sockd: Don't require that dhcp4 packets originate from port 67.

The RFC requires that clients receive on port UDP 68 and send
to servers at UDP 67, but it doesn't make further mandates.

So, although most servers and clients originate packets from 67 or 68,
it isn't required and thus the check here is too strict.

sockd.c

@@ -246,11 +246,12 @@ static int create_raw_listen_socket(bool *using_bpf)
         BPF_STMT(BPF_RET + BPF_K, 0),
         // Packet is UDP.  Advance X past the IP header.
         BPF_STMT(BPF_LDX + BPF_B + BPF_MSH, 0),
-        // Verify that the UDP client and server ports match that of the
-        // IANA-assigned DHCP ports.
-        BPF_STMT(BPF_LD + BPF_W + BPF_IND, 0),
+        // Verify that the UDP client port matches that of the
+        // IANA-assigned DHCP port; RFCs do not require the origin
+        // port to correspond to the DHCP_SERVER_PORT.
+        BPF_STMT(BPF_LD + BPF_H + BPF_IND, 2),
         BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K,
-                 (DHCP_SERVER_PORT << 16) + DHCP_CLIENT_PORT, 1, 0),
+                 DHCP_CLIENT_PORT, 1, 0),
         BPF_STMT(BPF_RET + BPF_K, 0),
         // Get the UDP length field and store it in X.
         BPF_STMT(BPF_LD + BPF_H + BPF_IND, 4),