breaker is added on `backend1` using the expression `NetworkErrorRatio() > 0.5`: watch error ratio over 10 second sliding window
 
+## Custom Error pages
+
+Custom error pages can be returned, in lieu of the default, according to frontend-configured ranges of HTTP Status codes.
+In the example below, if a 503 status is returned from the frontend "website", the custom error page at http://2.3.4.5/503.html is returned with the actual status code set in the HTTP header.
+Note, the 503.html page itself is not hosted on traefik, but some other infrastructure.   
+
+```toml
+[frontends]
+  [frontends.website]
+  backend = "website"
+  [errors]
+    [error.network]
+    status = ["500-599"]
+    backend = "error"
+    query = "/{status}.html"
+  [frontends.website.routes.website]
+  rule = "Host: website.mydomain.com"
+
+[backends]
+  [backends.website]
+    [backends.website.servers.website]
+    url = "https://1.2.3.4"
+  [backends.error]
+    [backends.error.servers.error]
+    url = "http://2.3.4.5"
+```
+
+In the above example, the error page rendered was based on the status code.
+Instead, the query parameter can also be set to some generic error page like so: `query = "/500s.html"`
+
+Now the 500s.html error page is returned for the configured code range.
+The configured status code ranges are inclusive; that is, in the above example, the 500s.html page will be returned for status codes 500 through, and including, 599.
+
 # Configuration
 
 Træfik's configuration has two parts: