/* SIP extension for NAT alteration.

   *
   * (C) 2005 by Christian Hentschel <chentschel@arnet.com.ar>
   * based on RR's ip_nat_ftp.c and other modules.

   * (C) 2007 United Security Providers

   * (C) 2007, 2008, 2011, 2012 Patrick McHardy <kaber@trash.net>

   *
   * This program is free software; you can redistribute it and/or modify
   * it under the terms of the GNU General Public License version 2 as
   * published by the Free Software Foundation.
   */
  
  #include <linux/module.h>
  #include <linux/skbuff.h>

  #include <linux/inet.h>

  #include <linux/udp.h>

  #include <linux/tcp.h>

  
  #include <net/netfilter/nf_nat.h>
  #include <net/netfilter/nf_nat_helper.h>

  #include <net/netfilter/nf_conntrack_helper.h>
  #include <net/netfilter/nf_conntrack_expect.h>

  #include <net/netfilter/nf_conntrack_seqadj.h>

  #include <linux/netfilter/nf_conntrack_sip.h>
  
  MODULE_LICENSE("GPL");
  MODULE_AUTHOR("Christian Hentschel <chentschel@arnet.com.ar>");
  MODULE_DESCRIPTION("SIP NAT helper");
  MODULE_ALIAS("ip_nat_sip");

  static unsigned int mangle_packet(struct sk_buff *skb, unsigned int protoff,
  				  unsigned int dataoff,

  				  const char **dptr, unsigned int *datalen,
  				  unsigned int matchoff, unsigned int matchlen,
  				  const char *buffer, unsigned int buflen)
  {
  	enum ip_conntrack_info ctinfo;
  	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);

  	struct tcphdr *th;
  	unsigned int baseoff;
  
  	if (nf_ct_protonum(ct) == IPPROTO_TCP) {

  		th = (struct tcphdr *)(skb->data + protoff);
  		baseoff = protoff + th->doff * 4;

  		matchoff += dataoff - baseoff;
  
  		if (!__nf_nat_mangle_tcp_packet(skb, ct, ctinfo,

  						protoff, matchoff, matchlen,

  						buffer, buflen, false))
  			return 0;
  	} else {

  		baseoff = protoff + sizeof(struct udphdr);

  		matchoff += dataoff - baseoff;
  
  		if (!nf_nat_mangle_udp_packet(skb, ct, ctinfo,

  					      protoff, matchoff, matchlen,

  					      buffer, buflen))
  			return 0;
  	}

  
  	/* Reload data pointer and adjust datalen value */

  	*dptr = skb->data + dataoff;

  	*datalen += buflen - matchlen;
  	return 1;
  }

  static int sip_sprintf_addr(const struct nf_conn *ct, char *buffer,
  			    const union nf_inet_addr *addr, bool delim)
  {
  	if (nf_ct_l3num(ct) == NFPROTO_IPV4)
  		return sprintf(buffer, "%pI4", &addr->ip);
  	else {
  		if (delim)
  			return sprintf(buffer, "[%pI6c]", &addr->ip6);
  		else
  			return sprintf(buffer, "%pI6c", &addr->ip6);
  	}
  }
  
  static int sip_sprintf_addr_port(const struct nf_conn *ct, char *buffer,
  				 const union nf_inet_addr *addr, u16 port)
  {
  	if (nf_ct_l3num(ct) == NFPROTO_IPV4)
  		return sprintf(buffer, "%pI4:%u", &addr->ip, port);
  	else
  		return sprintf(buffer, "[%pI6c]:%u", &addr->ip6, port);
  }

  static int map_addr(struct sk_buff *skb, unsigned int protoff,
  		    unsigned int dataoff,

  		    const char **dptr, unsigned int *datalen,
  		    unsigned int matchoff, unsigned int matchlen,

  		    union nf_inet_addr *addr, __be16 port)

  {

  	enum ip_conntrack_info ctinfo;

  	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);

  	enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);

  	struct nf_ct_sip_master *ct_sip_info = nfct_help_data(ct);

  	char buffer[INET6_ADDRSTRLEN + sizeof("[]:nnnnn")];

  	unsigned int buflen;

  	union nf_inet_addr newaddr;

  	__be16 newport;

  	if (nf_inet_addr_cmp(&ct->tuplehash[dir].tuple.src.u3, addr) &&

  	    ct->tuplehash[dir].tuple.src.u.udp.port == port) {

  		newaddr = ct->tuplehash[!dir].tuple.dst.u3;

  		newport = ct->tuplehash[!dir].tuple.dst.u.udp.port;

