Squeezing non-relays at the entry node

Damian Johnson atagar1 at gmail.com
Thu Dec 17 02:23:53 UTC 2009

I'm sure you've thought of this, but adversaries can replicate any
properties we're looking for to rate limit. In this case simply making
yourself a slow relay and routing client traffic through yourself
(being your own first hop) seems to get around the limitation. -Damian

On Sun, Dec 13, 2009 at 5:23 PM, Roger Dingledine <arma at mit.edu> wrote:
> Hi folks (Nick in particular),
> I've been pondering other performance improvements. One of them is to
> rate-limit client connections as they enter the network. Rate limiting
> in the Tor client itself would work better, but it's not a very stable
> equilibrium -- it encourages people to switch to security disasters
> like tortunnel.
> So I'm looking at rate-limiting non-relay OR connections. Here's the
> patch:
> diff --git a/src/or/connection_or.c b/src/or/connection_or.c
> index aa26bf8..3f984bf 100644
> --- a/src/or/connection_or.c
> +++ b/src/or/connection_or.c
> @@ -333,10 +333,24 @@ connection_or_init_conn_from_address(or_connection_t
> ,
>  {
>   or_options_t *options = get_options();
>   routerinfo_t *r = router_get_by_digest(id_digest);
> -  conn->bandwidthrate = (int)options->BandwidthRate;
> -  conn->read_bucket = conn->bandwidthburst =
>   connection_or_set_identity_digest(conn, id_digest);
> +  if (r || router_get_consensus_status_by_id(id_digest)) {
> +    /* It's in the consensus, or we have a descriptor for it meaning it
> +     * was probably in a recent consensus. It's a recognized relay:
> +     * give it full bandwidth. */
> +    conn->bandwidthrate = (int)options->BandwidthRate;
> +    conn->read_bucket = conn->bandwidthburst =
> +  } else { /* Not a recognized relay. Squeeze it down based on the
> +            * suggested bandwidth parameters in the consensus. */
> +    conn->bandwidthrate =
> +      (int)networkstatus_get_param(NULL, "bwconnrate",
> +                                   (int)options->BandwidthRate);
> +    conn->read_bucket = conn->bandwidthburst =
> +      (int)networkstatus_get_param(NULL, "bwconnburst",
> +                                   (int)options->BandwidthBurst);
> +  }
> +
>   conn->_base.port = port;
>   tor_addr_copy(&conn->_base.addr, addr);
>   tor_addr_copy(&conn->real_addr, addr);
> As you can see, I'm making it configurable inside the consensus, so we
> can experiment with it rather than rolling it out and then changing our
> minds later. I don't have a good sense of whether it will be a good move,
> but the only way I can imagine to find out is to try it.
> I'm imagining trying it out with a rate of 20KB and a burst of 500KB.
> As a nice side effect, we'll also be rolling out the infrastructure for
> one defense against Sambuddho's "approximating a global passive adversary"
> congestion attack, if the attack ever gets precise enough that we can
> try out our defense and compare.
> In the distant future, where we've deployed a design where not all relays
> get to see the unified networkstatus consensus, we'll have to stop voting
> for a modified bwconnrate and bwconnburst, since the relays won't be
> able to know (at least this way) if it's a genuine relay. Maybe part
> of that design will be to present a signed credential proving you're a
> public relay. In any case, we have a way to disable this feature when
> that distant future arrives.
> We're also squeezing down bridge relays by this feature, since the public
> relays can't tell the difference between a bridge and a client. At some
> point we should make sure that bridges send their client traffic over
> different TCP connections than their own traffic. That's a separate
> discussion though.
> It shouldn't impact bandwidth bootstrapping tests, since those aim to
> spread 500KB of traffic across 4 circuits.
> It would impact Mike's bwauthority tests. We'd want to make an exception
> for those Tors. I think we'd leave the torperf deployments alone, since
> after all their goal is to measure "realistic" client performance.
> It could also impact initial directory info bootstrapping -- if you try
> to fetch 1MB from a particular dir mirror, it would slow down the second
> half of that download. We'd want to keep an eye on how much that changes.
> A more thorough solution would be to rate-limit all the OR conns coming
> from a particular non-relay into the same bucket, to prevent people
> getting around the limits by opening multiple TCP connections. But it's
> actually not so easy to open multiple conns to the same destination in
> Tor; plus I'm aiming to solve this for the general case where people
> are overloading relays and don't even know it's a bad thing.
> My main concern here is that I wonder if we are being thorough enough at
> detecting "is a relay". It checks the consensus and the descriptor cache
> currently. So if the authorities think you're not Running, they won't put
> you in the consensus, and no relays will hear about you. If you go up and
> down, relays that serve dirport info will have your descriptor cached,
> so they'll recognize you so long as you were around in the past day or so.
> Relays that don't serve dirport info will stop fetching descriptors,
> but they'll continue to fetch the consensus. So they'll still mostly work.
> Are there any other cases that are going to be a problem? Are there better
> (simple, easy to deploy soon) ways to decide if the peer is a relay?
> --Roger
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