File pdnsd-fix-udppacketsize.diff of Package pdnsd

From: Stefan Seyfried <stefan.seyfried@sap.com>
Subject: fix invalid UDP packet in case of truncation

The UDP response packet pdnsd sends is invalid in case of a truncated packet:
The header fields ANCOUNT and NSCOUNT stil contains the "intended" number of
records, but the payload is actually truncated to udpmaxrespsize.
This might have worked if only additionals would overflow the packet size,
but fails definitely when regular records already exceed it.
Fix by always checking for space in the packet before actually adding any
record, be it regular or additional.

Index: b/src/dns_answer.c
===================================================================
--- a/src/dns_answer.c
+++ b/src/dns_answer.c
@@ -392,10 +392,12 @@ static int add_rr(dns_msg_t **ans, size_
 		rdlen=dlen;
 		*sz+=dlen;
 	}
-
-	if (udp && *sz>*udp && section==S_ADDITIONAL) /* only add the record if we do not increase the length over 512 */
-		*sz=osz;                              /* (or possibly more if the request used EDNS) in additionals for udp answer. */
-	else {
+	/* not only S_ADDITIONAL, skip everything if it does not fit into the response, sz+10 because we add 10 bytes below */
+	if (udp && *sz+10>*udp /*&& section==S_ADDITIONAL*/) {
+				/* only add the record if we do not increase the length over 512 */
+		*sz=osz;	/* (or possibly more if the request used EDNS) in additionals for udp answer. */
+		*udp=0;		/* this signals that the response is too big for the udp packet */
+	} else {
 		PUTINT16(type,rrht);
 		PUTINT16(C_IN,rrht);
 		PUTINT32(ttl,rrht);
@@ -1419,7 +1421,9 @@ static void *udp_answer_thread(void *dat
 	}
 	pthread_cleanup_push(free, resp);
 	if (rlen>udpmaxrespsize) {
-		rlen=udpmaxrespsize;
+		/* udpmaxrespsize == 0 is the flag for "we did not have enough space for the response */
+		if (udpmaxrespsize)
+			rlen=udpmaxrespsize;
 		resp->hdr.tc=1; /*set truncated bit*/
 	}
 	DEBUG_MSG("Outbound msg len %li, tc=%u, rc=\"%s\"\n",(long)rlen,resp->hdr.tc,get_ename(rcode));