init

Makefile

@@ -0,0 +1,3 @@
+
+all:
+	clang -O3 -target bpf -c src/main.c -o faker.o

src/main.c

@@ -0,0 +1,106 @@
+#include <linux/bpf.h>
+#define fuck_lsp
+#include <bpf/bpf_endian.h>
+#include <bpf/bpf_helpers.h>
+
+#define ARRAY_SIZE(a) sizeof(a) / sizeof(a[0])
+
+#define ETH_HEADER_SIZE 14
+
+#define ETHER_TYPE_FIELD_OFFSET 12
+
+#define ETHER_TYPE_IPV4 0x0800
+#define ETHER_TYPE_IPV6 0x86DD
+
+#define IPV4_PROTOCOL_OFFSET 9
+
+#define IPV6_NEXT_HEADER_OFFSET 6
+
+#define IPV6_HEADER_SIZE 40
+
+#define TCP_PROTO_ID 6
+
+#define TCP_DST_PORT_OFFSET 2
+
+#define TCP_FLAGS_OFFSET 13
+
+#define SYN_MASK 0b00000010
+
+#define LOW_PORT 4000
+const __u16 open_ports[] = {4444};
+#define HIGH_PORT 5000
+
+char _license[] SEC("license") = "GPL";
+
+static __always_inline const __u8 *get_tcp_data(const __u16 EtherType,
+                                                const __u8 *ip_packet_data,
+                                                const __u8 *data_end) {
+  if (EtherType == ETHER_TYPE_IPV4) {
+
+    // check protocol
+    const __u8 *protocol_id_ptr = ip_packet_data + IPV4_PROTOCOL_OFFSET;
+    if (protocol_id_ptr > data_end) {
+      return NULL;
+    }
+    if (*protocol_id_ptr == TCP_PROTO_ID) {
+      return ip_packet_data + ((*ip_packet_data & 0b00001111) * 4);
+    }
+
+  } else if (EtherType == ETHER_TYPE_IPV6) {
+    // check  Next Header
+    const __u8 *protocol_id_ptr = ip_packet_data + IPV6_NEXT_HEADER_OFFSET;
+    if (protocol_id_ptr > data_end) {
+      return NULL;
+    }
+    if (*protocol_id_ptr == TCP_PROTO_ID) {
+      return ip_packet_data + IPV6_HEADER_SIZE;
+    }
+  }
+  return NULL;
+}
+
+SEC("faker")
+int xdp_drop_prog(struct xdp_md *ctx) {
+  const __u8 *data_end = (__u8 *)(long)ctx->data_end;
+  const __u8 *data = (__u8 *)(long)ctx->data;
+
+  const __u16 *EtherType_ptr = (__u16 *)(data + ETHER_TYPE_FIELD_OFFSET);
+  if (EtherType_ptr > (__u16 *)data_end) {
+    return XDP_PASS;
+  }
+  const __u16 EtherType = bpf_ntohs(*EtherType_ptr);
+
+  const __u8 *ip_packet_data = data + ETH_HEADER_SIZE;
+
+  const __u8 *tcp_packet_data =
+      get_tcp_data(EtherType, ip_packet_data, data_end);
+  if (!tcp_packet_data) {
+    return XDP_PASS;
+  } // return if not tcp or IP
+
+  const __u16 dst_port = bpf_ntohs(*(tcp_packet_data + TCP_DST_PORT_OFFSET));
+
+  if (dst_port < LOW_PORT) {
+    return XDP_PASS;
+  }
+  if (dst_port > HIGH_PORT) {
+    return XDP_PASS;
+  }
+
+  for (__u16 i = 0; i < ARRAY_SIZE(open_ports); i++) {
+    if (dst_port == open_ports[i]) {
+      return XDP_PASS;
+    }
+  }
+
+  const __u8 tcp_flags = *(tcp_packet_data + TCP_FLAGS_OFFSET);
+
+  // check SYN
+  if (tcp_flags & SYN_MASK) {
+    if (bpf_get_prandom_u32() % 3 == 0) {
+      return XDP_DROP;
+    };
+  }
+
+  return XDP_PASS;
+}