[PATCH] netfilter: xtables: introduce xt_length revision 2

[Jan Engelhardt <jengelh@medozas.de>]

Introduce xt_length match revision 1. It adds support for layer-4,
layer-5 and layer-7 length matching. It is much easier than writing up
the according xt_u32 magic.

This can be used for packet scheduling; specific example are online
games where all data is transferred over the same port, but the regular
gameplay has a characteristically lower packet size than bulk downloads
of game maps. (Tested with Unreal Tournament 99.)

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
---
 include/linux/netfilter/xt_length.h |   18 +++
 net/netfilter/xt_length.c           |  230 ++++++++++++++++++++++++++++++++++-
 2 files changed, 245 insertions(+), 3 deletions(-)

diff --git a/include/linux/netfilter/xt_length.h b/include/linux/netfilter/xt_length.h
index b82ed7c..7eb7e56 100644
--- a/include/linux/netfilter/xt_length.h
+++ b/include/linux/netfilter/xt_length.h
@@ -8,4 +8,22 @@ struct xt_length_info {
     __u8	invert;
 };
 
+enum {
+	XT_LENGTH_INVERT = 1 << 0,
+
+	/* IP header plus payload */
+	XT_LENGTH_LAYER3 = 1 << 1,
+	/* Strip IP header: */
+	XT_LENGTH_LAYER4 = 1 << 2,
+	/* Strip TCP/UDP/etc. header */
+	XT_LENGTH_LAYER5 = 1 << 3,
+	/* TCP/UDP/SCTP payload */
+	XT_LENGTH_LAYER7 = 1 << 4,
+};
+
+struct xt_length_mtinfo2 {
+	__u32 min, max;
+	__u16 flags;
+};
+
 #endif /*_XT_LENGTH_H*/
diff --git a/net/netfilter/xt_length.c b/net/netfilter/xt_length.c
index 176e557..2092686 100644
--- a/net/netfilter/xt_length.c
+++ b/net/netfilter/xt_length.c
@@ -1,20 +1,31 @@
 /* Kernel module to match packet length. */
 /* (C) 1999-2001 James Morris <jmorros@intercode.com.au>
+ * Copyright © Jan Engelhardt <jengelh@medozas.de>, 2007 - 2010
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  */
-
+#include <linux/dccp.h>
 #include <linux/module.h>
 #include <linux/skbuff.h>
+#include <linux/icmp.h>
+#include <linux/ip.h>
 #include <linux/ipv6.h>
+#include <linux/sctp.h>
+#include <linux/tcp.h>
+#include <linux/udp.h>
 #include <net/ip.h>
-
-#include <linux/netfilter/xt_length.h>
+#include <net/ipv6.h>
 #include <linux/netfilter/x_tables.h>
+#include <linux/netfilter/xt_length.h>
+#include <linux/netfilter_ipv6/ip6_tables.h>
+#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
+#	define WITH_IPV6 1
+#endif
 
 MODULE_AUTHOR("James Morris <jmorris@intercode.com.au>");
+MODULE_AUTHOR("Jan Engelhardt <jengelh@medozas.de>");
 MODULE_DESCRIPTION("Xtables: Packet length (Layer3,4,5) match");
 MODULE_LICENSE("GPL");
 MODULE_ALIAS("ipt_length");
@@ -39,6 +50,201 @@ length_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 	return (pktlen >= info->min && pktlen <= info->max) ^ info->invert;
 }
 
