[NETFILTER]: nf_nat: add SNMP NAT helper port

Add nf_conntrack port of the SNMP NAT helper.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/Kconfig

@@ -477,6 +477,21 @@ config IP_NF_NAT_SNMP_BASIC
 
 	  To compile it as a module, choose M here.  If unsure, say N.
 
+config NF_NAT_SNMP_BASIC
+	tristate "Basic SNMP-ALG support (EXPERIMENTAL)"
+	depends on EXPERIMENTAL && NF_NAT
+	---help---
+
+	  This module implements an Application Layer Gateway (ALG) for
+	  SNMP payloads.  In conjunction with NAT, it allows a network
+	  management system to access multiple private networks with
+	  conflicting addresses.  It works by modifying IP addresses
+	  inside SNMP payloads to match IP-layer NAT mapping.
+
+	  This is the "basic" form of SNMP-ALG, as described in RFC 2962
+
+	  To compile it as a module, choose M here.  If unsure, say N.
+
 # If they want FTP, set to $CONFIG_IP_NF_NAT (m or y),
 # or $CONFIG_IP_NF_FTP (m or y), whichever is weaker.
 # From kconfig-language.txt:

net/ipv4/netfilter/Makefile

@@ -56,6 +56,7 @@ obj-$(CONFIG_NF_NAT_H323) += nf_nat_h323.o
 obj-$(CONFIG_NF_NAT_IRC) += nf_nat_irc.o
 obj-$(CONFIG_NF_NAT_PPTP) += nf_nat_pptp.o
 obj-$(CONFIG_NF_NAT_SIP) += nf_nat_sip.o
+obj-$(CONFIG_NF_NAT_SNMP_BASIC) += nf_nat_snmp_basic.o
 obj-$(CONFIG_NF_NAT_TFTP) += nf_nat_tftp.o
 
 # NAT protocols (nf_nat)

net/ipv4/netfilter/nf_nat_snmp_basic.c

+/*
+ * nf_nat_snmp_basic.c
+ *
+ * Basic SNMP Application Layer Gateway
+ *
+ * This IP NAT module is intended for use with SNMP network
+ * discovery and monitoring applications where target networks use
+ * conflicting private address realms.
+ *
+ * Static NAT is used to remap the networks from the view of the network
+ * management system at the IP layer, and this module remaps some application
+ * layer addresses to match.
+ *
+ * The simplest form of ALG is performed, where only tagged IP addresses
+ * are modified.  The module does not need to be MIB aware and only scans
+ * messages at the ASN.1/BER level.
+ *
+ * Currently, only SNMPv1 and SNMPv2 are supported.
+ *
+ * More information on ALG and associated issues can be found in
+ * RFC 2962
+ *
+ * The ASB.1/BER parsing code is derived from the gxsnmp package by Gregory
+ * McLean & Jochen Friedrich, stripped down for use in the kernel.
+ *
+ * Copyright (c) 2000 RP Internet (www.rpi.net.au).
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA
+ *
+ * Author: James Morris <jmorris@intercode.com.au>
+ *
+ * Updates:
+ * 2000-08-06: Convert to new helper API (Harald Welte).
+ *
+ */
+#include <linux/module.h>
+#include <linux/moduleparam.h>
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/in.h>
+#include <linux/ip.h>
+#include <linux/udp.h>
+#include <net/checksum.h>
+#include <net/udp.h>
+
+#include <net/netfilter/nf_nat.h>
+#include <net/netfilter/nf_conntrack_helper.h>
+#include <net/netfilter/nf_nat_helper.h>
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("James Morris <jmorris@intercode.com.au>");
+MODULE_DESCRIPTION("Basic SNMP Application Layer Gateway");
+MODULE_ALIAS("ip_nat_snmp_basic");
+
+#define SNMP_PORT 161
+#define SNMP_TRAP_PORT 162
+#define NOCT1(n) (*(u8 *)n)
+
+static int debug;
+static DEFINE_SPINLOCK(snmp_lock);
+
+/*
+ * Application layer address mapping mimics the NAT mapping, but
+ * only for the first octet in this case (a more flexible system
+ * can be implemented if needed).
+ */
+struct oct1_map
+{
+	u_int8_t from;
+	u_int8_t to;
+};
+
+
+/*****************************************************************************
+ *
+ * Basic ASN.1 decoding routines (gxsnmp author Dirk Wisse)
+ *
+ *****************************************************************************/
+
+/* Class */
+#define ASN1_UNI	0	/* Universal */
+#define ASN1_APL	1	/* Application */
+#define ASN1_CTX	2	/* Context */
+#define ASN1_PRV	3	/* Private */
+
+/* Tag */
+#define ASN1_EOC	0	/* End Of Contents */
+#define ASN1_BOL	1	/* Boolean */
+#define ASN1_INT	2	/* Integer */
+#define ASN1_BTS	3	/* Bit String */
+#define ASN1_OTS	4	/* Octet String */
+#define ASN1_NUL	5	/* Null */
+#define ASN1_OJI	6	/* Object Identifier  */
+#define ASN1_OJD	7	/* Object Description */
+#define ASN1_EXT	8	/* External */
+#define ASN1_SEQ	16	/* Sequence */
+#define ASN1_SET	17	/* Set */
+#define ASN1_NUMSTR	18	/* Numerical String */
+#define ASN1_PRNSTR	19	/* Printable String */
+#define ASN1_TEXSTR	20	/* Teletext String */
+#define ASN1_VIDSTR	21	/* Video String */
+#define ASN1_IA5STR	22	/* IA5 String */
+#define ASN1_UNITIM	23	/* Universal Time */
+#define ASN1_GENTIM	24	/* General Time */
+#define ASN1_GRASTR	25	/* Graphical String */
+#define ASN1_VISSTR	26	/* Visible String */
+#define ASN1_GENSTR	27	/* General String */
+
+/* Primitive / Constructed methods*/
+#define ASN1_PRI	0	/* Primitive */
+#define ASN1_CON	1	/* Constructed */
+
+/*
+ * Error codes.
