valid ip-id

src/main.c

@@ -16,6 +16,7 @@
 
 #define IPV4_TTL_OFFSET 8
 
+#define IPV4_ID_OFFSET 4
 #define IPV4_PROTOCOL_OFFSET 9
 #define IPV4_CHECKSUM_OFFSET 10
 #define IPV4_SRC_ADDR_OFFSET 12
@@ -34,6 +35,7 @@
 #define TCP_SRC_PORT_OFFSET 0
 
 #define TCP_SEQ_OFFSET 4
+#define TCP_ACK_NUM_OFFSET 8
 
 #define TCP_DATA_OFFSET_OFFSET 12
 #define TCP_DATA_OFFSET_SHIFT 4
@@ -230,6 +232,11 @@ int xdp_drop_prog(struct xdp_md *ctx) {
 
       *tcp_flags_ptr = tcp_flags | ACK_MASK;
 
+      // set ISN and ack number
+      *(__u32 *)(tcp_packet_data + TCP_ACK_NUM_OFFSET) = bpf_htonl(
+          bpf_ntohl(*(__u32 *)(tcp_packet_data + TCP_SEQ_OFFSET))+1);
+      *(__u32 *)(tcp_packet_data + TCP_SEQ_OFFSET) = bpf_get_prandom_u32();
+
       *(__u16 *)(tcp_packet_data + TCP_CHECKSUM_OFFSET) = 0; // must be zero for calculation
 
       const __u8 tcp_data_offset =
@@ -278,6 +285,10 @@ int xdp_drop_prog(struct xdp_md *ctx) {
             *ipv4_checksum_ptr = bpf_htons((__u16)new_checksum);
           }
 
+          // set ID 
+            *(__u16*)(ip_packet_data+IPV4_ID_OFFSET) = bpf_get_prandom_u32();
+          
+
           struct tcp_v4_pseudo_header pseudo_header = {
               *src_ip_ptr, *dst_ip_ptr, 0, TCP_PROTO_ID,
               bpf_htons(data_end - tcp_packet_data)};