resolved: implement client-side DNAME resolution

Most servers apparently always implicitly convert DNAME to CNAME, but
some servers don't, hence implement this properly, as this is required
by edns0.

4 files changed, 84 insertions, 12 deletions

diff --git a/src/resolve/resolved-dns-cache.c b/src/resolve/resolved-dns-cache.c
index 04f64022e0..d963ce6e00 100644
--- a/src/resolve/resolved-dns-cache.c
+++ b/src/resolve/resolved-dns-cache.c
@@ -20,8 +20,10 @@
 ***/
 
 #include "alloc-util.h"
+#include "dns-domain.h"
 #include "resolved-dns-cache.h"
 #include "resolved-dns-packet.h"
+#include "string-util.h"
 
 /* Never cache more than 1K entries */
 #define CACHE_MAX 1024
@@ -521,25 +523,53 @@ fail:
 
 static DnsCacheItem *dns_cache_get_by_key_follow_cname(DnsCache *c, DnsResourceKey *k) {
         _cleanup_(dns_resource_key_unrefp) DnsResourceKey *cname_key = NULL;
-        DnsCacheItem *i, *j;
+        DnsCacheItem *i;
+        const char *n;
+        int r;
 
         assert(c);
         assert(k);
 
+        /* If we hit some OOM error, or suchlike, we don't care too
+         * much, after all this is just a cache */
+
         i = hashmap_get(c->by_key, k);
-        if (i || k->type == DNS_TYPE_CNAME)
+        if (i || k->type == DNS_TYPE_CNAME || k->type == DNS_TYPE_DNAME)
                 return i;
 
-        /* check if we have a CNAME record instead */
+        /* Check if we have a CNAME record instead */
         cname_key = dns_resource_key_new_cname(k);
         if (!cname_key)
                 return NULL;
 
-        j = hashmap_get(c->by_key, cname_key);
-        if (j)
-                return j;
+        i = hashmap_get(c->by_key, cname_key);
+        if (i)
+                return i;
+
+        /* OK, let's look for cached DNAME records. */
+        n = DNS_RESOURCE_KEY_NAME(k);
+        for (;;) {
+                _cleanup_(dns_resource_key_unrefp) DnsResourceKey *dname_key = NULL;
+                char label[DNS_LABEL_MAX];
+
+                if (isempty(n))
+                        return NULL;
 
-        return i;
+                dname_key = dns_resource_key_new(k->class, DNS_TYPE_DNAME, n);
+                if (!dname_key)
+                        return NULL;
+
+                i = hashmap_get(c->by_key, dname_key);
+                if (i)
+                        return i;
+
+                /* Jump one label ahead */
+                r = dns_label_unescape(&n, label, sizeof(label));
+                if (r <= 0)
+                        return NULL;
+        }
+
+        return NULL;
 }
 
 int dns_cache_lookup(DnsCache *c, DnsResourceKey *key, int *rcode, DnsAnswer **ret) {
diff --git a/src/resolve/resolved-dns-question.c b/src/resolve/resolved-dns-question.c
index 75f2ed6042..9fb3038381 100644
--- a/src/resolve/resolved-dns-question.c
+++ b/src/resolve/resolved-dns-question.c
@@ -207,6 +207,7 @@ int dns_question_cname_redirect(DnsQuestion *q, const DnsResourceRecord *cname,
 
         assert(cname);
         assert(ret);
+        assert(IN_SET(cname->key->type, DNS_TYPE_CNAME, DNS_TYPE_DNAME));
 
         if (!q) {
                 n = dns_question_new(0);
@@ -219,7 +220,22 @@ int dns_question_cname_redirect(DnsQuestion *q, const DnsResourceRecord *cname,
         }
 
