From 007c5159f4cb7d7f7eabdbad53b0e8affd9986b8 Mon Sep 17 00:00:00 2001
From: Quentin McGaw <quentin.mcgaw@gmail.com>
Date: Thu, 19 Feb 2026 17:24:06 +0000
Subject: [PATCH] hotfix(pmtud): increase TCP margin from 150 to 300 compared
 to ICMP found MTU

---
 internal/pmtud/pmtud.go    | 7 ++++---
 internal/pmtud/test/mtu.go | 7 +++++--
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/internal/pmtud/pmtud.go b/internal/pmtud/pmtud.go
index 716f416f..edf07ca6 100644
--- a/internal/pmtud/pmtud.go
+++ b/internal/pmtud/pmtud.go
@@ -13,7 +13,7 @@ import (
 	"github.com/qdm12/gluetun/internal/pmtud/tcp"
 )
 
-var ErrPMTUDFailICMPAndTCP = errors.New("PMTUD failed with both ICMP and TCP")
+var ErrPMTUDFailICMPAndTCP = errors.New("PMTUD succeeded with ICMP but failed with TCP")
 
 // PathMTUDiscover discovers the maximum MTU using both ICMP and TCP.
 // Multiple ICMP addresses and TCP addresses can be specified for redundancy.
@@ -66,7 +66,7 @@ func PathMTUDiscover(ctx context.Context, icmpAddrs []netip.Addr, tcpAddrs []net
 		minMTU = constants.MinIPv6MTU
 	}
 	if icmpSuccess {
-		const mtuMargin = 150
+		const mtuMargin = 300
 		minMTU = max(maxPossibleMTU-mtuMargin, minMTU)
 	}
 	mtu, err = tcp.PathMTUDiscover(ctx, tcpAddrs, minMTU, maxPossibleMTU, tryTimeout, fw, logger)
@@ -77,7 +77,8 @@ func PathMTUDiscover(ctx context.Context, icmpAddrs []netip.Addr, tcpAddrs []net
 				return maxPossibleMTU, nil // only rely on ICMP PMTUD results
 			}
 		}
-		return 0, fmt.Errorf("%w", ErrPMTUDFailICMPAndTCP)
+		return 0, fmt.Errorf("%w - ignoring ICMP obtained MTU %d",
+			ErrPMTUDFailICMPAndTCP, maxPossibleMTU)
 	}
 	logger.Debugf("TCP path MTU discovery found maximum valid MTU %d", mtu)
 	return mtu, nil
diff --git a/internal/pmtud/test/mtu.go b/internal/pmtud/test/mtu.go
index e66976fb..9d3acaa0 100644
--- a/internal/pmtud/test/mtu.go
+++ b/internal/pmtud/test/mtu.go
@@ -1,6 +1,9 @@
 package test
 
-import "math"
+import (
+	"fmt"
+	"math"
+)
 
 // MakeMTUsToTest determines a slice of MTU values to test
 // between minMTU and maxMTU inclusive. It creates an MTU
@@ -17,7 +20,7 @@ func MakeMTUsToTest(minMTU, maxMTU uint32) (mtus []uint32) {
 	diff := maxMTU - minMTU
 	switch {
 	case minMTU > maxMTU:
-		panic("minMTU > maxMTU")
+		panic(fmt.Sprintf("minMTU %d is greater than maxMTU %d", minMTU, maxMTU))
 	case diff <= mtusLength:
 		mtus = make([]uint32, 0, diff)
 		for mtu := minMTU; mtu <= maxMTU; mtu++ {