Wintun NeighborDiscoverySupported

Brad Spencer bspencer at
Thu Sep 9 14:32:01 UTC 2021

We have noticed that Windows seems to try to send ARP requests over 
Wintun interfaces.  In our configurations, these don't go anywhere and 
get no responses, but the ARP table fills up with addresses.  For example:

$  arp -a -N |wc -l

Our Wintun interfaces are created with these properties:

PS C:\> Get-NetIPInterface -InterfaceIndex 69 |Select-Object -Property 
AddressFamily,NeighborDiscoverySupported,NeighborUnreachabilityDetection |fl

AddressFamily                   : IPv6
NeighborDiscoverySupported      : Yes
NeighborUnreachabilityDetection : Enabled

AddressFamily                   : IPv4
NeighborDiscoverySupported      : Yes
NeighborUnreachabilityDetection : Enabled

We _think_ that the NeighborDiscoverySupported property being Yes means 
that Windows issues ARP requests for addresses on the Wintun interface.  
(The second property controls another neighbour behaviour that perhaps 
could also be disabled for Wintun interfaces.)

Microsoft's documentation for SetIpInterfaceEntry() claims that these 
properties are set "by the network stack", but offers no hint as to what 
part of the network stack does this.  It does not seem possible to 
change these properties on an existing interface.

Are these properties that either the Wintun driver or user-space API is 
able to control?  Should they be set to No and Disabled?  I don't see 
any use of the driver variant of SetIpInterfaceEntry() or 
MIB_IPINTERFACE_ROW in Wintun's driver or API code, so I'm not sure when 
these properties get determined.

Brad Spencer

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