From c440609ca7fb8b85d83d528526b3f02c1a63376d Mon Sep 17 00:00:00 2001
From: RPRX <63339210+rprx@users.noreply.github.com>
Date: Mon, 26 Apr 2021 06:19:51 +0000
Subject: [PATCH] Add local response to ICMP PING requests (IPv4 & IPv6)
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https://github.com/2dust/v2rayNG/issues/1027

高效实现，十分感谢 @yuhan6665 协助测试
---
 misc/ipv4_proto.h     |  1 +
 misc/ipv6_proto.h     |  1 +
 tun2socks/tun2socks.c | 42 ++++++++++++++++++++++++++++++++++++++----
 3 files changed, 40 insertions(+), 4 deletions(-)

diff --git a/misc/ipv4_proto.h b/misc/ipv4_proto.h
index fea726015..ba59b2315 100644
--- a/misc/ipv4_proto.h
+++ b/misc/ipv4_proto.h
@@ -42,6 +42,7 @@
 #include <misc/packed.h>
 #include <misc/read_write_int.h>
 
+#define IPV4_PROTOCOL_ICMP 1
 #define IPV4_PROTOCOL_IGMP 2
 #define IPV4_PROTOCOL_UDP 17
 
diff --git a/misc/ipv6_proto.h b/misc/ipv6_proto.h
index b25554138..0b03b3807 100644
--- a/misc/ipv6_proto.h
+++ b/misc/ipv6_proto.h
@@ -39,6 +39,7 @@
 
 #define IPV6_NEXT_IGMP 2
 #define IPV6_NEXT_UDP 17
+#define IPV6_NEXT_ICMP 58
 
 B_START_PACKED
 struct ipv6_header {
diff --git a/tun2socks/tun2socks.c b/tun2socks/tun2socks.c
index b4352bcf9..f154614fc 100644
--- a/tun2socks/tun2socks.c
+++ b/tun2socks/tun2socks.c
@@ -1428,10 +1428,27 @@ int process_device_udp_packet (uint8_t *data, int data_len)
 
     switch (ip_version) {
         case 4: {
-            // ignore non-UDP packets
-            if (data_len < sizeof(struct ipv4_header) || data[offsetof(struct ipv4_header, protocol)] != IPV4_PROTOCOL_UDP) {
+            // ignore non-UDP-nor-ICMP packets
+            if (data_len < sizeof(struct ipv4_header)) {
                 goto fail;
             }
+            switch (data[offsetof(struct ipv4_header, protocol)]) {
+                case IPV4_PROTOCOL_UDP: break;
+                case IPV4_PROTOCOL_ICMP: {
+                    if (data[0] == 69 && data_len >= 28 && data[20] == 8 && data[21] == 0) { // ping
+                        uint8_t ip_source[4];
+                        memcpy(ip_source, &data[12], 4);
+                        memcpy(&data[12], &data[16], 4);
+                        memcpy(&data[16], ip_source, 4);
+                        data[20] = 0; // response
+                        data[22] += 8; // checksum
+                        if (data[22] < 8) data[23] += 1;
+                        BTap_Send(&device, data, data_len);
+                    }
+                    return 1;
+                }
+                default: goto fail;
+            }
 
             // parse IPv4 header
             struct ipv4_header ipv4_header;
@@ -1483,10 +1500,27 @@ int process_device_udp_packet (uint8_t *data, int data_len)
                 goto fail;
             }
 
-            // ignore non-UDP packets
-            if (data_len < sizeof(struct ipv6_header) || data[offsetof(struct ipv6_header, next_header)] != IPV6_NEXT_UDP) {
+            // ignore non-UDP-nor-ICMP packets
+            if (data_len < sizeof(struct ipv6_header)) {
                 goto fail;
             }
+            switch (data[offsetof(struct ipv6_header, next_header)]) {
+                case IPV6_NEXT_UDP: break;
+                case IPV6_NEXT_ICMP: {
+                    if (data_len >= 48 && data[40] == 128 && data[41] == 0) { // ping6
+                        uint8_t ip_source[16];
+                        memcpy(ip_source, &data[8], 16);
+                        memcpy(&data[8], &data[24], 16);
+                        memcpy(&data[24], ip_source, 16);
+                        data[40] = 129; // response
+                        data[42] -= 1; // checksum
+                        if (data[42] == 255) data[43] -= 1;
+                        BTap_Send(&device, data, data_len);
+                    }
+                    return 1;
+                }
+                default: goto fail;
+            }
 
             // parse IPv6 header
             struct ipv6_header ipv6_header;