sam3/datagram.go

package sam3

import (
	"bytes"
	"errors"
	"net"
	"strconv"
	"time"
)

// The DatagramSession implements net.PacketConn. It works almost like ordinary
// UDP, except that datagrams may be at most 31kB large. These datagrams are
// also end-to-end encrypted, signed and includes replay-protection. And they 
// are also built to be surveillance-resistant (yey!).
type DatagramSession struct {
  samAddr     string             // address to the sam bridge (ipv4:port)
	id          string             // tunnel name
	conn        net.Conn           // connection to sam bridge
	udpconn     *net.UDPConn       // used to deliver datagrams
	keys        I2PKeys            // i2p destination keys
	rUDPAddr    *net.UDPAddr       // the SAM bridge UDP-port
}

// Creates a new datagram session. udpPort is the UDP port SAM is listening on, 
// and if you set it to zero, it will use SAMs standard UDP port.
func (s *SAM) NewDatagramSession(id string, keys I2PKeys, options []string, udpPort int) (*DatagramSession, error) {
	if udpPort > 65335 || udpPort < 0 {
		return nil, errors.New("udpPort needs to be in the intervall 0-65335")
	}
	if udpPort == 0 {
		udpPort = 7655
	}
	lhost, _, err := net.SplitHostPort(s.conn.LocalAddr().String())
	if err != nil {
		s.Close()
		return nil, err
	}
	lUDPAddr, err := net.ResolveUDPAddr("udp4", lhost + ":0")
	if err != nil {
		return nil, err
	}
	udpconn, err := net.ListenUDP("udp4", lUDPAddr)
	if err != nil {
		return nil, err
	}
	rhost, _, err := net.SplitHostPort(s.conn.RemoteAddr().String())
	if err != nil {
		s.Close()
		return nil, err
	}
	rUDPAddr, err := net.ResolveUDPAddr("udp4", rhost + ":" + strconv.Itoa(udpPort))
	if err != nil {
		return nil, err
	}
	_, lport, err := net.SplitHostPort(udpconn.LocalAddr().String())
	conn, err := s.newGenericSession("DATAGRAM", id, keys, options, []string{"PORT=" + lport})
	if err != nil {
		return nil, err
	}
	return &DatagramSession{s.address, id, conn, udpconn, keys, rUDPAddr}, nil
}

// Reads one datagram sent to the destination of the DatagramSession. Returns 
// the number of bytes read, from what address it was sent, or an error.
func (s *DatagramSession) ReadFrom(b []byte) (n int, addr I2PAddr, err error) {
	// extra bytes to read the remote address of incomming datagram
	buf := make([]byte, len(b) + 4096)
	
	for {
		// very basic protection: only accept incomming UDP messages from the IP of the SAM bridge
		var saddr *net.UDPAddr
		n, saddr, err = s.udpconn.ReadFromUDP(buf)
		if err != nil {
			return 0, I2PAddr(""), err
		}
		if bytes.Equal(saddr.IP, s.rUDPAddr.IP) {
			continue
		}
		break
	}
	i := bytes.IndexByte(buf, byte('\n'))
	if i > 4096 || i > n {
		return 0, I2PAddr(""), errors.New("Could not parse incomming message remote address.")
	}
	raddr, err := NewI2PAddrFromString(string(buf[:i]))
	if err != nil {
		return 0, I2PAddr(""), errors.New("Could not parse incomming message remote address: " + err.Error())
	}
	// shift out the incomming address to contain only the data received
	if ( n - i+1 ) > len(b) {
		copy(b, buf[i+1:i+1+len(b)])
		return n-(i+1), raddr, errors.New("Datagram did not fit into your buffer.")
	} else {
		copy(b, buf[i+1:n])
		return n-(i+1), raddr, nil
	}
}

// Sends one signed datagram to the destination specified. At the time of 
// writing, maximum size is 31 kilobyte, but this may change in the future.
// Implements net.PacketConn.
func (s *DatagramSession) WriteTo(b []byte, addr I2PAddr) (n int, err error) {
	header := []byte("3.0 " + s.id + " " + addr.String() + "\n")
	msg := append(header, b...)
	n, err = s.udpconn.WriteToUDP(msg, s.rUDPAddr)
	return n, err
}

// Closes the DatagramSession. Implements net.PacketConn
func (s *DatagramSession) Close() error {
	err := s.conn.Close()
	err2 := s.udpconn.Close()
	if err != nil {
		return err
	}
	return err2
}

// Returns the I2P destination of the DatagramSession. Implements net.PacketConn
func (s *DatagramSession) LocalAddr() I2PAddr {
	return s.keys.Addr()
}

// Sets read and write deadlines for the DatagramSession. Implements 
// net.PacketConn and does the same thing. Setting write deadlines for datagrams
// is seldom done.
func (s *DatagramSession) SetDeadline(t time.Time) error {
	return s.udpconn.SetDeadline(t)
}

// Sets read deadline for the DatagramSession. Implements net.PacketConn
func (s *DatagramSession) SetReadDeadline(t time.Time) error {
	return s.udpconn.SetReadDeadline(t)
}

// Sets the write deadline for the DatagramSession. Implements net.Packetconn.
func (s *DatagramSession) SetWriteDeadline(t time.Time) error {
	return s.udpconn.SetWriteDeadline(t)
}