hotfix(pmtud): detect IPv6 usage in VPN connection

internal/pmtud/ip/source.go

@@ -4,7 +4,6 @@ import (
 	"errors"
 	"fmt"
 	"net/netip"
-	"strings"
 
 	"github.com/jsimonetti/rtnetlink"
 	"github.com/qdm12/gluetun/internal/pmtud/constants"
@@ -28,10 +27,7 @@ func SrcAddr(dst netip.AddrPort, proto int) (src netip.AddrPort, cleanup func(),
 	return netip.AddrPortFrom(srcAddr, srcPort), cleanup, nil
 }
 
-var (
-	errNoRoute            = errors.New("no route to destination")
-	ErrNetworkUnreachable = errors.New("network unreachable")
-)
+var errNoRoute = errors.New("no route to destination")
 
 func srcIP(dst netip.Addr) (netip.Addr, error) {
 	conn, err := rtnetlink.Dial(nil)
@@ -54,9 +50,6 @@ func srcIP(dst netip.Addr) (netip.Addr, error) {
 	}
 	messages, err := conn.Route.Get(requestMessage)
 	if err != nil {
-		if strings.Contains(err.Error(), "network is unreachable") {
-			err = ErrNetworkUnreachable
-		}
 		return netip.Addr{}, fmt.Errorf("getting routes to %s: %w", dst, err)
 	}
 

internal/pmtud/tcp/mss.go

@@ -43,9 +43,6 @@ func findHighestMSSDestination(ctx context.Context, familyToFD map[int]fileDescr
 			case err != nil: // error already occurred for another findMSS goroutine
 			case errors.Is(result.err, iptables.ErrMarkMatchModuleMissing):
 				err = fmt.Errorf("finding MSS for %s: %w", result.dst, result.err)
-			case dst.Addr().Is6() && errors.Is(result.err, ip.ErrNetworkUnreachable):
-				// silently discard IPv6 network unreachable errors since they are common
-				// and expected when the host doesn't have IPv6 connectivity
 			default: // another error not due to the match module missing
 				logger.Debugf("finding MSS for %s failed: %s", result.dst, result.err)
 			}

internal/pmtud/vpn.go

@@ -1,21 +1,19 @@
 package pmtud
 
 import (
-	"net/netip"
-
 	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/constants/vpn"
 	pconstants "github.com/qdm12/gluetun/internal/pmtud/constants"
 )
 
 // MaxTheoreticalVPNMTU returns the theoretical maximum MTU for a VPN tunnel
-// given the VPN type, network protocol, and VPN gateway IP address.
+// given the VPN type, network protocol, and whether IPv6 is used.
 // This is notably useful to skip testing MTU values higher than this value.
 // The function panics if the network or VPN type is unknown.
-func MaxTheoreticalVPNMTU(vpnType, network string, vpnGateway netip.Addr) uint32 {
+func MaxTheoreticalVPNMTU(vpnType, network string, ipv6 bool) uint32 {
 	const physicalLinkMTU = pconstants.MaxEthernetFrameSize
 	vpnLinkMTU := physicalLinkMTU
-	if vpnGateway.Is4() {
+	if !ipv6 {
 		vpnLinkMTU -= pconstants.IPv4HeaderLength
 	} else {
 		vpnLinkMTU -= pconstants.IPv6HeaderLength