echo.go

package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	for scanner.Scan() {
		if scanner.Text() == "plz flag" {
			fmt.Println("flag{xxx}")
			return
		}
		fmt.Println(scanner.Text())
	}
	if err := scanner.Err(); err != nil {
		fmt.Fprintln(os.Stderr, "reading standard input:", err)
	}
}

echo_srv.go

package main

import (
	"bufio"
	"fmt"
	"log"
	"net"
	"strconv"
)

const PORT = 12345

func main() {
	server, err := net.Listen("tcp", ":"+strconv.Itoa(PORT))
	if server == nil {
		log.Fatal(err)
	}
	conns := clientConns(server)
	for {
		go handleConn(<-conns)
	}
}

func clientConns(listener net.Listener) chan net.Conn {
	ch := make(chan net.Conn)
	i := 0
	go func() {
		for {
			client, err := listener.Accept()
			if client == nil {
				log.Fatal(err)
				continue
			}
			i++
			fmt.Printf("%d: %v <-> %v\n", i, client.LocalAddr(), client.RemoteAddr())
			ch <- client
		}
	}()
	return ch
}

func handleConn(client net.Conn) {
	flag := []byte("flag{xxx}")
	b := bufio.NewReader(client)
	for {
		line, err := b.ReadBytes('\n')
		if string(line) == "plz flag\n" {
			client.Write(flag)
			continue
		}
		if err != nil { // EOF, or worse
			break
		}
		client.Write(line)
	}
}