How to use with QUIC without running into DATAGRAM frame too large error

[LeonardWalter]

Hi,
I am trying to build a DoQ client combined with a masque proxy.
Currently I am using this code to connect to my DoQ server at myurl:8853:

	template := uritemplate.MustNew("https://myurl.org:9443/masque?h={target_host}&p={target_port}")
	target := "myurl.org:8853"

	raddr, err := net.ResolveUDPAddr("udp", target)
	if err != nil {
		log.Fatalf("failed to resolve udp addr: %v", err)
		return
	}

	cl := masque.Client{
		QUICConfig: &quic.Config{
			EnableDatagrams:   true,
			InitialPacketSize: 1350,
		},
	}

	pconn, _, err := cl.Dial(context.Background(), template, raddr)
	if err != nil {
		log.Fatalf("failed to dial masque proxy: %v", err)
		return
	}

	qc := new(quic.Config)
	qc.DisablePathMTUDiscovery = true
	tc := new(tls.Config)
	tc.ServerName = "myurl.org"
	tc.NextProtos = []string{"doq"}

	earlyConn, err := quic.DialEarly(context.Background(), pconn, raddr, tc, qc)
	if err != nil {
		log.Fatalf("failed to dial target via masque proxy: %v", err)
		return
	}

Everything seems to work fine with the client, but on the proxy I always receive following error, which causes the quic.DialEarly to fail.
2024/12/05 13:57:38 proxying receive side to [2a02:****:5d33]:8853 failed: DATAGRAM frame too large

I already tried different quic.Config InitialPacketSize values for both inner quic and outer masque stream but I cant get it to work.

[marten-seemann]

Is this an error that is sent over the connection or is it the local error returned. Unfortunately, they currently have the same string representation:
https://github.com/quic-go/quic-go/blob/108b6603c877807f77a89b472e5fd2fef44fafd4/errors.go#L65-L75

and:
https://github.com/quic-go/quic-go/blob/108b6603c877807f77a89b472e5fd2fef44fafd4/connection.go#L1640-L1649

[LeonardWalter]

Both, depending on the InitialPacketSize.

When I test it with InitialPacketSize: 1350,
This error shows up locally:
failed to dial target via masque proxy: INTERNAL_ERROR (local): EOF
and this on the proxy:
proxying receive side to [2a02:xxxx:5d33]:8853 failed: DATAGRAM frame too large

Now when I decrease the InitialPacketSize: 1250,
This error shows up locally:
failed to dial target via masque proxy: INTERNAL_ERROR (local): DATAGRAM frame too large
and this on the proxy:
proxying send side to [2a02:xxxx:5d33]:8853 failed: timeout: no recent network activity

[LeonardWalter]

Just wanted to update that I managed to solve this by setting InitialPacketSize: 1400 on the client and InitialPacketSize: 1500 on the proxy. Manually setting it, and setting it to a higher value on the proxy than on the client was essential here for me.