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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
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@ -102,6 +102,7 @@
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#include "feature/stats/predict_ports.h"
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#include "feature/stats/rephist.h"
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#include "lib/buf/buffers.h"
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#include "lib/crypt_ops/crypto_rand.h"
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#include "lib/crypt_ops/crypto_util.h"
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#include "core/or/cell_st.h"
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@ -498,6 +499,21 @@ clip_dns_ttl(uint32_t ttl)
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return MAX_DNS_TTL;
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}
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/** Given a TTL (in seconds), determine what TTL an exit relay should use by
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* first clipping as usual and then adding some randomness which is sampled
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* uniformly at random from [-FUZZY_DNS_TTL, FUZZY_DNS_TTL]. This facilitates
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* fuzzy TTLs, which makes it harder to infer when a website was visited via
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* side-channels like DNS (see "Website Fingerprinting with Website Oracles").
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*
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* Note that this can't underflow because FUZZY_DNS_TTL < MIN_DNS_TTL.
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*/
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uint32_t
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clip_dns_fuzzy_ttl(uint32_t ttl)
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{
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return clip_dns_ttl(ttl) +
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crypto_rand_uint(1 + 2*FUZZY_DNS_TTL) - FUZZY_DNS_TTL;
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}
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/** Send a relay end cell from stream <b>conn</b> down conn's circuit, and
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* remember that we've done so. If this is not a client connection, set the
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* relay end cell's reason for closing as <b>reason</b>.
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@ -188,11 +188,9 @@ void connection_ap_warn_and_unmark_if_pending_circ(
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entry_connection_t *entry_conn,
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const char *where);
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/** Lowest value for DNS ttl that a server should give or a client should
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* believe. */
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/** Lowest value for DNS ttl clipping excluding the random addition. */
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#define MIN_DNS_TTL (5*60)
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/** Highest value for DNS ttl that a server should give or a client should
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* believe. */
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/** Highest value for DNS ttl clipping excluding the random addition. */
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#define MAX_DNS_TTL (60*60)
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/** How long do we keep DNS cache entries before purging them (regardless of
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* their TTL)? */
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@ -200,8 +198,16 @@ void connection_ap_warn_and_unmark_if_pending_circ(
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/** How long do we cache/tell clients to cache DNS records when no TTL is
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* known? */
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#define DEFAULT_DNS_TTL (30*60)
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/** How much should we +- each TTL to make it fuzzy with uniform sampling at
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* exits? The value 4 minutes was chosen so that the lowest possible clip is
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* 60s. Such low clips were used in the past for all TTLs due to a bug in Tor,
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* see "The effect of DNS on Tor's Anonymity" by Greschbach et al. In other
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* words, sampling such low clips is unlikely to cause any breakage at exits.
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*/
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#define FUZZY_DNS_TTL (4*60)
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uint32_t clip_dns_ttl(uint32_t ttl);
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uint32_t clip_dns_fuzzy_ttl(uint32_t ttl);
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int connection_half_edge_is_valid_data(const smartlist_t *half_conns,
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streamid_t stream_id);
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@ -1637,7 +1637,7 @@ evdns_callback(int result, char type, int count, int ttl, void *addresses,
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}
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if (result != DNS_ERR_SHUTDOWN)
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dns_found_answer(string_address, orig_query_type,
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result, &addr, hostname, clip_dns_ttl(ttl));
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result, &addr, hostname, clip_dns_fuzzy_ttl(ttl));
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tor_free(arg_);
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}
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@ -90,6 +90,36 @@ test_dns_clip_ttl(void *arg)
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return;
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}
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static void
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test_dns_clip_fuzzy_ttl(void *arg)
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{
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(void)arg;
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/* Case 0: check that the fuzzy TTL constant is valid
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*/
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tt_int_op(FUZZY_DNS_TTL, OP_LE, MIN_DNS_TTL);
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tt_int_op(FUZZY_DNS_TTL, OP_LE, MAX_DNS_TTL);
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/* Case 1: low clips
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*/
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for (int i = 0; i < 1024; i++) {
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int fuzzy_ttl = clip_dns_fuzzy_ttl(MIN_DNS_TTL - 1);
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tt_int_op(fuzzy_ttl, OP_GE, MIN_DNS_TTL-FUZZY_DNS_TTL);
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tt_int_op(fuzzy_ttl, OP_LE, MIN_DNS_TTL+FUZZY_DNS_TTL);
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}
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/* Case 2: high clips
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*/
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for (int i = 0; i < 1024; i++) {
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int fuzzy_ttl = clip_dns_fuzzy_ttl(MIN_DNS_TTL);
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tt_int_op(fuzzy_ttl, OP_GE, MAX_DNS_TTL-FUZZY_DNS_TTL);
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tt_int_op(fuzzy_ttl, OP_LE, MAX_DNS_TTL+FUZZY_DNS_TTL);
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}
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done:
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return;
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}
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static int resolve_retval = 0;
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static int resolve_made_conn_pending = 0;
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static char *resolved_name = NULL;
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@ -779,6 +809,7 @@ struct testcase_t dns_tests[] = {
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TT_FORK, NULL, NULL },
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#endif
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{ "clip_ttl", test_dns_clip_ttl, TT_FORK, NULL, NULL },
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{ "clip_fuzzy_ttl", test_dns_clip_fuzzy_ttl, TT_FORK, NULL, NULL },
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{ "resolve", test_dns_resolve, TT_FORK, NULL, NULL },
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{ "impl_addr_is_ip", test_dns_impl_addr_is_ip, TT_FORK, NULL, NULL },
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{ "impl_non_exit", test_dns_impl_non_exit, TT_FORK, NULL, NULL },
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