+ */
+#define ASN1_ERR_NOERROR		0
+#define ASN1_ERR_DEC_EMPTY		2
+#define ASN1_ERR_DEC_EOC_MISMATCH	3
+#define ASN1_ERR_DEC_LENGTH_MISMATCH	4
+#define ASN1_ERR_DEC_BADVALUE		5
+
+/*
+ * ASN.1 context.
+ */
+struct asn1_ctx
+{
+	int error;			/* Error condition */
+	unsigned char *pointer;		/* Octet just to be decoded */
+	unsigned char *begin;		/* First octet */
+	unsigned char *end;		/* Octet after last octet */
+};
+
+/*
+ * Octet string (not null terminated)
+ */
+struct asn1_octstr
+{
+	unsigned char *data;
+	unsigned int len;
+};
+
+static void asn1_open(struct asn1_ctx *ctx,
+                      unsigned char *buf,
+                      unsigned int len)
+{
+	ctx->begin = buf;
+	ctx->end = buf + len;
+	ctx->pointer = buf;
+	ctx->error = ASN1_ERR_NOERROR;
+}
+
+static unsigned char asn1_octet_decode(struct asn1_ctx *ctx, unsigned char *ch)
+{
+	if (ctx->pointer >= ctx->end) {
+		ctx->error = ASN1_ERR_DEC_EMPTY;
+		return 0;
+	}
+	*ch = *(ctx->pointer)++;
+	return 1;
+}
+
+static unsigned char asn1_tag_decode(struct asn1_ctx *ctx, unsigned int *tag)
+{
+	unsigned char ch;
+
+	*tag = 0;
+
+	do
+	{
+		if (!asn1_octet_decode(ctx, &ch))
+			return 0;
+		*tag <<= 7;
+		*tag |= ch & 0x7F;
+	} while ((ch & 0x80) == 0x80);
+	return 1;
+}
+
+static unsigned char asn1_id_decode(struct asn1_ctx *ctx,
+                                    unsigned int *cls,
+                                    unsigned int *con,
+                                    unsigned int *tag)
+{
+	unsigned char ch;
+
+	if (!asn1_octet_decode(ctx, &ch))
+		return 0;
+
+	*cls = (ch & 0xC0) >> 6;
+	*con = (ch & 0x20) >> 5;
+	*tag = (ch & 0x1F);
+
+	if (*tag == 0x1F) {
+		if (!asn1_tag_decode(ctx, tag))
+			return 0;
+	}
+	return 1;
+}
+
+static unsigned char asn1_length_decode(struct asn1_ctx *ctx,
+                                        unsigned int *def,
+                                        unsigned int *len)
+{
+	unsigned char ch, cnt;
+
+	if (!asn1_octet_decode(ctx, &ch))
+		return 0;
+
+	if (ch == 0x80)
+		*def = 0;
+	else {
+		*def = 1;
+
+		if (ch < 0x80)
+			*len = ch;
+		else {
+			cnt = (unsigned char) (ch & 0x7F);
+			*len = 0;
+
+			while (cnt > 0) {
+				if (!asn1_octet_decode(ctx, &ch))
+					return 0;
+				*len <<= 8;
+				*len |= ch;
+				cnt--;
+			}
+		}
+	}
+	return 1;
+}
+
+static unsigned char asn1_header_decode(struct asn1_ctx *ctx,
+                                        unsigned char **eoc,
+                                        unsigned int *cls,
+                                        unsigned int *con,
+                                        unsigned int *tag)
+{
+	unsigned int def, len;
+
+	if (!asn1_id_decode(ctx, cls, con, tag))
+		return 0;
+
+	def = len = 0;
+	if (!asn1_length_decode(ctx, &def, &len))
+		return 0;
+
+	if (def)
+		*eoc = ctx->pointer + len;
+	else
+		*eoc = NULL;
+	return 1;
+}
+
+static unsigned char asn1_eoc_decode(struct asn1_ctx *ctx, unsigned char *eoc)
+{
+	unsigned char ch;
+
+	if (eoc == 0) {
+		if (!asn1_octet_decode(ctx, &ch))
+			return 0;
+
+		if (ch != 0x00) {
+			ctx->error = ASN1_ERR_DEC_EOC_MISMATCH;
+			return 0;
+		}
+
+		if (!asn1_octet_decode(ctx, &ch))
+			return 0;
+
+		if (ch != 0x00) {
+			ctx->error = ASN1_ERR_DEC_EOC_MISMATCH;
+			return 0;
+		}
+		return 1;
+	} else {
+		if (ctx->pointer != eoc) {
+			ctx->error = ASN1_ERR_DEC_LENGTH_MISMATCH;
+			return 0;
+		}
+		return 1;
+	}
+}
+
+static unsigned char asn1_null_decode(struct asn1_ctx *ctx, unsigned char *eoc)
+{
+	ctx->pointer = eoc;
+	return 1;
+}
+
+static unsigned char asn1_long_decode(struct asn1_ctx *ctx,
+                                      unsigned char *eoc,
+                                      long *integer)
+{
+	unsigned char ch;
+	unsigned int  len;
+
+	if (!asn1_octet_decode(ctx, &ch))
+		return 0;
+
+	*integer = (signed char) ch;
+	len = 1;
+
+	while (ctx->pointer < eoc) {
+		if (++len > sizeof (long)) {
+			ctx->error = ASN1_ERR_DEC_BADVALUE;
+			return 0;
+		}
+
+		if (!asn1_octet_decode(ctx, &ch))
+			return 0;
+
+		*integer <<= 8;
+		*integer |= ch;
+	}
+	return 1;
+}
+
+static unsigned char asn1_uint_decode(struct asn1_ctx *ctx,
+                                      unsigned char *eoc,
