Date:	Mon, 20 Aug 2012 10:57:54 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...i.de>
To:	Patrick McHardy <kaber@...sh.net>
cc:	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 06/18] netfilter: add protocol independant NAT core


On Monday 2012-08-20 05:39, Patrick McHardy wrote:
> 
> enum ctattr_nat {
> 	CTA_NAT_UNSPEC,
>-	CTA_NAT_MINIP,
>-	CTA_NAT_MAXIP,
>+	CTA_NAT_V4_MINIP,
>+#define CTA_NAT_MINIP CTA_NAT_V4_MINIP
>+	CTA_NAT_V4_MAXIP,
>+#define CTA_NAT_MAXIP CTA_NAT_V4_MAXIP
> 	CTA_NAT_PROTO,
> 	__CTA_NAT_MAX
> };

One could also

enum ctattr_nat {
   ...
   __CTA_NAT_MAX,

  CTA_NAT_MINIP = CTA_NAT_V4_MINIP,
  CTA_NAT_MAXIP = CTA_NAT_V4_MAXIP,
};

to provide the old names.


>diff --git a/net/ipv4/netfilter/Kconfig b/net/ipv4/netfilter/Kconfig
>index fcc543c..33372a1 100644
>--- a/net/ipv4/netfilter/Kconfig
>+++ b/net/ipv4/netfilter/Kconfig
[...]
> 
>-config NF_NAT_NEEDED
>-	bool
>-	depends on NF_NAT
>-	default y
>-

Could add "if NF_NAT_IPV4".."endif" block as appropriate around here,
to save on all the extra "depend on NF_NAT_IPV4" clauses.

> config IP_NF_TARGET_MASQUERADE
> 	tristate "MASQUERADE target support"
>-	depends on NF_NAT
>+	depends on NF_NAT_IPV4
> 	default m if NETFILTER_ADVANCED=n
> 	help
> 	  Masquerading is a special case of NAT: all outgoing connections are




>+static int nf_nat_ipv4_in_range(const struct nf_conntrack_tuple *t,
>+				const struct nf_nat_range *range)
>+{
>+	return ntohl(t->src.u3.ip) >= ntohl(range->min_addr.ip) &&
>+	       ntohl(t->src.u3.ip) <= ntohl(range->max_addr.ip);
>+}

static bool ..

>+static bool nf_nat_ipv4_manip_pkt(struct sk_buff *skb,
>+				  unsigned int iphdroff,
>+				  const struct nf_nat_l4proto *l4proto,
>+				  const struct nf_conntrack_tuple *target,
>+				  enum nf_nat_manip_type maniptype)
>+{
>+	struct iphdr *iph;
>+	unsigned int hdroff;
>+
>+	if (!skb_make_writable(skb, iphdroff + sizeof(*iph)))
>+		return false;
>+
>+	iph = (void *)skb->data + iphdroff;

Is iph = ip_hdr(skb), hdroff = iphdroff+skb_iphdrlen(iph) not usable here?

>+	hdroff = iphdroff + iph->ihl * 4;
>+
>+	if (!l4proto->manip_pkt(skb, &nf_nat_l3proto_ipv4, iphdroff, hdroff,
>+				target, maniptype))
>+		return false;
>+	iph = (void *)skb->data + iphdroff;

Is trying to avoid some GNU extensions a worthwhile goal? If so,
iph = (struct iphdr *)(skb->data + iphdroff) should be used, like in:

>+static void nf_nat_ipv4_csum_update(struct sk_buff *skb,
>+				    unsigned int iphdroff, __sum16 *check,
>+				    const struct nf_conntrack_tuple *t,
>+				    enum nf_nat_manip_type maniptype)
>+{
>+	struct iphdr *iph = (struct iphdr *)(skb->data + iphdroff);
>[...]
>+}



