UDP packet bypassing routing

[sidezzz]

Packet connection (udp) only respect the first packet it received. For example if i set a rule to reject specific ip address or protocol it can be bypassed by sending a dummy packet to a valid address through UDP socket and then send a different packet to a different address through the same socket. Found out that bufio.copyPacketWaitWithPool blindly send any packets to any destination without applying routing rules.
if i set a breakpoint to the line inside bufio.copyPacketWaitWithPool with destinationConn.WritePacket(buffer, destination) i can clearly see that the sing-box itself is sending those packets. Also it's sad that it is not logged at all, even with trace level, the log contains only

INFO[0013] [3169077631 0ms] inbound/tun[tun-in]: inbound packet connection from 172.19.16.1:17852
INFO[0013] [3169077631 0ms] inbound/tun[tun-in]: inbound packet connection to 1.2.3.4:100
INFO[0013] [3169077631 0ms] outbound/direct[direct]: outbound packet connection

Example c# code to replicate, with having a remote udp listener on a 100.100.100.100:8585

using var udpClient = new UdpClient();
var legitEndpoint = new IPEndPoint(IPAddress.Parse("1.2.3.4"), 100);
var bannedEndpoint = new IPEndPoint(IPAddress.Parse("100.100.100.100"), 8585);
await udpClient.SendAsync(new byte[] { 100 }, fakeEndpoint);
await udpClient.SendAsync(new byte[] { 200 }, bannedEndpoint );

{
	"log": {
		"level": "trace"
	},
	"inbounds": [
		{
			"type": "tun",
			"tag": "tun-in",
			"interface_name": "tun0",
			"address": "172.19.16.1/30",
			"stack": "gvisor",
			"mtu": 9000,
			"auto_route": true
		}
	],
	"outbounds": [
		{
			"type": "direct",
			"tag": "direct"
		}
	],
	"route": {
		"final": "direct",
		"auto_detect_interface": true,
		"rules": [
			{
				"ip_cidr": "100.100.100.100/32",
				"action": "reject"
			}
		]
	}
}

[dyhkwong]

This is the expected behavior but not a bug. It is a traffic router that routes traffic to different outbounds rather than a firewall. Unlike a typical firewall with boolean "block" and "allow" only, you can't re-route the traffic to another outbound if destination address changes from the same socket.

[sidezzz]

Well, at least it would be nice to have a note about it in the documentation. For me it was a big mistery that despite having bittorrent sniffer filter, the traffic still goes through unwanted outbound.

[dyhkwong]

Documentation should mention sing-box specific issues only. All popular proxy software work like this (imagine what if STUN packets from the same socket are routed to different servers). And I don't think uTP reuses the same socket for different destination addresses.

[sidezzz]

Well it is shared at least on qBittorrent

[dyhkwong]

As blocking traffic is now a route action rather than a special outbound, it may need extra attention.

[nekohasekai]

Perhaps we could add an option for inbounds to use dnat instead of snat.