Specifying link-local address in rc.conf

[Michael Gmelin]

On Tue, 7 Jul 2020 12:08:35 +0200
Michael Gmelin <freebsd at grem.de> wrote:

> On Mon, 6 Jul 2020 22:58:54 +0200
> Niclas Zeising <zeising+freebsd at daemonic.se> wrote:
> 
> > On 2020-07-06 13:05, Niclas Zeising wrote:  
> > > Hi!
> > > Is it possible to specify a link-local address in rc.conf, and get
> > > only that link-local address?
> > > 
> > > When I add a specific link-local address, such as fe80::1/64, to
> > > an interface from the command line, and then add a global uincast
> > > address, it works as expected, I do not get an additional
> > > link-local address. When I try to do the same in rc.conf however,
> > > it does not work. I have added the following:
> > > ifconfig_vtnet0_inet6="inet6 fe80::1/64"
> > > ifconfig_vtnet0_alias0="2001:6b8::1/64"
> > > to rc.conf, but when restarting, I get both fe80::1 and a
> > > EUI64-based link-local address assigned to the interface.  It does
> > > not matter which order I specify the entries in rc.conf.
> > > I have tried changing the sysctl net.inet6.ip6.auto_linklocal to 0
> > > in /etc/sysctl.conf, but that does not work (it seems it takes
> > > effect too late, the interface still has the AUTO_LINKLOCAL flag
> > > set according to ifconfig).
> > > I tried adding -auto_linklocal to the ifconfig_* lines in
> > > /etc/rc.conf, and while this removes the AUTO_LINKLOCAL flag from
> > > the interface, a EUI64 based link-local address is still assigned
> > > to the interface.
> > > 
> > > While there might not be much of a problem having mulitple
> > > link-local addresses, this is surprising.  Adding a specific
> > > link-local address to an interface, even from rc.conf, shouldn't
> > > mean that the EUI64-one is created.  Perhaps something like
> > > ifconfig_IF_linklocal="" should be added, or this is a bug in
> > > rc.network.  I tried looking through rc.network and related rc
> > > files, but I didn't find anything obvious, and I couldn't really
> > > figure out how addresses are added to interfaces during boot.
> > > 
> > > This is causing issues when I want for instance rtadvd to use the 
> > > fe80::1/64 address as source for router advertisements, since it
> > > picks the EUI64 link-local address as source (which means this
> > > also ends up in client routing tables as the default gateway).  I
> > > can work around this problem by adding fe80::1/64 with
> > > prefer_source, but I would prefer if there only was one
> > > link-local address on an interface.
> > > 
> > > All of this is on FreeBSD 12.1.
> > > 
> > > Perhaps I'm just missing something, but this behavior feels a
> > > little surprising.
> > > 
> > > I can share more detailed configuration, and perhaps the whole
> > > virtual machine I'm testing on, if needed.
> > > 
> > > Regards    
> > 
> > I did some more digging.  It only happens when there's also an 
> > ifconfig_IF for IPv4 in the /etc/rc.conf.
> > 
> > Not working configuration (/etc/rc.conf)
> > ifconfig_vtnet0="192.168.0.1/24"
> > ifconfig_vtnet0_ipv6="inet6 fe80::1/64"
> > ifconfig_vtnet0_alias0="inet6 2001:6b8::/64"
> > 
> > This configuration creates an EUI64-based link-local addess on
> > vtnet0 as well.
> > 
> > Working configuration (apart from no IPv4)
> > ifconfig_vtnet0_ipv6="inet6 fe80::1/64"
> > ifconfig_vtnet0_alias0="inet6 2001:6b8::/64""  
> 
> >   
> 
> This works:
> 
> ifconfig_vtnet0="inet6 -auto_linklocal"
> ifconfig_vtnet0_ipv6="up"
> ifconfig_vtnet0_aliases="\
>   inet 192.168.0.1/24 inet6 fe80::1/64 inet6 2001:6b8::/64"
> 
> # ifconfig vtnet0
> vtnet0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST>
> metric 0 mtu 1500 options=80028<VLAN_MTU,JUMBO_MTU,LINKSTATE>
>         ether 58:9c:fc:0b:99:97
>         inet 192.168.0.1 netmask 0xffffff00 broadcast 192.168.0.255
>         inet6 fe80::1%vtnet0 prefixlen 64 scopeid 0x2
>         inet6 2001:6b8:: prefixlen 64
>         media: Ethernet 10Gbase-T <full-duplex>
>         status: active
>         nd6 options=1<PERFORMNUD>
> 
> -m
> 

This works too:

ifconfig_vtnet0="inet"
ifconfig_vtnet0_ipv6="inet6 -auto_linklocal"
ifconfig_vtnet0_aliases="\
  inet 192.168.0.1/24 inet6 fe80::1/64 inet6 2001:6b8::/64"

As well as this:

ifconfig_vtnet0="inet"
ifconfig_vtnet0_ipv6="inet6 -auto_linklocal fe80::1/64"
ifconfig_vtnet0_aliases="inet 192.168.0.1/24 inet6 2001:6b8::/64"

Using `ifconfig_vtnet0="inet 192.168.0.1/24"` adds the auto-generated
link-local address (which is what you seem to try to avoid).

Cheers,
Michael

-- 
Michael Gmelin