dhcp4: Don't use the non-binding DHCP Conn silently on linux.

pixiecore: Plumb NewSnooperConn into Server as an advanced option.
pixiecore/cli: Plumb DHCPNoBind to the CLI.

Now, by default, Pixiecore will bind() to the DHCP server port in a way
that shows up in `netstat` by default. If you want to enable the "coexist
with my existing DHCP server" mode, you need to explicitly pass
--dhcp-no-bind.

dhcp4/conn.go

@@ -26,7 +26,6 @@ import (
 // defined as a var so tests can override it.
 var (
 	dhcpClientPort = 68
-	platformConn   func(string) (conn, error)
 )
 
 // txType describes how a Packet should be sent on the wire.
@@ -68,13 +67,6 @@ type Conn struct {
 
 // NewConn creates a Conn bound to the given UDP ip:port.
 func NewConn(addr string) (*Conn, error) {
-	if platformConn != nil {
-		c, err := platformConn(addr)
-		if err == nil {
-			return &Conn{c}, nil
-		}
-	}
-	// Always try falling back to the portable implementation
 	c, err := newPortableConn(addr)
 	if err != nil {
 		return nil, err

dhcp4/conn_linux.go

@@ -36,8 +36,16 @@ type linuxConn struct {
 	conn *ipv4.RawConn
 }
 
-func init() {
-	platformConn = newLinuxConn
+// NewSnooperConn creates a Conn that listens on the given UDP ip:port.
+//
+// Unlike NewConn, NewSnooperConn does not bind to the ip:port,
+// enabling the Conn to coexist with other services on the machine.
+func NewSnooperConn(addr string) (*Conn, error) {
+	c, err := newLinuxConn(addr)
+	if err != nil {
+		return nil, err
+	}
+	return &Conn{c}, nil
 }
 
 func newLinuxConn(addr string) (conn, error) {

dhcp4/conn_unsupported.go

@@ -0,0 +1,27 @@
+// Copyright 2016 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//+build !linux
+
+package dhcp4
+
+import "errors"
+
+// NewSnooperConn creates a Conn that listens on the given UDP ip:port.
+//
+// Unlike NewConn, NewSnooperConn does not bind to the ip:port,
+// enabling the Conn to coexist with other services on the machine.
+func NewSnooperConn(addr string) (*Conn, error) {
+	return nil, errors.New("snooper Conns not supported on this OS")
+}

pixiecore/cli/cli.go

@@ -73,9 +73,10 @@ func serverConfigFlags(cmd *cobra.Command) {
 	cmd.Flags().BoolP("log-timestamps", "t", false, "Add a timestamp to each log line")
 	cmd.Flags().IPP("listen-addr", "l", net.IPv4zero, "IPv4 address to listen on")
 	cmd.Flags().IntP("port", "p", 80, "Port to listen on for HTTP")
-	cmd.Flags().String("ipxe-bios", "", "path to an iPXE binary for BIOS/UNDI")
-	cmd.Flags().String("ipxe-efi32", "", "path to an iPXE binary for 32-bit UEFI")
-	cmd.Flags().String("ipxe-efi64", "", "path to an iPXE binary for 64-bit UEFI")
+	cmd.Flags().Bool("dhcp-no-bind", false, "Handle DHCP traffic without binding to the DHCP server port")
+	cmd.Flags().String("ipxe-bios", "", "Path to an iPXE binary for BIOS/UNDI")
+	cmd.Flags().String("ipxe-efi32", "", "Path to an iPXE binary for 32-bit UEFI")
+	cmd.Flags().String("ipxe-efi64", "", "Path to an iPXE binary for 64-bit UEFI")
 }
 
 func mustFile(path string) []byte {
@@ -104,6 +105,10 @@ func serverFromFlags(cmd *cobra.Command) *pixiecore.Server {
 	if err != nil {
 		fatalf("Error reading flag: %s", err)
 	}
+	dhcpNoBind, err := cmd.Flags().GetBool("dhcp-no-bind")
+	if err != nil {
+		fatalf("Error reading flag: %s", err)
+	}
 	ipxeBios, err := cmd.Flags().GetString("ipxe-bios")
 	if err != nil {
 		fatalf("Error reading flag: %s", err)
@@ -125,9 +130,10 @@ func serverFromFlags(cmd *cobra.Command) *pixiecore.Server {
 	}
 
 	ret := &pixiecore.Server{
-		Ipxe:     map[pixiecore.Firmware][]byte{},
-		Log:      logWithStdFmt,
-		HTTPPort: httpPort,
+		Ipxe:       map[pixiecore.Firmware][]byte{},
+		Log:        logWithStdFmt,
+		HTTPPort:   httpPort,
+		DHCPNoBind: dhcpNoBind,
 	}
 	for fwtype, bs := range Ipxe {
 		ret.Ipxe[fwtype] = bs

pixiecore/pixiecore.go

@@ -154,6 +154,12 @@ type Server struct {
 	TFTPPort int
 	PXEPort  int
 
+	// Listen for DHCP traffic without binding to the DHCP port. This
+	// enables coexistence of Pixiecore with another DHCP server.
+	//
+	// Currently only supported on Linux.
+	DHCPNoBind bool
+
 	errs chan error
 }
 
@@ -173,7 +179,12 @@ func (s *Server) Serve() error {
 		s.HTTPPort = portHTTP
 	}
 
-	dhcp, err := dhcp4.NewConn(fmt.Sprintf("%s:%d", s.Address, s.DHCPPort))
+	newDHCP := dhcp4.NewConn
+	if s.DHCPNoBind {
+		newDHCP = dhcp4.NewSnooperConn
+	}
+
+	dhcp, err := newDHCP(fmt.Sprintf("%s:%d", s.Address, s.DHCPPort))
 	if err != nil {
 		return err
 	}