         for (i = 0; i < q->n_keys; i++) {
-                r = dns_name_equal(DNS_RESOURCE_KEY_NAME(q->keys[i]), cname->cname.name);
+                _cleanup_free_ char *destination = NULL;
+                const char *d;
+
+                if (cname->key->type == DNS_TYPE_CNAME)
+                        d = cname->cname.name;
+                else {
+                        r = dns_name_change_suffix(DNS_RESOURCE_KEY_NAME(q->keys[i]), DNS_RESOURCE_KEY_NAME(cname->key), cname->dname.name, &destination);
+                        if (r < 0)
+                                return r;
+                        if (r == 0)
+                                continue;
+
+                        d = destination;
+                }
+
+                r = dns_name_equal(DNS_RESOURCE_KEY_NAME(q->keys[i]), d);
                 if (r < 0)
                         return r;
 
diff --git a/src/resolve/resolved-dns-rr.c b/src/resolve/resolved-dns-rr.c
index 636e07dea9..9b264900fc 100644
--- a/src/resolve/resolved-dns-rr.c
+++ b/src/resolve/resolved-dns-rr.c
@@ -57,10 +57,33 @@ DnsResourceKey* dns_resource_key_new_cname(const DnsResourceKey *key) {
 }
 
 DnsResourceKey* dns_resource_key_new_redirect(const DnsResourceKey *key, const DnsResourceRecord *cname) {
+        int r;
+
         assert(key);
         assert(cname);
 
-        return dns_resource_key_new(key->class, key->type, cname->cname.name);
+        assert(IN_SET(cname->key->type, DNS_TYPE_CNAME, DNS_TYPE_DNAME));
+
+        if (cname->key->type == DNS_TYPE_CNAME)
+                return dns_resource_key_new(key->class, key->type, cname->cname.name);
+        else {
+                DnsResourceKey *k;
+                char *destination = NULL;
+
+                r = dns_name_change_suffix(DNS_RESOURCE_KEY_NAME(key), DNS_RESOURCE_KEY_NAME(cname->key), cname->dname.name, &destination);
+                if (r < 0)
+                        return NULL;
+                if (r == 0)
+                        return dns_resource_key_ref((DnsResourceKey*) key);
+
+                k = dns_resource_key_new_consume(key->class, key->type, destination);
+                if (!k) {
+                        free(destination);
+                        return NULL;
+                }
+
+                return k;
+        }
 }
 
 DnsResourceKey* dns_resource_key_new_consume(uint16_t class, uint16_t type, char *name) {
@@ -142,10 +165,12 @@ int dns_resource_key_match_cname(const DnsResourceKey *key, const DnsResourceRec
         if (rr->key->class != key->class && key->class != DNS_CLASS_ANY)
                 return 0;
 
-        if (rr->key->type != DNS_TYPE_CNAME)
+        if (rr->key->type == DNS_TYPE_CNAME)
+                return dns_name_equal(DNS_RESOURCE_KEY_NAME(key), DNS_RESOURCE_KEY_NAME(rr->key));
+        else if (rr->key->type == DNS_TYPE_DNAME)
+                return dns_name_endswith(DNS_RESOURCE_KEY_NAME(key), DNS_RESOURCE_KEY_NAME(rr->key));
+        else
                 return 0;
-
-        return dns_name_equal(DNS_RESOURCE_KEY_NAME(rr->key), DNS_RESOURCE_KEY_NAME(key));
 }
 
 static void dns_resource_key_hash_func(const void *i, struct siphash *state) {
diff --git a/src/resolve/resolved-dns-rr.h b/src/resolve/resolved-dns-rr.h
index c02f2ec00f..c1601fb694 100644
--- a/src/resolve/resolved-dns-rr.h
+++ b/src/resolve/resolved-dns-rr.h
@@ -186,6 +186,7 @@ static inline const char* DNS_RESOURCE_KEY_NAME(const DnsResourceKey *key) {
 
 DnsResourceKey* dns_resource_key_new(uint16_t class, uint16_t type, const char *name);
 DnsResourceKey* dns_resource_key_new_cname(const DnsResourceKey *key);
+DnsResourceKey* dns_resource_key_new_dname(const DnsResourceKey *key);
 DnsResourceKey* dns_resource_key_new_redirect(const DnsResourceKey *key, const DnsResourceRecord *cname);
 DnsResourceKey* dns_resource_key_new_consume(uint16_t class, uint16_t type, char *name);
 DnsResourceKey* dns_resource_key_ref(DnsResourceKey *key);