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Re: How does IWD handle setting MAC address?

Hi Thomas,


I am in favor of address randomization even while it has
limited
affect, but at least for background scanning it is useful.
However
doing this via RTNL is causing a weird layer violation and
all
sorts
of potential races and issues. This needs to be done with
full
awareness of cfg80211 and thus via nl80211. So iwd should
do
it.
And
iwd should just expose an on/off switch for WiFi Privacy.


TL;DR: the policy of which MAC address to use (and when) is
flexible
and present in NetworkManager configuration. And it's more
then a
simple randomize on/off switch.

===
A smaller reason is, that some people have strong opinions
and
consider
important which bits of the address to scramble (and choose a
well
known manufacturer OUI)[1].
I personally don't agree with the importance of such
considerations,
but I'd like NetworkManager to be the first choice for people
with
this
particular need -- regardless of whether this need is real or
only
perceived.
In NM you can configure how the bits are scrambled very
flexible.
Both
while scanning[2] and while being associated[3].

More interesting is, I don't only want to have a random MAC
address
while scanning, but also while being associated. My permanent
MAC
address should never ever be reveiled.
But a new random MAC address on each new association isn't
exactly
what you
want either, because then I get a new IP address from DHCP
each
time and have
to redo captive portal login.
So, I want for each of my Wi-Fi profiles a different, stable
MAC
address. Actually, for public networks like a hotel, I want
to
use
a
stable MAC address for a limited amount of time. The example
in
[4]
show how to do that in NM.
===


I have nothing against an option that says generate a new MAC
address
for this SSID and keep using it from that time forward.


If I understand correctly, you agree that the MAC address depends
on
the profile.



It is a bit counterproductive if nl80211 doesn’t allow to
specify
the
MAC address for association. Since powering down WiFi, changing
the
address and powering back up is something that I am strictly
against.

So if these things are what people really want, then neither NM
nor
iwd should actually do the heavy lifting for it. It should be
done
by
the wireless stack in the kernel.


Ok, whatever works best.



That said iwd should cope Ok with the MAC address changing
behind
its
back if it receives the RTNL notification (RTM_NEWLINK) if
it
isn't
connected.  It always updates it's copy of the address on a
RTM_NEWLINK so the race condition shouldn't be present I
suppose.


I would think so too. NM change the MAC address via RTNL only
while
scanning, early during activation, and late during
deactivation.
As the wireless daemon does/should not autoactivate the
device
against
NM's wish and NM determines that the device is deactivated
only
after
an event from iwd.
Hence, there shouldn't be a race of NM interfering while
being
connected. The race is only while scanning and iwd should
just
cope
with that.

Alternatively/additionally, a SetMacAddress() D-Bus call
would
avoid
any race and allow to leave the decision which address to
user to
somebody closer to the user.


It will not be as simple as that. You need to leave iwd with
the
decision making for connecting to known WiFi networks. It just
isn’t
as dumb as wpa_supplicant and from a NM perspective, you should
be
doing as little as you do with BlueZ or oFono.

This means iwd needs to be told what to do and not just an
address.
It doesn’t matter if it is via a D-Bus call or RTNL. iwd
remembers
known networks and will connect to them if they are in range,
roam
automatically and also switch networks if it makes sense. That
means
any randomization policy would have to be executed inside iwd
and
not
outside. As stated above, if you want different MAC addresses
per
SSID, then that needs to be inside iwd.

So many things in the wpa_supplicant design led to “hacks”
outside
to
add features and that really has to stop. It is not
maintainable
and
the corner cases and race condition this architecture causes is
just
crazy.



For NM, at each moment not all its connection profiles are
candidate
for connecting automatically. The list of which profiles can be
autoactivated depends on NM internal state, for example
- is the profile even configured to allow autoactivation?
- is the user owning the connection logged in (if it's
restricted
 to a user)?
- if the profile requires secrets, is somebody previledged
around
 to potentially provide them?
- was the connection previously manually disconnected by the
user 
 (which marks it as blocked from autoconnecting again)
- did a previous connection attempt fail, e.g. no DHCP lease.
If  
 it failed $configurable times, it will be blocked for a few 
 minutes.

