ndb/dns: filter dns answers avoiding cache poisoning

only cache what we asked for or need to resolve the
query. filter out everything else.

1 files changed, 39 insertions, 24 deletions

diff --git a/sys/src/cmd/ndb/dn.c b/sys/src/cmd/ndb/dn.c
index fcf262259..0628f8ee5 100644
--- a/sys/src/cmd/ndb/dn.c
+++ b/sys/src/cmd/ndb/dn.c
@@ -1085,54 +1085,69 @@ rrcat(RR **start, RR *rp)
 	return *start;
 }
 
-/*
- *  remove negative cache rr's from an rr list
- */
 RR*
-rrremneg(RR **l)
+rrremfilter(RR **l, int (*filter)(RR*, void*), void *arg)
 {
-	RR **nl, *rp;
-	RR *first;
+	RR *first, *rp;
+	RR **nl;
 
 	first = nil;
 	nl = &first;
 	while(*l != nil){
 		rp = *l;
-		if(rp->negative){
+		if((*filter)(rp, arg)){
 			*l = rp->next;
 			*nl = rp;
 			nl = &rp->next;
 			*nl = nil;
 		} else
-			l = &rp->next;
+			l = &(*l)->next;
 	}
 
 	return first;
 }
 
+static int
+filterneg(RR *rp, void*)
+{
+	return rp->negative;
+}
+static int
+filtertype(RR *rp, void *arg)
+{
+	return rp->type == *((int*)arg);
+}
+static int
+filterowner(RR *rp, void *arg)
+{
+	return rp->owner == (DN*)arg;
+}
+
+/*
+ *  remove negative cache rr's from an rr list
+ */
+RR*
+rrremneg(RR **l)
+{
+	return rrremfilter(l, filterneg, nil);
+}
+
 /*
  *  remove rr's of a particular type from an rr list
  */
 RR*
 rrremtype(RR **l, int type)
 {
-	RR *first, *rp;
-	RR **nl;
-
-	first = nil;
-	nl = &first;
-	while(*l != nil){
-		rp = *l;
-		if(rp->type == type){
-			*l = rp->next;
-			*nl = rp;
-			nl = &rp->next;
-			*nl = nil;
-		} else
-			l = &(*l)->next;
-	}
+	return rrremfilter(l, filtertype, &type);
+}
 
-	return first;
+/*
+ *  remove rr's of a particular owner from an rr list
+ */
+RR*
+rrremowner(RR **l, DN *owner)
+{
+	return rrremfilter(l, filterowner, owner);
 }
 
 static char *