(freedom).PacketReader not work, but buf.PacketReader work

[patterniha]

im trying to solve #4800 (comment).

After using buf.PacketReader instead of (freedom)PacketReader everything is Ok.

when we use "redirect" setting, UDPOverride is set and we use buf.PacketReader instead of (freedom)PacketReader, this is because when we use "redirect" everything is OK.

///

I still can't find what's wrong with freedom(PacketReader).

[patterniha]

found the problem...

[dyhkwong]

I already told you this is caused by #3351. And do NOT try to change the code without reading the context of #3734.

[patterniha]

I already told you this is caused by #3351. And do NOT try to change the code without reading the context of #3734.

thx, i read it.

i debugged the problem, and i found what is the problem.

I will open PR with a detailed explanation and fix soon.

[patterniha]

#4816

[dyhkwong]

UDP does not work like this and this change will break the case of a single UDP socket reads from/writes to multiple different domain name addresses. You need to maintain a per-socket mapping of IP-domain when writing and recover the corresponding domain name from IP address when reading, and figure out how to deal with the unfixable case of conflict mappings (an IP may correspond to different domain names). And, you also need to fix the logic in common/buf/override.go.

[patterniha]

this change will break

break??? It doesn't even work for one domain right now.

///

single UDP socket reads from/writes to multiple different domain name addresses.

this is impossible, suppose we have two domain example-1.com and example-2.com, and both resolved to "2.2.2.2" ip, so if we receive response from "2.2.2.2" this is for example-1.com or example-2.com ???

this is not related to Xray-core, this is logically impossible.

///

also, when we have multiple-domain for single-UDP??? for example if we use mux, after demux each request use separate-UDP-connection.

[dyhkwong]

It is theoretically possible although practically apps are not going to do this, e.g. if fake DNS is used, an app thinks a.example.com and b.example.com are different IP addresses, and is unaware that they are actually different IP addresses or not. It is of course possible to talk to 198.18.0.1 and 198.18.0.2 on the same UDP socket.
198.18.0.1-a.example.com-23.192.228.80
198.18.0.2-b.example.com-23.192.228.80

And take STUN as an example, if fake DNS is used, the first request address will be a domain name and the second will be an IP address. For normal NAT behavior, the first response address needs to be that domain name, and the second needs to be that IP address.

[patterniha]

Fixed: #4816