Asynchronous networking @ GopherCon 2018

An introduction to the net package, livecoding a TLS-aware TCP proxy.

Read the code! → https://github.com/FiloSottile/mostly-harmless/tree/master/talks/asyncnet

Transcript

1. ASYNCHRONOUS NETWORKING GOPHERCON, DENVER — 28 AUGUST 2018 Filippo Valsorda Google @FiloSottile
2. Threading servers Thread Request 2 Request 1 Thread Thread Request 3
3. The event loop Request 1 Thread Request 1 Request 2 Request 3 Request 3 Request 2
4. The event loop Request 1 Thread Request 1 Request 2 Request 3 Request 3 Pop from event queue Monitor for events Request 2
5. Goroutines go Request 2 Request 1 go Request 1 go Request 3 Block Block Request 3 Request 2 Block
6. Accepting connections SECTION ONE
7. Let's write code!
9. ACCEPTING CONNECTIONS — RECAP 01. ACCEPTING CONNECTIONS l, err := net.Listen("tcp", "localhost:4242") if err != nil { log.Fatal(err) } Open a net.Listener. Never block in the Accept loop. for { conn, err := l.Accept() if err != nil { log.Fatal(err) } go serviceConn(conn) } Call Accept in a loop, and start new goroutines for each Conn.
10. A simple proxy SECTION TWO
11. Proxying a connection 02. A SIMPLE PROXY Proxy gophercon.com:443 localhost:4242 Browser
12. "splice(2) moves data between two file descriptors without copying between kernel address space and user address space."
13. A SIMPLE PROXY — RECAP 02. A SIMPLE PROXY upstream, err := net.Dial("tcp", addr) if err != nil { log.Println(err) return } defer upstream.Close() Connect to upstream. go io.Copy(upstream, conn) _, err = io.Copy(conn, upstream) Copy both directions. Close cancels both io.Copy.
14. Parsing TLS SECTION THREE
19. PARSING A TLS CLIENT HELLO — RECAP 03. PARSING TLS conn.SetDeadline(time.Now().Add(30 * time.Second)) var buf bytes.Buffer if _, err := io.CopyN(&buf, conn, 1+2+2); err != nil { return } length := binary.BigEndian.Uint16(buf.Bytes()[3:5]) if _, err := io.CopyN(&buf, conn, int64(length)); err != nil { return } Set a timeout and
 read the message. ch, ok := ParseClientHello(buf.Bytes()) if ok { log.Printf("Received connection for %q!", ch.SNI) } Parse TLS.
20. Pre-read connection 03. PARSING TLS Client Hello Already Read() up to here rest of the connection Would only proxy this part In bytes.Buffer
22. PROXYING A PRE-READ CONNECTION — RECAP 03. PARSING TLS type prefixConn struct { io.Reader net.Conn } func (c prefixConn) Read(b []byte) (int, error) { return c.Reader.Read(b) } Make a net.Conn wrapper. proxyConn(prefixConn{ Reader: io.MultiReader(&buf, conn), Conn: conn, }, "gophercon.com:https") Use MultiReader.
23. Serving TLS SECTION FOUR
24. ! ! tls.Conn is a net.Conn wrapper 04. SERVING TLS
26. SERVING A TLS CONNECTION — RECAP 04. SERVING TLS c := tls.Server(prefixConn{ Reader: io.MultiReader(&buf, conn), Conn: conn, }, config) Wrap the net.Conn with tls.Server. proxyConn(c, "gophercon.com:http") Service the wrapped plaintext connection. tls.Conn is a net.Conn wrapper. It takes care of handshake and de/encryption.
27. Thank you! https://git.io/fAYGy Filippo Valsorda Google @FiloSottile