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go build

Generate CA, server, and client certs

./grpc-authenticated-greeter gencerts

Start the server on port 7676:

./grpc-authenticated-greeter server \
  --ca ca_cert.pem --cert server_cert.pem --key server_key.pem \
  --binding :7676

In another terminal, run a client:

./grpc-authenticated-greeter client \
  --ca ca_cert.pem --cert client1_cert.pem --key client1_key.pem \
  --serveraddress 127.0.0.1:7676 --servername server \
  --message "Read me"

The client should log the response from the server, such as:

INFO[0000] got response   response="Hello, client1. You said 'Read me'"

Follow these instructions to install protoc and the grpc plugin.

Run the following to regenerate the *.pb.go file, protocol/HelloService.pb.go

go generate ./...

• go-arg is very cool! Just feed it a struct and it'll parse command line arguments. It's very flexible and intuitive. Even embedding common arguments into a "command struct" did what I expected

type ClientServerArgs struct {
	Ca   string `arg:"required" help:"PEM file containing the CA cert shared by server and clients"`
	Key  string `arg:"required" help:"PEM file containing private key"`
	Cert string `arg:"required" help:"PEM file containing public certificate"`
}

type ServerCmd struct {
	ClientServerArgs
	Binding string `arg:"required" help:"host:port of server binding where host is optional"`
}

• Generating a self-signed CA cert and then signing client and server certs is quite doable in Go. Check out certs/generate.go to see how that's done
• The authenticating client info is a little bit buried when implementing a server handler, but peer.FromContext was the key to cracking that open. server/server.go is where that is used.
• To do full mTLS authentication, be sure to configure the server's TLS to require and verify the client cert:

	transportCreds := credentials.NewTLS(&tls.Config{
		Certificates: []tls.Certificate{cert},
		ClientAuth:   tls.RequireAndVerifyClientCert,
		ClientCAs:    clientCertPool,
	})

• The Go package can be configured for the code generated from the protobuf declarations. The following in protocol/HelloService.proto avoids the awkwardness of a package named for the proto file:

option go_package = "protocol";