Re: IPv6 address scope not set to operator-configured value

[Guillaume Nault <gnault@redhat.com>]

[ Cc: David Ahern, who might have an opinion on this]

On Fri, Sep 08, 2023 at 06:02:00PM +0100, Tj wrote:
> Using iproute2 and kernel v6.5.0 with Debian 12 Bookworm amd64
> (tested also with v6.136 nixos) setting scope on an IPv6 fails
> silently with no indications as to why and the address is configured
> with what appears to be a scope based on the prefix (usually 0 but
> for fe80::/16 addresses scope is set to 253). Doesn't matter whether
> using scope names (from /etc/iproute2/rt_scopes) or numbers. Similar
> command for IPv4 succeeds.

I don't think we could let the user manually set the scope of IPv6
addresses. As you realised, with IPv6, the scope is automatically
derived from the address.

I had a patch to reject IPv6 netlink commands that have a scope
attribute. I've never sent it though, as I was affraid to break
existing users. But now that I read your message, I feel that maybe
we'd better explicitely reject such commands instead of silently
ignoring the scope. That'd avoid this kind of misunderstanding.

If anyone feels we should reject the scope attribute when adding
IPv6 routes or addresses, just let me know and I'll send this patch.

> ip address add fddc::2/64 scope 200 dev PUBLIC
> ip -N -6 address show dev PUBLIC
> ...
> inet6 fddc::2/64 scope 0
> 
> I used `gdb` to trace this expecting somehow the scope was not being read correctly but it is:
> 
> 2577            if (!scoped && cmd != RTM_DELADDR)
> (gdb) p scoped
> $22 = <optimized out>
> (gdb) p cmd
> $23 = <optimized out>
> (gdb) n
> 2580            req.ifa.ifa_index = ll_name_to_index(d);
> (gdb) p req.ifa.ifa_scope
> $24 = 200 '\310'
> ...
> 2607            if (echo_request)
> (gdb) n
> 2610                    ret = rtnl_talk(&rth, &req.n, NULL);
> (gdb) p req.n
> $25 = {nlmsg_len = 64, nlmsg_type = 20, nlmsg_flags = 1537, nlmsg_seq = 0, nlmsg_pid = 0}
> (gdb) p rth
> $26 = {fd = 3, local = {nl_family = 16, nl_pad = 0, nl_pid = 2381950, nl_groups = 0}, peer = {nl_family = 0, nl_pad = 0, nl_pid = 0, nl_groups = 0}, seq = 1694191286,
>   dump = 0, proto = 0, dump_fp = 0x0, flags = 4}
> (gdb) s
> rtnl_talk (rtnl=0x5555555f7020 <rth>, n=n@entry=0x7fffffffe140, answer=answer@entry=0x0) at ./lib/libnetlink.c:1170
> 1170    {
> ...
> ipaddr_modify (cmd=<optimized out>, flags=<optimized out>, argc=<optimized out>, argv=0x7fffffffe478) at ./ip/ipaddress.c:2612
> 2612            if (ret)
> (gdb) p ret
> $27 = 0
> 
> 
> 
> 
> 
>