Re: [v6ops] Comments on <draft-horley-v6ops-lab>

On Sat, 31 Jul 2021 at 07:42, Brian E Carpenter
<brian.e.carpenter@gmail.com> wrote:
>
> On 31-Jul-21 05:20, Ca By wrote:
> >
> >     Global Filtering:
> >
> >     The draft says that this prefix should be added the list of non-routable prefixes.  I note that this means that every ISP in the world would need to do this.    That’s a big ask and may not actually happen operationally.
> >
> >
> > I agree with Bob. Don’t do this to us operators.
> >
> > Broadly, i oppose the i-d. It is not needed. Any lab can use gua or ula
> or bogon
>
> I agree. However, I did see one comment earlier: that the RFC6724 precedence table needs to be tweaked in all hosts to avoid special handling for fc00::/7. Is there a good way to do that remotely?
>

I've been wondering if that is really the case, or there is something
else going on in the example that Nick posted to the list.

The only way for IPv4 to be preferred over ULA per the precedence
table is if IPv4 addresses are represented as IPv4-mapped addresses
(per RFC6724, 3.2).

The example we've seen on the list was with SSH under Ubunto, so using
glibc getaddrinfo().

AI_V4MAPPED | AI_ADDRCONFIG are the default flags for getaddrinfo() if
NULL is specified for the 'hints' structure for the getaddrinfo()
call.

However, the call to getaddrinfo() in openssh-portable in the current
github version (ssh.c, resolve_host()) always supplies a 'hints'
structure, and doesn't ever set AI_V4MAPPED.

Here's a getaddrinfo() test using the same flags that SSH uses by default:

    memset(&hints, 0, sizeof(hints));

    hints.ai_family = AF_UNSPEC;        /* Allow IPv4 or IPv6 */
    hints.ai_socktype = SOCK_STREAM;    /* Stream socket */

    s = getaddrinfo(argv[1], NULL, &hints, &result);

looking up a domain name with only IPv4 and IPv6 ULA DNS records. The
output order shown is the order getaddrinfo() supplied:

[mark@opy getaddrinfo]$ ./gai v6opstest.nosense.org

getaddrinfo(v6opstest.nosense.org)

    AF_INET6, fd68:1e02:dc1a:ffff::3, sin6_scope_id = 0
    AF_INET, 10.255.255.3

[mark@opy getaddrinfo]$

So the ULA address should be preferred as a DA over IPv4.

I'm wondering if perhaps the preference of IPv4 over IPv6 ULA was
instead caused by source address selection. Perhaps Nick's test host
only had IPv4 and IPv6 GUA addresses, so IPv4 became the SA choice
when there was only IPv6 ULA and IPv4 DAs. OTOH, if the DA choices
were IPv4 and IPv6 GUA, then IPv6 GUA was the chosen source address.

So perhaps source address selection made it appear that IPv4 was
preferred over ULA for destination address selection.

Regards,
Mark.

> There's also an argument in the draft that RFC4193 specifies pseudo-random bits in the prefix. In the lab context, that's a requirement than can be (and often is) ignored.
>
>     Brian
>
> _______________________________________________
> v6ops mailing list
> v6ops@ietf.org
> https://www.ietf.org/mailman/listinfo/v6ops