netfilter: nf_ct_helper: better logging for dropped packets

Connection tracking helpers have to drop packets under exceptional
situations. Currently, the user gets the following logging message
in case that happens:

	nf_ct_%s: dropping packet ...

However, depending on the helper, there are different reasons why a
packet can be dropped.

This patch modifies the existing code to provide more specific
error message in the scope of each helper to help users to debug
the reason why the packet has been dropped, ie:

	nf_ct_%s: dropping packet: reason ...

Thanks to Joe Perches for many formatting suggestions.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 6 insertions, 2 deletions

diff --git a/net/netfilter/nf_nat_amanda.c b/net/netfilter/nf_nat_amanda.c
index 42d337881171..3b67c9d11273 100644
--- a/net/netfilter/nf_nat_amanda.c
+++ b/net/netfilter/nf_nat_amanda.c
@@ -56,15 +56,19 @@ static unsigned int help(struct sk_buff *skb,
 		}
 	}
 
-	if (port == 0)
+	if (port == 0) {
+		nf_ct_helper_log(skb, exp->master, "all ports in use");
 		return NF_DROP;
+	}
 
 	sprintf(buffer, "%u", port);
 	ret = nf_nat_mangle_udp_packet(skb, exp->master, ctinfo,
 				       protoff, matchoff, matchlen,
 				       buffer, strlen(buffer));
-	if (ret != NF_ACCEPT)
+	if (ret != NF_ACCEPT) {
+		nf_ct_helper_log(skb, exp->master, "cannot mangle packet");
 		nf_ct_unexpect_related(exp);
+	}
 	return ret;
 }