+                                      unsigned int *integer)
+{
+	unsigned char ch;
+	unsigned int  len;
+
+	if (!asn1_octet_decode(ctx, &ch))
+		return 0;
+
+	*integer = ch;
+	if (ch == 0) len = 0;
+	else len = 1;
+
+	while (ctx->pointer < eoc) {
+		if (++len > sizeof (unsigned int)) {
+			ctx->error = ASN1_ERR_DEC_BADVALUE;
+			return 0;
+		}
+
+		if (!asn1_octet_decode(ctx, &ch))
+			return 0;
+
+		*integer <<= 8;
+		*integer |= ch;
+	}
+	return 1;
+}
+
+static unsigned char asn1_ulong_decode(struct asn1_ctx *ctx,
+                                       unsigned char *eoc,
+                                       unsigned long *integer)
+{
+	unsigned char ch;
+	unsigned int  len;
+
+	if (!asn1_octet_decode(ctx, &ch))
+		return 0;
+
+	*integer = ch;
+	if (ch == 0) len = 0;
+	else len = 1;
+
+	while (ctx->pointer < eoc) {
+		if (++len > sizeof (unsigned long)) {
+			ctx->error = ASN1_ERR_DEC_BADVALUE;
+			return 0;
+		}
+
+		if (!asn1_octet_decode(ctx, &ch))
+			return 0;
+
+		*integer <<= 8;
+		*integer |= ch;
+	}
+	return 1;
+}
+
+static unsigned char asn1_octets_decode(struct asn1_ctx *ctx,
+                                        unsigned char *eoc,
+                                        unsigned char **octets,
+                                        unsigned int *len)
+{
+	unsigned char *ptr;
+
+	*len = 0;
+
+	*octets = kmalloc(eoc - ctx->pointer, GFP_ATOMIC);
+	if (*octets == NULL) {
+		if (net_ratelimit())
+			printk("OOM in bsalg (%d)\n", __LINE__);
+		return 0;
+	}
+
+	ptr = *octets;
+	while (ctx->pointer < eoc) {
+		if (!asn1_octet_decode(ctx, (unsigned char *)ptr++)) {
+			kfree(*octets);
+			*octets = NULL;
+			return 0;
+		}
+		(*len)++;
+	}
+	return 1;
+}
+
+static unsigned char asn1_subid_decode(struct asn1_ctx *ctx,
+                                       unsigned long *subid)
+{
+	unsigned char ch;
+
+	*subid = 0;
+
+	do {
+		if (!asn1_octet_decode(ctx, &ch))
+			return 0;
+
+		*subid <<= 7;
+		*subid |= ch & 0x7F;
+	} while ((ch & 0x80) == 0x80);
+	return 1;
+}
+
+static unsigned char asn1_oid_decode(struct asn1_ctx *ctx,
+                                     unsigned char *eoc,
+                                     unsigned long **oid,
+                                     unsigned int *len)
+{
+	unsigned long subid;
+	unsigned int  size;
+	unsigned long *optr;
+
+	size = eoc - ctx->pointer + 1;
+	*oid = kmalloc(size * sizeof(unsigned long), GFP_ATOMIC);
+	if (*oid == NULL) {
+		if (net_ratelimit())
+			printk("OOM in bsalg (%d)\n", __LINE__);
+		return 0;
+	}
+
+	optr = *oid;
+
+	if (!asn1_subid_decode(ctx, &subid)) {
+		kfree(*oid);
+		*oid = NULL;
+		return 0;
+	}
+
+	if (subid < 40) {
+		optr [0] = 0;
+		optr [1] = subid;
+	} else if (subid < 80) {
+		optr [0] = 1;
+		optr [1] = subid - 40;
+	} else {
+		optr [0] = 2;
+		optr [1] = subid - 80;
+	}
+
+	*len = 2;
+	optr += 2;
+
+	while (ctx->pointer < eoc) {
+		if (++(*len) > size) {
+			ctx->error = ASN1_ERR_DEC_BADVALUE;
+			kfree(*oid);
+			*oid = NULL;
+			return 0;
+		}
+
+		if (!asn1_subid_decode(ctx, optr++)) {
+			kfree(*oid);
+			*oid = NULL;
+			return 0;
+		}
+	}
+	return 1;
+}
+
+/*****************************************************************************
+ *
+ * SNMP decoding routines (gxsnmp author Dirk Wisse)
+ *
+ *****************************************************************************/
+
+/* SNMP Versions */
+#define SNMP_V1				0
+#define SNMP_V2C			1
+#define SNMP_V2				2
+#define SNMP_V3				3
+
+/* Default Sizes */
+#define SNMP_SIZE_COMM			256
+#define SNMP_SIZE_OBJECTID		128
+#define SNMP_SIZE_BUFCHR		256
+#define SNMP_SIZE_BUFINT		128
+#define SNMP_SIZE_SMALLOBJECTID		16
+
+/* Requests */
+#define SNMP_PDU_GET			0
+#define SNMP_PDU_NEXT			1
+#define SNMP_PDU_RESPONSE		2
+#define SNMP_PDU_SET			3
+#define SNMP_PDU_TRAP1			4
+#define SNMP_PDU_BULK			5
+#define SNMP_PDU_INFORM			6
+#define SNMP_PDU_TRAP2			7
+
+/* Errors */
+#define SNMP_NOERROR			0
+#define SNMP_TOOBIG			1
+#define SNMP_NOSUCHNAME			2
+#define SNMP_BADVALUE			3
+#define SNMP_READONLY			4
+#define SNMP_GENERROR			5
+#define SNMP_NOACCESS			6
+#define SNMP_WRONGTYPE			7
+#define SNMP_WRONGLENGTH		8
+#define SNMP_WRONGENCODING		9
+#define SNMP_WRONGVALUE			10
