diff options
Diffstat (limited to 'src/or/dns.c')
| -rw-r--r-- | src/or/dns.c | 159 |
1 files changed, 64 insertions, 95 deletions
diff --git a/src/or/dns.c b/src/or/dns.c index c7adfbc971..0ad4c0f505 100644 --- a/src/or/dns.c +++ b/src/or/dns.c @@ -9,6 +9,42 @@ * This is implemented as a wrapper around Adam Langley's eventdns.c code. * (We can't just use gethostbyname() and friends because we really need to * be nonblocking.) + * + * There are three main cases when a Tor relay uses dns.c to launch a DNS + * request: + * <ol> + * <li>To check whether the DNS server is working more or less correctly. + * This happens via dns_launch_correctness_checks(). The answer is + * reported in the return value from later calls to + * dns_seems_to_be_broken(). + * <li>When a client has asked the relay, in a RELAY_BEGIN cell, to connect + * to a given server by hostname. This happens via dns_resolve(). + * <li>When a client has asked the rela, in a RELAY_RESOLVE cell, to look + * up a given server's IP address(es) by hostname. This also happens via + * dns_resolve(). + * </ol> + * + * Each of these gets handled a little differently. + * + * To check for correctness, we look up some hostname we expect to exist and + * have real entries, some hostnames which we expect to definitely not exist, + * and some hostnames that we expect to probably not exist. If too many of + * the hostnames that shouldn't exist do exist, that's a DNS hijacking + * attempt. If too many of the hostnames that should exist have the same + * addresses as the ones that shouldn't exist, that's a very bad DNS hijacking + * attempt, or a very naughty captive portal. And if the hostnames that + * should exist simply don't exist, we probably have a broken nameserver. + * + * To handle client requests, we first check our cache for answers. If there + * isn't something up-to-date, we've got to launch A or AAAA requests as + * appropriate. How we handle responses to those in particular is a bit + * complex; see dns_lookup() and set_exitconn_info_from_resolve(). + * + * When a lookup is finally complete, the inform_pending_connections() + * function will tell all of the streams that have been waiting for the + * resolve, by calling connection_exit_connect() if the client sent a + * RELAY_BEGIN cell, and by calling send_resolved_cell() or + * send_hostname_cell() if the client sent a RELAY_RESOLVE cell. **/ #define DNS_PRIVATE @@ -27,61 +63,8 @@ #include "router.h" #include "ht.h" #include "sandbox.h" -#ifdef HAVE_EVENT2_DNS_H #include <event2/event.h> #include <event2/dns.h> -#else -#include <event.h> -#include "eventdns.h" -#ifndef HAVE_EVDNS_SET_DEFAULT_OUTGOING_BIND_ADDRESS -#define HAVE_EVDNS_SET_DEFAULT_OUTGOING_BIND_ADDRESS -#endif -#endif - -#ifndef HAVE_EVENT2_DNS_H -struct evdns_base; -struct evdns_request; -#define evdns_base_new(x,y) tor_malloc(1) -#define evdns_base_clear_nameservers_and_suspend(base) \ - evdns_clear_nameservers_and_suspend() -#define evdns_base_search_clear(base) evdns_search_clear() -#define evdns_base_set_default_outgoing_bind_address(base, a, len) \ - evdns_set_default_outgoing_bind_address((a),(len)) -#define evdns_base_resolv_conf_parse(base, options, fname) \ - evdns_resolv_conf_parse((options), (fname)) -#define evdns_base_count_nameservers(base) \ - evdns_count_nameservers() -#define evdns_base_resume(base) \ - evdns_resume() -#define evdns_base_config_windows_nameservers(base) \ - evdns_config_windows_nameservers() -#define evdns_base_set_option_(base, opt, val) \ - evdns_set_option((opt),(val),DNS_OPTIONS_ALL) -/* Note: our internal eventdns.c, plus Libevent 1.4, used a 1 return to - * signify failure to launch a resolve. Libevent 2.0 uses a -1 return to - * signify a failure on a resolve, though if we're on Libevent 2.0, we should - * have event2/dns.h and never hit these macros. Regardless, 0 is success. */ -#define evdns_base_resolve_ipv4(base, addr, options, cb, ptr) \ - ((evdns_resolve_ipv4((addr), (options), (cb), (ptr))!=0) \ - ? NULL : ((void*)1)) -#define evdns_base_resolve_ipv6(base, addr, options, cb, ptr) \ - ((evdns_resolve_ipv6((addr), (options), (cb), (ptr))!