TUN inbound: Reply fake pong to ICMP ping (#6015)

https://github.com/XTLS/Xray-core/pull/6015#issuecomment-4321525342

proxy/tun/README.md

@@ -41,10 +41,12 @@ Here is simple Xray config snippet to enable the inbound:
 
 - IPv4 and IPv6
 - TCP and UDP
+- ICMP Echo (ping)
 
 ## LIMITATION
 
-- No ICMP support
+- Only ICMP Echo request/reply is supported; other ICMP message types are ignored
+- ICMP Echo replies are generated locally by the TUN stack; they do not validate real remote ICMP reachability
 - Connections are established to any host, as connection success is only a mark of successful accepting packet for proxying. Hosts that are not accepting connections or don't even exists, will look like they opened a connection (SYN-ACK), and never send back a single byte, closing connection (RST) after some time. This is the side effect of the whole process actually being a proxy, and not real network layer 3 vpn
 
 ## CONSIDERATIONS
@@ -248,4 +250,4 @@ Set the environment variable `xray.tun.fd` (or `XRAY_TUN_FD`) to the fd number b
 Build using gomobile for iOS framework integration:
 ```
 gomobile bind -target=ios
-```
+```