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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>
 */
#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 *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)) {
            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 *skb, unsigned int protoff,
            struct nf_conn *ct,
            enum ip_conntrack_info ctinfo)
{
      int dir = CTINFO2DIR(ctinfo);
      unsigned int ret;
      struct iphdr *iph = ip_hdr(skb);
      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) != skb->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(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 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,
};

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,
};

/*****************************************************************************
 *
 * 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);

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