diff options
| -rw-r--r-- | changes/ticket19769 | 7 | ||||
| -rw-r--r-- | src/or/dns.c | 30 | ||||
| -rw-r--r-- | src/or/dns.h | 14 | ||||
| -rw-r--r-- | src/or/or.h | 12 | ||||
| -rw-r--r-- | src/test/test_dns.c | 28 |
5 files changed, 33 insertions, 58 deletions
diff --git a/changes/ticket19769 b/changes/ticket19769 new file mode 100644 index 0000000000..9fc05c3e9e --- /dev/null +++ b/changes/ticket19769 @@ -0,0 +1,7 @@ + o Major features (security): + - Change the algorithm used to decide DNS TTLs on client and server side, + to better resist DNS-based correlation attacks like the DefecTor attack + of Greschbach, Pulls, Roberts, Winter, and Feamster). Now + relays only return one of two possible DNS TTL values, and clients + are willing to believe DNS TTL values up to 3 hours long. + Closes ticket 19769. diff --git a/src/or/dns.c b/src/or/dns.c index 5f9813b912..41a6dfd0a4 100644 --- a/src/or/dns.c +++ b/src/or/dns.c @@ -243,29 +243,19 @@ has_dns_init_failed(void) } /** Helper: Given a TTL from a DNS response, determine what TTL to give the - * OP that asked us to resolve it. */ + * OP that asked us to resolve it, and how long to cache that record + * ourselves. */ uint32_t dns_clip_ttl(uint32_t ttl) { - if (ttl < MIN_DNS_TTL) - return MIN_DNS_TTL; - else if (ttl > MAX_DNS_TTL) - return MAX_DNS_TTL; - else - return ttl; -} - -/** Helper: Given a TTL from a DNS response, determine how long to hold it in - * our cache. */ -STATIC uint32_t -dns_get_expiry_ttl(uint32_t ttl) -{ - if (ttl < MIN_DNS_TTL) - return MIN_DNS_TTL; - else if (ttl > MAX_DNS_ENTRY_AGE) - return MAX_DNS_ENTRY_AGE; + /* This logic is a defense against "DefectTor" DNS-based traffic + * confirmation attacks, as in https://nymity.ch/tor-dns/tor-dns.pdf . + * We only give two values: a "low" value and a "high" value. + */ + if (ttl < MIN_DNS_TTL_AT_EXIT) + return MIN_DNS_TTL_AT_EXIT; else - return ttl; + return MAX_DNS_TTL_AT_EXIT; } /** Helper: free storage held by an entry in the DNS cache. */ @@ -1317,7 +1307,7 @@ make_pending_resolve_cached(cached_resolve_t *resolve) resolve->ttl_hostname < ttl) ttl = resolve->ttl_hostname; - set_expiry(new_resolve, time(NULL) + dns_get_expiry_ttl(ttl)); + set_expiry(new_resolve, time(NULL) + dns_clip_ttl(ttl)); } assert_cache_ok(); diff --git a/src/or/dns.h b/src/or/dns.h index b14f7dd29c..951a2a3467 100644 --- a/src/or/dns.h +++ b/src/or/dns.h @@ -12,6 +12,18 @@ #ifndef TOR_DNS_H #define TOR_DNS_H +/** Lowest value for DNS ttl that a server will give. */ +#define MIN_DNS_TTL_AT_EXIT (5*60) +/** Highest value for DNS ttl that a server will give. */ +#define MAX_DNS_TTL_AT_EXIT (60*60) + +/** How long do we keep DNS cache entries before purging them (regardless of + * their TTL)? */ +#define MAX_DNS_ENTRY_AGE (3*60*60) +/** How long do we cache/tell clients to cache DNS records when no TTL is + * known? */ +#define DEFAULT_DNS_TTL (30*60) + int dns_init(void); int has_dns_init_failed(void); void dns_free_all(void); @@ -31,8 +43,6 @@ void dump_dns_mem_usage(int severity); #ifdef DNS_PRIVATE #include "dns_structs.h" -STATIC uint32_t dns_get_expiry_ttl(uint32_t ttl); - MOCK_DECL(STATIC int,dns_resolve_impl,(edge_connection_t *exitconn, int is_resolve,or_circuit_t *oncirc, char **hostname_out, int *made_connection_pending_out, cached_resolve_t **resolve_out)); diff --git a/src/or/or.h b/src/or/or.h index 66717792b4..e48cd92f03 100644 --- a/src/or/or.h +++ b/src/or/or.h @@ -143,18 +143,6 @@ /** Maximum size of a single extrainfo document, as above. */ #define MAX_EXTRAINFO_UPLOAD_SIZE 50000 -/** How long do we keep DNS cache entries before purging them (regardless of - * their TTL)? */ -#define MAX_DNS_ENTRY_AGE (30*60) -/** How long do we cache/tell clients to cache DNS records when no TTL is - * known? */ -#define DEFAULT_DNS_TTL (30*60) -/** How long can a TTL be before we stop believing it? */ -#define MAX_DNS_TTL (3*60*60) -/** How small can a TTL be before we stop believing it? Provides rudimentary - * pinning. */ -#define MIN_DNS_TTL 60 - /** How often do we rotate onion keys? */ #define MIN_ONION_KEY_LIFETIME (7*24*60*60) /** How often do we rotate TLS contexts? */ diff --git a/src/test/test_dns.c b/src/test/test_dns.c index 8346c0a33f..6a8e92cb47 100644 --- a/src/test/test_dns.c +++ b/src/test/test_dns.c @@ -16,30 +16,11 @@ NS(test_main)(void *arg) { (void)arg; - uint32_t ttl_mid = MIN_DNS_TTL / 2 + MAX_DNS_TTL / 2; + uint32_t ttl_mid = MIN_DNS_TTL_AT_EXIT / 2 + MAX_DNS_TTL_AT_EXIT / 2; - tt_int_op(dns_clip_ttl(MIN_DNS_TTL - 1),==,MIN_DNS_TTL); - tt_int_op(dns_clip_ttl(ttl_mid),==,ttl_mid); - tt_int_op(dns_clip_ttl(MAX_DNS_TTL + 1),==,MAX_DNS_TTL); - - done: - return; -} - -#undef NS_SUBMODULE - -#define NS_SUBMODULE expiry_ttl - -static void -NS(test_main)(void *arg) -{ - (void)arg; - - uint32_t ttl_mid = MIN_DNS_TTL / 2 + MAX_DNS_ENTRY_AGE / 2; - - tt_int_op(dns_get_expiry_ttl(MIN_DNS_TTL - 1),==,MIN_DNS_TTL); - tt_int_op(dns_get_expiry_ttl(ttl_mid),==,ttl_mid); - tt_int_op(dns_get_expiry_ttl(MAX_DNS_ENTRY_AGE + 1),==,MAX_DNS_ENTRY_AGE); + tt_int_op(dns_clip_ttl(MIN_DNS_TTL_AT_EXIT - 1),==,MIN_DNS_TTL_AT_EXIT); + tt_int_op(dns_clip_ttl(ttl_mid),==,MAX_DNS_TTL_AT_EXIT); + tt_int_op(dns_clip_ttl(MAX_DNS_TTL_AT_EXIT + 1),==,MAX_DNS_TTL_AT_EXIT); done: return; @@ -749,7 +730,6 @@ NS(test_main)(void *arg) struct testcase_t dns_tests[] = { TEST_CASE(clip_ttl), - TEST_CASE(expiry_ttl), TEST_CASE(resolve), TEST_CASE_ASPECT(resolve_impl, addr_is_ip_no_need_to_resolve), TEST_CASE_ASPECT(resolve_impl, non_exit), |