tailscale/net/tstun/mtu.go

34 lines
926 B
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package tstun
import "tailscale.com/envknob"
const (
maxMTU uint32 = 65536
defaultMTU uint32 = 1280
)
// DefaultMTU returns either the constant default MTU of 1280, or the value set
// in TS_DEBUG_MTU clamped to a maximum of 65536.
func DefaultMTU() uint32 {
// DefaultMTU is the Tailscale default MTU for now.
//
// wireguard-go defaults to 1420 bytes, which only works if the
// "outer" MTU is 1500 bytes. This breaks on DSL connections
// (typically 1492 MTU) and on GCE (1460 MTU?!).
//
// 1280 is the smallest MTU allowed for IPv6, which is a sensible
// "probably works everywhere" setting until we develop proper PMTU
// discovery.
tunMTU := defaultMTU
if mtu, ok := envknob.LookupUintSized("TS_DEBUG_MTU", 10, 32); ok {
mtu := uint32(mtu)
if mtu > maxMTU {
mtu = maxMTU
}
tunMTU = mtu
}
return tunMTU
}