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Makefile

@@ -1,3 +1,5 @@
 
 all:
 	clang -std=gnu23 -O3 -target bpf -c src/main.c -o faker.o
+debug:
+	clang -g -std=gnu23 -O3 -target bpf -c src/main.c -o faker.o

src/main.c

@@ -12,19 +12,36 @@
 #define ETHER_TYPE_IPV4 0x0800
 #define ETHER_TYPE_IPV6 0x86DD
 
+#define IPV4_TTL_OFFSET 8
+
 #define IPV4_PROTOCOL_OFFSET 9
+#define IPV4_CHECKSUM_OFFSET 10
+#define IPV4_SRC_ADDR_OFFSET 12
+#define IPV4_DST_ADDR_OFFSET 16
 
 #define IPV6_NEXT_HEADER_OFFSET 6
+#define IPV6_HOP_LIMIT_OFFSET 7
+#define IPV6_SRC_ADDR_OFFSET 8
+#define IPV6_DST_ADDR_OFFSET 24
 
 #define IPV6_HEADER_SIZE 40
 
 #define TCP_PROTO_ID 6
 
 #define TCP_DST_PORT_OFFSET 2
+#define TCP_SRC_PORT_OFFSET 0
+
+#define TCP_SEQ_OFFSET 4
+
+#define TCP_DATA_OFFSET_OFFSET 12
+#define TCP_DATA_OFFSET_SHIFT 4
 
 #define TCP_FLAGS_OFFSET 13
 
+#define TCP_CHECKSUM_OFFSET 16
+
 #define SYN_MASK 0b00000010
+#define ACK_MASK 0b00010000
 
 #define LOW_PORT 4000
 const __u16 open_ports[] = {4444};
@@ -38,6 +55,23 @@ __u64 time_seed = 0xffffffffffffffff;
 
 #define DAY_LENGHT_IN_NS 86400000000000
 
+#define DEFAULT_TTL 64
+
+struct __attribute__((packed)) tcp_v4_pseudo_header { // BIG endian
+  __u32 src_ip;
+  __u32 dst_ip;
+  __u8 zero_byte;
+  __u8 protocol_number; // TCP_PROTO_ID
+  __u16 tcp_segment_lenght;
+};
+struct __attribute__((packed)) tcp_v6_pseudo_header { // BIG endian
+  __u128 src_ip;
+  __u128 dst_ip;
+  __u32 upper_layer_packet_lenght;
+  __u8 zero[3];
+  __u8 next_header_value; // TCP_PROTO_ID
+};
+
 __u32 mulberry32_at(__u32 seed, __u32 index) {
   __u32 z = seed + index * 0x6D2B79F5u;
   z = (z ^ (z >> 15)) * (z | 1u);
@@ -111,6 +145,29 @@ static __always_inline const __u8 *get_tcp_data(const __u16 EtherType,
   return NULL;
 }
 
+static __u16 tcp_checksum_fold(__u32 sum) {
+  sum = (sum & 0xFFFF) + (sum >> 16);
+  sum = (sum & 0xFFFF) + (sum >> 16);
+  sum = (sum & 0xFFFF) + (sum >> 16);
+  return (__u16)~sum;
+}
+
+static __u32 tcp_checksum_add(const void *data, const void *data_end,
+                              __u32 sum) {
+  const __u8 *p = data;
+  for (int i = 0; i < 740; i++) { // MTU/2, upper bound for the verifier
+    if (p + 2 > (const __u8 *)data_end) {
+      break;
+    }
+    sum += (p[0] << 8) | p[1];
+    p += 2;
+  }
+  if (p + 1 <= (const __u8 *)data_end) {
+    sum += p[0] << 8;
+  }
+  return sum;
+}
+
 SEC("faker")
 int xdp_drop_prog(struct xdp_md *ctx) {
   if (bpf_ktime_get_ns() - time_seed > DAY_LENGHT_IN_NS) {
@@ -126,6 +183,9 @@ int xdp_drop_prog(struct xdp_md *ctx) {
   const __u16 EtherType = bpf_ntohs(*(__u16 *)(data + ETHER_TYPE_FIELD_OFFSET));
 
   const __u8 *ip_packet_data = data + ETH_HEADER_SIZE;
+  if (ip_packet_data + 20 > data_end) { // it wont be smaller than this
+    return XDP_PASS;
+  }
 
   const __u8 *tcp_packet_data =
       get_tcp_data(EtherType, ip_packet_data, data_end);
@@ -133,12 +193,12 @@ int xdp_drop_prog(struct xdp_md *ctx) {
     return XDP_PASS;
   }
 
-  if (tcp_packet_data + TCP_FLAGS_OFFSET + 1 > data_end) {
+  if (tcp_packet_data + TCP_CHECKSUM_OFFSET + 2 > data_end) {
     return XDP_PASS;
   }
 
-  const __u16 dst_port =
+  __u16 *dst_port_ptr = (__u16 *)(tcp_packet_data + TCP_DST_PORT_OFFSET);
-      bpf_ntohs(*(__u16 *)(tcp_packet_data + TCP_DST_PORT_OFFSET));
+  const __u16 dst_port = bpf_ntohs(*dst_port_ptr);
 
   if (dst_port < LOW_PORT) {
     return XDP_PASS;
@@ -151,15 +211,104 @@ int xdp_drop_prog(struct xdp_md *ctx) {
   if (binary_search(open_ports, ARRAY_SIZE(open_ports), dst_port)) {
     return XDP_PASS;
   }
-
+  __u8 *tcp_flags_ptr = (__u8 *)(tcp_packet_data + TCP_FLAGS_OFFSET);
-  const __u8 tcp_flags = *(tcp_packet_data + TCP_FLAGS_OFFSET);
+  const __u8 tcp_flags = *tcp_flags_ptr;
 
