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envknob,net/tstun,wgengine: use TS_DEBUG_MTU consistently

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Noted on #5915 TS_DEBUG_MTU was not used consistently everywhere.
Extract the default into a function that can apply this centrally and
use it everywhere.

Added envknob.Lookup{Int,Uint}Sized to make it easier to keep CodeQL
happy when using converted values.

Updates #5915

Signed-off-by: James Tucker <james@tailscale.com>
This commit is contained in:
James Tucker
2023-03-30 10:37:06 -07:00
committed by James Tucker
parent 8ed4fd1dbc
commit 40fa2a420c
7 changed files with 105 additions and 25 deletions
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15
net/tstun/constants.go
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@@ -1,15 +0,0 @@
// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package tstun
// 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.
const DefaultMTU = 1280

33
net/tstun/mtu.go Normal file
View File

@@ -0,0 +1,33 @@
// 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
}

28
net/tstun/mtu_test.go Normal file
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@@ -0,0 +1,28 @@
// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package tstun
import (
"os"
"testing"
)
func TestDefaultMTU(t *testing.T) {
orig := os.Getenv("TS_DEBUG_MTU")
defer os.Setenv("TS_DEBUG_MTU", orig)
os.Setenv("TS_DEBUG_MTU", "")
if DefaultMTU() != 1280 {
t.Errorf("DefaultMTU() = %d, want 1280", DefaultMTU())
}
os.Setenv("TS_DEBUG_MTU", "9000")
if DefaultMTU() != 9000 {
t.Errorf("DefaultMTU() = %d, want 9000", DefaultMTU())
}
os.Setenv("TS_DEBUG_MTU", "123456789")
if DefaultMTU() != maxMTU {
t.Errorf("DefaultMTU() = %d, want %d", DefaultMTU(), maxMTU)
}
}

7
net/tstun/tun.go
View File

@@ -14,7 +14,6 @@ import (
"time"
"github.com/tailscale/wireguard-go/tun"
"tailscale.com/envknob"
"tailscale.com/types/logger"
)
@@ -45,11 +44,7 @@ func New(logf logger.Logf, tunName string) (tun.Device, string, error) {
}
dev, err = createTAP(tapName, bridgeName)
} else {
tunMTU := DefaultMTU
if mtu, ok := envknob.LookupInt("TS_DEBUG_MTU"); ok {
tunMTU = mtu
}
dev, err = tun.CreateTUN(tunName, tunMTU)
dev, err = tun.CreateTUN(tunName, int(DefaultMTU()))
}
if err != nil {
return nil, "", err