resolved: add ResolveService() bus call for resolving SRV and DNS-SD services

This also adds client-side support for this to systemd-resolve-host.

Note that the ResolveService() API can deal both with DNS-SD service
(consisting of service name, type and domain), as well as classic SRV
services (consisting just of a type and a domain), all exposed in the
same call.

This patch also reworks CNAME handling in order to reuse it between
hostname, RR and service lookups.

In contrast to Avahi and Bonjour, this new API will actually reolve the
A/AAAA RRs the SRV RRs point to in one go (unless this is explicitly
disabled). This normally comes for free, as these RRs are sent along
the SRV responses anyway, hence let's make use of that. This makes the
API considerably easier to use, as a single ResolveService() invocation
will return all necessary data to pick a server and connect() to it.

Note that this only implements the DNS-SD resolving step, it does not
implement DNS-SD browsing, as that makes sense primarily on mDNS, due to
its continuous nature.

1 files changed, 126 insertions, 0 deletions

diff --git a/src/resolve/resolved-dns-question.c b/src/resolve/resolved-dns-question.c
index 48951221dc..868658652d 100644
--- a/src/resolve/resolved-dns-question.c
+++ b/src/resolve/resolved-dns-question.c
@@ -301,3 +301,129 @@ int dns_question_cname_redirect(DnsQuestion *q, const DnsResourceRecord *cname,
 
         return 1;
 }
+
+const char *dns_question_name(DnsQuestion *q) {
+        assert(q);
+
+        if (q->n_keys < 1)
+                return NULL;
+
+        return DNS_RESOURCE_KEY_NAME(q->keys[0]);
+}
+
+int dns_question_new_address(DnsQuestion **ret, int family, const char *name) {
+        _cleanup_(dns_question_unrefp) DnsQuestion *q = NULL;
+        int r;
+
+        assert(ret);
+        assert(name);
+
+        if (!IN_SET(family, AF_INET, AF_INET6, AF_UNSPEC))
+                return -EAFNOSUPPORT;
+
+        q = dns_question_new(family == AF_UNSPEC ? 2 : 1);
+        if (!q)
+                return -ENOMEM;
+
+        if (family != AF_INET6) {
+                _cleanup_(dns_resource_key_unrefp) DnsResourceKey *key = NULL;
+
+                key = dns_resource_key_new(DNS_CLASS_IN, DNS_TYPE_A, name);
+                if (!key)
+                        return -ENOMEM;
+
+                r = dns_question_add(q, key);
+                if (r < 0)
+                        return r;
+        }
+
+        if (family != AF_INET) {
+                _cleanup_(dns_resource_key_unrefp) DnsResourceKey *key = NULL;
+
+                key = dns_resource_key_new(DNS_CLASS_IN, DNS_TYPE_AAAA, name);
+                if (!key)
+                        return -ENOMEM;
+
+                r = dns_question_add(q, key);
+                if (r < 0)
+                        return r;
+        }
+
+        *ret = q;
+        q = NULL;
+
+        return 0;
+}
+
+int dns_question_new_reverse(DnsQuestion **ret, int family, const union in_addr_union *a) {
+        _cleanup_(dns_resource_key_unrefp) DnsResourceKey *key = NULL;
+        _cleanup_(dns_question_unrefp) DnsQuestion *q = NULL;
+        _cleanup_free_ char *reverse = NULL;
+        int r;
+
+        assert(ret);
+        assert(a);
+
+        if (!IN_SET(family, AF_INET, AF_INET6, AF_UNSPEC))
+                return -EAFNOSUPPORT;
+
+        r = dns_name_reverse(family, a, &reverse);
+        if (r < 0)
+                return r;
+
+        q = dns_question_new(1);
+        if (!q)
+                return -ENOMEM;
+
+        key = dns_resource_key_new_consume(DNS_CLASS_IN, DNS_TYPE_PTR, reverse);
+        if (!key)
+                return -ENOMEM;
+
+        reverse = NULL;
+
+        r = dns_question_add(q, key);
+        if (r < 0)
+                return r;
+
+        *ret = q;
+        q = NULL;
+
+        return 0;
+}
+
+int dns_question_new_service(DnsQuestion **ret, const char *name, bool with_txt) {
+        _cleanup_(dns_resource_key_unrefp) DnsResourceKey *key = NULL;
+        _cleanup_(dns_question_unrefp) DnsQuestion *q = NULL;
+        int r;
+
+        assert(ret);
+        assert(name);
+
+        q = dns_question_new(1 + with_txt);
+        if (!q)
+                return -ENOMEM;
+
+        key = dns_resource_key_new(DNS_CLASS_IN, DNS_TYPE_SRV, name);
+        if (!key)
+                return -ENOMEM;
+
+        r = dns_question_add(q, key);
+        if (r < 0)
+                return r;
+
+        if (with_txt) {
+                dns_resource_key_unref(key);
+                key = dns_resource_key_new(DNS_CLASS_IN, DNS_TYPE_TXT, name);
+                if (!key)
+                        return -ENOMEM;
+
+                r = dns_question_add(q, key);
+                if (r < 0)
+                        return r;
+        }
+
+        *ret = q;
+        q = NULL;
+
+        return 0;
+}