[WIP] Added DHCPv4 server (#178)

* Added DHCPv4 server
* Added more modifiers
* Fixed some bugs

1 files changed, 154 insertions, 0 deletions

diff --git a/dhcpv4/server.go b/dhcpv4/server.go
new file mode 100644
index 0000000..7db4abe
--- /dev/null
+++ b/dhcpv4/server.go
@@ -0,0 +1,154 @@
+package dhcpv4
+
+import (
+	"fmt"
+	"log"
+	"net"
+	"sync"
+	"time"
+)
+
+/*
+  To use the DHCPv4 server code you have to call NewServer with two arguments:
+  - a handler function, that will be called every time a valid DHCPv4 packet is
+    received, and
+  - an address to listen on.
+
+  The handler is a function that takes as input a packet connection, that can be
+  used to reply to the client; a peer address, that identifies the client sending
+  the request, and the DHCPv4 packet itself. Just implement your custom logic in
+  the handler.
+
+  The address to listen on is used to know IP address, port and optionally the
+  scope to create and UDP socket to listen on for DHCPv4 traffic.
+
+  Example program:
+
+
+package main
+
+import (
+	"log"
+	"net"
+
+	"github.com/insomniacslk/dhcp/dhcpv4"
+)
+
+func handler(conn net.PacketConn, peer net.Addr, m dhcpv4.DHCPv4) {
+	// this function will just print the received DHCPv4 message, without replying
+	log.Print(m.Summary())
+}
+
+func main() {
+	laddr := net.UDPAddr{
+		IP:   net.ParseIP("127.0.0.1"),
+		Port: 67,
+	}
+	server := dhcpv4.NewServer(laddr, handler)
+
+	defer server.Close()
+	if err := server.ActivateAndServe(); err != nil {
+		log.Panic(err)
+	}
+}
+
+*/
+
+// Handler is a type that defines the handler function to be called every time a
+// valid DHCPv4 message is received
+type Handler func(conn net.PacketConn, peer net.Addr, m *DHCPv4)
+
+// Server represents a DHCPv4 server object
+type Server struct {
+	conn       net.PacketConn
+	connMutex  sync.Mutex
+	shouldStop chan bool
+	Handler    Handler
+	localAddr  net.UDPAddr
+}
+
+// LocalAddr returns the local address of the listening socket, or nil if not
+// listening
+func (s *Server) LocalAddr() net.Addr {
+	s.connMutex.Lock()
+	defer s.connMutex.Unlock()
+	if s.conn == nil {
+		return nil
+	}
+	return s.conn.LocalAddr()
+}
+
+// ActivateAndServe starts the DHCPv4 server
+func (s *Server) ActivateAndServe() error {
+	s.connMutex.Lock()
+	if s.conn == nil {
+		conn, err := net.ListenUDP("udp4", &s.localAddr)
+		if err != nil {
+			s.connMutex.Unlock()
+			return err
+		}
+		s.conn = conn
+	}
+	s.connMutex.Unlock()
+	defer s.Close()
+	var (
+		pc *net.UDPConn
+		ok bool
+	)
+	if pc, ok = s.conn.(*net.UDPConn); !ok {
+		return fmt.Errorf("Error: not an UDPConn")
+	}
+	if pc == nil {
+		return fmt.Errorf("ActivateAndServe: Invalid nil PacketConn")
+	}
+	log.Printf("Server listening on %s", pc.LocalAddr())
+	log.Print("Ready to handle requests")
+	for {
+		select {
+		case <-s.shouldStop:
+			break
+		case <-time.After(time.Millisecond):
+		}
+		pc.SetReadDeadline(time.Now().Add(time.Second))
+		rbuf := make([]byte, 4096) // FIXME this is bad
+		n, peer, err := pc.ReadFrom(rbuf)
+		if err != nil {
+			switch err.(type) {
+			case net.Error:
+				// silently skip and continue
+			default:
+				// complain and continue
+				log.Printf("Error reading from packet conn: %v", err)
+			}
+			continue
+		}
+		log.Printf("Handling request from %v", peer)
+		m, err := FromBytes(rbuf[:n])
+		if err != nil {
+			log.Printf("Error parsing DHCPv4 request: %v", err)
+			continue
+		}
+		s.Handler(pc, peer, m)
+	}
+	return nil
+}
+
+// Close sends a termination request to the server, and closes the UDP listener
+func (s *Server) Close() error {
+	s.shouldStop <- true
+	s.connMutex.Lock()
+	defer s.connMutex.Unlock()
+	if s.conn != nil {
+		return s.conn.Close()
+	}
+	return nil
+}
+
+// NewServer initializes and returns a new Server object
+func NewServer(addr net.UDPAddr, handler Handler) *Server {
+	return &Server{
+		localAddr:  addr,
+		Handler:    handler,
+		shouldStop: make(chan bool, 1),
+	}
+}