Wintun NeighborDiscoverySupported

[Brad Spencer]

On 2021-09-09 5:23 p.m., Alan Graham wrote:
> Adding the 0.0.0.0/0 (or ":::") to the config is what is causing some
> growth of the arp table, but it is not growing indefinitely.  After
> looking around for SupportsNeighborDiscovery and finding nothing, I
> decided to check the repro.  When not routing all traffic through an
> interface the arp cache is basically static:

Thanks for looking into this, too.

I also suspect having the gateway set like this is probably necessary 
for Windows to start adding ARP entries.  How were you able to determine 
that it is also sufficient?  My guess is that if it is possible to 
indicate to Windows that the interface does not support neighbour 
discovery in general, doing so likely prevents ARP entries regardless of 
the gateway values.

BTW, how did you determine that it does not grow indefinitely?

> So I tend to agree with Jason that this is "harmless" and shouldn't
> cause any serious problems.  It would be nice for Microsoft to fix
> Set-NetIPInterface, it looks like a bug that SupportsNeighborDiscovery
> can't be set.

One "harm" might be that the OS keeping an easy-to-query list of all 
(recent?) destinations in the ARP table, which could be undesirable.

I'm not sure it's a bug, per se.  It seems by design that you cannot 
change the value of SupportsNeighborDiscovery after the interface is 
created.  The documentation for SetIpInterfaceEntry()[1] says:

> The MaxReassemblySize, MinRouterAdvertisementInterval, 
> MaxRouterAdvertisementInterval , Connected, SupportsWakeUpPatterns, 
> SupportsNeighborDiscovery, SupportsRouterDiscovery, ReachableTime, 
> TransmitOffload, and ReceiveOffload members of the MIB_IPINTERFACE_ROW 
> structure pointed to by the Row are ignored when the 
> SetIpInterfaceEntry function is called. These members are set by the 
> network stack and cannot be changed using the SetIpInterfaceEntry 
> function.

I noticed that the Cmdlet in PowerShell seems to treat the 
-NeighborDiscoverySupported option as an input filter vs. a value that 
you can set.  While this surprised me, it is at least consistent with 
the Win32 API docs.

1. 
https://docs.microsoft.com/en-us/windows/win32/api/netioapi/nf-netioapi-setipinterfaceentry#remarks 


-- 
Brad Spencer