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Fallback Certificate

Status: Draft

Contour supports virtual host based routing over TLS and utilizes SNI which allows multiple fqdn's to be used on the same network endpoint. Unfortunately, some requests are sent and do not have the SNI server name set. When this happens, the request fails since the request does not match any routing rules applied to Envoy.

This design doc looks to enable a fallback certificate, such that when a request is received at Envoy, it will still route to the proper set of endpoints even though standard SNI logic isn't applied.

Goals

  • Allow fallback certificate to be defined
  • Enable the fallback cert only for specific vhosts

Non Goals

  • Allowing multiple default certs to be defined

Background

Contour provides virtual host based routing, so that the TLS request is routed to the appropriate service, based on the hostname specified in the HTTPS’s HOST header. As HOST Header is encrypted during TLS handshake, it can’t be used for virtual host based routing unless client sends HTTPS request specifying hostname using the SNI or the request is first decrypted using a default TLS certificate. Some users need to support clients which may not be using SNI. When an HTTPS request is received, Envoy needs to first decrypt the request using a default TLS certificate and then based on the HOST header, route it to appropriate service. As of now, Contour only provides certificate at virtual host level in an HTTPProxy and there is no way to define a default TLS certificate in Contour.

High-Level Design

Contour will add a new argument to contour serve named --fallback-certificate which references a secret which is namespaced with a name (e.g. namespace/name). This same configuration will be available in the Contour configuration file in the tls.fallback-certificate location. Secondly, a new field will be added to the HTTPProxy.Spec.VirtualHost.TLS named EnableFallbackCertificate to allow virtual hosts to opt into this functionality. This last point is important as by default, all vhosts will not be enabled for this feature.

Detailed Design

Envoy API

Contour defines FilterChainMatches (https://www.envoyproxy.io/docs/envoy/v1.14.1/api-v2/api/v2/listener/listener_components.proto.html?highlight=filterchainmatch#listener-filterchainmatch) on SNI names which allows for a single Envoy listener to proxy multiple vhosts over TLS. This feature will add a new filter chain match on TransportProtocol: tls which will match any request which is TLS but does not match a pre-configured SNI defined in the previous step. Envoy processes FilterChainMataches with SNI matches before transport protocol.

Next this catch-all filter chain takes a route_config_name reference in the envoy.http_connection_manager. For all non-http requests, an Envoy RDS config named ingress_http is configured with all the routes. For each virtual host that has enabled the EnableFallbackCertificate flag a new RDS route table will be created which will contain all the routes for vhosts which have opted into the fallback certificate.

Example fallback route:

{
     "version_info": "2",
     "route_config": {
      "@type": "type.googleapis.com/envoy.api.v2.RouteConfiguration",
      "name": "ingress_fallback",
      "virtual_hosts": [
       {
        "name": "containersteve.com",
        "domains": [
         "containersteve.com",
         "containersteve.com:*"
        ],
        "routes": [
         {
          "match": {
           "prefix": "/"
          },
          "redirect": {
           "https_redirect": true
          }
         }
        ]
       },
       {
        "name": "demo.projectcontour.io",
        "domains": [
         "demo.projectcontour.io",
         "demo.projectcontour.io:*"
        ],
        "routes": [
         {
          "match": {
           "prefix": "/secure"
          },
          "redirect": {
           "https_redirect": true
          }
         }
        ]
       }
      ],
     "last_updated": "2020-04-22T17:25:41.290Z"
    }
   ]
  }

New catch-all filter chain

 &envoy_api_v2_listener.FilterChain{
    Filters: filters,
     FilterChainMatch: &envoy_api_v2_listener.FilterChainMatch{
         TransportProtocol: "tls",
     },
 }

Contour API

The new argument to contour serve must be in the format of namespace/name.

The HTTPProxy spec will add a new field named EnableFallbackCertificate and will default to false:

type TLS struct {
	// required, the name of a secret in the current namespace
	SecretName string `json:"secretName,omitempty"`
	// Minimum TLS version this vhost should negotiate
	// +optional
	MinimumProtocolVersion string `json:"minimumProtocolVersion,omitempty"`
	// If Passthrough is set to true, the SecretName will be ignored
	// and the encrypted handshake will be passed through to the
	// backing cluster.
	// +optional
	Passthrough bool `json:"passthrough,omitempty"`
    
    // +optional
    EnableFallbackCertificate bool `json:"enableFallbackCertificate,omitempty""`
}