server6: Create UDP conn manually for more control

Similar to server4 where the UDP connection is manually created using
the socket interfaces, this creates a connection with adequate options:

 * SO_BINDTODEVICE or equivalent if an interface is requested
 * V6ONLY when supported by the operating system
 * Allows binding to a multicast address specifically instead of
   falling back to wildcard

Signed-off-by: Anatole Denis <natolumin@unverle.fr>

1 files changed, 62 insertions, 0 deletions

diff --git a/dhcpv6/server6/conn.go b/dhcpv6/server6/conn.go
new file mode 100644
index 0000000..08c54e8
--- /dev/null
+++ b/dhcpv6/server6/conn.go
@@ -0,0 +1,62 @@
+package server6
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"os"
+
+	"github.com/insomniacslk/dhcp/interfaces"
+	"golang.org/x/sys/unix"
+)
+
+// NewIPv6UDPConn returns a UDPv6-only connection bound to both the interface and port
+// given based on a IPv6 DGRAM socket.
+// As a bonus, you can actually listen on a multicast address instead of being punted to the wildcard
+//
+// The interface must already be configured.
+func NewIPv6UDPConn(iface string, addr *net.UDPAddr) (net.PacketConn, error) {
+	fd, err := unix.Socket(unix.AF_INET6, unix.SOCK_DGRAM, unix.IPPROTO_UDP)
+	if err != nil {
+		return nil, fmt.Errorf("cannot get a UDP socket: %v", err)
+	}
+	f := os.NewFile(uintptr(fd), "")
+	// net.FilePacketConn dups the FD, so we have to close this in any case.
+	defer f.Close()
+
+	// Allow broadcasting.
+	if err := unix.SetsockoptInt(fd, unix.IPPROTO_IPV6, unix.IPV6_V6ONLY, 1); err != nil {
+		if errno, ok := err.(unix.Errno); !ok {
+			return nil, fmt.Errorf("unexpected socket error: %v", err)
+		} else if errno != unix.ENOPROTOOPT { // Unsupported on some OSes (but in that case v6only is default), so we ignore ENOPROTOOPT
+			return nil, fmt.Errorf("cannot bind socket v6only %v", err)
+		}
+	}
+	// Allow reusing the addr to aid debugging.
+	if err := unix.SetsockoptInt(fd, unix.SOL_SOCKET, unix.SO_REUSEADDR, 1); err != nil {
+		return nil, fmt.Errorf("cannot set reuseaddr on socket: %v", err)
+	}
+	if len(iface) != 0 {
+		// Bind directly to the interface.
+		if err := interfaces.BindToInterface(fd, iface); err != nil {
+			if errno, ok := err.(unix.Errno); ok && errno == unix.EACCES {
+				// Return a more helpful error message in this (fairly common) case
+				return nil, errors.New("Cannot bind to interface without CAP_NET_RAW or root permissions. " +
+					"Restart with elevated privilege, or run without specifying an interface to bind to all available interfaces.")
+			}
+			return nil, fmt.Errorf("cannot bind to interface %s: %v", iface, err)
+		}
+	}
+
+	if addr == nil {
+		return nil, errors.New("An address to listen on needs to be specified")
+	}
+	// Bind to the port.
+	saddr := unix.SockaddrInet6{Port: addr.Port}
+	copy(saddr.Addr[:], addr.IP)
+	if err := unix.Bind(fd, &saddr); err != nil {
+		return nil, fmt.Errorf("cannot bind to address %v: %v", addr, err)
+	}
+
+	return net.FilePacketConn(f)
+}