[Here's a new proposal from Andrea. See ticket 4581 for implementation details.]

Filename: 258-dirauth-dos.txt Title: Denial-of-service resistance for directory authorities Author: Andrea Shepard Created: 2015-10-27 Status: Open

1. Problem statement

The directory authorities are few in number and vital for the functioning of the Tor network; threats of denial of service attacks against them have occurred in the past. They should be more resistant to unreasonably large connection volumes.

2. Design overview

There are two possible ways a new connection to a directory authority can be established, directly by a TCP connection to the DirPort, or tunneled inside a Tor circuit and initiated with a begindir cell. The client can originate the former as direct connections or from a Tor exit, and the latter either as fully anonymized circuits or one-hop links to the dirauth's ORPort.

The dirauth will try to heuristically classify incoming requests as one of these four indirection types, and then in the two non-anonymized cases further sort them into hash buckets on the basis of source IP. It will use an exponentially-weighted moving average to measure the rate of connection attempts in each bucket, and also separately limit the number of begindir cells permitted on each circuit. It will periodically scan the hash tables and forget counters which have fallen below a threshold to prevent memory exhaustion.

3. Classification of incoming connections

Clients can originate connections as one of four indirection types:

- DIRIND_ONEHOP: begindir cell on a single-hop Tor circuit - DIRIND_ANONYMOUS: begindir cell on a fully anonymized Tor circuit - DIRIND_DIRECT_CONN: direct TCP connection to dirport - DIRIND_ANON_DIRPORT: TCP connection to dirport from an exit relay

The directory authority can always tell a dirport connection from a begindir, but it must use its knowledge of the current consensus and exit policies to disambiguate whether the connection is anonymized.

It should treat a begindir as DIRIND_ANONYMOUS when the previous hop in the circuit it appears on is in the current consensus, and as DIRIND_ONEHOP otherwise; it should treat a dirport connection as DIRIND_ANON_DIRPORT if the source address appears in the consensus and allows exits to the dirport in question, or as DIRIND_DIRECT_CONN otherwise. In the case of relays which also act as clients, these heuristics may falsely classify direct/onehop connections as anonymous, but will never falsely classify anonymous connections as direct/onehop.

4. Exponentially-weighted moving average counters and hash table

The directory authority implements a set of exponentially-weighted moving averages to measure the rate of incoming connections in each bucket. The two anonymous connection types are each a single bucket, but the two non- anonymous cases get a single bucket per source IP each, stored in a hash table. The directory authority must periodically scan this hash table for counters which have decayed close to zero and free them to avoid permitting memory exhaustion.

This introduces five new configuration parameters:

- DirDoSFilterEWMATimeConstant: the time for an EWMA counter to decay by a factor of 1/e, in seconds.

- DirDoSFilterMaxAnonConnectRate: the threshold to trigger the DoS filter on DIRIND_ANONYMOUS connections.

- DirDoSFilterMaxAnonDirportConnectRate: the threshold to trigger the DoS filter on DIRIND_ANON_DIRPORT connections.

- DirDoSFilterMaxBegindirRatePerIP: the threshold per source IP to trigger the DoS filter on DIRIND_ONEHOP connections.

- DirDoSFilterMaxDirectConnRatePerIP: the threshold per source IP to trigger the DoS filter on DIRIND_DIRECT_CONN connections.

When incrementing a counter would put it over the relevant threshold, the filter is said to be triggered. In this case, the directory authority does not update the counter, but instead suppresses the incoming request. In the DIRIND_ONEHOP and DIRIND_ANONYMOUS cases, the directory authority must kill the circuit rather than merely refusing the request, to prevent an unending stream of client retries on the same circuit.

5. Begindir cap

Directory authorities limit the number of begindir cells permitted in the lifetime of a particular circuit, separately from the EWMA counters. This can only affect the DIRIND_ANONYMOUS and DIRIND_ONEHOP connetion types. A sixth configuration variable, DirDoSFilterMaxBegindirPerCircuit, controls this feature.

6. Limitations

Widely distributed DoS attacks with many source IPs may still be able to avoid raising any single DIRIND_ONEHOP or DIRIND_DIRECT_CONN counter above threshold.