+#define SNMP_NOCREATION			11
+#define SNMP_INCONSISTENTVALUE		12
+#define SNMP_RESOURCEUNAVAILABLE	13
+#define SNMP_COMMITFAILED		14
+#define SNMP_UNDOFAILED			15
+#define SNMP_AUTHORIZATIONERROR		16
+#define SNMP_NOTWRITABLE		17
+#define SNMP_INCONSISTENTNAME		18
+
+/* General SNMP V1 Traps */
+#define SNMP_TRAP_COLDSTART		0
+#define SNMP_TRAP_WARMSTART		1
+#define SNMP_TRAP_LINKDOWN		2
+#define SNMP_TRAP_LINKUP		3
+#define SNMP_TRAP_AUTFAILURE		4
+#define SNMP_TRAP_EQPNEIGHBORLOSS	5
+#define SNMP_TRAP_ENTSPECIFIC		6
+
+/* SNMPv1 Types */
+#define SNMP_NULL                0
+#define SNMP_INTEGER             1    /* l  */
+#define SNMP_OCTETSTR            2    /* c  */
+#define SNMP_DISPLAYSTR          2    /* c  */
+#define SNMP_OBJECTID            3    /* ul */
+#define SNMP_IPADDR              4    /* uc */
+#define SNMP_COUNTER             5    /* ul */
+#define SNMP_GAUGE               6    /* ul */
+#define SNMP_TIMETICKS           7    /* ul */
+#define SNMP_OPAQUE              8    /* c  */
+
+/* Additional SNMPv2 Types */
+#define SNMP_UINTEGER            5    /* ul */
+#define SNMP_BITSTR              9    /* uc */
+#define SNMP_NSAP               10    /* uc */
+#define SNMP_COUNTER64          11    /* ul */
+#define SNMP_NOSUCHOBJECT       12
+#define SNMP_NOSUCHINSTANCE     13
+#define SNMP_ENDOFMIBVIEW       14
+
+union snmp_syntax
+{
+	unsigned char uc[0];	/* 8 bit unsigned */
+	char c[0];		/* 8 bit signed */
+	unsigned long ul[0];	/* 32 bit unsigned */
+	long l[0];		/* 32 bit signed */
+};
+
+struct snmp_object
+{
+	unsigned long *id;
+	unsigned int id_len;
+	unsigned short type;
+	unsigned int syntax_len;
+	union snmp_syntax syntax;
+};
+
+struct snmp_request
+{
+	unsigned long id;
+	unsigned int error_status;
+	unsigned int error_index;
+};
+
+struct snmp_v1_trap
+{
+	unsigned long *id;
+	unsigned int id_len;
+	unsigned long ip_address;	/* pointer  */
+	unsigned int general;
+	unsigned int specific;
+	unsigned long time;
+};
+
+/* SNMP types */
+#define SNMP_IPA    0
+#define SNMP_CNT    1
+#define SNMP_GGE    2
+#define SNMP_TIT    3
+#define SNMP_OPQ    4
+#define SNMP_C64    6
+
+/* SNMP errors */
+#define SERR_NSO    0
+#define SERR_NSI    1
+#define SERR_EOM    2
+
+static inline void mangle_address(unsigned char *begin,
+                                  unsigned char *addr,
+                                  const struct oct1_map *map,
+                                  __sum16 *check);
+struct snmp_cnv
+{
+	unsigned int class;
+	unsigned int tag;
+	int syntax;
+};
+
+static struct snmp_cnv snmp_conv [] =
+{
+	{ASN1_UNI, ASN1_NUL, SNMP_NULL},
+	{ASN1_UNI, ASN1_INT, SNMP_INTEGER},
+	{ASN1_UNI, ASN1_OTS, SNMP_OCTETSTR},
+	{ASN1_UNI, ASN1_OTS, SNMP_DISPLAYSTR},
+	{ASN1_UNI, ASN1_OJI, SNMP_OBJECTID},
+	{ASN1_APL, SNMP_IPA, SNMP_IPADDR},
+	{ASN1_APL, SNMP_CNT, SNMP_COUNTER},	/* Counter32 */
+	{ASN1_APL, SNMP_GGE, SNMP_GAUGE},	/* Gauge32 == Unsigned32  */
+	{ASN1_APL, SNMP_TIT, SNMP_TIMETICKS},
+	{ASN1_APL, SNMP_OPQ, SNMP_OPAQUE},
+
+	/* SNMPv2 data types and errors */
+	{ASN1_UNI, ASN1_BTS, SNMP_BITSTR},
+	{ASN1_APL, SNMP_C64, SNMP_COUNTER64},
+	{ASN1_CTX, SERR_NSO, SNMP_NOSUCHOBJECT},
+	{ASN1_CTX, SERR_NSI, SNMP_NOSUCHINSTANCE},
+	{ASN1_CTX, SERR_EOM, SNMP_ENDOFMIBVIEW},
+	{0,       0,       -1}
+};
+
+static unsigned char snmp_tag_cls2syntax(unsigned int tag,
+                                         unsigned int cls,
+                                         unsigned short *syntax)
+{
+	struct snmp_cnv *cnv;
+
+	cnv = snmp_conv;
+
+	while (cnv->syntax != -1) {
+		if (cnv->tag == tag && cnv->class == cls) {
+			*syntax = cnv->syntax;
+			return 1;
+		}
+		cnv++;
+	}
+	return 0;
+}
+
+static unsigned char snmp_object_decode(struct asn1_ctx *ctx,
+                                        struct snmp_object **obj)
+{
+	unsigned int cls, con, tag, len, idlen;
+	unsigned short type;
+	unsigned char *eoc, *end, *p;
+	unsigned long *lp, *id;
+	unsigned long ul;
+	long l;
+
+	*obj = NULL;
+	id = NULL;
+
+	if (!asn1_header_decode(ctx, &eoc, &cls, &con, &tag))
+		return 0;
+
+	if (cls != ASN1_UNI || con != ASN1_CON || tag != ASN1_SEQ)
+		return 0;
+
+	if (!asn1_header_decode(ctx, &end, &cls, &con, &tag))
+		return 0;
