mirror of https://github.com/tailscale/tailscale/
Revert "wgengine,net,ipn,disco: split up and define different types of MTU"
This reverts commit 059051c58a.
Signed-off-by: Val <valerie@tailscale.com>
pull/9519/head
Val
8 months ago
committed by
valscale
parent
059051c58a
commit
6cc5b272d8
@ -1,154 +1,33 @@
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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 (
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"tailscale.com/envknob"
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)
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// The MTU (Maximum Transmission Unit) of a network interface is the largest
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// packet that can be sent or received through that interface, including all
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// headers above the link layer (e.g. IP headers, UDP headers, Wireguard
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// headers, etc.). We have to think about several different values of MTU:
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//
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// Wire MTU: The MTU of an interface underneath the tailscale TUN, e.g. an
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// Ethernet network card will default to a 1500 byte MTU. The user may change
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// this MTU at any time.
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//
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// TUN MTU: The current MTU of the tailscale TUN. This MTU is adjusted downward
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// to make room for the wireguard/tailscale headers. For example, if the
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// underlying network interface's MTU is 1500 bytes, the maximum size of a
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// packet entering the tailscale TUN is 1420 bytes. The user may change this MTU
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// at any time via the OS's tools (ifconfig, ip, etc.).
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//
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// User configured initial MTU: The MTU the tailscale TUN should be created
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// with, set by the user via TS_DEBUG_MTU. It should be adjusted down from the
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// underlying interface MTU by 80 bytes to make room for the wireguard
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// headers. This envknob is mostly for debugging. This value is used once at TUN
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// creation and ignored thereafter.
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//
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// User configured current MTU: The MTU set via the OS's tools (ifconfig, ip,
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// etc.). This MTU can change at any time. Setting the MTU this way goes through
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// the MTU() method of tailscale's TUN wrapper.
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//
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// Maximum probed MTU: This is the largest MTU size that we send probe packets
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// for.
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//
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// Safe MTU: If the tailscale TUN MTU is set to this value, almost all packets
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// will get to their destination. Tailscale defaults to this MTU in the absence
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// of path MTU probe information or user MTU configuration. We may occasionally
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// find a path that needs a smaller MTU but it is very rare.
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//
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// Peer MTU: This is the path MTU to a peer's current best endpoint. It defaults
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// to the Safe MTU unless we have path MTU probe results that tell us otherwise.
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//
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// Initial MTU: This is the MTU tailscaled creates the TUN with. In order of
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// priority, it is:
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//
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// 1. If set, the value of TS_DEBUG_MTU clamped to a maximum of 65536
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// 2. If TS_DEBUG_ENABLE_PMTUD is set, the maximum size MTU we probe, minus wg
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// overhead
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// 3. If TS_DEBUG_ENABLE_PMTUD is not set, the Safe MTU
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//
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// Current MTU: This the MTU of the tailscale TUN at any given moment
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// after TUN creation. In order of priority, it is:
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//
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// 1. The MTU set by the user via the OS, if it has ever been set
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// 2. If TS_DEBUG_ENABLE_PMTUD is set, the maximum size MTU we probe, minus wg
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// overhead
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// 4. If TS_DEBUG_ENABLE_PMTUD is not set, the Safe MTU
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// TUNMTU is the MTU for the tailscale TUN.
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type TUNMTU uint32
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// WireMTU is the MTU for the underlying network devices.
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type WireMTU uint32
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import "tailscale.com/envknob"
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const (
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// maxTUNMTU is the largest MTU we will consider for the Tailscale
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// TUN. This is inherited from wireguard-go and can be surprisingly
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// small; on Windows it is currently 2048 - 32 bytes and iOS it is 1700
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// - 32 bytes.
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// TODO(val,raggi): On Windows this seems to derive from RIO driver
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// constraints in Wireguard but we don't use RIO so could probably make
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// this bigger.
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maxTUNMTU TUNMTU = TUNMTU(MaxPacketSize)
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// safeTUNMTU is the default "safe" MTU for the Tailscale TUN that we
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// use in the absence of other information such as path MTU probes.
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safeTUNMTU TUNMTU = 1280
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maxMTU uint32 = 65536
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defaultMTU uint32 = 1280
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)
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// MaxProbedWireMTU is the largest MTU we will test for path MTU
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// discovery.
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var MaxProbedWireMTU WireMTU = 9000
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func init() {
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if MaxProbedWireMTU > WireMTU(maxTUNMTU) {
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MaxProbedWireMTU = WireMTU(maxTUNMTU)
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}
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}
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// wgHeaderLen is the length of all the headers Wireguard adds to a packet
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// in the worst case (IPv6). This constant is for use when we can't or
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// shouldn't use information about the IP version of a specific packet
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// (e.g., calculating the MTU for the Tailscale interface.
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//
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// A Wireguard header includes:
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//
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// - 20-byte IPv4 header or 40-byte IPv6 header
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// - 8-byte UDP header
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// - 4-byte type
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// - 4-byte key index
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// - 8-byte nonce
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// - 16-byte authentication tag
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const wgHeaderLen = 40 + 8 + 4 + 4 + 8 + 16
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// TUNToWireMTU takes the MTU that the Tailscale TUN presents to the user and
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// returns the on-the-wire MTU necessary to transmit the largest packet that
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// will fit through the TUN, given that we have to add wireguard headers.
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func TUNToWireMTU(t TUNMTU) WireMTU {
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return WireMTU(t + wgHeaderLen)
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}
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// WireToTUNMTU takes the MTU of an underlying network device and returns the
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// largest possible MTU for a Tailscale TUN operating on top of that device,
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// given that we have to add wireguard headers.
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func WireToTUNMTU(w WireMTU) TUNMTU {
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if w < wgHeaderLen {
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return 0
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}
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return TUNMTU(w - wgHeaderLen)
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}
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// DefaultTUNMTU returns the MTU we use to set the Tailscale TUN
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// MTU. It is also the path MTU that we default to if we have no
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// information about the path to a peer.
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//
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// 1. If set, the value of TS_DEBUG_MTU clamped to a maximum of MaxTunMTU
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// 2. If TS_DEBUG_ENABLE_PMTUD is set, the maximum size MTU we probe, minus wg overhead
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// 3. If TS_DEBUG_ENABLE_PMTUD is not set, the Safe MTU
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func DefaultTUNMTU() TUNMTU {
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if m, ok := envknob.LookupUintSized("TS_DEBUG_MTU", 10, 32); ok {
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return min(TUNMTU(m), maxTUNMTU)
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}
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debugPMTUD, _ := envknob.LookupBool("TS_DEBUG_ENABLE_PMTUD")
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if debugPMTUD {
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return WireToTUNMTU(MaxProbedWireMTU)
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}
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return safeTUNMTU
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}
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// Temporary workaround for code on corp that uses this function name.
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// TODO(val): Remove as soon as corp OSS is updated.
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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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return uint32(DefaultTUNMTU())
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}
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// DefaultWireMTU returns the default TUN MTU, adjusted for wireguard
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// overhead.
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func DefaultWireMTU() WireMTU {
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return TUNToWireMTU(DefaultTUNMTU())
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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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