Re: [v6ops] enable-ula.py

Hi all,
It was fine to run the script by a geek who understands his environment.
IMHO: It is not suitable for the default configuration.

The link could have routers not in sync on purpose. For example, one for internal communication, and another for external.
If one router would announce ULA PIO (and route to this ULA)
But another router could not route to this ULA
then the communication would be broken.
Because the second router would have a good chance to be chosen as the default for ULA that it does not understand.
It is a very similar situation that Ted has shown on the last IETF.
The root cause is that RFC 6724 assumes that the next hop is chosen before the source. It SHOULD be the opposite.

Of course, it is possible to patch the second router by the source routing configuration
Then the second router would be just a redundant hop for every second flow.
But it is for the geeks. It is not normal to have such a default assumption.

Eduard
-----Original Message-----
From: v6ops [mailto:v6ops-bounces@ietf.org] On Behalf Of Brian E Carpenter
Sent: Wednesday, May 11, 2022 1:23 AM
To: v6ops@ietf.org
Subject: Re: [v6ops] enable-ula.py

Hi,

I've been asked off-list whether this script should be run as a cron job, since a new ULA prefix could be announced by a RA/PIO at any time.

Good point, but my script is explicitly written to be used by a human to avoid blunders. However, what I think should happen is that the IPv6 stack should automatically do what the script does, whenever a new ULA is configured on a host. Namely, add the corresponding /48 prefix to the active precedence table. At the right place in the code, it should only be a few instructions.

And of course, delete it when the last ULA under that /48 goes away.

Any Linux core programmers here?

Regards
    Brian Carpenter
On 08-May-22 15:21, Brian E Carpenter wrote:
> Hi,
> 
> So, how hard is it to automagically set ULA precedence for a given /48, as suggested in section 10.6 of RFC 6724?
> 
> Quite easy for Windows, as it turns out, and quite hard for Linux.
> 
> In fact, if I wasn't being polite, I'd say that the Linux 
> implementation is a mess. For more details, see Karl Auer's blog post from ten years ago, which explains it as best it can be explained: http://biplane.com.au/blog/?p=122 . As far as I can tell, my quite recent Linux (5.4.0-109-generic x86_64) is still like that and mainly stuck in RFC-3484-land.
> 
> So I wrote a little Python program which (a) detects if the host it's running in has any ULAs, (b) extracts the corresponding /48 prefix(es), and (c) sets the corresponding label and precedence for such prefix(es) according to section 10.6 of RFC 6724.
> 
> On Linux, the program also forces all the RFC 6724 defaults. It does that by overwriting /etc/gai.conf, which is more drastic than Karl's script in his blog post.
> 
> Sadly, both Windows and Linux need this treatment after every reboot. Someone with deeper knowledge of the operating systems might be able to get round this. And the program doesn't know what to do for other POSIX compliant systems. But it's open source, so contributions are welcome.
> 
> As the program itself says "This is experimental software that might disturb network access." So far, it hasn't disturbed either my Windows or my Linux laptop. However, if you want to try it, it's at your own risk and I strongly recommend using a spare machine.
> 
> enable-ula.py is at https://github.com/becarpenter/misc
> 
> Regards
>       Brian
> 

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