+
+	if (cls != ASN1_UNI || con != ASN1_PRI || tag != ASN1_OJI)
+		return 0;
+
+	if (!asn1_oid_decode(ctx, end, &id, &idlen))
+		return 0;
+
+	if (!asn1_header_decode(ctx, &end, &cls, &con, &tag)) {
+		kfree(id);
+		return 0;
+	}
+
+	if (con != ASN1_PRI) {
+		kfree(id);
+		return 0;
+	}
+
+	type = 0;
+	if (!snmp_tag_cls2syntax(tag, cls, &type)) {
+		kfree(id);
+		return 0;
+	}
+
+	l = 0;
+	switch (type) {
+		case SNMP_INTEGER:
+			len = sizeof(long);
+			if (!asn1_long_decode(ctx, end, &l)) {
+				kfree(id);
+				return 0;
+			}
+			*obj = kmalloc(sizeof(struct snmp_object) + len,
+			               GFP_ATOMIC);
+			if (*obj == NULL) {
+				kfree(id);
+				if (net_ratelimit())
+					printk("OOM in bsalg (%d)\n", __LINE__);
+				return 0;
+			}
+			(*obj)->syntax.l[0] = l;
+			break;
+		case SNMP_OCTETSTR:
+		case SNMP_OPAQUE:
+			if (!asn1_octets_decode(ctx, end, &p, &len)) {
+				kfree(id);
+				return 0;
+			}
+			*obj = kmalloc(sizeof(struct snmp_object) + len,
+			               GFP_ATOMIC);
+			if (*obj == NULL) {
+				kfree(id);
+				if (net_ratelimit())
+					printk("OOM in bsalg (%d)\n", __LINE__);
+				return 0;
+			}
+			memcpy((*obj)->syntax.c, p, len);
+			kfree(p);
+			break;
+		case SNMP_NULL:
+		case SNMP_NOSUCHOBJECT:
+		case SNMP_NOSUCHINSTANCE:
+		case SNMP_ENDOFMIBVIEW:
+			len = 0;
+			*obj = kmalloc(sizeof(struct snmp_object), GFP_ATOMIC);
+			if (*obj == NULL) {
+				kfree(id);
+				if (net_ratelimit())
+					printk("OOM in bsalg (%d)\n", __LINE__);
+				return 0;
+			}
+			if (!asn1_null_decode(ctx, end)) {
+				kfree(id);
+				kfree(*obj);
+				*obj = NULL;
+				return 0;
+			}
+			break;
+		case SNMP_OBJECTID:
+			if (!asn1_oid_decode(ctx, end, (unsigned long **)&lp, &len)) {
+				kfree(id);
+				return 0;
+			}
+			len *= sizeof(unsigned long);
+			*obj = kmalloc(sizeof(struct snmp_object) + len, GFP_ATOMIC);
+			if (*obj == NULL) {
+				kfree(lp);
+				kfree(id);
+				if (net_ratelimit())
+					printk("OOM in bsalg (%d)\n", __LINE__);
+				return 0;
+			}
+			memcpy((*obj)->syntax.ul, lp, len);
+			kfree(lp);
+			break;
+		case SNMP_IPADDR:
+			if (!asn1_octets_decode(ctx, end, &p, &len)) {
+				kfree(id);
+				return 0;
+			}
+			if (len != 4) {
+				kfree(p);
+				kfree(id);
+				return 0;
+			}
+			*obj = kmalloc(sizeof(struct snmp_object) + len, GFP_ATOMIC);
+			if (*obj == NULL) {
+				kfree(p);
+				kfree(id);
+				if (net_ratelimit())
+					printk("OOM in bsalg (%d)\n", __LINE__);
+				return 0;
+			}
+			memcpy((*obj)->syntax.uc, p, len);
+			kfree(p);
+			break;
+		case SNMP_COUNTER:
+		case SNMP_GAUGE:
+		case SNMP_TIMETICKS:
+			len = sizeof(unsigned long);
+			if (!asn1_ulong_decode(ctx, end, &ul)) {
+				kfree(id);
+				return 0;
+			}
+			*obj = kmalloc(sizeof(struct snmp_object) + len, GFP_ATOMIC);
+			if (*obj == NULL) {
+				kfree(id);
+				if (net_ratelimit())
+					printk("OOM in bsalg (%d)\n", __LINE__);
+				return 0;
+			}
+			(*obj)->syntax.ul[0] = ul;
+			break;
+		default:
+			kfree(id);
+			return 0;
+	}
+
+	(*obj)->syntax_len = len;
+	(*obj)->type = type;
+	(*obj)->id = id;
+	(*obj)->id_len = idlen;
+
+	if (!asn1_eoc_decode(ctx, eoc)) {
+		kfree(id);
+		kfree(*obj);
+		*obj = NULL;
+		return 0;
+	}
+	return 1;
+}
+
+static unsigned char snmp_request_decode(struct asn1_ctx *ctx,
+                                         struct snmp_request *request)
+{
+	unsigned int cls, con, tag;
+	unsigned char *end;
+
+	if (!asn1_header_decode(ctx, &end, &cls, &con, &tag))
+		return 0;
+
+	if (cls != ASN1_UNI || con != ASN1_PRI || tag != ASN1_INT)
+		return 0;
+
+	if (!asn1_ulong_decode(ctx, end, &request->id))
+		return 0;
+
+	if (!asn1_header_decode(ctx, &end, &cls, &con, &tag))
+		return 0;
+
+	if (cls != ASN1_UNI || con != ASN1_PRI || tag != ASN1_INT)
+		return 0;
+
+	if (!asn1_uint_decode(ctx, end, &request->error_status))
+		return 0;
+
+	if (!asn1_header_decode(ctx, &end, &cls, &con, &tag))
+		return 0;
+
+	if (cls != ASN1_UNI || con != ASN1_PRI || tag != ASN1_INT)
+		return 0;
+
+	if (!asn1_uint_decode(ctx, end, &request->error_index))
+		return 0;
+
+	return 1;
+}
+
+/*
+ * Fast checksum update for possibly oddly-aligned UDP byte, from the
+ * code example in the draft.
