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dns: ignore spam addresses from cname answers

--- a/sys/src/cmd/ndb/dnresolve.c

+++ b/sys/src/cmd/ndb/dnresolve.c

@@ -1091,8 +1091,20 @@

 	unique(mp->ns);

 	unique(mp->ar);

-	if(mp->an)

+	if(mp->an){

+		/*

+		 * only use cname answer when returned. some dns servers

+		 * attach spam address records which poisons the cache.

+		 */

+		if((tp = rrremtype(&mp->an, Tcname)) != 0){

+			if(mp->an){

+				dnslog("removing spam %Q for %Q from %I", mp->an, tp, srcip);

+				rrfreelist(mp->an);

+			}

+			mp->an = tp;

+		}

 		rrattach(mp->an, (mp->flags & Fauth) != 0);

+	}

 	if(mp->ar)

 		rrattach(mp->ar, Notauthoritative);

 	if(mp->ns && !cfg.justforw){

-- 

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