8000 fix: stop selecting direct connections with too-small MTU by spikecurtis · Pull Request #85 · coder/tailscale · GitHub
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fix: stop selecting direct connections with too-small MTU #85

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Merged
merged 8 commits into from
Jun 10, 2025
Merged

fix: stop selecting direct connections with too-small MTU #85

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bede4fb
feat: pad disco ping/pong and enable path mtu on Windows
spikecurtis Jun 9, 2025
1bab46f
feat: set path MTU discover on Linux
spikecurtis Jun 9, 2025
ef3f81d
feat: set IP_DONTFRAG on macOS
spikecurtis Jun 9, 2025
c89c6b7
logs and comments
spikecurtis Jun 9, 2025
4686156
fix: check Disco Ping/Pong length in unit tests
spikecurtis Jun 10, 2025
6923084
restyle constant defs on Windows
spikecurtis Jun 10, 2025
b824a9c
make ping/pong test explicit
spikecurtis Jun 10, 2025
4ec7ef5
import ordering
spikecurtis Jun 10, 2025
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@spikecurtis
spikecurtis committed Jun 9, 2025
commit c89c6b7551639ca48bff53754c9c5367bf68cfcf
5 changes: 4 additions & 1 deletion disco/disco.go
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Expand Up @@ -54,7 +54,10 @@ const v1 = byte(1)

// paddedPayloadLen is the desired length we want to pad Ping and Pong payloads
// to so that they are the maximum size of a Wireguard packet we would
// subsequently send.
// subsequently send. This ensures that any UDP paths we discover will actually
// support the packet sizes the net stack will send over those paths. Any peers
// behind a small-MTU link will have to depend on DERP.
// c.f. https://github.com/coder/coder/issues/15523
// Our inner IP packets can be up to 1280 bytes, with the Wireguard header of
// 30 bytes, that is 1310. The final 2 is the inner payload header's type and version.
const paddedPayloadLen = 1310 - len(Magic) - keyLen - NonceLen - box.Overhead - 2
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7 changes: 6 additions & 1 deletion wgengine/magicsock/magicsock.go
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Expand Up @@ -2377,7 +2377,12 @@ func (c *Conn) bindSocket(ruc *RebindingUDPConn, network string, curPortFate cur
continue
}
trySetSocketBuffer(pconn, c.logf)
trySetPathMTUDiscover(pconn, c.logf, network)
// CODER: https://github.com/coder/coder/issues/15523
// Attempt to tell the OS not to fragment packets over this interface. We pad disco Ping and Pong packets to the
// size of the direct UDP packets that get sent for direct connections. Thus, any interfaces or paths that
// cannot fully support direct connections due to MTU limitations will not be selected. If no direct paths meet
// the MTU requirements for a peer, we will fall back to DERP for that peer.
tryPreventFragmentation(pconn, c.logf, network)
// Success.
if debugBindSocket() {
c.logf("magicsock: bindSocket: successfully listened %v port %d", network, port)
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12 changes: 6 additions & 6 deletions wgengine/magicsock/magicsock_darwin.go
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Expand Up @@ -8,11 +8,11 @@ import (
"tailscale.com/types/nettype"
)

func trySetPathMTUDiscover(pconn nettype.PacketConn, logf logger.Logf, network string) {
func tryPreventFragmentation(pconn nettype.PacketConn, logf logger.Logf, network string) {
if c, ok := pconn.(*net.UDPConn); ok {
s, err := c.SyscallConn()
if err != nil {
logf("magicsock: failed to set Path MTU Discover: get syscall conn: %v", err)
logf("magicsock: dontfrag: failed to get syscall conn: %v", err)
}
level := unix.IPPROTO_IP
option := unix.IP_DONTFRAG
Expand All @@ -23,14 +23,14 @@ func trySetPathMTUDiscover(pconn nettype.PacketConn, logf logger.Logf, network s
err = s.Control(func(fd uintptr) {
err := unix.SetsockoptInt(int(fd), level, option, 1)
if err != nil {
logf("magicsock: failed to set Path MTU Discover: SetsockoptInt failed: %v", err)
logf("magicsock: dontfrag: SetsockoptInt failed: %v", err)
}
})
if err != nil {
logf("magicsock: failed to set Path MTU Discover: control connection: %v", err)
10000 logf("magicsock: dontfrag: control connection failed: %v", err)
}
logf("magicsock: successfully set Path MTU Discover on %s", pconn.LocalAddr().String())
logf("magicsock: dontfrag: success on %s", pconn.LocalAddr().String())
return
}
logf("magicsock: failed to set Path MTU Discover: not a UDPConn")
logf("magicsock: dontfrag: failed because it was not a UDPConn")
}
12 changes: 6 additions & 6 deletions wgengine/magicsock/magicsock_linux.go
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Expand Up @@ -405,11 +405,11 @@ func init() {
controlMessageSize = unix.CmsgSpace(2)
}

