Specifying link-local address in rc.conf

[Niclas Zeising]

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"

Regards
-- 
Niclas