coredns/plugin/traffic/README.md

traffic

Name

traffic - handout addresses according to assignments from Envoy's xDS.

Description

The traffic plugin is a balancer that allows traffic steering, weighted responses and draining of clusters.

The cluster information is retrieved from a service discovery manager that implements the service discovery protocols from Envoy implements. It connects to the manager using the Aggregated Discovery Service (ADS) protocol. Endpoints and clusters are discovered every 10 seconds. The plugin hands out responses that adhere to these assignments. Only endpoints that are healthy are handed out.

If traffic's locality has been set the answers can be localized.

A cluster in Envoy is defined as: "A group of logically similar endpoints that Envoy connects to." Each cluster has a name, which traffic extends to be a domain name. See "Naming Clusters" below.

The use case for this plugin is when a cluster has endpoints running in multiple (Kubernetes?) clusters and you need to steer traffic to (or away) from these endpoints, i.e. endpoint A needs to be upgraded, so all traffic to it is drained. Or the entire Kubernetes needs to upgraded, and all endpoints need to be drained from it.

For A and AAAA queries each DNS response contains a single IP address that's considered the best one. The TTL on these answer is set to 5s. It will only return successful responses either with an answer or otherwise a NODATA response. Queries for non-existent clusters get a NXDOMAIN, where the minimal TTL is also set to 5s.

For SRV queries all healthy backends will be returned - assuming the client doing the query is smart enough to select the best one. When SRV records are returned, the endpoint DNS names are synthesized endpoint-<N>.<cluster>.<zone> that carries the IP address. Querying for these synthesized names works as well.

Load reporting is not supported for the following reason: A DNS query is done by a resolver. Behind this resolver (which can also cache) there may be many clients that will use this reply. The responding server (CoreDNS) has no idea how many clients use this resolver. So reporting a load of +1 on the CoreDNS side can results in anything from 1 to 1000+ of queries on the endpoint, making the load reporting from traffic highly inaccurate.

Traffic implements version 3 of the xDS API.

Syntax

traffic TO...

This enabled the traffic plugin, with a default node ID of coredns and no TLS.

• TO... are the control plane endpoints to connect to. These must start with grpc://. The port number defaults to 443, if not specified.

The extended syntax is available if you want more control.

traffic TO... {
    node ID
    locality REGION[,ZONE[,SUBZONE]] [REGION[,ZONE[,SUBZONE]]]...
    tls CERT KEY CA
    tls_servername NAME
    ignore_health
}

• node ID is how traffic identifies itself to the control plane. This defaults to coredns.

• locality has a list of REGION,ZONE,SUBZONE sets. These tell traffic where its running and what should be considered local traffic. Each REGION,ZONE,SUBZONE set will be used to match clusters against while generating responses. The list should descend in proximity. ZONE or ZONE and SUBZONE may be omitted. This signifies a wild card match. I.e. when there are 3 regions, US, EU, ASIA, and this CoreDNS is running in EU, you can use: locality EU US ASIA. Each list must be separated using spaces. The elements within a set should be separated with only a comma.

• tls CERT KEY CA define the TLS properties for gRPC connection. If this is omitted an insecure connection is attempted. From 0 to 3 arguments can be provided with the meaning as described below

  • tls - no client authentication is used, and the system CAs are used to verify the server certificate

  • tls CA - no client authentication is used, and the file CA is used to verify the server certificate

  • tls CERT KEY - client authentication is used with the specified cert/key pair. The server certificate is verified with the system CAs.

  • tls CERT KEY CA - client authentication is used with the specified cert/key pair. The server certificate is verified using the specified CA file.

• tls_servername NAME allows you to set a server name in the TLS configuration. This is needed because traffic connects to an IP address, so it can't infer the server name from it.

• ignore_health can be enabled to ignore endpoint health status, this can aid in debugging.

Naming Clusters

When a cluster is named this usually consists out of a single word, i.e. "cluster-v0", or "web". The traffic plugins uses the name(s) specified in the Server Block to create fully qualified domain names. For example if the Server Block specifies lb.example.org as one of the names, and "cluster-v0" is one of the load balanced cluster, traffic will respond to query asking for cluster-v0.lb.example.org. and the same goes for web; web.lb.example.org.

Localized Endpoints

Endpoints can be grouped by location, this location information is used if the locality property is used in the configuration.

Matching Algorithm

How are clients match against the data we receive from xDS endpoint? Ignoring locality for now, it will go through the following steps:

1. Does the cluster exist? If not return NXDOMAIN, otherwise continue.

2. Run through the endpoints, discard any endpoints that are not HEALTHY. If we are left with no endpoint return a NODATA response, otherwise continue.

3. If weights are assigned use those to pick an endpoint, otherwise randomly pick one and return a response to the client.

If locality has been specified there is an extra step between 2 and 3.

2a. Match the endpoints using the locality that groups several of them, it's the most specific match from left to right in the locality list; if no REGION,ZONE,SUBZONE matches then try REGION,ZONE and then REGION. If still not match, move on the to next one. If we found none, we continue with step 4 above, ignoring any locality.

Metrics

If monitoring is enabled (via the prometheus plugin) then the following metric are exported:

• coredns_traffic_cluster_tracked{} the number of tracked clusters.
• coredns_traffic_endpoint_tracked{} the number of tracked clusters.

Ready

This plugin report readiness to the ready plugin. This will happen after a gRPC stream has been established to the control plane.

Examples

lb.example.org {
    traffic grpc://127.0.0.1:18000 {
        node test-id
    }
    debug
    log
}

This will load balance any names under lb.example.org using the data from the manager running on localhost on port 18000. The node ID will be test-id and no TLS will be used.

Bugs

Priority and locality information from ClusterLoadAssignments is not used. Multiple TO addresses is not implemented. Credentials are not implemented.

TODO

Node may only be set on the first request.