feat: set path MTU discover on Linux

spikecurtis · spikecurtis · commit 1bab46fdb716 · 2025-06-09T06:28:27.000Z
diff --git a/wgengine/magicsock/magicsock_darwin.go b/wgengine/magicsock/magicsock_darwin.go
@@ -1,6 +1,36 @@
 package magicsock
 
+import (
+	"net"
+
+	"golang.org/x/sys/unix"
+	"tailscale.com/types/logger"
+	"tailscale.com/types/nettype"
+)
+
 func trySetPathMTUDiscover(pconn nettype.PacketConn, logf logger.Logf, network string) {
-	// TODO: implement
-	logf("magicsock: failed to set Path MTU Discover: not implemented")
+	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)
+		}
+		level := unix.IPPROTO_IP
+		option := unix.IP_MTU_DISCOVER
+		if network == "udp6" {
+			level = unix.IPPROTO_IPV6
+			option = unix.IPV6_MTU_DISCOVER
+		}
+		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)
+			}
+		})
+		if err != nil {
+			logf("magicsock: failed to set Path MTU Discover: control connection: %v", err)
+		}
+		logf("magicsock: successfully set Path MTU Discover on %s", pconn.LocalAddr().String())
+		return
+	}
+	logf("magicsock: failed to set Path MTU Discover: not a UDPConn")
 }
diff --git a/wgengine/magicsock/magicsock_linux.go b/wgengine/magicsock/magicsock_linux.go
@@ -406,6 +406,28 @@ func init() {
 }
 
 func trySetPathMTUDiscover(pconn nettype.PacketConn, logf logger.Logf, network string) {
-	// TODO: implement
-	logf("magicsock: failed to set Path MTU Discover: not implemented")
+	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)
+		}
+		level := unix.IPPROTO_IP
+		option := unix.IP_MTU_DISCOVER
+		if network == "udp6" {
+			level = unix.IPPROTO_IPV6
+			option = unix.IPV6_MTU_DISCOVER
+		}
+		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)
+			}
+		})
+		if err != nil {
+			logf("magicsock: failed to set Path MTU Discover: control connection: %v", err)
+		}
+		logf("magicsock: successfully set Path MTU Discover on %s", pconn.LocalAddr().String())
+		return
+	}
+	logf("magicsock: failed to set Path MTU Discover: not a UDPConn")
 }
diff --git a/wgengine/magicsock/magicsock_windows.go b/wgengine/magicsock/magicsock_windows.go
@@ -22,7 +22,6 @@ const (
 )
 
 func trySetPathMTUDiscover(pconn nettype.PacketConn, logf logger.Logf, network string) {
-	logf("setting Path MTU Discover on %s", pconn.LocalAddr().String())
 	if c, ok := pconn.(*net.UDPConn); ok {
 		s, err := c.SyscallConn()
 		if err != nil {
@@ -41,7 +40,7 @@ func trySetPathMTUDiscover(pconn nettype.PacketConn, logf logger.Logf, network s
 		if err != nil {
 			logf("magicsock: failed to set Path MTU Discover: control connection: %v", err)
 		}
-		logf("sucessfully set Path MTU Discover on %s", pconn.LocalAddr().String())
+		logf("magicsock: successfully set Path MTU Discover on %s", pconn.LocalAddr().String())
 		return
 	}
 	logf("magicsock: failed to set Path MTU Discover: not a UDPConn")

Original file line number	Diff line number	Diff line change
`@@ -22,7 +22,6 @@ const (`
`22`	`22`	`)`
`23`	`23`
`24`	`24`	`func trySetPathMTUDiscover(pconn nettype.PacketConn, logf logger.Logf, network string) {`
`25`		`- logf("setting Path MTU Discover on %s", pconn.LocalAddr().String())`
`26`	`25`	`if c, ok := pconn.(*net.UDPConn); ok {`
`27`	`26`	`s, err := c.SyscallConn()`
`28`	`27`	`if err != nil {`
`@@ -41,7 +40,7 @@ func trySetPathMTUDiscover(pconn nettype.PacketConn, logf logger.Logf, network s`
`41`	`40`	`if err != nil {`
`42`	`41`	`logf("magicsock: failed to set Path MTU Discover: control connection: %v", err)`
`43`	`42`	`}`
`44`		`- logf("sucessfully set Path MTU Discover on %s", pconn.LocalAddr().String())`
	`43`	`+ logf("magicsock: successfully set Path MTU Discover on %s", pconn.LocalAddr().String())`
`45`	`44`	`return`
`46`	`45`	`}`
`47`	`46`	`logf("magicsock: failed to set Path MTU Discover: not a UDPConn")`