+ */
+static void fast_csum(__sum16 *csum,
+                      const unsigned char *optr,
+                      const unsigned char *nptr,
+                      int offset)
+{
+	unsigned char s[4];
+
+	if (offset & 1) {
+		s[0] = s[2] = 0;
+		s[1] = ~*optr;
+		s[3] = *nptr;
+	} else {
+		s[1] = s[3] = 0;
+		s[0] = ~*optr;
+		s[2] = *nptr;
+	}
+
+	*csum = csum_fold(csum_partial(s, 4, ~csum_unfold(*csum)));
+}
+
+/*
+ * Mangle IP address.
+ * 	- begin points to the start of the snmp messgae
+ *      - addr points to the start of the address
+ */
+static inline void mangle_address(unsigned char *begin,
+                                  unsigned char *addr,
+                                  const struct oct1_map *map,
+                                  __sum16 *check)
+{
+	if (map->from == NOCT1(addr)) {
+		u_int32_t old;
+
+		if (debug)
+			memcpy(&old, (unsigned char *)addr, sizeof(old));
+
+		*addr = map->to;
+
+		/* Update UDP checksum if being used */
+		if (*check) {
+			fast_csum(check,
+			          &map->from, &map->to, addr - begin);
+
+		}
+
+		if (debug)
+			printk(KERN_DEBUG "bsalg: mapped %u.%u.%u.%u to "
+			       "%u.%u.%u.%u\n", NIPQUAD(old), NIPQUAD(*addr));
+	}
+}
+
+static unsigned char snmp_trap_decode(struct asn1_ctx *ctx,
+                                      struct snmp_v1_trap *trap,
+                                      const struct oct1_map *map,
+                                      __sum16 *check)
+{
+	unsigned int cls, con, tag, len;
+	unsigned char *end;
+
+	if (!asn1_header_decode(ctx, &end, &cls, &con, &tag))
+		return 0;
+
+	if (cls != ASN1_UNI || con != ASN1_PRI || tag != ASN1_OJI)
+		return 0;
+
+	if (!asn1_oid_decode(ctx, end, &trap->id, &trap->id_len))
+		return 0;
+
+	if (!asn1_header_decode(ctx, &end, &cls, &con, &tag))
+		goto err_id_free;
+
+	if (!((cls == ASN1_APL && con == ASN1_PRI && tag == SNMP_IPA) ||
+	      (cls == ASN1_UNI && con == ASN1_PRI && tag == ASN1_OTS)))
+		goto err_id_free;
+
+	if (!asn1_octets_decode(ctx, end, (unsigned char **)&trap->ip_address, &len))
+		goto err_id_free;
+
+	/* IPv4 only */
+	if (len != 4)
+		goto err_addr_free;
+
+	mangle_address(ctx->begin, ctx->pointer - 4, map, check);
+
+	if (!asn1_header_decode(ctx, &end, &cls, &con, &tag))
+		goto err_addr_free;
+
+	if (cls != ASN1_UNI || con != ASN1_PRI || tag != ASN1_INT)
+		goto err_addr_free;
+
+	if (!asn1_uint_decode(ctx, end, &trap->general))
+		goto err_addr_free;
+
+	if (!asn1_header_decode(ctx, &end, &cls, &con, &tag))
+		goto err_addr_free;
+
+	if (cls != ASN1_UNI || con != ASN1_PRI || tag != ASN1_INT)
+		goto err_addr_free;
+
+	if (!asn1_uint_decode(ctx, end, &trap->specific))
+		goto err_addr_free;
+
+	if (!asn1_header_decode(ctx, &end, &cls, &con, &tag))
+		goto err_addr_free;
+
+	if (!((cls == ASN1_APL && con == ASN1_PRI && tag == SNMP_TIT) ||
+	      (cls == ASN1_UNI && con == ASN1_PRI && tag == ASN1_INT)))
+		goto err_addr_free;
+
+	if (!asn1_ulong_decode(ctx, end, &trap->time))
+		goto err_addr_free;
+
+	return 1;
+
+err_addr_free:
+	kfree((unsigned long *)trap->ip_address);
+
+err_id_free:
+	kfree(trap->id);
+
+	return 0;
+}
+
+/*****************************************************************************
+ *
+ * Misc. routines
+ *
+ *****************************************************************************/
+
+static void hex_dump(unsigned char *buf, size_t len)
+{
+	size_t i;
+
+	for (i = 0; i < len; i++) {
+		if (i && !(i % 16))
+			printk("\n");
+		printk("%02x ", *(buf + i));
+	}
+	printk("\n");
+}
+
+/*
+ * Parse and mangle SNMP message according to mapping.
+ * (And this is the fucking 'basic' method).
