proxy/trojan: fix writing UDP packet

[dyhkwong]

#2440 #1795

Every UDP packet should have some headers.

[Vigilans]

Ref #2336 (comment), maybe related?

3. In fact, the author made attempt to deal with the issue mentioned above in trojan outbound:
   

   v2ray-core/proxy/trojan/protocol.go

   Lines 102 to 114 in 83ea1ba

   	func (w *PacketWriter) WriteMultiBuffer(mb buf.MultiBuffer) error {
   	b := make([]byte, maxLength)
   	for !mb.IsEmpty() {
   	var length int
   	mb, length = buf.SplitBytes(mb, b)
   	if _, err := w.writePacket(b[:length], w.Target); err != nil {
   	buf.ReleaseMulti(mb)
   	return err
   	}
   	}
   	return nil
   	}

   
   It 1) Creates a buffer of length 8192 (maxLength = 8192); 2) Read the MultiBuffer 8192 bytes by 8192 bytes, then encode each (at most 8192 bytes) buffer into trojan UDP payloads.

   This in a sense fixes the problem that trojan inbound writes the UDP payload 2048 bytes by 2048 bytes. Since a UDP payload will not exceed 8192 bytes (max 4096 bytes as you mentioned), the split UDP packet in trojan inbound will be reunited in trojan outbound.

   However, even if scoped in trojan inbound -> trojan outbound, this solution is still problematic. How to ensure that the passed mb buf.MultiBuffer only contains payload of 1 UDP packet? In fact this is not guaranteed, so the UDP packet in outbound may contains payload of more than 1 UDP packets from inbound, causing misbehavior.

[dyhkwong]

Ref #2336 (comment), maybe related?

Indeed. So, this PR also can't handle UDP payload larger than 2048 bytes. Can this be fixed by modifying Trojan's PacketWriter and PacketReader to make use of buf.NewWithSize(), like packetstream in #2337?

[Vigilans]

Ref #2336 (comment), maybe related?

Indeed. So, this PR also can't handle UDP payload larger than 2048 bytes. Can this be fixed by modifying Trojan's PacketWriter and PacketReader to make use of buf.NewWithSize(), like packetstream in #2337?

I think it will fix. I remembered that I was referencing Trojan's PacketWriter and PacketReader to implement packetstream ones, though I did not have the time and usecase to fix and test for trojan.

[codecov-commenter]

Codecov Report

Patch coverage: 42.85% and no project coverage change.

Comparison is base (9b52628) 39.05% compared to head (5b6e0fd) 39.06%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2446   +/-   ##
=======================================
  Coverage   39.05%   39.06%           
=======================================
  Files         617      617           
  Lines       37036    37028    -8     
=======================================
- Hits        14465    14464    -1     
+ Misses      20981    20970   -11     
- Partials     1590     1594    +4     

Impacted Files	Coverage Δ
proxy/trojan/protocol.go	42.22% <42.85%> (-3.24%)	⬇️

... and 9 files with indirect coverage changes

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[xiaokangwang]

I have summarily approved this merge request. Thanks for your contribution.

It is worth noting that UDP packet requires segmentation is not widely supported and should be considered edge case.