server4: Only allow IPv4 addresses

IPv6 addresses would not cause a crash, but would silently listen on the
wildcard address instead of the passed address, which is surprising at
best. Instead check for the address family and reject non-v4 addresses

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

2 files changed, 14 insertions, 0 deletions

diff --git a/dhcpv4/server4/conn.go b/dhcpv4/server4/conn.go
index d62a5ac..3e49669 100644
--- a/dhcpv4/server4/conn.go
+++ b/dhcpv4/server4/conn.go
@@ -43,6 +43,9 @@ func NewIPv4UDPConn(iface string, addr *net.UDPAddr) (*net.UDPConn, error) {
 	}
 	// Bind to the port.
 	saddr := unix.SockaddrInet4{Port: addr.Port}
+	if addr.IP != nil && addr.IP.To4() == nil {
+		return nil, fmt.Errorf("wrong address family (expected v4) for %s", addr.IP)
+	}
 	copy(saddr.Addr[:], addr.IP.To4())
 	if err := unix.Bind(fd, &saddr); err != nil {
 		return nil, fmt.Errorf("cannot bind to port %d: %v", addr.Port, err)
diff --git a/dhcpv4/server4/server_test.go b/dhcpv4/server4/server_test.go
index da2b199..43314ad 100644
--- a/dhcpv4/server4/server_test.go
+++ b/dhcpv4/server4/server_test.go
@@ -116,3 +116,14 @@ func TestServer(t *testing.T) {
 		require.Equal(t, ifaces[0].HardwareAddr, p.ClientHWAddr)
 	}
 }
+
+func TestBadAddrFamily(t *testing.T) {
+	saddr := &net.UDPAddr{
+		IP:   net.IPv6loopback,
+		Port: 0,
+	}
+	_, err := NewServer("", saddr, DORAHandler)
+	if err == nil {
+		t.Fatal("Expected server4.NewServer to fail with an IPv6 address")
+	}
+}