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Set the QUIC InitialPacketSize to 1200 bytes (#7886)

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quic-go defaults the `InitialPacketSize` to 1280 bytes. This is used for
the size of the payload plus UDP header. When quic-go creates its
initial handshake packet, it pads it to this size as an optimization
for the anti-amplification limit (which does not really apply to
Caddy's server connections).

Tailscale uses an MTU of 1280 (the minimum IPv6 MTU) because it is a
tunnel operating in unknown environments, possibly inside other
tunnels.

When wrapped in an IP header (either v4 or v6), the 1280 byte QUIC packet
constructed by quic-go exceeds the Tailscale MTU and gets dropped. This
makes it impossible to respond to an HTTP3 connection attempt over
Tailscale when using the default `InitialPacketSize`.

We explicitly set the `InitialPacketSize` to 1200 (its minimum) to support
HTTP3 connections over Tailscale and other low MTU connections.

Once the connection is established, quic-go can perform MTU discovery to
increase the packet size to the maximum supported by the connection, so
any throughput loss due to the lowered `InitialPacketSize` is
short-lived.
This commit is contained in:
Salynn 2026-07-25 17:48:25 -05:00 • committed by GitHub
parent ff6da1216b
commit c5b66cf8ea
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GPG key ID: B5690EEEBB952194
2 changed files with 6 additions and 4 deletions
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5
listeners.go
View file

@ -480,8 +480,9 @@ func (na NetworkAddress) ListenQUIC(ctx context.Context, portOffset uint, config
earlyLn, err := tr.ListenEarly(
http3.ConfigureTLSConfig(quicTlsConfig),
&quic.Config{
Allow0RTT: allow0rtt,
Tracer: h3qlog.DefaultConnectionTracer,
InitialPacketSize: 1200,
Allow0RTT: allow0rtt,
Tracer: h3qlog.DefaultConnectionTracer,
},
)
if err != nil {

5
modules/caddyhttp/server.go
View file

@ -825,8 +825,9 @@ func (s *Server) serveHTTP3(addr caddy.NetworkAddress, tlsCfg *tls.Config) error
TLSConfig: tlsCfg,
MaxHeaderBytes: s.MaxHeaderBytes,
QUICConfig: &quic.Config{
Versions: []quic.Version{quic.Version1, quic.Version2},
Tracer: h3qlog.DefaultConnectionTracer,
Versions: []quic.Version{quic.Version1, quic.Version2},
InitialPacketSize: 1200,
Tracer: h3qlog.DefaultConnectionTracer,
},
IdleTimeout: time.Duration(s.IdleTimeout),
}