Re: Dynamic link-local address not assigned after collision

["Muggeridge, Matt" <matt.muggeridge2@xxxxxxx>]

> -----Original Message-----
> From: Andrei Borzenkov <arvidjaar@xxxxxxxxx>
> Sent: Tuesday, 7 January 2025 4:53 PM
> To: Muggeridge, Matt <matt.muggeridge2@xxxxxxx>; systemd-
> devel@xxxxxxxxxxxxxxxxxxxxx
> Subject: Re:  Dynamic link-local address not assigned after
> collision
> 
> 07.01.2025 06:01, Muggeridge, Matt wrote:
> > I have two systems where I need them to assign link-local addresses in a
> 169.254.4/24 subnet, regardless of any other addresses configured. I'm using a
> /24 subnet because I have multiple interfaces on the system that require link-
> local addresses; I was placing them in their own subnets.
> >
> >  From this
> comment<https://urldefense.com/v3/__https://www.freedesktop.org/software
> /systemd/man/latest/systemd.network.html*:*:text=If*20an*20IPv4*20link*2D
> local*20address*20(169.254.0.0/16)*20is*20specified*2C*20IPv4*20Address*
> 20Conflict*20Detection*20(RFC*205227)*20is*20enabled*20for*20the*20ad
> dress__;I34lJSUlJSUlJSUlJSUlJSUlJSUlJQ!!NpxR!mUKARk40sTkVB6_bBo-
> PwV_8cNH1BnXhG-Rw5RKPlgiLC-wcytrCwj-s_ecVsyLGqy2aUGKa1TplpOp-
> JEL3dA$ .> in the man page, I was expecting I could configure the interface
> address identically on both systems and leave it to collision detection to pick an
> appropriate available address.
> >
> > The 1st interface assigns its link-local address (169.254.4.2/24). Unfortunately,
> the next interface to start doesn't assign any link-local address. I've tried to get
> more debug information by increasing log-level to debug, but I get no indications
> around the address assignments.
> >
> > As an experiment, I change the 2nd system to use a different host address in
> the same subnet, and it gets assigned correctly.  I guess that collision detection is
> preventing the address assignment.
> >
> > How do I achieve this or how can I get more debug information?
> >
> > The relevant snippet from the network config file is:
> >
> > [Network]
> > Address=169.254.4.2/24
> >
> 
> This is not "dynamic link-local address". It is the fixed address you assigned. If
> this address is not available, why do you expect systemd-networkd to silently
> substitute it with another one?

It behaves this way when using IPv4LLStartAddress. If the address is in use, it
chooses another from the 169.254/16 range.

The man page for "[Network]" section "Address=" property also states that
if it's assigning an IPv4LL address, then it uses IPv4 Address Conflict Detection
(RFC5227).  That RFC talks about trying a new address.  Here's an extract:

     The configuring
     software may choose to cease network operation, or it may
     automatically select a new address so that the host may re-establish
     IP connectivity as soon as possible.

So networkd could choose to select a new address.  Currently, it chooses
not to try a new address, leaving the interface unusable.

> 
> Link local addressing is more than just the specific prefix. It is a separate
> procedure to configure interface that follows well defined rules for selecting
> addresses to use. You cannot really mix it with fixed address assignment.
> 
> What could be done is supporting something like
> 
> Address=169.254.4.0/24
> 
> with the semantic "pick any unused address in this range".
> 

Yes, that's exactly what I'm after. I've been modifying ipv4ll_pick_address()
in src/libsystemd-network/sd-ipv4ll.c to achieve this. Hopefully, I'll have some
results today.

> >
> > Full contents of the network configuration follows:
> >
> > $ networkctl cat 10-eno0.network
> > # /etc/systemd/network/10-eno0.network
> > [Match]
> > KernelCommandLine=!nfsroot
> > Name=eno0
> >
> > [DHCP]
> > ClientIdentifier=mac
> > RouteMetric=10
> > UseDomains=yes
> > UseHostname=yes
> > UseMTU=yes
> >
> > [IPv6AcceptRA]
> > UseRedirect=no
> >
> > [Link]
> > RequiredForOnline=no
> >
> > [Network]
> > DHCP=yes
> > Domains=labs.company.net
> > NTP=10.10.135.123
> > # LinkLocalAddressing=yes
> > # IPv4LLStartAddress=169.254.1.1
> > Address=169.254.4.2/24
> >
> > # /usr/lib/systemd/network/10-eno0.network.d/10-network.conf
> > [Network]
> > MulticastDNS=yes
> >
> > Regards,
> > Matt.
> >
> >