From: Lennart Poettering <lennart@poettering.net>
Date: Tue, 29 Jul 2014 23:47:48 +0000 (+0200)
Subject: resolved: follow more closely the recommend timeouts and TTLs from the LLMNR spec
X-Git-Tag: v216~385
X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=elogind.git;a=commitdiff_plain;h=0c903ae7db52af8ecad0619ec0da021f951c9c1c

resolved: follow more closely the recommend timeouts and TTLs from the LLMNR spec
---

diff --git a/src/resolve/resolved-dns-query.c b/src/resolve/resolved-dns-query.c
index 857025152..14ae68395 100644
--- a/src/resolve/resolved-dns-query.c
+++ b/src/resolve/resolved-dns-query.c
@@ -24,12 +24,27 @@
 #include "resolved-dns-query.h"
 #include "resolved-dns-domain.h"
 
-#define TRANSACTION_TIMEOUT_USEC (5 * USEC_PER_SEC)
+/* After how much time to repeat classic DNS requests */
+#define DNS_TRANSACTION_TIMEOUT_USEC (5 * USEC_PER_SEC)
+
+/* After how much time to repeat LLMNR requests, see RFC 4795 Section 7 */
+#define LLMNR_TRANSACTION_TIMEOUT_USEC (1 * USEC_PER_SEC)
+
+/* How long to wait for the query in total */
 #define QUERY_TIMEOUT_USEC (30 * USEC_PER_SEC)
-#define ATTEMPTS_MAX 8
+
+/* Maximum attempts to send DNS requests, across all DNS servers */
+#define DNS_TRANSACTION_ATTEMPTS_MAX 8
+
+/* Maximum attempts to send LLMNR requests, see RFC 4795 Section 2.7 */
+#define LLMNR_TRANSACTION_ATTEMPTS_MAX 3
+
 #define CNAME_MAX 8
 #define QUERIES_MAX 2048
 
+#define TRANSACTION_TIMEOUT_USEC(p) ((t)->scope->protocol == DNS_PROTOCOL_LLMNR ? LLMNR_TRANSACTION_TIMEOUT_USEC : DNS_TRANSACTION_TIMEOUT_USEC)
+#define TRANSACTION_ATTEMPTS_MAX(p) ((t)->scope->protocol == DNS_PROTOCOL_LLMNR ? LLMNR_TRANSACTION_ATTEMPTS_MAX : DNS_TRANSACTION_ATTEMPTS_MAX)
+
 static int dns_query_transaction_go(DnsQueryTransaction *t);
 
 DnsQueryTransaction* dns_query_transaction_free(DnsQueryTransaction *t) {
@@ -411,10 +426,13 @@ static int dns_query_make_packet(DnsQueryTransaction *t) {
 }
 
 static int dns_query_transaction_go(DnsQueryTransaction *t) {
+        bool had_stream;
         int r;
 
         assert(t);
 
+        had_stream = !!t->stream;
+
         dns_query_transaction_stop(t);
 
         log_debug("Beginning transaction on scope %s on %s/%s",
@@ -422,7 +440,14 @@ static int dns_query_transaction_go(DnsQueryTransaction *t) {
                   t->scope->link ? t->scope->link->name : "*",
                   t->scope->family == AF_UNSPEC ? "*" : af_to_name(t->scope->family));
 
-        if (t->n_attempts >= ATTEMPTS_MAX) {
+        if (t->n_attempts >= TRANSACTION_ATTEMPTS_MAX(t)) {
+                dns_query_transaction_complete(t, DNS_QUERY_ATTEMPTS_MAX);
+                return 0;
+        }
+
+        if (t->scope->protocol == DNS_PROTOCOL_LLMNR && had_stream) {
+                /* If we already tried via a stream, then we don't
+                 * retry on LLMNR. See RFC 4795, Section 2.7. */
                 dns_query_transaction_complete(t, DNS_QUERY_ATTEMPTS_MAX);
                 return 0;
         }
@@ -482,7 +507,7 @@ static int dns_query_transaction_go(DnsQueryTransaction *t) {
                 return dns_query_transaction_go(t);
         }
 
-        r = sd_event_add_time(t->scope->manager->event, &t->timeout_event_source, CLOCK_MONOTONIC, now(CLOCK_MONOTONIC) + TRANSACTION_TIMEOUT_USEC, 0, on_transaction_timeout, t);
+        r = sd_event_add_time(t->scope->manager->event, &t->timeout_event_source, CLOCK_MONOTONIC, now(CLOCK_MONOTONIC) + TRANSACTION_TIMEOUT_USEC(t), 0, on_transaction_timeout, t);
         if (r < 0)
                 return r;
 
diff --git a/src/resolve/resolved-link.c b/src/resolve/resolved-link.c
index 6ac7c5be9..341cb5aea 100644
--- a/src/resolve/resolved-link.c
+++ b/src/resolve/resolved-link.c
@@ -25,7 +25,8 @@
 #include "strv.h"
 #include "resolved-link.h"
 
-#define DEFAULT_TTL (10)
+/* RFC 4795 Section 2.8. suggests a TTL of 30s by default */
+#define LLMNR_DEFAULT_TTL (30)
 
 static void link_address_add_rrs(LinkAddress *a);
 
@@ -354,7 +355,7 @@ static void link_address_add_rrs(LinkAddress *a) {
                         }
 
                         a->llmnr_address_rr->a.in_addr = a->in_addr.in;
-                        a->llmnr_address_rr->ttl = DEFAULT_TTL;
+                        a->llmnr_address_rr->ttl = LLMNR_DEFAULT_TTL;
                 }
 
                 if (!a->llmnr_ptr_rr) {
@@ -362,7 +363,7 @@ static void link_address_add_rrs(LinkAddress *a) {
                         if (r < 0)
                                 goto fail;
 
-                        a->llmnr_ptr_rr->ttl = DEFAULT_TTL;
+                        a->llmnr_ptr_rr->ttl = LLMNR_DEFAULT_TTL;
                 }
 
                 if (link_address_relevant(a)) {
@@ -397,7 +398,7 @@ static void link_address_add_rrs(LinkAddress *a) {
                         }
 
                         a->llmnr_address_rr->aaaa.in6_addr = a->in_addr.in6;
-                        a->llmnr_address_rr->ttl = DEFAULT_TTL;
+                        a->llmnr_address_rr->ttl = LLMNR_DEFAULT_TTL;
                 }
 
                 if (!a->llmnr_ptr_rr) {
@@ -405,7 +406,7 @@ static void link_address_add_rrs(LinkAddress *a) {
                         if (r < 0)
                                 goto fail;
 
-                        a->llmnr_ptr_rr->ttl = DEFAULT_TTL;
+                        a->llmnr_ptr_rr->ttl = LLMNR_DEFAULT_TTL;
                 }
 
                 if (link_address_relevant(a)) {