Nick Mathewson nickm@torproject.org writes:
Hi, all!
I noticed that proposal 236 doesn't mention directory guards. (See proposal 207, implemented in Tor 0.2.4.) I think that we should consider retaining multiple directory guards while going to a single guard for multi-hop circuits.
My rationale here is that when we have only a single directory guard, it can more easily perform hard-to-detect route biasing attacks by pretending not to have descriptors for nodes it doesn't like. Its ability to do this is limited by fact that we won't build circuits unless 95% of all paths are buildable (see get_frac_paths_needed_for_circuits() and its users). But still, trusting a single source for the completeness and freshness of your directory info is suboptimal.
Hi, all!
I noticed that proposal 236 doesn't mention directory guards. (See proposal 207, implemented in Tor 0.2.4.) I think that we should consider retaining multiple directory guards while going to a single guard for multi-hop circuits.
My rationale here is that when we have only a single directory guard, it can more easily perform hard-to-detect route biasing attacks by pretending not to have descriptors for nodes it doesn't like. Its ability to do this is limited by fact that we won't build circuits unless 95% of all paths are buildable (see get_frac_paths_needed_for_circuits() and its users). But still, trusting a single source for the completeness and freshness of your directory info is suboptimal.
I discussed this a bit with Roger yesterday, who helped me understand the issue.
It seems to me that from a security prespective it makes sense to use our circuit guard as our directory guard (assuming that it's a DirCache). This is to minimize our network fingerprint, and we also trust our guard to not selectively deny EXTEND requests anyway (which is a form of a path bias attack).
So, IIUC, the path bias attack Nick is worrying about is: a) Client fetches consensus from dirguard.
b) Client walks consensus and asks the dirguard for router microdescriptors in batches.
c) The attack is that a dirguard can now selectively claim that it doesn't have some microdescriptors, in an attempt to influence the path selection of the client.
The attack is not a big problem in the current architecture, since the client will go ahead and ask another directory guard for the missing descriptors. However, it becomes a problem when we have a single directory guard since there is no one else to talk to.
IIUC, the problem stems from the fact that even honest directory servers might not have the microdescriptors that a client asks for:
This might happen when a dirserv fetches a new consensus from the auths, the new consensus contains some unknown routers, and the dirserver hasn't fetched the descriptors for these unknown routers yet. So there is a race condition between the dirserv fetching a consensus, and the dirserv downloading all the missing descriptors, during which it will serve a consensus containing routers that it doesn't have the descriptors for.
Are there other legitimate cases where an honest directory server can claim to not have a descriptor?
If the above is the only case, we can fix it by making sure that directory servers start serving a consensus _only after_ they have downloaded the descriptors of all the routers mentioned in that consensus.
If the above behavior is applied, it means that honest directory servers don't have any excuse for not having a descriptor mentioned in a consensus they just served. Hence, a directory server that claims that, is an evil directory server.
But that's too easy, isn't it? I wonder how the above idea interacts when multiple consensuses are valid at the same time. And also if the above idea gets implemented, non-upgraded directory servers will be flagged as evil by upgraded clients.
.oO(I wonder how frequent it is for an honest dirguard to not have the descriptors a client is asking for.)