Get subject DN from clients certificate in Go gRPC handler

Question

I'm using Golang gRPC with mutual tls. Is it possible to get client's certificate subject DN from rpc method?

// ...
func main() {
    // ...
    creds := credentials.NewTLS(&tls.Config{
        ClientAuth:   tls.RequireAndVerifyClientCert,
        Certificates: []tls.Certificate{certificate},
        ClientCAs:    certPool,
        MinVersion:   tsl.VersionTLS12,
    })
    s := NewMyService()
    gs := grpc.NewServer(grpc.Creds(creds))
    RegisterGRPCZmqProxyServer(gs, s)
    er := gs.Serve(lis)
    // ...
}

// ...
func (s *myService) Foo(ctx context.Context, req *FooRequest) (*FooResonse, error) {
    $dn := // What should be here?
    // ...
}

Is it possible?

Answer 1

You can use peer.Peer from ctx context.Context for access to the OID registry in x509.Certificate.

func (s *myService) Foo(ctx context.Context, req *FooRequest) (*FooResonse, error) {
    p, ok := peer.FromContext(ctx)
        if ok {
            tlsInfo := p.AuthInfo.(credentials.TLSInfo)
            subject := tlsInfo.State.VerifiedChains[0][0].Subject
            // do something ...
        }
}

Subject it is pkix.Name and in docs write:

Name represents an X.509 distinguished name

And I used code from this answer and it well work.