[NETFILTER]: nf_nat_sip: only perform RTP DNAT if SIP session was SNATed

DNAT of the the RTP session is only necessary if the SIP session has
been SNATed.
Signed-off-by: NJerome Borsboom <j.borsboom@erasmusmc.nl>
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/netfilter/nf_nat_sip.c

@@ -260,7 +260,11 @@ static unsigned int ip_nat_sdp(struct sk_buff **pskb,
 	DEBUGP("ip_nat_sdp():\n");
 
 	/* Connection will come from reply */
-	newip = ct->tuplehash[!dir].tuple.dst.u3.ip;
+	if (ct->tuplehash[dir].tuple.src.u3.ip ==
+	    ct->tuplehash[!dir].tuple.dst.u3.ip)
+		newip = exp->tuple.dst.u3.ip;
+	else
+		newip = ct->tuplehash[!dir].tuple.dst.u3.ip;
 
 	exp->saved_ip = exp->tuple.dst.u3.ip;
 	exp->tuple.dst.u3.ip = newip;