/*
 * 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>
 *
 * Copyright (c) 2006-2010 Patrick McHardy <kaber@trash.net>
 */
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/slab.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_expect.h>
#include <net/netfilter/nf_conntrack_helper.h>
#include <net/netfilter/nf_nat_helper.h>
#include <linux/netfilter/nf_conntrack_snmp.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 = ch & 0x7F;
                        *len = 0;

                        while (cnt > 0) {
                                if (!asn1_octet_decode(ctx, &ch))
                                        return 0;
                                *len <<= 8;
                                *len |= ch;
                                cnt--;
                        }
                }
        }

        /* don't trust len bigger than ctx buffer */
        if (*len > ctx->end - ctx->pointer)
                return 0;

        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;

        /* primitive shall be definite, indefinite shall be constructed */
        if (*con == ASN1_PRI && !def)
                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 == NULL) {
                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)
                return 0;

        ptr = *octets;
        while (ctx->pointer < eoc) {
                if (!asn1_octet_decode(ctx, 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 long *optr;
        size_t size;

        size = eoc - ctx->pointer + 1;

        /* first subid actually encodes first two subids */
        if (size < 2 || size > ULONG_MAX/sizeof(unsigned long))
                return 0;

        *oid = kmalloc(size * sizeof(unsigned long), GFP_ATOMIC);
        if (*oid == NULL)
                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 const 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)
{
        const 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);
                        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(p);
                        kfree(id);
                        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);
                        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, &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);
                        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);
                        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);
                        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] = ~0;
                s[1] = ~*optr;
                s[2] = 0;
                s[3] = *nptr;
        } else {
                s[0] = ~*optr;
                s[1] = ~0;
                s[2] = *nptr;
                s[3] = 0;
        }

        *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, 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 %pI4 to %pI4\n",
                               &old, 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(const 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) {
                static const unsigned char *const 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;

        while (!asn1_eoc_decode(&ctx, eoc)) {
                unsigned int i;

                if (!snmp_object_decode(&ctx, &obj)) {
                        if (obj) {
                                kfree(obj->id);
                                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);
        }

        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 *skb)
{
        struct iphdr *iph = ip_hdr(skb);
        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)) {
                net_warn_ratelimited("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 *skb, unsigned int protoff,
                struct nf_conn *ct,
                enum ip_conntrack_info ctinfo)
{
        int dir = CTINFO2DIR(ctinfo);
        unsigned int ret;
        const struct iphdr *iph = ip_hdr(skb);
        const struct udphdr *udph = (struct udphdr *)((__be32 *)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) != skb->len - (iph->ihl << 2)) {
                net_warn_ratelimited("SNMP: dropping malformed packet src=%pI4 dst=%pI4\n",
                                     &iph->saddr, &iph->daddr);
                 return NF_DROP;
        }

        if (!skb_make_writable(skb, skb->len))
                return NF_DROP;

        spin_lock_bh(&snmp_lock);
        ret = snmp_translate(ct, ctinfo, skb);
        spin_unlock_bh(&snmp_lock);
        return ret;
}

static const struct nf_conntrack_expect_policy snmp_exp_policy = {
        .max_expected   = 0,
        .timeout        = 180,
};

static struct nf_conntrack_helper snmp_helper __read_mostly = {
        .me                     = THIS_MODULE,
        .help                   = help,
        .expect_policy          = &snmp_exp_policy,
        .name                   = "snmp",
        .tuple.src.l3num        = AF_INET,
        .tuple.src.u.udp.port   = cpu_to_be16(SNMP_PORT),
        .tuple.dst.protonum     = IPPROTO_UDP,
};

static struct nf_conntrack_helper snmp_trap_helper __read_mostly = {
        .me                     = THIS_MODULE,
        .help                   = help,
        .expect_policy          = &snmp_exp_policy,
        .name                   = "snmp_trap",
        .tuple.src.l3num        = AF_INET,
        .tuple.src.u.udp.port   = cpu_to_be16(SNMP_TRAP_PORT),
        .tuple.dst.protonum     = IPPROTO_UDP,
};

/*****************************************************************************
 *
 * Module stuff.
 *
 *****************************************************************************/

static int __init nf_nat_snmp_basic_init(void)
{
        int ret = 0;

        BUG_ON(nf_nat_snmp_hook != NULL);
        RCU_INIT_POINTER(nf_nat_snmp_hook, help);

        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)
{
        RCU_INIT_POINTER(nf_nat_snmp_hook, NULL);
        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);