   // check SYN
   if (tcp_flags & SYN_MASK) {
     bpf_printk("packet is SYN\n");
-    if (mulberry32_at((__u32)time_seed, dst_port) % 3== 0) {
+    if (mulberry32_at((__u32)time_seed, dst_port) % 3 == 0) {
       bpf_printk("DROPING!\n");
-      return XDP_DROP;
+      __u16 *src_port_ptr = (__u16 *)(tcp_packet_data + TCP_SRC_PORT_OFFSET);
+      const __u16 src_port = bpf_ntohs(*src_port_ptr);
+
+      *src_port_ptr = bpf_htons(dst_port);
+      *dst_port_ptr = bpf_htons(src_port);
+
+      *tcp_flags_ptr = tcp_flags | ACK_MASK;
+
+      const __u8 tcp_data_offset =
+          bpf_ntohs(((*(__u8 *)(tcp_packet_data + TCP_DATA_OFFSET_OFFSET))) >>
+                    TCP_DATA_OFFSET_SHIFT) *
+          sizeof(__u32);
+
+      __u16 tcp_checksum; // big endian
+
+      {
+        __u32 tmp_sum = 0;
+        if (tcp_packet_data + tcp_data_offset > data_end) {
+          return XDP_PASS;
+        }
+        if (EtherType == ETHER_TYPE_IPV4) {
+          __u32 *src_ip_ptr = (__u32 *)(ip_packet_data + IPV4_SRC_ADDR_OFFSET);
+          __u32 *dst_ip_ptr = (__u32 *)(ip_packet_data + IPV4_DST_ADDR_OFFSET);
+
+          { // swap src and dst ip
+            __u32 src_ip_BE = *src_ip_ptr;
+            *src_ip_ptr = *dst_ip_ptr;
+            *dst_ip_ptr = *src_ip_ptr;
+          }
+
+          *(__u8 *)(ip_packet_data + IPV4_TTL_OFFSET) = DEFAULT_TTL;
+
+          { // calculate ipv4 header checksum
+            __u16 *ipv4_checksum_ptr =
+                (__u16 *)(ip_packet_data + IPV4_CHECKSUM_OFFSET);
+            *ipv4_checksum_ptr = 0; // must be zero for calculation
+            __u16 new_checksum = 0;
+            const __u8 *p = ip_packet_data;
+            for (int i = 0; i < 30; i++) {
+              if (p + 2 > data_end || p + 2 > tcp_packet_data) {
+                break;
+              }
+              new_checksum += bpf_ntohs(*(const __u16 *)p);
+              p += 2;
+            }
+            *ipv4_checksum_ptr = bpf_htons(new_checksum);
+          }
+
+          struct tcp_v4_pseudo_header pseudo_header = {
+              *src_ip_ptr, *dst_ip_ptr, 0, TCP_PROTO_ID,
+              bpf_htons(data_end - tcp_packet_data)};
+
+          tmp_sum = tcp_checksum_add(
+              &pseudo_header,
+              (const __u8 *)&pseudo_header + sizeof(pseudo_header), tmp_sum);
+        } else {
+          __u128 *src_ip_ptr =
+              (__u128 *)(ip_packet_data + IPV6_SRC_ADDR_OFFSET);
+          __u128 *dst_ip_ptr =
+              (__u128 *)(ip_packet_data + IPV6_DST_ADDR_OFFSET);
+
+          { // swap src and dst ip
+            __u128 src_ip_BE = *src_ip_ptr;
+            *src_ip_ptr = *dst_ip_ptr;
+            *dst_ip_ptr = *src_ip_ptr;
+          }
+
+          *(__u8 *)(ip_packet_data + IPV6_HOP_LIMIT_OFFSET) = DEFAULT_TTL;
+
+          struct tcp_v6_pseudo_header pseudo_header = {
+              *(__u128 *)(ip_packet_data + IPV6_SRC_ADDR_OFFSET),
+              *(__u128 *)(ip_packet_data + IPV6_DST_ADDR_OFFSET),
+              bpf_htonl(data_end - tcp_packet_data),
+              {0, 0, 0},
+              TCP_PROTO_ID};
+
+          tmp_sum = tcp_checksum_add(
+              &pseudo_header,
+              (const __u8 *)&pseudo_header + sizeof(pseudo_header), tmp_sum);
+        }
+
+        tmp_sum = tcp_checksum_add(tcp_packet_data,
+                                   tcp_packet_data + tcp_data_offset, tmp_sum);
+        tmp_sum = tcp_checksum_add(tcp_packet_data + tcp_data_offset, data_end,
+                                   tmp_sum);
+        tcp_checksum = bpf_htons(tcp_checksum_fold(tmp_sum));
+      }
+      *(__u16 *)(tcp_packet_data + TCP_CHECKSUM_OFFSET) = tcp_checksum;
+
+      return XDP_TX;
     }
   } else {
     bpf_printk("packet is  not SYN\n");