+ */
+static int snmp_parse_mangle(unsigned char *msg,
+                             u_int16_t len,
+                             const struct oct1_map *map,
+                             __sum16 *check)
+{
+	unsigned char *eoc, *end;
+	unsigned int cls, con, tag, vers, pdutype;
+	struct asn1_ctx ctx;
+	struct asn1_octstr comm;
+	struct snmp_object **obj;
+
+	if (debug > 1)
+		hex_dump(msg, len);
+
+	asn1_open(&ctx, msg, len);
+
+	/*
+	 * Start of SNMP message.
+	 */
+	if (!asn1_header_decode(&ctx, &eoc, &cls, &con, &tag))
+		return 0;
+	if (cls != ASN1_UNI || con != ASN1_CON || tag != ASN1_SEQ)
+		return 0;
+
+	/*
+	 * Version 1 or 2 handled.
+	 */
+	if (!asn1_header_decode(&ctx, &end, &cls, &con, &tag))
+		return 0;
+	if (cls != ASN1_UNI || con != ASN1_PRI || tag != ASN1_INT)
+		return 0;
+	if (!asn1_uint_decode (&ctx, end, &vers))
+		return 0;
+	if (debug > 1)
+		printk(KERN_DEBUG "bsalg: snmp version: %u\n", vers + 1);
+	if (vers > 1)
+		return 1;
+
+	/*
+	 * Community.
+	 */
+	if (!asn1_header_decode (&ctx, &end, &cls, &con, &tag))
+		return 0;
+	if (cls != ASN1_UNI || con != ASN1_PRI || tag != ASN1_OTS)
+		return 0;
+	if (!asn1_octets_decode(&ctx, end, &comm.data, &comm.len))
+		return 0;
+	if (debug > 1) {
+		unsigned int i;
+
+		printk(KERN_DEBUG "bsalg: community: ");
+		for (i = 0; i < comm.len; i++)
+			printk("%c", comm.data[i]);
+		printk("\n");
+	}
+	kfree(comm.data);
+
+	/*
+	 * PDU type
+	 */
+	if (!asn1_header_decode(&ctx, &eoc, &cls, &con, &pdutype))
+		return 0;
+	if (cls != ASN1_CTX || con != ASN1_CON)
+		return 0;
+	if (debug > 1) {
+		unsigned char *pdus[] = {
+			[SNMP_PDU_GET] = "get",
+			[SNMP_PDU_NEXT] = "get-next",
+			[SNMP_PDU_RESPONSE] = "response",
+			[SNMP_PDU_SET] = "set",
+			[SNMP_PDU_TRAP1] = "trapv1",
+			[SNMP_PDU_BULK] = "bulk",
+			[SNMP_PDU_INFORM] = "inform",
+			[SNMP_PDU_TRAP2] = "trapv2"
+		};
+
+		if (pdutype > SNMP_PDU_TRAP2)
+			printk(KERN_DEBUG "bsalg: bad pdu type %u\n", pdutype);
+		else
+			printk(KERN_DEBUG "bsalg: pdu: %s\n", pdus[pdutype]);
+	}
+	if (pdutype != SNMP_PDU_RESPONSE &&
+	    pdutype != SNMP_PDU_TRAP1 && pdutype != SNMP_PDU_TRAP2)
+		return 1;
+
+	/*
+	 * Request header or v1 trap
+	 */
+	if (pdutype == SNMP_PDU_TRAP1) {
+		struct snmp_v1_trap trap;
+		unsigned char ret = snmp_trap_decode(&ctx, &trap, map, check);
+
+		if (ret) {
+			kfree(trap.id);
+			kfree((unsigned long *)trap.ip_address);
+		} else
+			return ret;
+
+	} else {
+		struct snmp_request req;
+
+		if (!snmp_request_decode(&ctx, &req))
+			return 0;
+
+		if (debug > 1)
+			printk(KERN_DEBUG "bsalg: request: id=0x%lx error_status=%u "
+			"error_index=%u\n", req.id, req.error_status,
+			req.error_index);
+	}
+
+	/*
+	 * Loop through objects, look for IP addresses to mangle.
+	 */
+	if (!asn1_header_decode(&ctx, &eoc, &cls, &con, &tag))
+		return 0;
+
+	if (cls != ASN1_UNI || con != ASN1_CON || tag != ASN1_SEQ)
+		return 0;
+
+	obj = kmalloc(sizeof(struct snmp_object), GFP_ATOMIC);
+	if (obj == NULL) {
+		if (net_ratelimit())
+			printk(KERN_WARNING "OOM in bsalg(%d)\n", __LINE__);
+		return 0;
+	}
+
+	while (!asn1_eoc_decode(&ctx, eoc)) {
+		unsigned int i;
+
+		if (!snmp_object_decode(&ctx, obj)) {
+			if (*obj) {
+				kfree((*obj)->id);
+				kfree(*obj);
+			}
+			kfree(obj);
+			return 0;
+		}
+
+		if (debug > 1) {
+			printk(KERN_DEBUG "bsalg: object: ");
+			for (i = 0; i < (*obj)->id_len; i++) {
+				if (i > 0)
+					printk(".");
+				printk("%lu", (*obj)->id[i]);
+			}
+			printk(": type=%u\n", (*obj)->type);
+
+		}
+
+		if ((*obj)->type == SNMP_IPADDR)
+			mangle_address(ctx.begin, ctx.pointer - 4 , map, check);
+
+		kfree((*obj)->id);
+		kfree(*obj);
+	}
+	kfree(obj);
+
+	if (!asn1_eoc_decode(&ctx, eoc))
+		return 0;
+
+	return 1;
+}
+
+/*****************************************************************************
+ *
+ * NAT routines.
+ *
+ *****************************************************************************/
+
+/*
+ * SNMP translation routine.
