From 9d9cd98de42dcc2e8b80aa08fa20125e354b6950 Mon Sep 17 00:00:00 2001
From: Andrew Ritchie <aritchie@akamai.com>
Date: Wed, 11 Mar 2026 21:39:19 +0100
Subject: [PATCH] Add support for MTU option overrides

---
 main.go | 19 +++++++++++++++++++
 ra.go   |  7 ++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/main.go b/main.go
index 77f7d62..af08622 100644
--- a/main.go
+++ b/main.go
@@ -12,7 +12,17 @@ import (
 	"github.com/vishvananda/netlink"
 )
 
+const (
+	omitMTU   = 0
+	deviceMTU = -1
+)
+
 var (
+	flagMTU = flag.Int(
+		"mtu",
+		omitMTU,
+		fmt.Sprintf("configure the MTU value that is sent.  %d (default) = disable MTU option, %d = use MTU of the interface device, >0 = send the provided value as the MTU option.", omitMTU, deviceMTU),
+	)
 	flagLifeTime = flag.Duration("lifetime", (30 * time.Minute), "Lifetime (prefix valid time will be 3x lifetime).")
 	flagInterval = flag.Duration("interval", (5 * time.Minute), "Frequency of *un*solicitated RAs.")
 	errRetry     = errors.New("retry")
@@ -81,6 +91,15 @@ func main() {
 	ll.Infof("Loglevel '%s'", ll.GetLevel())
 	ll.Infof("Sending RAs valid for %v every %v on interfaces matching %s", *flagLifeTime, *flagInterval, *flagTapRegex)
 	ll.Infof("Excluding %s from RAs", exclude)
+	if *flagMTU == deviceMTU {
+		ll.Infoln("MTU Option: interface determined")
+	} else if *flagMTU == omitMTU {
+		ll.Infoln("MTU Option disabled")
+	} else if *flagMTU > omitMTU {
+		ll.Infof("MTU Option: %v", *flagMTU)
+	} else {
+		ll.Infof("MTU Option: invalid value %d", *flagMTU)
+	}
 
 	if flagLifeTime.Seconds() < 3*(flagInterval.Seconds()) {
 		ll.Warnf(
diff --git a/ra.go b/ra.go
index 20e3457..cb0fc66 100644
--- a/ra.go
+++ b/ra.go
@@ -28,7 +28,12 @@ func (t Tap) sendLoop(ctx context.Context, c *ndp.Conn) error {
 			Direction: ndp.Source,
 			Addr:      t.Ifi.HardwareAddr,
 		},
-		ndp.NewMTU(uint32(t.Ifi.MTU)),
+	}
+
+	if *flagMTU == deviceMTU {
+		options = append(options, ndp.NewMTU(uint32(t.Ifi.MTU)))
+	} else if *flagMTU > omitMTU {
+		options = append(options, ndp.NewMTU(uint32(*flagMTU)))
 	}
 
 	if t.Prefix != nil {