[Networkmanager] IPv6 Prefix Length

Robert Nichols rnicholsNOSPAM at comcast.net
Fri Mar 10 23:21:06 UTC 2023

On 3/10/23 16:12, Chris Adams wrote:
> Once upon a time, Robert Nichols <rnicholsNOSPAM at comcast.net> said:
>> On a host that is acting as a router, how do I get NetworkManager to request an IPv6 address block larger than a single /128 address? By fiddling with various options in the Network Connections GUI, I was just once able to get a /64 prefix. That happened when I set "Method" to "Shared with other computers", which saved "IPV6_AUTOCONF=shared" (a totally undocumented setting) in the interface's ifcfg file. But, that /64 prefix did not survive a reboot, and now I can't get it back at all.
>> A /64 prefix is bad enough (I'd like to confine some devices to their own subnet), but it's a lot better than a /128, which would force me to use IPv6 NAT, defeating the primary purpose of IPv6. (Comcast/Xfinity now appears to limit residential customers to a /64 -- no subnetting.)
> I haven't had Comcast for several years, but in the past, they supported
> delegating a /60 prefix, but you have to supply the hint to request it.
> And, after you request a /64, their side remembers that, so you have to
> change your DHCPv6 ID to request something new.
> The last time I looked, NM basically isn't correct for this use.  It has
> no way to set the prefix delegation request size, it has no way to set
> the assignment of delegated prefixes, it's not hooked to the dispatcher,
> and more.  As it stands, NM is not a solution for an IPv6 gateway, you
> need to use something else.
> I opened an issue for the things I thought needed to be added to make
> this functional, but it was never touched and then auto-closed.  I poked
> around the code a little bit, but never had time to get familiar enough
> with it and work up any patches.
> https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/942

Thanks for the response. No wonder I couldn't get it to work. Lovely -- more manual configuration to do, and since the same interface is my IPv4 connection, I'll have to do that manually as well!

Yes, Comcast used to supply a /60 prefix to residential customers, but there are several gripes in the user support forum about now getting only a /64. It seems you need a "business" account to get more, and since many TV shows are licensed for residential viewing only, you don't get cable TV service on a business account.

Bob Nichols     "NOSPAM" is really part of my email address.
                 Do NOT delete it.

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