[tor-dev] Proposal 297: Relaxing the protover-based shutdown rules
nickm at torproject.org
Wed Sep 19 16:20:33 UTC 2018
Title: Relaxing the protover-based shutdown rules
Author: Nick Mathewson
In proposal 264 (now implemented) we introduced the subprotocol
versioning mechanism to better handle forward-compatibility in
the Tor network. Included was a mechanism for safely disabling
obsolete versions of Tor that no longer ran any supported
protocols. If a version of Tor receives a consensus that lists
as "required" any protocol version that it cannot speak, Tor will
not start--even if the consensus is in its cache.
The intended use case for this is that once some protocol has
been provided by all supported versions for a long time, the
authorities can mark it as "required". We had thought about the
"adding a requirement" case mostly.
This past weekend, though, we found an unwanted side-effect: it
is hard to safely *un*-require a currently required protocol.
Here's what happened:
- Long ago, we created the LinkAuth=1 protocol, which required
direct access to the ClientRandom and ServerRandom fields.
- Later, once we implemented Ed25519 identity keys, we added
an improved LinkAuth=3 protocol, which uses the RFC5705 "key
export" mechanism. (0.3.0.1-alpha)
- When we added the subprotocols mechanism, we listed
LinkAuth=1 as required. (backported to 0.2.9.x)
- While porting Tor to NSS, we found that LinkAuth=1 couldn't
be supported, because NSS wisely declines to expose the TLS
fields it uses. So we removed "LinkAuth=1" from the
required list (backported to 0.3.2.x), and got a bunch of
authorities to upgrade.
- In 0.3.5.1-alpha, once enough authorities had upgraded, we
removed "LinkAuth=1" from the supported subprotocols list
when Tor is running with NSS. [*]
- We found, however, that this change could cause a bug when
Tor+NSS started with a cached consensus that was created before
LinkAuth=1 was removed from the requirements. Tor would
decline to start, because the (old) consensus told it that
LinkAuth=1 was required.
This proposal discusses two alternatives for making it safe to
remove required subprotocol versions in the future.
[*] There was actually a bug here where OpenSSL removed LinkAuth=1
too, but that's mostly beside the point for this timeline, other
than the fact it would have made things waaay worse if people
hadn't caught it.
2. Recommended change: consider the consensus date.
I propose that when deciding whether to shut down because of
subprotocol requirements, a Tor implementation should only shut
down if the consensus is dated to some time after the
implementation's release date.
With this change, an old cached consensus cannot cause the
implementation to shut down, but a newer one can. This makes it
safe to put out a release that does not support a formerly
required protocol, so long as the authorities have upgraded to
stop requiring that protocol.
(It is safe to use the *scheduled* release date for the
implementation, plus a few months -- just so long as we don't
plan to start requiring a subprotocol that's not supported by the
latest version of Tor.)
3. Not-recommended change: ignore the cached consensus.
Was it a mistake to have Tor consider a cached consensus when
deciding whether to shut down?
The rationale for considering the cached consensus was that when
a Tor implementation is obsolete, we don't want it hammering on
the network, probing for new consensuses, and possibly
reconnecting aggressively as its handshakes fail. That still
seems compelling to me, though it's possible that if we find some
problem with the methodology from section 2 above, we'll need to
find some other way to achieve this goal.
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