On Fri, 2018-01-05 at 14:58 +0100, Thomas Haller wrote: > On Thu, 2018-01-04 at 20:22 +0100, Marcel Holtmann wrote: > > 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. Dan _______________________________________________ networkmanager-list mailing list networkmanager-list@gnome.org https://mail.gnome.org/mailman/listinfo/networkmanager-list
