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