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Use syscall.SO_BINDTODEVICE to only bind on specific interface #31

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Use syscall.SO_BINDTODEVICE to only bind on specific interface #31

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43 changes: 35 additions & 8 deletions serverif.go
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Original file line number Diff line number Diff line change
Expand Up @@ -14,9 +14,6 @@ type serveIfConn struct {

func (s *serveIfConn) ReadFrom(b []byte) (n int, addr net.Addr, err error) {
n, s.cm, addr, err = s.conn.ReadFrom(b)
if s.cm != nil && s.cm.IfIndex != s.ifIndex { // Filter all other interfaces
n = 0 // Packets < 240 are filtered in Serve().
}
return
}

Expand All @@ -38,8 +35,7 @@ func (s *serveIfConn) WriteTo(b []byte, addr net.Addr) (n int, err error) {
// import outside the std library. Serving DHCP over multiple interfaces will
// require your own dhcp4.ServeConn, as listening to broadcasts utilises all
// interfaces (so you cannot have more than on listener).
func ServeIf(ifIndex int, conn net.PacketConn, handler Handler) error {
p := ipv4.NewPacketConn(conn)
func ServeIf(ifIndex int, p *ipv4.PacketConn, handler Handler) error {
if err := p.SetControlMessage(ipv4.FlagInterface, true); err != nil {
return err
}
Expand All @@ -54,10 +50,41 @@ func ListenAndServeIf(interfaceName string, handler Handler) error {
if err != nil {
return err
}
l, err := net.ListenPacket("udp4", ":67")
ip := net.ParseIP("0.0.0.0")
p, err := broadcastOpen(ip, 67, interfaceName)
if err != nil {
return err
}
defer l.Close()
return ServeIf(iface.Index, l, handler)
defer p.Close()
return ServeIf(iface.Index, p, handler)
}

func broadcastOpen(bindAddr net.IP, port int, ifname string) (*ipv4.PacketConn, error) {
s, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_DGRAM, syscall.IPPROTO_UDP)
if err != nil {
log.Fatal(err)
}
if err := syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1); err != nil {
log.Fatal(err)
}
if err := syscall.SetsockoptString(s, syscall.SOL_SOCKET, syscall.SO_BINDTODEVICE, ifname); err != nil {
log.Fatal(err)
}

lsa := syscall.SockaddrInet4{Port: port}
copy(lsa.Addr[:], bindAddr.To4())

if err := syscall.Bind(s, &lsa); err != nil {
syscall.Close(s)
log.Fatal(err)
}
f := os.NewFile(uintptr(s), "")
c, err := net.FilePacketConn(f)
f.Close()
if err != nil {
log.Fatal(err)
}
p := ipv4.NewPacketConn(c)

return p, nil
}