Commit 15efc25f authored by Rasmus Dahlberg's avatar Rasmus Dahlberg Committed by David Goulet
Browse files

dns: Make TTLs fuzzy at exit relays 

This change mitigates DNS-based website oracles by making the time that
a domain name is cached uncertain (+- 4 minutes of what's measurable).

Resolves TROVE-2021-009.

Fixes #40674
parent be7afe65

src/core/or/connection_edge.c

+16 −0
Original line number Diff line number Diff line
@@ -102,6 +102,7 @@ 
#include "feature/stats/predict_ports.h"

#include "feature/stats/rephist.h"

#include "lib/buf/buffers.h"

#include "lib/crypt_ops/crypto_rand.h"

#include "lib/crypt_ops/crypto_util.h"



#include "core/or/cell_st.h"
@@ -498,6 +499,21 @@ clip_dns_ttl(uint32_t ttl) 
    return MAX_DNS_TTL;

}



/** Given a TTL (in seconds), determine what TTL an exit relay should use by

 * first clipping as usual and then adding some randomness which is sampled

 * uniformly at random from [-FUZZY_DNS_TTL, FUZZY_DNS_TTL].  This facilitates

 * fuzzy TTLs, which makes it harder to infer when a website was visited via

 * side-channels like DNS (see "Website Fingerprinting with Website Oracles").

 *

 * Note that this can't underflow because FUZZY_DNS_TTL < MIN_DNS_TTL.

 */

uint32_t

clip_dns_fuzzy_ttl(uint32_t ttl)

{

  return clip_dns_ttl(ttl) +

    crypto_rand_uint(1 + 2*FUZZY_DNS_TTL) - FUZZY_DNS_TTL;

}



/** Send a relay end cell from stream <b>conn</b> down conn's circuit, and

 * remember that we've done so.  If this is not a client connection, set the

 * relay end cell's reason for closing as <b>reason</b>.

src/core/or/connection_edge.h

+10 −4
Original line number Diff line number Diff line
@@ -188,11 +188,9 @@ void connection_ap_warn_and_unmark_if_pending_circ( 
                                             entry_connection_t *entry_conn,

                                             const char *where);



/** Lowest value for DNS ttl that a server should give or a client should

 * believe. */

/** Lowest value for DNS ttl clipping excluding the random addition. */

#define MIN_DNS_TTL (5*60)

/** Highest value for DNS ttl that a server should give or a client should

 * believe. */

/** Highest value for DNS ttl clipping excluding the random addition. */

#define MAX_DNS_TTL (60*60)

/** How long do we keep DNS cache entries before purging them (regardless of

 * their TTL)? */
@@ -200,8 +198,16 @@ void connection_ap_warn_and_unmark_if_pending_circ( 
/** How long do we cache/tell clients to cache DNS records when no TTL is

 * known? */

#define DEFAULT_DNS_TTL (30*60)

/** How much should we +- each TTL to make it fuzzy with uniform sampling at

 * exits?  The value 4 minutes was chosen so that the lowest possible clip is

 * 60s.  Such low clips were used in the past for all TTLs due to a bug in Tor,

 * see "The effect of DNS on Tor's Anonymity" by Greschbach et al.  In other

 * words, sampling such low clips is unlikely to cause any breakage at exits.

 */

#define FUZZY_DNS_TTL (4*60)



uint32_t clip_dns_ttl(uint32_t ttl);

uint32_t clip_dns_fuzzy_ttl(uint32_t ttl);



int connection_half_edge_is_valid_data(const smartlist_t *half_conns,

                                       streamid_t stream_id);

src/feature/relay/dns.c

+1 −1
Original line number Diff line number Diff line
@@ -1637,7 +1637,7 @@ evdns_callback(int result, char type, int count, int ttl, void *addresses, 
  }

  if (result != DNS_ERR_SHUTDOWN)

    dns_found_answer(string_address, orig_query_type,

                     result, &addr, hostname, clip_dns_ttl(ttl));

                     result, &addr, hostname, clip_dns_fuzzy_ttl(ttl));



  tor_free(arg_);

}

src/test/test_dns.c

+31 −0
Original line number Diff line number Diff line
@@ -90,6 +90,36 @@ test_dns_clip_ttl(void *arg) 
  return;

}



static void

test_dns_clip_fuzzy_ttl(void *arg)

{

  (void)arg;



  /* Case 0: check that the fuzzy TTL constant is valid

   */

  tt_int_op(FUZZY_DNS_TTL, OP_LE, MIN_DNS_TTL);

  tt_int_op(FUZZY_DNS_TTL, OP_LE, MAX_DNS_TTL);



  /* Case 1: low clips

   */

  for (int i = 0; i < 1024; i++) {

    int fuzzy_ttl = clip_dns_fuzzy_ttl(MIN_DNS_TTL - 1);

    tt_int_op(fuzzy_ttl, OP_GE, MIN_DNS_TTL-FUZZY_DNS_TTL);

    tt_int_op(fuzzy_ttl, OP_LE, MIN_DNS_TTL+FUZZY_DNS_TTL);

  }



  /* Case 2: high clips

   */

  for (int i = 0; i < 1024; i++) {

    int fuzzy_ttl = clip_dns_fuzzy_ttl(MIN_DNS_TTL);

    tt_int_op(fuzzy_ttl, OP_GE, MAX_DNS_TTL-FUZZY_DNS_TTL);

    tt_int_op(fuzzy_ttl, OP_LE, MAX_DNS_TTL+FUZZY_DNS_TTL);

  }



  done:

  return;

}



static int resolve_retval = 0;

static int resolve_made_conn_pending = 0;

static char *resolved_name = NULL;
@@ -779,6 +809,7 @@ struct testcase_t dns_tests[] = { 
     TT_FORK, NULL, NULL },

#endif

   { "clip_ttl", test_dns_clip_ttl, TT_FORK, NULL, NULL },

   { "clip_fuzzy_ttl", test_dns_clip_fuzzy_ttl, TT_FORK, NULL, NULL },

   { "resolve", test_dns_resolve, TT_FORK, NULL, NULL },

   { "impl_addr_is_ip", test_dns_impl_addr_is_ip, TT_FORK, NULL, NULL },

   { "impl_non_exit", test_dns_impl_non_exit, TT_FORK, NULL, NULL },