=0) \ - ? NULL : ((void*)1)) -#define evdns_base_resolve_reverse(base, addr, options, cb, ptr) \ - ((evdns_resolve_reverse((addr), (options), (cb), (ptr))!=0) \ - ? NULL : ((void*)1)) -#define evdns_base_resolve_reverse_ipv6(base, addr, options, cb, ptr) \ - ((evdns_resolve_reverse_ipv6((addr), (options), (cb), (ptr))!=0) \ - ? NULL : ((void*)1)) - -#elif defined(LIBEVENT_VERSION_NUMBER) && LIBEVENT_VERSION_NUMBER < 0x02000303 -#define evdns_base_set_option_(base, opt, val) \ - evdns_base_set_option((base), (opt),(val),DNS_OPTIONS_ALL) - -#else -#define evdns_base_set_option_ evdns_base_set_option - -#endif /** How long will we wait for an answer from the resolver before we decide * that the resolver is wedged? */ @@ -260,29 +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. */ @@ -353,7 +326,7 @@ cached_resolve_add_answer(cached_resolve_t *resolve, resolve->result_ipv4.err_ipv4 = dns_result; resolve->res_status_ipv4 = RES_STATUS_DONE_ERR; } - + resolve->ttl_ipv4 = ttl; } else if (query_type == DNS_IPv6_AAAA) { if (resolve->res_status_ipv6 != RES_STATUS_INFLIGHT) return; @@ -368,6 +341,7 @@ cached_resolve_add_answer(cached_resolve_t *resolve, resolve->result_ipv6.err_ipv6 = dns_result; resolve->res_status_ipv6 = RES_STATUS_DONE_ERR; } + resolve->ttl_ipv6 = ttl; } } @@ -548,6 +522,7 @@ send_resolved_cell,(edge_connection_t *conn, uint8_t answer_type, answer_type = RESOLVED_TYPE_ERROR; /* fall through. */ } + /* Falls through. */ case RESOLVED_TYPE_ERROR_TRANSIENT: case RESOLVED_TYPE_ERROR: { @@ -846,8 +821,14 @@ dns_resolve_impl,(edge_connection_t *exitconn, int is_resolve, } /** Given an exit connection <b>exitconn</b>, and a cached_resolve_t - * <b>resolve</b> whose DNS lookups have all succeeded or failed, update the - * appropriate fields (address_ttl and addr) of <b>exitconn</b>. + * <b>resolve</b> whose DNS lookups have all either succeeded or failed, + * update the appropriate fields (address_ttl and addr) of <b>exitconn</b>. + * + * The logic can be complicated here, since we might have launched both + * an A lookup and an AAAA lookup, and since either of those might have + * succeeded or failed, and since we want to answer a RESOLVE cell with + * a full answer but answer a BEGIN cell with whatever answer the client + * would accept <i>and</i> we could still connect to. * * If this is a reverse lookup, set *<b>hostname_out</b> to a newly allocated * copy of the name resulting hostname. @@ -1190,7 +1171,12 @@ dns_found_answer(const char *address, uint8_t query_type, /** Given a pending cached_resolve_t that we just finished resolving, * inform every connection that was waiting for the outcome of that - * resolution. */ + * resolution. + * + * Do this by sending a RELAY_RESOLVED cell (if the pending stream had sent us + * RELAY_RESOLVE cell), or by launching an exit connection (if the pending + * stream had send us a RELAY_BEGIN cell). + */ static void inform_pending_connections(cached_resolve_t *resolve) { @@ -1323,7 +1309,7 @@ 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(); @@ -1373,23 +1359,6 @@ configure_nameservers(int force) } } -#ifdef HAVE_EVDNS_SET_DEFAULT_OUTGOING_BIND_ADDRESS - if (! tor_addr_is_null(&options->OutboundBindAddressIPv4_)) { - int socklen; - struct sockaddr_storage ss; - socklen = tor_addr_to_sockaddr(&options->OutboundBindAddressIPv4_, 0, - (struct sockaddr *)&ss, sizeof(ss)); - if (socklen <= 0) { - log_warn(LD_BUG, "Couldn't convert outbound bind address to sockaddr." - " Ignoring."); - } else { - evdns_base_set_default_outgoing_bind_address(the_evdns_base, - (struct sockaddr *)&ss, - socklen); - } - } -#endif - evdns_set_log_fn(evdns_log_cb); if (conf_fname) { log_debug(LD_FS, "stat()ing %s", conf_fname); @@ -1454,7 +1423,7 @@ configure_nameservers(int force) } #endif -#define SET(k,v) evdns_base_set_option_(the_evdns_base, (k), (v)) +#define SET(k,v) evdns_base_set_option(the_evdns_base, (k), (v)) if (evdns_base_count_nameservers(the_evdns_base) == 1) { SET("max-timeouts:", "16"); |