chore(pmtud): clarify debug logs and fix log error message

internal/pmtud/icmp/icmp.go

@@ -20,17 +20,17 @@ func PathMTUDiscover(ctx context.Context, ip netip.Addr,
 	physicalLinkMTU uint32, timeout time.Duration, logger Logger,
 ) (mtu uint32, err error) {
 	if ip.Is4() {
-		logger.Debug("finding IPv4 next hop MTU")
+		logger.Debugf("finding IPv4 next hop MTU to %s", ip)
 		mtu, err = findIPv4NextHopMTU(ctx, ip, physicalLinkMTU, timeout, logger)
 		switch {
 		case err == nil:
 			return mtu, nil
 		case errors.Is(err, errTimeout) || errors.Is(err, ErrCommunicationAdministrativelyProhibited): // blackhole
 		default:
-			return 0, fmt.Errorf("finding IPv4 next hop MTU: %w", err)
+			return 0, fmt.Errorf("finding IPv4 next hop MTU to %s: %w", ip, err)
 		}
 	} else {
-		logger.Debug("requesting IPv6 ICMP packet-too-big reply")
+		logger.Debugf("requesting IPv6 ICMP packet-too-big reply from %s", ip)
 		mtu, err = getIPv6PacketTooBig(ctx, ip, physicalLinkMTU, timeout, logger)
 		switch {
 		case err == nil:
@@ -43,7 +43,7 @@ func PathMTUDiscover(ctx context.Context, ip netip.Addr,
 
 	// Fall back method: send echo requests with different packet
 	// sizes and check which ones succeed to find the maximum MTU.
-	logger.Debug("falling back to sending different sized echo packets")
+	logger.Debugf("falling back to sending different sized echo packets to %s", ip)
 	minMTU := constants.MinIPv4MTU
 	if ip.Is6() {
 		minMTU = constants.MinIPv6MTU

internal/pmtud/pmtud.go

@@ -13,7 +13,10 @@ import (
 	"github.com/qdm12/gluetun/internal/pmtud/tcp"
 )
 
-var ErrPMTUDFailICMPAndTCP = errors.New("PMTUD succeeded with ICMP but failed with TCP")
+var (
+	ErrICMPOkTCPFail   = errors.New("PMTUD succeeded with ICMP but failed with TCP")
+	ErrICMPFailTCPFail = errors.New("PMTUD failed with both ICMP and TCP")
+)
 
 // PathMTUDiscover discovers the maximum MTU using both ICMP and TCP.
 // Multiple ICMP addresses and TCP addresses can be specified for redundancy.
@@ -77,8 +80,11 @@ func PathMTUDiscover(ctx context.Context, icmpAddrs []netip.Addr, tcpAddrs []net
 				return maxPossibleMTU, nil // only rely on ICMP PMTUD results
 			}
 		}
-		return 0, fmt.Errorf("%w - ignoring ICMP obtained MTU %d",
-			ErrPMTUDFailICMPAndTCP, maxPossibleMTU)
+		if icmpSuccess {
+			return 0, fmt.Errorf("%w - discarding ICMP obtained MTU %d",
+				ErrICMPOkTCPFail, maxPossibleMTU)
+		}
+		return 0, fmt.Errorf("%w", ErrICMPFailTCPFail)
 	}
 	logger.Debugf("TCP path MTU discovery found maximum valid MTU %d", mtu)
 	return mtu, nil