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Revert to VPN original MTU (set by WIREGUARD_MTU for example) if ICMP fails

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This commit is contained in:
Quentin McGaw
2025-10-03 14:16:47 +00:00
parent b6e873cf25
commit c0ebd180cb
2 changed files with 6 additions and 4 deletions
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9
internal/vpn/tunnelup.go
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@@ -95,6 +95,8 @@ func updateToMaxMTU(ctx context.Context, vpnInterface string,
return fmt.Errorf("getting VPN interface by name: %w", err)
}
originalMTU := link.MTU
// Note: no point testing for an MTU of 1500, it will never work due to the VPN
// protocol overhead, so start lower than 1500 according to the protocol used.
const physicalLinkMTU = 1500
@@ -123,10 +125,9 @@ func updateToMaxMTU(ctx context.Context, vpnInterface string,
case err == nil:
logger.Infof("Setting VPN interface %s MTU to maximum valid MTU %d", vpnInterface, vpnLinkMTU)
case errors.Is(err, pmtud.ErrMTUNotFound):
const conservativeMTU = 1300
vpnLinkMTU = conservativeMTU
logger.Infof("Setting VPN interface %s MTU to a conservative MTU of %d (due to: %s)",
vpnInterface, conservativeMTU, err)
vpnLinkMTU = int(originalMTU)
logger.Infof("Reverting VPN interface %s MTU to %d (due to: %s)",
vpnInterface, originalMTU, err)
default:
return fmt.Errorf("path MTU discovering: %w", err)
}