From 5b1dc295fea6d0f51c73798e92e0a98b914b94f1 Mon Sep 17 00:00:00 2001
From: Quentin McGaw <quentin.mcgaw@gmail.com>
Date: Thu, 2 Oct 2025 16:47:31 +0000
Subject: [PATCH] Return an error if all MTUs failed to test

---
 internal/pmtud/pmtud.go | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/internal/pmtud/pmtud.go b/internal/pmtud/pmtud.go
index 1cdabe2c..9ad90e35 100644
--- a/internal/pmtud/pmtud.go
+++ b/internal/pmtud/pmtud.go
@@ -12,6 +12,8 @@ import (
 	"golang.org/x/net/icmp"
 )
 
+var ErrICMPAllMTUsFailed = errors.New("all MTU sizes tested failed")
+
 // PathMTUDiscover discovers the maximum MTU for the path to the given ip address.
 // If the physicalLinkMTU is zero, it defaults to 1500 which is the ethernet standard MTU.
 // If the pingTimeout is zero, it defaults to 1 second.
@@ -138,7 +140,7 @@ func pmtudMultiSizes(ctx context.Context, ip netip.Addr,
 		}
 
 		if tests[0].mtu == minMTU+1 { // All MTUs failed.
-			return minMTU, nil
+			return 0, fmt.Errorf("%w", ErrICMPAllMTUsFailed)
 		}
 		// Re-test with MTUs between the minimum MTU
 		// and the smallest next MTU we tested.