TCP Proxying and Protocol Detection

Linkerd is capable of proxying all TCP traffic, including TLS’d connections, WebSockets, and HTTP tunneling.

Linkerd performs protocol detection to determine whether traffic is HTTP or HTTP/2 (including gRPC). If Linkerd detects that a connection is using HTTP or HTTP/2, Linkerd will automatically provide HTTP-level metrics and routing without configuration from the user. (See HTTP, HTTP/2, and gRPC Proxying for more.)

If Linkerd cannot determine that a connection is using HTTP or HTTP/2, Linkerd will proxy the connection, but will only be able to provide byte-level metrics. Note that this also applies to TLS’d HTTP connections if the application initiates the TLS, as Linkerd will not be able to observe the HTTP transactions in this connection.

Configuring Protocol Detection

In some cases Linkerd’s protocol detection requires configuration. Currently, this is required for unencrypted “server-speaks-first” protocols, or protocols where the server sends data before the client sends data. In these cases, Linkerd cannot automatically recognize the protocol used on the connection. (Note that TLS-enabled connections work as normal, because TLS itself is a client-speaks-first protocol.)

The following protocols are known to be server-speaks-first:

  • 25 - SMTP
  • 3306 - MySQL
  • 8086 - InfluxDB
  • 4222 - NATS
  • 27017 - MongoDB

If you’re working with a protocol that can’t be automatically recognized by Linkerd, use the --skip-inbound-ports and --skip-outbound-ports flags when running linkerd inject.

For example, if your application makes requests to a MySQL database running on port 4406, use the command:

linkerd inject deployment.yml --skip-outbound-ports=4406 \
  | kubectl apply -f -

Likewise if your application runs an SMTP server that accepts incoming requests on port 35, use the command:

linkerd inject deployment.yml --skip-inbound-ports=35 \
  | kubectl apply -f -

Plaintext MySQL and SMTP

For MySQL and SMTP, if you are using Linkerd to proxy plaintext connections on their default ports (3306 and 25, respectively), then Linkerd will currently identify these protocols based on the port, and will not attempt to perform protocol detection. Thus, no extra configuration is necessary for plaintext MySQL and SMTP connections.