func trySetPathMTUDiscover(pconn nettype.PacketConn, logf logger.Logf, network string) {
func tryPreventFragmentation(pconn nettype.PacketConn, logf logger.Logf, network string) {
if c, ok := pconn.(*net.UDPConn); ok {
s, err := c.SyscallConn()
if err != nil {
logf("magicsock: failed to set Path MTU Discover: get syscall conn: %v", err)
logf("magicsock: dontfrag: failed to get syscall conn: %v", err)
}
level := unix.IPPROTO_IP
option := unix.IP_MTU_DISCOVER
Expand All @@ -420,14 +420,14 @@ func trySetPathMTUDiscover(pconn nettype.PacketConn, logf logger.Logf, network s
err = s.Control(func(fd uintptr) {
err := unix.SetsockoptInt(int(fd), level, option, unix.IP_PMTUDISC_DO)
if err != nil {
logf("magicsock: failed to set Path MTU Discover: SetsockoptInt failed: %v", err)
logf("magicsock: dontfrag: SetsockoptInt failed: %v", err)
}
})
if err != nil {
logf("magicsock: failed to set Path MTU Discover: control connection: %v", err)
logf("magicsock: dontfrag: control connection failed: %v", err)
}
logf("magicsock: successfully set Path MTU Discover on %s", pconn.LocalAddr().String())
logf("magicsock: dontfrag: success on %s", pconn.LocalAddr().String())
return
}
logf("magicsock: failed to set Path MTU Discover: not a UDPConn")
logf("magicsock: dontfrag: failed because it was not a UDPConn")
}
12 changes: 6 additions & 6 deletions wgengine/magicsock/magicsock_windows.go
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8655
Original file line number Diff line number Diff line change
Expand Up @@ -21,11 +21,11 @@ const (
IP_PMTUDISC_MAX
)

func trySetPathMTUDiscover(pconn nettype.PacketConn, logf logger.Logf, network string) {
func tryPreventFragmentation(pconn nettype.PacketConn, logf logger.Logf, network string) {
if c, ok := pconn.(*net.UDPConn); ok {
s, err := c.SyscallConn()
if err != nil {
logf("magicsock: failed to set Path MTU Discover: get syscall conn: %v", err)
logf("magicsock: dontfrag: failed to get syscall conn: %v", err)
}
level := windows.IPPROTO_IP
if network == "udp6" {
Expand All @@ -34,14 +34,14 @@ func trySetPathMTUDiscover(pconn nettype.PacketConn, logf logger.Logf, network s
err = s.Control(func(fd uintptr) {
err := windows.SetsockoptInt(windows.Handle(fd), level, IP_MTU_DISCOVER, IP_PMTUDISC_DO)
if err != nil {
logf("magicsock: failed to set Path MTU Discover: SetsockoptInt failed: %v", err)
logf("magicsock: dontfrag: SetsockoptInt failed: %v", err)
}
})
if err != nil {
logf("magicsock: failed to set Path MTU Discover: control connection: %v", err)
logf("magicsock: dontfrag: control connection failed: %v", err)
}
logf("magicsock: successfully set Path MTU Discover on %s", pconn.LocalAddr().String())
logf("magicsock: dontfrag: success on %s", pconn.LocalAddr().String())
return
}
logf("magicsock: failed to set Path MTU Discover: not a UDPConn")
logf("magicsock: dontfrag: failed because it was not a UDPConn")
}
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