With supplicant, NM intersects the list of autoconnect candidates
with
the list from the scan-list, and decides which to (auto)
activate. As
far as supplicant is concerned, this is not happening
automatically,
and there is no race.

If I understand you, the reason to let iwd automatically pick a
network, is because iwd knows better.

But in case there are multiple autoconnect candidates that could
be
activated, then NM chooses the candidate which
- has the highest autoconnect priority (configurable)
- was used the least long ago.
Indeed, NM doesn't consider the signal strength and other Wi-Fi
properties. It's a missing feature.

How is iwd choosing automatically? Choosing based on signal
strength
and encryption parameters would be a nice feature, but what about
non-
Wi-Fi related factors.
How will iwd allow NM to contribute to that decision?


Note that choosing based solely on signal strength can be
problematic. 
It works great if you are somewhere that has only one AP you've
connected to before.  But the moment you have multiple different
SSIDs
that you've connected to before, it starts to have issues.

An example case was the old Red Hat (or was it Mozilla, I forget,
they
were right down the street from each other) office in Mountain
View,
which was just upstairs from a Starbucks.  Depending on where you
were
in the office, Starbuck's APs could be stronger than the office
ones. 
These days even "public" APs have strong encryption with automatic
login (HotSpot 2.0, EAP-SIM, etc) too.

---

Looking at the iwd code, it appears to:

1) only autoconnect to networks that have been successful at least
once
(see comment in network.c::network_rankmod())

2) BSSs are ranked according to factors in
scan.c::scan_bss_compute_rank() which is heavily biased towards
signal
strength.  After that, better encryption, 5G, and low utilization
(read
from an IE) is preferred.

3) then the BSS is added to its network object; network objects are
tracked in a list and the most recently connected networks since
IWD
has been running are first; the rest are in reverse-order-seen (see
network_info_get()).

4) when generating the autoconnect list, the BSS's rank from #2 is
multiplied by a "rankmod" number (<=1) that depends on where the
BSS's
network is in the list from #3 (device.c::process_bss()).  So BSSs
that
were previously connected to have a lower rank, and BSSs that
haven't
been connected to yet this IWD run could be even lower.

However, since the BSSs have ranks themselves, this modifier
appears to
allow situations where IWD would switch from SSID A to SSID B, even
if
A was still visible, when there is a much-stronger SSID B AP.  I
could
be wrong of course.  But this would break expectations around how
NM
currently works, where it holds on to the current SSID until the
connection is broken.

Perhaps this is desirable, maybe it allows the dual-channel AP
situation where for example you are on 5GHz SSID A and move to
another
room, so A becomes low signal, but the 2.4GHz SSID B is now much
stronger so IWD reconnects to that one.  However, this could result
in
an IP address change depending on how your AP works, which would
break
existing connections.  Which is one reason NM doesn't normally
switch
between SSIDs.

I'm sure Marcel will correct anything I've gotten wrong above.


a lot of these can be changed or fine-tuned while we are making iwd
better. However the big point is that iwd knowns about the known
networks and stores them. So we need to work with basic premise of
this. Same as BlueZ knowns its PAN devices and oFono knows its SIM
cards and APNs. That really has to be the assumption first and
foremost.


That BlueZ remembers PAN devices makes sense, because these devices
were paired outside of NetworkManager, using bluetooth tools.

BlueZ/oFono autonoumously connects? I didn't think that is the case, it
it? AFAIS, it's always NetworkManager which initiates the activation.


actually with iwd you can also use iwd tools to connect your WiFi, you do not need to go through NM. And we 
need to support that kind of interaction as well.

As I said before, I full realize that wpa_supplicant made you do everything, but with iwd that is no longer 
needed. For example you can have a dead simple UI element that just trigger WPS based connection. You do 
that via iwd and then move on with life. NM will pick up the new known network and its connection. 
Everybody will be happy.

iwd is different than wpa_supplicant and it is a change for the better :)


And yes, I know wpa_supplicant dealt everybody a bad hand and told
you to deal with it. However we need to change this mantra towards
something clean and modern. Especially since there are so many WiFi
extensions that will allow you to make decision that wpa_supplicant
will never give you access to. So lets figure out what is needed and
tune around that.




