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Provider-local addresses?

  • From: Brian Candler
  • Date: Thu Mar 09 13:20:47 2000


Having watched the 'route filtering panel' discussion from the most recent
NANOG meeting, I have an idea which I would like to float.

One of the concerns raised was that small ISPs are not eligible for
provider-independent blocks; unfortunately this forces them to renumber if
they change upstream provider. It was pointed out that this in itself is not
a good reason for getting PI space, because then everybody would want it.

Now, renumbering is tedious but in principle not difficult. The major
difficulties are with DNS - in particular, changing the DNS settings in all
the clients, which may well have the IP addresses of the ISP's caching
nameservers hardcoded in. [1]

My suggestion is that a small amount of unrouted space is allocated for
"provider-local" use. For sake of example, let's say 200.0.0/24.

Then we can assign say and as well-known addresses for
local primary and secondary DNS caches. These are configured as loopback
interfaces on the ISP's DNS servers, and static /32 routes injected into the
provider's IGP to reach them. [2]

The ISP would not need to divulge the 'real' addresses of its DNS caches,
and could thus renumber at will. It would also benefit customers who roam
between multiple ISPs, as they wouldn't have to keep changing their client
settings. Notice it should work for multiple levels of service-provider
hierarchy: sending a packet to would follow defaultroute until it
hit the first ISP who implements a server on this address.

These addresses might be useful for other services too: e.g. outgoing SMTP
relays, NTP servers, etc. In principle all these should have DNS names which
customers use, but experience shows that customers _do_ end up hardcoding IP
addresses into their configurations, which makes it very difficult to
renumber them.

The allocation would only need to be a single /24 or smaller, and it could
come from any useless bit of 'toxic waste' because it would not actually be
routed on the Internet.

Does anybody see any value in this proposal? Of course, it's not much help
to an ISP with downstream leased-line customers, because the customers will
have to renumber as well.


Brian Candler.
[this posting is from my personal address and not on behalf of my employer]

[1] There are MS extensions to PPP to negotiate primary/secondary DNS cache,
but not everybody uses them.

[2] We can also preallocate space in a way suitable for subnetting:   Primary DNS cache   Primary NTP server
    ... Secondary DNS cache Secondary NTP server
This allows people to implement this proposal by bringing up interface
aliases, without messing with host routes in IGP, and still have some
resilience by putting the primary/secondary services on different physical