+static bool
+xtlength_layer5_tcp(unsigned int *length, const struct sk_buff *skb,
+		    unsigned int offset)
+{
+	const struct tcphdr *tcph;
+	struct tcphdr buf;
+
+	tcph = skb_header_pointer(skb, offset, sizeof(buf), &buf);
+	if (tcph == NULL)
+		return false;
+
+	*length = skb->len - offset;
+	if (*length >= 4 * tcph->doff)
+		*length -= 4 * tcph->doff;
+	return true;
+}
+
+static bool
+xtlength_layer5_dccp(unsigned int *length, const struct sk_buff *skb,
+		     unsigned int offset)
+{
+	const struct dccp_hdr *dh;
+	struct dccp_hdr dhbuf;
+
+	dh = skb_header_pointer(skb, offset, sizeof(dhbuf), &dhbuf);
+	if (dh == NULL)
+		return false;
+
+	*length = skb->len - offset;
+	if (*length >= 4 * dh->dccph_doff)
+		*length -= 4 * dh->dccph_doff;
+	return true;
+}
+
+static inline bool
+xtlength_layer5(unsigned int *length, const struct sk_buff *skb,
+		unsigned int prot, unsigned int offset)
+{
+	switch (prot) {
+	case IPPROTO_TCP:
+		return xtlength_layer5_tcp(length, skb, offset);
+	case IPPROTO_UDP:
+	case IPPROTO_UDPLITE:
+		*length = skb->len - offset - sizeof(struct udphdr);
+		return true;
+	case IPPROTO_SCTP:
+		*length = skb->len - offset - sizeof(struct sctphdr);
+		return true;
+	case IPPROTO_DCCP:
+		return xtlength_layer5_dccp(length, skb, offset);
+	case IPPROTO_ICMP:
+		*length = skb->len - offset - sizeof(struct icmphdr);
+		return true;
+	case IPPROTO_ICMPV6:
+		*length = skb->len - offset -
+		          offsetof(struct icmp6hdr, icmp6_dataun);
+		return true;
+	case IPPROTO_AH:
+		*length = skb->len - offset - sizeof(struct ip_auth_hdr);
+		return true;
+	case IPPROTO_ESP:
+		*length = skb->len - offset - sizeof(struct ip_esp_hdr);
+		return true;
+	}
+	return false;
+}
+
+static bool
+xtlength_layer7_sctp(unsigned int *length, const struct sk_buff *skb,
+		     unsigned int offset)
+{
+	const struct sctp_chunkhdr *ch;
+	struct sctp_chunkhdr chbuf;
+	unsigned int pos;
+
+	*length = 0;
+	for (pos = sizeof(struct sctphdr); pos < skb->len;
+	     pos += ntohs(ch->length))
+	{
+		ch = skb_header_pointer(skb, offset + pos,
+		     sizeof(chbuf), &chbuf);
+		if (ch == NULL)
+			return false;
+		if (ch->type != SCTP_CID_DATA)
+			continue;
+		*length += ntohs(ch->length);
+	}
+	return true;
+}
+
+static bool xtlength_layer7(unsigned int *length, const struct sk_buff *skb,
+			    unsigned int proto, unsigned int offset)
+{
+	switch (proto) {
+	case IPPROTO_SCTP:
+		return xtlength_layer7_sctp(length, skb, offset);
+	default:
+		return xtlength_layer5(length, skb, proto, offset);
+	}
+}
+
+static bool
+length2_mt(const struct sk_buff *skb, struct xt_action_param *par)
+{
+	const struct xt_length_mtinfo2 *info = par->matchinfo;
+	const struct iphdr *iph = ip_hdr(skb);
+	unsigned int len = 0;
+	bool hit = true;
+
+	if (info->flags & XT_LENGTH_LAYER3)
+		len = ntohs(iph->tot_len);
+	else if (info->flags & XT_LENGTH_LAYER4)
+		len = ntohs(iph->tot_len) - par->thoff;
+	else if (info->flags & XT_LENGTH_LAYER5)
+		hit = xtlength_layer5(&len, skb, iph->protocol, par->thoff);
+	else if (info->flags & XT_LENGTH_LAYER7)
