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I'm trying to tune my IPv6 in OpenWrt, if tuning is the right word.

My IPv6 settings in OpenWrt 19.07.3

My IPv6 settings in OpenWrt 19.07.3

This question is about SLAAC (wikipedia), which I presume is the stateless + ... in the list, so I was thinking to narrow it to stateful-only, but I have no idea what pros and cons it would bring?

Note: My IPv6 operates normal to date. I merely want to turn off some useless options now.

Transcript from IPv6 test

Done from Vivaldi browser.

  • IPv6: Supported

  • Address: [censored]

  • Type: Native IPv6

  • SLAAC: No

  • ICMP: Reachable

Vlastimil Burián
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1 Answers1

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This seems to correspond to the 'ra_management' option:

  • "stateless only" (0: no M-Flag but A-Flag)
  • "stateless + stateful" (1: both M and A flags)
  • "stateful only" (2: M but not A flag)

So yes, "stateless" actually corresponds to the Router Advertisement flag that allows clients to use SLAAC (with optionally using DHCPv6 Information Requests to get DNS). If you choose "stateful only", clients will be required to use DHCPv6 to acquire an address lease as well.

But note that this isn't something that changes how your DHCPv6 server behaves – it changes information sent in RAs, which tells clients whether and how to interact with the DHCPv6 server in the first place. When stateless mode is allowed (A-flag is set), client devices might not even start DHCPv6 at all – they might be fine with the information already received in RAs.

With that in mind, a big disadvantage of "stateful only" mode is that it will cause issues for clients which don't support DHCPv6. It is possible that currently you have clients (e.g. Android devices) which have no DHCPv6 support and configure themselves entirely through Router Advertisements.

Those clients will no longer have an IPv6 address if you switch to stateful-only mode.

Note that IPv6-Test's website is slightly outdated with its "SLAAC" label. What it really checks there is whether the IP address was generated from your MAC (EUI-48) address, by looking for the ff:fe infix – but that is not actually the only way to use SLAAC anymore.

Modern operating systems will instead use RFC 7217 or a similar scheme to generate their permanent IPv6 address, which is still "SLAAC" but cannot be externally recognized as such.

Temporary addresses (the older "Privacy Extensions") are also assigned using the SLAAC mechanism but with a completely random interface identifier.

u1686_grawity
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  • Are you suggesting to leave default? I'm perfectly fine with that in any case. Thanks for taking your time. – Vlastimil Burián Aug 09 '20 at 08:11
  • I would say keep it. You'll still need RAs to be broadcast (for default gateway) even if SLAAC is disabled, so it doesn't improve network performance or anything. – u1686_grawity Aug 09 '20 at 08:43