+ */
+static int snmp_translate(struct nf_conn *ct,
+                          enum ip_conntrack_info ctinfo,
+                          struct sk_buff **pskb)
+{
+	struct iphdr *iph = (*pskb)->nh.iph;
+	struct udphdr *udph = (struct udphdr *)((__be32 *)iph + iph->ihl);
+	u_int16_t udplen = ntohs(udph->len);
+	u_int16_t paylen = udplen - sizeof(struct udphdr);
+	int dir = CTINFO2DIR(ctinfo);
+	struct oct1_map map;
+
+	/*
+	 * Determine mappping for application layer addresses based
+	 * on NAT manipulations for the packet.
+	 */
+	if (dir == IP_CT_DIR_ORIGINAL) {
+		/* SNAT traps */
+		map.from = NOCT1(&ct->tuplehash[dir].tuple.src.u3.ip);
+		map.to = NOCT1(&ct->tuplehash[!dir].tuple.dst.u3.ip);
+	} else {
+		/* DNAT replies */
+		map.from = NOCT1(&ct->tuplehash[dir].tuple.src.u3.ip);
+		map.to = NOCT1(&ct->tuplehash[!dir].tuple.dst.u3.ip);
+	}
+
+	if (map.from == map.to)
+		return NF_ACCEPT;
+
+	if (!snmp_parse_mangle((unsigned char *)udph + sizeof(struct udphdr),
+	                       paylen, &map, &udph->check)) {
+		if (net_ratelimit())
+			printk(KERN_WARNING "bsalg: parser failed\n");
+		return NF_DROP;
+	}
+	return NF_ACCEPT;
+}
+
+/* We don't actually set up expectations, just adjust internal IP
+ * addresses if this is being NATted */
+static int help(struct sk_buff **pskb, unsigned int protoff,
+		struct nf_conn *ct,
+		enum ip_conntrack_info ctinfo)
+{
+	int dir = CTINFO2DIR(ctinfo);
+	unsigned int ret;
+	struct iphdr *iph = (*pskb)->nh.iph;
+	struct udphdr *udph = (struct udphdr *)((u_int32_t *)iph + iph->ihl);
+
+	/* SNMP replies and originating SNMP traps get mangled */
+	if (udph->source == htons(SNMP_PORT) && dir != IP_CT_DIR_REPLY)
+		return NF_ACCEPT;
+	if (udph->dest == htons(SNMP_TRAP_PORT) && dir != IP_CT_DIR_ORIGINAL)
+		return NF_ACCEPT;
+
+	/* No NAT? */
+	if (!(ct->status & IPS_NAT_MASK))
+		return NF_ACCEPT;
+
+	/*
+	 * Make sure the packet length is ok.  So far, we were only guaranteed
+	 * to have a valid length IP header plus 8 bytes, which means we have
+	 * enough room for a UDP header.  Just verify the UDP length field so we
+	 * can mess around with the payload.
+	 */
+	if (ntohs(udph->len) != (*pskb)->len - (iph->ihl << 2)) {
+		 if (net_ratelimit())
+			 printk(KERN_WARNING "SNMP: dropping malformed packet "
+				"src=%u.%u.%u.%u dst=%u.%u.%u.%u\n",
+				NIPQUAD(iph->saddr), NIPQUAD(iph->daddr));
+		 return NF_DROP;
+	}
+
+	if (!skb_make_writable(pskb, (*pskb)->len))
+		return NF_DROP;
+
+	spin_lock_bh(&snmp_lock);
+	ret = snmp_translate(ct, ctinfo, pskb);
+	spin_unlock_bh(&snmp_lock);
+	return ret;
+}
+
+static struct nf_conntrack_helper snmp_helper __read_mostly = {
+	.max_expected		= 0,
+	.timeout		= 180,
+	.me			= THIS_MODULE,
+	.help			= help,
+	.name			= "snmp",
+	.tuple.src.l3num	= AF_INET,
+	.tuple.src.u.udp.port	= __constant_htons(SNMP_PORT),
+	.tuple.dst.protonum	= IPPROTO_UDP,
+	.mask.src.l3num		= 0xFFFF,
+	.mask.src.u.udp.port	= __constant_htons(0xFFFF),
+	.mask.dst.protonum	= 0xFF,
+};
+
+static struct nf_conntrack_helper snmp_trap_helper __read_mostly = {
+	.max_expected		= 0,
+	.timeout		= 180,
+	.me			= THIS_MODULE,
+	.help			= help,
+	.name			= "snmp_trap",
+	.tuple.src.l3num	= AF_INET,
+	.tuple.src.u.udp.port	= __constant_htons(SNMP_TRAP_PORT),
+	.tuple.dst.protonum	= IPPROTO_UDP,
+	.mask.src.l3num		= 0xFFFF,
+	.mask.src.u.udp.port	= __constant_htons(0xFFFF),
+	.mask.dst.protonum	= 0xFF,
+};
+
+/*****************************************************************************
+ *
+ * Module stuff.
+ *
+ *****************************************************************************/
+
+static int __init nf_nat_snmp_basic_init(void)
+{
+	int ret = 0;
+
+	ret = nf_conntrack_helper_register(&snmp_helper);
+	if (ret < 0)
+		return ret;
+	ret = nf_conntrack_helper_register(&snmp_trap_helper);
+	if (ret < 0) {
+		nf_conntrack_helper_unregister(&snmp_helper);
+		return ret;
+	}
+	return ret;
+}
+
+static void __exit nf_nat_snmp_basic_fini(void)
+{
+	nf_conntrack_helper_unregister(&snmp_helper);
+	nf_conntrack_helper_unregister(&snmp_trap_helper);
+}
+
+module_init(nf_nat_snmp_basic_init);
+module_exit(nf_nat_snmp_basic_fini);
+
+module_param(debug, int, 0600);