+		hit = xtlength_layer7(&len, skb, iph->protocol, par->thoff);
+	if (!hit)
+		return false;
+
+	return (len >= info->min && len <= info->max) ^
+	       !!(info->flags & XT_LENGTH_INVERT);
+}
+
+#ifdef WITH_IPV6
+/**
+ * llayer4_proto - figure out the L4 protocol in an IPv6 packet
+ * @skb:	skb pointer
+ * @offset:	position at which L4 starts (equal to 'protoff' in IPv4 code)
+ * @hotdrop:	hotdrop pointer
+ *
+ * Searches for a recognized L4 header. On success, fills in @offset and
+ * returns the protocol number. If not found, %NEXTHDR_MAX is returned.
+ * On error, @hotdrop is set.
+ */
+static unsigned int
+llayer4_proto(const struct sk_buff *skb, unsigned int *offset, bool *hotdrop)
+{
+	/*
+	 * Do encapsulation first so that %NEXTHDR_TCP does not hit the TCP
+	 * part in an IPv6-in-IPv6 encapsulation.
+	 */
+	static const unsigned int types[] =
+		{IPPROTO_IPV6, IPPROTO_IPIP, IPPROTO_ESP, IPPROTO_AH,
+		IPPROTO_ICMP, IPPROTO_TCP, IPPROTO_UDP, IPPROTO_UDPLITE,
+		IPPROTO_SCTP, IPPROTO_DCCP};
+	unsigned int i;
+	int err;
+
+	for (i = 0; i < ARRAY_SIZE(types); ++i) {
+		err = ipv6_find_hdr(skb, offset, types[i], NULL);
+		if (err >= 0)
+			return types[i];
+		if (err != -ENOENT) {
+			*hotdrop = true;
+			break;
+		}
+	}
+	return NEXTHDR_MAX;
+}
+
+static bool
+length2_mt6(const struct sk_buff *skb, struct xt_action_param *par)
+{
+	const struct xt_length_mtinfo2 *info = par->matchinfo;
+	const struct ipv6hdr *iph = ipv6_hdr(skb);
+	unsigned int len = 0, l4proto;
+	unsigned int thoff = par->thoff;
+	bool hit = true;
+
+	if (info->flags & XT_LENGTH_LAYER3) {
+		if (iph->payload_len == 0)
+			/* Jumbograms */
+			len = skb->len;
+		else
+			len = sizeof(struct ipv6hdr) + ntohs(iph->payload_len);
+	} else {
+		l4proto = llayer4_proto(skb, &thoff, &par->hotdrop);
+		if (l4proto == NEXTHDR_MAX)
+			return false;
+		if (info->flags & XT_LENGTH_LAYER4)
+			len = skb->len - thoff;
+		else if (info->flags & XT_LENGTH_LAYER5)
+			hit = xtlength_layer5(&len, skb, l4proto, thoff);
+		else if (info->flags & XT_LENGTH_LAYER7)
+			hit = xtlength_layer7(&len, skb, l4proto, thoff);
+	}
+	if (!hit)
+		return false;
+
+	return (len >= info->min && len <= info->max) ^
+	       !!(info->flags & XT_LENGTH_INVERT);
+}
+#endif
+
 static struct xt_match length_mt_reg[] __read_mostly = {
 	{
 		.name		= "length",
@@ -54,6 +260,24 @@ static struct xt_match length_mt_reg[] __read_mostly = {
 		.matchsize	= sizeof(struct xt_length_info),
 		.me		= THIS_MODULE,
 	},
+	{
+		.name           = "length2",
+		.revision       = 2,
+		.family         = NFPROTO_IPV4,
+		.match          = length2_mt,
+		.matchsize      = sizeof(struct xt_length_mtinfo2),
+		.me             = THIS_MODULE,
+	},
+#ifdef WITH_IPV6
+	{
+		.name           = "length2",
+		.revision       = 2,
+		.family         = NFPROTO_IPV6,
+		.match          = length2_mt6,
+		.matchsize      = sizeof(struct xt_length_mtinfo2),
+		.me             = THIS_MODULE,
+	},
+#endif
 };
 
 static int __init length_mt_init(void)
-- 
1.7.1

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