Experimenting with YARP: A Reverse Proxy

YARP is a reverse proxy toolkit for building fast proxy servers in .NET using the infrastructure from ASP.NET and .NET.

Published on 16 September 2020

A Route is the inbound URL which the reverse proxy is going to act on. The cluster is a list of potential destination URLs.

 "ReverseProxy": {
    "Routes": [
      {
        "RouteId": "LoginServiceRoute",
        "ClusterId": "clusterLoginService",
        "Match": {
          "Path": "/loginservice/{*remainder}"
        },
        "Transforms": [
          {
            "PathRemovePrefix": "/loginservice"
          }
        ]
      }
    ],
    "Clusters": {
      "clusterLoginService": {
        "Destinations": {
          "clusterLoginService/destination1": {
            "Address": "https://localhost:1116/"
          }
        }
      }
    }
  }

This would be similar to this NGINX virtual server configuration.

server {
    listen 80;
    server_name localhost;

    location /loginservice/ {
        proxy_pass              https://localhost:1116;
    }
  }

Take care when adding a transformation to a route, do not add a single transformation, it must be wrapped in [] or you will get no transformations and lots of confusion.

My mistake looked like this

{
  "RouteId": "TestServiceRoute",
  "ClusterId": "clusterTestService",
  "Match": {
    "Path": "/testservice/{*remainder}"
  },
  "Transforms": 
  {
    "PathRemovePrefix": "/testservice"
  }
},

It took debugging into the YARP source to figure out my mistake which resulted in 503 and 404 errors due to URLs like https://localhost:1116/loginservice/hc instead of the correct https://localhost:1116/hc.

{
  "RouteId": "TestServiceRoute",
  "ClusterId": "clusterTestService",
  "Match": {
    "Path": "/testservice/{*remainder}"
  },
  "Transforms":  [ // <-- was missing
    {
      "PathRemovePrefix": "/testservice"
    }
  ] // <-- was missing
},

It is easier to see the issue as soon as you start adding additional transformations. In preview 5 for example it is possible to transform route values to querystring parameters, now it is clear that Transformations must be an array.

      {
        "RouteId": "TestPatternServiceRoute",
        "ClusterId": "clusterTestService",
        "Match": {
          "Path": "/testpatternservice/{*remainder}"
        },
        "Transforms": [
          {
            "PathPattern": "/search/"
          },
          {
            "QueryRouteParameter": "q",
            "Append": "remainder"
          }
        ]
      },