Implement DNS with a temporary solution

The DNS resolution implemented with this commit uses getaddrinfo() from
the standard library to get an IPv4 address for the domainpart of the
user-given Jabber ID (JID).

This is by far not the process described in [RFC 6120][1]. This XMPP
core standard prefers SRV entry lookups. Taking A records like in this
implementation is referenced as "fallback process".

Furthermore this implementation just takes the first A record it finds
which could also be improved. It could for example be that IPv4 is
blocked in the referenced network and a AAAA IPv6 record is given and
functional. In this case this implementation would needlessly fail to connect.

[1]: https://www.rfc-editor.org/rfc/rfc6120

1 files changed, 15 insertions, 4 deletions

diff --git a/xmpp.c b/xmpp.c
index dda49dd..9db1574 100644
--- a/xmpp.c
+++ b/xmpp.c
@@ -22,6 +22,7 @@
 #include <string.h>
 #include <sys/types.h>
 #include <sys/socket.h>
+#include <arpa/inet.h>
 #include <netdb.h>
 
 
@@ -62,11 +63,17 @@ char *get_domainpart(char *jid)
  * This handles DNS resolution. It returns the first valid addrinfo which is
  * returned by getaddrinfo. Mind that the addrinfo could use IPv6 instead of
  * IPv4.
+ *
+ * FIXME this just takes the first received IPv4 address (by far not compliant
+ *       to RFC 6120!)
+ * FIXME This function contains a memory leak! Free the linked list servinfo!
  */
 struct addrinfo *get_addrinfo(char *domain)
 {
 	struct addrinfo hints;
 	struct addrinfo *servinfo;
+	void *addr;
+	char ipstr[INET6_ADDRSTRLEN];
 	memset(&hints, 0, sizeof hints);
 	hints.ai_family = AF_UNSPEC;
 	hints.ai_socktype = SOCK_STREAM;
@@ -77,14 +84,18 @@ struct addrinfo *get_addrinfo(char *domain)
 	struct addrinfo *p;
 	for(p=servinfo; p!=NULL; p=p->ai_next) {
 		if (p->ai_family == AF_INET) {
-			printf("an IPv4!\n");  // TODO
-
+			struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
+			addr = &(ipv4->sin_addr);
+			inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);
+			printf("Got compatible IPv4 address %s from DNS.\n", ipstr);
+			return p;
 		} else if (p->ai_family == AF_INET6) {
-			printf("an IPv6!\n");  // TODO
+			printf("Ignored IPv6 DNS entry which is not supported.\n");
 		} else {
-			printf("Unknown addrinfo address type.\n");
+			printf("Ignored unknown addrinfo address type.\n");
 		}
 	}
+	printf("No suitable IP address found via DNS!\n");
 	return NULL;
 }