For the IP address part, I will assume that iwd will actually start
doing DHCP itself soon. That is just needed if you look at some of
the features that tell you about IP address during association or the
brain-dead things like P2P. We are toying with this, but I almost
certain this will go in this direction. Similar on how cellular
modems actually do it. The IP address is a property of the WiFi
daemon and not the daemon that manages the network connections.


With WWan/ModemManager, pppoe/pppd, VPNs, the IP addressing is also
negotiated outside of NM.
Also, supplicant supports DHCP
( https://w1.fi/cgit/hostap/tree/src/ap/fils_hlp.c#n150 )
-- although NM doesn't support that. It's a missing Wi-Fi feature, but
I don't see a fundamental issue with NM+supplicant+FILS).

But while these components negotiate IP addresses one way or another,
they only report the address/routes to NM, and NM might them.

Would iwd actively configure addresses/routes? If not, that is fine
and
not different from e.g. WWAN.


Routes is a clear no. That is never part of the interface itself. For the IP address that is something we 
need to discuss. So far we have stayed away assigning IP addresses in the technology daemon and just told 
the managing entity above what these were or to run DHCP.

So we think that DHCP needs to be in iwd (and for P2P that means client+server). We also arrived at the 
conclusion that BlueZ and oFono should do DHCP by themselves if no static IP addresses can be read. For 
DHCP to function nicely and efficiently however it is important that the IP address also gets configured on 
the interface. So I think that eventually we need to move towards that technology daemon controls the 
interface and its addresses.

This needs a bit more thinking and research on who configures the IP, but the DHCP part is clearly moving 
into iwd.


I think iwd configuring addresses is wrong. Because this affects
routing, which very much determines the system-wide behavior and needs
to interplay with the interfaces.
For example, in NetworkManager you can:
- Configure ipv4.route-metric. For example, if you connect to your
  home network both via cable and Wi-Fi, (configurably) cable will 
  be preferred.
  Or if you activate WWAN and Wi-Fi at the same time, the default-
  route gets a metric based on the device priority (configurably).
  (in some cases, the route-metric might even be determined the 
  moment when starting associating. In combination with iwd 
  autonomously connecting, you couldn't even configure the desired 
  route-metric in the iwd profile).


The route metric is clearly part of NM. That should not be in control of a technology daemon.


- configure ipv4.never-default: controls whether the interface will 
  get the default route.


See no problem here. Since iwd would never touch any routes.


- configure additional manual routes for that  interface. If iwd and
  NM both configure routes, this is racy.


No interest in routes.


- Configure ipv4.route-table. An uncommon feature, where you can 
  put the routes from that interface in a separate routing table for 
  policy routing.


Same as above, no interest in routes.


- protect routes on other interfaces so that a malicious DHCP server 
  cannot hijack traffic
  (https://bugzilla.gnome.org/show_bug.cgi?id=749376 ). While this is
  not implemented yet and hard to get right conceptually, it would
  be a great feature.


See also no conflict there.

As I said, the routes and all its details is really not iwd business. So we are clear and agree on that. 
The IP address itself and its configuration is something where my current thinking is that this is owned by 
the technology daemon. And yes, I realize the issue with IP conflicts and overlaps. This needs a bit more 
thinking and looking into what RTNL actually provides and lets you control.


actually I take this back a little. Having spent the weekend reading up on VRF and policy routing, I think 
that iwd should actually maintain the routes for its wlan0 interface (including default route or other routes 
learned via DHCP). However it should do that in a separate routing table. I think that NM should tell iwd 
what routing table id it wants it to use and then iwd owns the IP configuration of its wlan0 interface and 
manages routes for wlan0 in the given routing table with that id.

Having the routes in a separate routing table then gives NM an easy way to utilize the, either with routing 
policy rules or with VRF. While the VRF and the l3mdev + cgroups seems to be super powerful if you want to 
say have one app go via LTE and the other via WiFi. Or even confine a Miracast app into the P2P network. Just 
some ideas to think about.

Regards

Marcel
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