server{4,6}: Return UDPConn from NewIPv*UDPConn

The concrete type under the interface is known here since we create the
connection in the same function. Since *net.UDPConn implements
net.PacketConn anyway, returning the concrete type here is more powerful
and less risky than having downstream users cast the value themselves

There should be no code change for downstream users, with the exception
of explicit casts (`udpc := conn.(*net.UDPConn)`), which can simply be
removed

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

2 files changed, 21 insertions, 4 deletions

diff --git a/dhcpv4/server4/conn.go b/dhcpv4/server4/conn.go
index 84d6aa3..0a4c73a 100644
--- a/dhcpv4/server4/conn.go
+++ b/dhcpv4/server4/conn.go
@@ -1,6 +1,7 @@
 package server4
 
 import (
+	"errors"
 	"fmt"
 	"net"
 	"os"
@@ -13,7 +14,7 @@ import (
 // given based on a IPv4 DGRAM socket. The UDP connection allows broadcasting.
 //
 // The interface must already be configured.
-func NewIPv4UDPConn(iface string, port int) (net.PacketConn, error) {
+func NewIPv4UDPConn(iface string, port int) (*net.UDPConn, error) {
 	fd, err := unix.Socket(unix.AF_INET, unix.SOCK_DGRAM, unix.IPPROTO_UDP)
 	if err != nil {
 		return nil, fmt.Errorf("cannot get a UDP socket: %v", err)
@@ -41,5 +42,13 @@ func NewIPv4UDPConn(iface string, port int) (net.PacketConn, error) {
 		return nil, fmt.Errorf("cannot bind to port %d: %v", port, err)
 	}
 
-	return net.FilePacketConn(f)
+	conn, err := net.FilePacketConn(f)
+	if err != nil {
+		return nil, err
+	}
+	udpconn, ok := conn.(*net.UDPConn)
+	if !ok {
+		return nil, errors.New("BUG(dhcp4): incorrect socket type, expected UDP")
+	}
+	return udpconn, nil
 }
diff --git a/dhcpv6/server6/conn.go b/dhcpv6/server6/conn.go
index 08c54e8..cbf72b4 100644
--- a/dhcpv6/server6/conn.go
+++ b/dhcpv6/server6/conn.go
@@ -15,7 +15,7 @@ import (
 // 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) {
+func NewIPv6UDPConn(iface string, addr *net.UDPAddr) (*net.UDPConn, 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)
@@ -58,5 +58,13 @@ func NewIPv6UDPConn(iface string, addr *net.UDPAddr) (net.PacketConn, error) {
 		return nil, fmt.Errorf("cannot bind to address %v: %v", addr, err)
 	}
 
-	return net.FilePacketConn(f)
+	conn, err := net.FilePacketConn(f)
+	if err != nil {
+		return nil, err
+	}
+	udpconn, ok := conn.(*net.UDPConn)
+	if !ok {
+		return nil, errors.New("BUG(dhcp6): incorrect socket type, expected UDP")
+	}
+	return udpconn, nil
 }