Dan,
Correct me if I'm wrong, but if I understand the problem here correctly,
this bug can be avoided if at least one of the new victim.dom
nameservers are not in the victim.dom domain but rather in a domain with
uncached or unchanged nameservers. This way the caching server would
retain correct information for one of the nameservers, resolve to the
server, and get valid addresses for any servers in the victim.dom domain
(and at the same time it could (would?) pick up authoritative NS records
for the domain, entirely replacing the old info in the cache).
Of course, moving a nameserver out of the domain might be as much
trouble as moving the domain to begin with... oh well. Plan ahead so
you can wait until the cache records time out, right?
My take on this is that it sounds like the root of the moving domain
problem is not in the seemingly irrelevent programming decision as much
as it is in the choice of who is the authority on the NS records. (IMO
at least) the .dom servers hold the authority for the NS records, not
the victim.dom nameservers. If the lookup request to the .dom servers
returned authoritative NS records for the victim.dom domain (below) then
the obsolete records in the cache would be replaced by these and valid
lookups could be performed (initially using the new, unauthoritative A
records from the .dom server).
victim.dom 259200 NS ns1.victim.dom
victim.dom 259200 NS ns2.victim.dom
Of course, this can't happen because the whole DNS system isn't set up
with authority for the NS records in the "right" place (or even the
ability to have it set up that way). Bummer.
-- Ken Gourlay
"D. J. Bernstein" wrote:
>
> Summary: If you're running BIND 8.2.2, and you have the victim.dom name
> servers in your cache, and victim.dom changes its server names, then any
> user who can make recursive queries through your cache can break your
> victim.dom lookups until the old records time out. The complete attack
> is one brief burst of legitimate packets.
>
> This is, of course, not as disastrous as BIND's next buffer overflow,
> but it's still an interesting example of how an attacker can use BIND's
> bogus ``credibility'' mechanism to exacerbate the effects of a seemingly
> minor bug and a seemingly irrelevant programming decision.
>
> There's also a race condition here that will allow a similar attack, at
> the expense of a low-bandwidth flood, when victim.dom isn't changing its
> server names. I'll leave this as an exercise for the reader.
>
> Details: Let's say the old victim.dom name servers were sun37.victim.dom
> (1.2.3.4) and pc5.victim.dom (5.6.7.8). The new servers are
> ns1.victim.dom (1.2.3.5) and ns2.victim.dom (5.6.7.9).
>
> After setting up the new servers, the administrator tells the .dom
> registrar to change the NS records. Of course, he leaves the old servers
> running for a while.
>
> Eventually the .dom registrar changes the victim.dom information:
>
> victim.dom 259200 NS ns1.victim.dom
> victim.dom 259200 NS ns2.victim.dom
> ns1.victim.dom 259200 A 1.2.3.5
> ns2.victim.dom 259200 A 5.6.7.9
>
> Meanwhile, your cache still has the old information:
>
> victim.dom 258437 NS sun37.victim.dom
> victim.dom 258437 NS pc5.victim.dom
> sun37.victim.dom 258437 A 1.2.3.4
> pc5.victim.dom 258437 A 5.6.7.8
>
> Now the attacker swings into action. All he has to do is ask your cache
> for the addresses of sun37.victim.dom and pc5.victim.dom a few hundred
> times. BIND assigns a ``credibility'' level of ``additional records'' to
> these addresses, and reduces the TTLs by 5% for each query:
>
> victim.dom 258435 NS sun37.victim.dom
> victim.dom 258435 NS pc5.victim.dom
> sun37.victim.dom 5 A 1.2.3.4
> pc5.victim.dom 5 A 5.6.7.8
>
> A few seconds later, the address records expire, leaving only the NS
> records, which will remain in your cache for a few days.
>
> An innocent user now asks your cache for the address of blah.victim.dom.
> Your cache sees that it can get the answer from the .victim.dom servers,
> sun37.victim.dom and pc5.victim.dom. But, oops, the addresses aren't
> available; your cache has to look them up.
>
> The seemingly minor bug is that BIND drops the blah.victim.dom query at
> this point. It hopes to have the sun37 and pc5 information cached by the
> time the user's stub resolver retries the query, so that it can resolve
> blah.victim.dom successfully.
>
> How, then, does your cache find the addresses of sun37.victim.dom and
> pc5.victim.dom? It could get the answer from the .victim.dom servers...
> Fortunately, the cache is smart enough to recognize this loop; it
> ignores the useless NS records and falls back to the .dom servers.
>
> The seemingly irrelevant programming decision is that BIND doesn't
> actually discard the useless NS records from the cache. It simply
> ignores them for the moment.
>
> The .dom servers provide a ``non-authoritative'' response with the new
> NS records and A records:
>
> victim.dom 259200 NS ns1.victim.dom
> victim.dom 259200 NS ns2.victim.dom
> ns1.victim.dom 259200 A 1.2.3.5
> ns2.victim.dom 259200 A 5.6.7.9
>
> BIND assigns a ``credibility'' level of ``authority records from a
> non-authoritative response'' to the new NS records, and ``authority
> records from an authoritative response'' to the useless NS records in
> its cache, so it discards the new NS records. It sticks to the old NS
> records until they time out. Meanwhile, it doesn't have the sun37 and
> pc5 addresses that it needs.
>
> ---Dan
This archive was generated by hypermail 2b30 : Fri Apr 13 2001 - 15:27:55 PDT