[Networkmanager] IPv6 Prefix Length

Chris Adams linux at cmadams.net
Fri Mar 10 22:12:06 UTC 2023

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.


Chris Adams <linux at cmadams.net>

More information about the Networkmanager mailing list