>+static void nf_nat_ipv4_csum_recalc(struct sk_buff *skb,
>+				    u8 proto, void *data, __sum16 *check,
>+				    int datalen, int oldlen)
>+{
>+	const struct iphdr *iph = ip_hdr(skb);
>+	struct rtable *rt = skb_rtable(skb);
>+
>+	if (skb->ip_summed != CHECKSUM_PARTIAL) {
>+		if (!(rt->rt_flags & RTCF_LOCAL) &&
>+		    (!skb->dev || skb->dev->features & NETIF_F_V4_CSUM)) {
>+			skb->ip_summed = CHECKSUM_PARTIAL;
>+			skb->csum_start = skb_headroom(skb) +
>+					  skb_network_offset(skb) +
>+					  ip_hdrlen(skb);
>+			skb->csum_offset = (void *)check - data;
>+			*check = ~csum_tcpudp_magic(iph->saddr, iph->daddr,
>+						    datalen, proto, 0);
>+		} else {
>+			*check = 0;
>+			*check = csum_tcpudp_magic(iph->saddr, iph->daddr,
>+						   datalen, proto,
>+						   csum_partial(data, datalen,
>+								0));
>+			if (proto == IPPROTO_UDP && !*check)
>+				*check = CSUM_MANGLED_0;
>+		}
>+	} else
>+		inet_proto_csum_replace2(check, skb,
>+					 htons(oldlen), htons(datalen), 1);
>+}

Here is a style factory trick: invert the condition such that the
simple case is first, and the big one becomes an else if
with a reduced indent:

	if (skb->ip_summed == CHECKSUM_PARTIAL) {
		inet_proto_csum_replace2(check, skb,
					 htons(oldlen), htons(datalen), 1);
	} else if (!(rt->rt_flags & RTCF_LOCAL) &&
		    (!skb->dev || skb->dev->features & NETIF_F_V4_CSUM)) {
		skb->ip_summed = CHECKSUM_PARTIAL;
		skb->csum_start = skb_headroom(skb) +
				  skb_network_offset(skb) +
				  ip_hdrlen(skb);
		skb->csum_offset = (void *)check - data;
		*check = ~csum_tcpudp_magic(iph->saddr, iph->daddr,
					    datalen, proto, 0);
	} else {
		*check = 0;
		*check = csum_tcpudp_magic(iph->saddr, iph->daddr,
					   datalen, proto,
					   csum_partial(data, datalen,
							0));
		if (proto == IPPROTO_UDP && !*check)
			*check = CSUM_MANGLED_0;
	}


>+static void __exit nf_nat_l3proto_ipv4_exit(void)
>+{
>+	nf_nat_l3proto_unregister(&nf_nat_l3proto_ipv4);
>+	nf_nat_l4proto_unregister(NFPROTO_IPV4, &nf_nat_l4proto_icmp);
>+}
>+
>+MODULE_LICENSE("GPL");
>+MODULE_ALIAS("nf-nat-" __stringify(AF_INET));

Technically, this would have to be NFPROTO_IPV4, though GNU C has yet
to gain an extension to stringify enum constants..


>+	/* 1) If this srcip/proto/src-proto-part is currently mapped,
>+	 * and that same mapping gives a unique tuple within the given
>+	 * range, use that.
>+	 *
>+	 * This is only required for source (ie. NAT/masq) mappings.
>+	 * So far, we don't do local source mappings, so multiple
>+	 * manips not an issue.
           manips are not an issue.


>-		/* nf_conntrack_alter_reply might re-allocate extension area */
>+		/* nf_conntrack_alter_reply might re-allocate exntension aera */

extension was correct :)

>+	{
>+		.name		= "SNAT",
>+		.revision	= 1,
>+		.target		= xt_snat_target_v1,
>+		.targetsize	= sizeof(struct nf_nat_range),
>+		.table		= "nat",
>+		.hooks		= (1 << NF_INET_POST_ROUTING) |
>+				  (1 << NF_INET_LOCAL_OUT),
>+		.me		= THIS_MODULE,

 .family = NFPROTO_UNSPEC,

Just for completeness.
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