[Cerowrt-devel] Routed LANs vs WOL & Windows troubles

Kevin Darbyshire-Bryant kevin at darbyshire-bryant.me.uk
Sun Apr 12 14:31:48 EDT 2015

On 12/04/2015 15:18, Alan Jenkins wrote:
> On 11/04/15 18:01, Kevin Darbyshire-Bryant wrote:
>> 1)  I've a central Windows based Home Server (WHS) with a Wake On Lan
>> facility - it dozes until a client appears on LAN/WLAN, sends a WOL
>> Magic packet.  Unfortunately the WOL Magic packets don't cross subnets
>> and the vast majority of clients are of the wireless variety.  Some sort
>> of WOL forwarding/proxying on the router would seem the way to go.  Has
>> anyone been here/solved it already?
> In theory wol is (optionally) used over udp.  I guess you need to set
> a static arp entry on the router.  Otherwise it will forget what the
> MAC address was for the IP you're sending to.  I've used `arp -f
> /etc/ethers` at boot on debian.  Not tested it on openwrt, or for wol
> and I know wol can be annoying.
> Apple (at least used to, I read some complaints recently) got wol
> working with Mac servers, Airport routers and mdns magic; I think the
> client didn't need to know wol (maybe just required some part of
> standardized mdns or dns-sd?).  I don't expect cross-subnet clients
> was a big selling point, but it sounded like there was potential there.
> Problem is home servers didn't really take off in a big way before
> Cloud, and modern products (like NAS, or my plug-computer) are lower
> power things that don't benefit so much from wol.  When/if subnetting
> gets popular I'm not sure anyone's going to be fixing up wol.  Outside
> of managing corporate PCs, hmm, where some form of "wol relay agent"
> sounds plausible enough it'll exist already.
It acts not just as a file store but also as a Windows update server,
consolidating & updating a 5 or so Windows boxes.
>> 2) I have a 'WSD' printer/multifunction device on the LAN, an Epson
>> something or other.  It can communicate across subnets (ping) without
>> issue but it always appears 'offline' as a WSD printer.  I can use the
>> scanner functionality no problem at all :-)
> pass :).
This appears to have been solved - removed & readded the device.  My
guess is that despite using dns (via dnsmasq) that windows doesn't like
the underlying IP changing.  Requires a bit more investigating on the 4
other windows clients to see if the issue is replicated.
>> 3) Windows and its firewall.  Windows likes its firewall on.  It only
>> likes to talk to things on the local attached subnet.  Windows by
>> default won't reply to pings across subnets and it certainly doesn't
>> like doing file sharing.  It would be wonderful if there was a nice easy
>> way (via DHCP?) of telling it 'trust 172.30.42/24' (or even my IPV6
>> equivalent /56)  Has anyone else fallen in to this?  Solved it?
> You only need to "unblock" on the server side, right?  Which is
> annoying, but shouldn't be too bad for someone who does WHS?  I assume
> you found a way to configure this manually, that's not your question. 
> I think there isn't a fully-automatic way to "unblock" for a home
> network.
Actually not just server side.  Client to client file sharing across
subnets is broken.  So the quick'n'dirty solution is to turn windows
firewall off to evaluate if things will work if other impediments are
solved (samba provided WINS on the router)

I've both Samba & avahi running on the router, in theory configured to
do the required SMB/WINS name collecting/forwarding.  Similar with Avahi
for mDNS stuff.

I'm still struggling but will put more effort in. 
> Discovery & name resolution is a potential issue.
> The "easy answer" is to forget discovery aka "network neighborhood"
> between different subnets.  Just use IP addresses or DNS names. 
> dnsmasq seems to take care of name resolution nicely for me, I get DNS
> names for my hosts without manually configuring dnsmasq.  If you don't
> use DHCP for the WHS (i.e. purely static IP), you'll need to add a
> manual entry to dnsmasq.  You don't get p2p name resolution (LLMNR
> nowadays) between different subnets.
> Discovery is done in "enterprise", so there must be a modern
> mechanism.  I'd expect using DNS, although I think there's some
> craziness about making different things visible to different users.  I
> don't know how hard it is to admin and/or whether samba serves it.
> The slightly harder answer: Samba says there's a hack for
> discovery[1], but that the best solution is to run a WINS server.  You
> then set a WINS server option in DHCP.[2]  I expect it works, but
> myself I've only really used it for name resolution in a single
> subnet.  (So I could disable a bunch of windows name-resolution
> broadcasts with regedit). Don't know if your WHS will do WINS for you,
> it's kinda deprecated, you could always run samba on the router.
> [1]
> https://www.samba.org/samba/docs/man/Samba-HOWTO-Collection/NetworkBrowsing.html#id2585378
> [2]
> http://wiki.openwrt.org/doc/howto/dhcp.dnsmasq#configuring_dnsmasq_to_broadcast_wins_server_information
The Samba WINS server is almost working, seems to be advertising every
other box...except the server.  So close!
>> 4) (A bonus Monty Python question)  I've a second wireless access point
>> at the other end of the garden, attached by a suitable length of Cat 6.
>> Devices at mid travel point ideally roam from House wifi to Shed
>> wifi...but now they change IP address as well.  To be honest I'm not
>> sure how this actually works in a bridged environment either since the
>> MAC now migrates from local wireless bridge interface to local wired
>> interface and potentially back again as I wander around the garden...how
>> does it really know where to send frames to this magically roaming
>> device?
> Yes they can't keep the same IP address on a different subnet :). 
> There are common cases where you don't notice and it wouldn't matter.
> There are references for bridging.  Basically it's an optimization
> over flooding packets to every single port (old-style dumb hub).  As
> soon as you send a frame from your MAC, all the bridges/switches in
> between "learn" where you are now.  If the target isn't known yet, the
> frame is just flooded.
> Maybe this helps: http://computer.howstuffworks.com/ethernet12.htm
Toke has given some instruction on this.  After some sleep I may even
understand it :-)
>> It appears a lot of 'it just works' functionality is designed for
>> bridged LAN/WLAN scenarios and hates routed but maybe I've got the wrong
>> end of a stick.
> I think you're right & it's all built on sand :).  It's not obvious to
> me either though.  If I actually used any network devices, like you
> do, I probably wouldn't be bothering.  Not outside of r&d.
> It'd be interesting if we had a simple writeup to show how more
> efficient discovery should be, and the impact on wireless to justify
> the change.  You can see in principle e.g. resolving names through
> dnsmasq instead of mdns can avoid broadcasting to everyone to get data
> that's already known by the router.  But the impact that has on
> wireless is less obvious - the broadcasts have to use a minimal
> wireless rate, lower by orders of magnitude.  And that it affects
> everyone in range sharing that channel.
> And when people want to just make it work across subnets without extra
> development, they just re-implement flooding over IP.  Cough, I think
> I have Avahi configured that way on my router, for linux service
> discovery...  Optimistically, someone will get it right, standardize
> it (DNS), and then vendors _have_ to use the efficient protocol
> because that's what the routers implement.

Discovered that a couple of iphone based apps for my Sky set top box,
Yamaha AV Receiver & TV won't do device discovery either.

Battling on,


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