[systemd-devel] network unit Match by router advertisement?

[Tom Gundersen]

On Wed, May 11, 2016 at 7:31 PM, Lennart Poettering <lennart at poettering.net>
wrote:

> On Wed, 11.05.16 11:32, Brian Kroth (bpkroth at gmail.com) wrote:
>
> > Hi again all,
> >
> > TL;DR: would it be possible (or make sense) to have systemd Match rules
> for
> > network units that could match on some artifact of the network the link
> is
> > attached to like vlan tag, router advertisement, wireless access point or
> > gateway mac, etc.?
>
> Well, .network files contain the definition how to set up a network
> interface, i.e. how to place it into UP state so that packets can be
> received and how to configure IP routing so that communication further
> on works. Hence: it uses relatively static properties of the device
> that are already available when the device is offline, to find the
> right .network file to read the dynamic configuration to apply in
> order to put it online. The router advertisment info and things like
> the gateway mac are pieces of information that are only available when
> the network is already up, when the network configuration is already
> applied. Hence using that as match for the configuration can't work:
> at the time we could use that information we already would have had to
> apply it. And if we don't apply it, we would never get the information
> to acquire...
>
> The VLAN tag is a different case though: it's assigned when the
> VLAN networkd device is created, and configured in the .netdev
> configuration file for that. Thus, it's already set the moment the
> network device pops up, and it could be used nicely for the
> matching. So yupp, added a MatchVLANId= or so, might make
> sense. Please file an RFE issue on github about this, if you'd like to
> see this implemented.
>
> Matching by AP could work. Iirc today's WLAN drivers actually will
> create virtual links for the network you connect to, and the ESSID for
> each would be set before networkd would take notice of it, hence this
> is probably something we could do. Note however, that networkd does
> not interface with the WLAN stack at all at this point, a WLAN device
> is treated like any other Ethernet device atm


It was always my intention to extend the Match logic to be able to do this
kind of matching on the environment. But as Lennart says, it would have to
be only based on things we can passively observe. I think it would possibly
be acceptable to eagerly UP an interface if it matches all but the
"environment" matches (and that is essentially how WLAN must work I guess,
though the UP might in that case be done by someone other than us), thought
that stuff definitely requires some thinking. I agree that VLANId is
totally fine to match on, and that ESSID (and probably similar things for
Bluetooth etc) would be fine if they are anyway available. But yeah,
whether to match on things that requires us to UP the interface would need
to be discussed some more.


> > However, the missing bit then would be network address assignment for the
> > various instances to the right interfaces.  Ideally, I'd just stamp out
> > network unit files and have the apache instance units depend upon that,
> but
> > the trouble is that traditionally NIC naming hasn't always been
> consistent
> > in the past.
> >
> > I've read through [1], but it doesn't really provide what I'm looking
> for.
> > Physical layout of the nic-port-types is semi interesting and perhaps
> > consistent, but network operator error may result in a misassigned vlan
> > port, or simply the wrong cable to the wrong port (which can be true for
> > physical or virtual realms unfortunately), etc.
> >
> > What I did in the past to work around that was to use ndisc6 or something
> > similar to verify that the expected interface had the expected network
> > properties - in this case a router advertisement.
>
> Hmm, schemes like this sound a bit dangerous, no? I mean, if you base
> your decision whether to apply the relatively open "internal LAN"
> config to an interface or the restricted "internet" config on the
> traffic you see on the port, then you make yourself vulnerable to
> people sending you rogue IP packets...
>
> I see your usecase though, but I don't really have any good suggestion
> what to do in this case I must say...
>
> Maybe adding something like a RequireDHCPServer= setting or so, that
> allows configuration of a DHCP server address, and when set would
> result in logged warnings if DHCP leases are offered from other
> servers thatn the configured one, might be an option? i.e. so that you
> at least get a loggable event when some .network file is applied to
> the wrong iface?
>
> But dunno, maybe Tom has an idea about this? Tom?
>

I'm very skeptical about these kind of schemes. We can really not promise
anything about where DHCP/NDisc come form. If someone has access to the
local lan, they can spoof absolutely anything, so we better not make the
impression that we can guarantee anything we can't. If you want some sort
of sanity checking, that might make sense, but probably as some external
ad-hoc tool rather than inside networkd.

>
> > [2] Sidenote: In the past I've used an old trick of setting the
> > preferred_lft to 0 for IPv6 addresses that I wanted to be available to
> > services, but not selected for outbound connections from the host.  This
> > was basically to help influence the usual source address selection
> criteria
> > which tries to avoid "deprecated" addresses.  I didn't see a way to
> specify
> > that in the systemd.network man page.  Is there one that I'm missing, or
> is
> > that another case for an Exec... statement?
>
> This has been added very recently to systemd, see #3102, #2166,
> b5834a0b38c1aa7d6975d76971cd75c07455d129. It will be available with
> the next release.
>
> Lennart
>
> --
> Lennart Poettering, Red Hat
>
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