1 // SPDX-License-Identifier: GPL-2.0
2 // Copyright (c) 2018 Facebook
3 
4 #include <linux/stddef.h>
5 #include <linux/bpf.h>
6 #include <sys/socket.h>
7 
8 #include "bpf_helpers.h"
9 #include "bpf_endian.h"
10 
11 #define SRC1_IP4		0xAC100001U /* 172.16.0.1 */
12 #define SRC2_IP4		0x00000000U
13 #define SRC_REWRITE_IP4		0x7f000004U
14 #define DST_IP4			0xC0A801FEU /* 192.168.1.254 */
15 #define DST_REWRITE_IP4		0x7f000001U
16 #define DST_PORT		4040
17 #define DST_REWRITE_PORT4	4444
18 
19 int _version SEC("version") = 1;
20 
21 SEC("cgroup/sendmsg4")
22 int sendmsg_v4_prog(struct bpf_sock_addr *ctx)
23 {
24 	if (ctx->type != SOCK_DGRAM)
25 		return 0;
26 
27 	/* Rewrite source. */
28 	if (ctx->msg_src_ip4 == bpf_htonl(SRC1_IP4) ||
29 	    ctx->msg_src_ip4 == bpf_htonl(SRC2_IP4)) {
30 		ctx->msg_src_ip4 = bpf_htonl(SRC_REWRITE_IP4);
31 	} else {
32 		/* Unexpected source. Reject sendmsg. */
33 		return 0;
34 	}
35 
36 	/* Rewrite destination. */
37 	if ((ctx->user_ip4 >> 24) == (bpf_htonl(DST_IP4) >> 24) &&
38 	     ctx->user_port == bpf_htons(DST_PORT)) {
39 		ctx->user_ip4 = bpf_htonl(DST_REWRITE_IP4);
40 		ctx->user_port = bpf_htons(DST_REWRITE_PORT4);
41 	} else {
42 		/* Unexpected source. Reject sendmsg. */
43 		return 0;
44 	}
45 
46 	return 1;
47 }
48 
49 char _license[] SEC("license") = "GPL";
50