  	} else if (nf_inet_addr_cmp(&ct->tuplehash[dir].tuple.dst.u3, addr) &&

  		   ct->tuplehash[dir].tuple.dst.u.udp.port == port) {

  		newaddr = ct->tuplehash[!dir].tuple.src.u3;

  		newport = ct_sip_info->forced_dport ? :
  			  ct->tuplehash[!dir].tuple.src.u.udp.port;

  	} else
  		return 1;

  	if (nf_inet_addr_cmp(&newaddr, addr) && newport == port)

  		return 1;

  	buflen = sip_sprintf_addr_port(ct, buffer, &newaddr, ntohs(newport));

  	return mangle_packet(skb, protoff, dataoff, dptr, datalen,
  			     matchoff, matchlen, buffer, buflen);

  }

  static int map_sip_addr(struct sk_buff *skb, unsigned int protoff,
  			unsigned int dataoff,

  			const char **dptr, unsigned int *datalen,

  			enum sip_header_types type)

  {
  	enum ip_conntrack_info ctinfo;
  	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);
  	unsigned int matchlen, matchoff;

  	union nf_inet_addr addr;
  	__be16 port;

  	if (ct_sip_parse_header_uri(ct, *dptr, NULL, *datalen, type, NULL,
  				    &matchoff, &matchlen, &addr, &port) <= 0)

  		return 1;

  	return map_addr(skb, protoff, dataoff, dptr, datalen,
  			matchoff, matchlen, &addr, port);

  }

  static unsigned int nf_nat_sip(struct sk_buff *skb, unsigned int protoff,

  			       unsigned int dataoff,

  			       const char **dptr, unsigned int *datalen)

  {

  	enum ip_conntrack_info ctinfo;
  	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);

  	enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);

  	struct nf_ct_sip_master *ct_sip_info = nfct_help_data(ct);

  	unsigned int coff, matchoff, matchlen;

  	enum sip_header_types hdr;

  	union nf_inet_addr addr;
  	__be16 port;

  	int request, in_header;

  	/* Basic rules: requests and responses. */

  	if (strncasecmp(*dptr, "SIP/2.0", strlen("SIP/2.0")) != 0) {

  		if (ct_sip_parse_request(ct, *dptr, *datalen,

  					 &matchoff, &matchlen,
  					 &addr, &port) > 0 &&

  		    !map_addr(skb, protoff, dataoff, dptr, datalen,

  			      matchoff, matchlen, &addr, port)) {
  			nf_ct_helper_log(skb, ct, "cannot mangle SIP message");

  			return NF_DROP;

  		}

  		request = 1;
  	} else
  		request = 0;

  	if (nf_ct_protonum(ct) == IPPROTO_TCP)
  		hdr = SIP_HDR_VIA_TCP;
  	else
  		hdr = SIP_HDR_VIA_UDP;

  	/* Translate topmost Via header and parameters */
  	if (ct_sip_parse_header_uri(ct, *dptr, NULL, *datalen,

  				    hdr, NULL, &matchoff, &matchlen,

  				    &addr, &port) > 0) {

  		unsigned int olen, matchend, poff, plen, buflen, n;

  		char buffer[INET6_ADDRSTRLEN + sizeof("[]:nnnnn")];

  
  		/* We're only interested in headers related to this
  		 * connection */
  		if (request) {

  			if (!nf_inet_addr_cmp(&addr,
  					&ct->tuplehash[dir].tuple.src.u3) ||

  			    port != ct->tuplehash[dir].tuple.src.u.udp.port)
  				goto next;
  		} else {

  			if (!nf_inet_addr_cmp(&addr,
  					&ct->tuplehash[dir].tuple.dst.u3) ||

  			    port != ct->tuplehash[dir].tuple.dst.u.udp.port)
  				goto next;
  		}

  		olen = *datalen;

  		if (!map_addr(skb, protoff, dataoff, dptr, datalen,

  			      matchoff, matchlen, &addr, port)) {
  			nf_ct_helper_log(skb, ct, "cannot mangle Via header");

  			return NF_DROP;

  		}

  		matchend = matchoff + matchlen + *datalen - olen;

  
  		/* The maddr= parameter (RFC 2361) specifies where to send
  		 * the reply. */
  		if (ct_sip_parse_address_param(ct, *dptr, matchend, *datalen,
  					       "maddr=", &poff, &plen,

  					       &addr, true) > 0 &&

  		    nf_inet_addr_cmp(&addr, &ct->tuplehash[dir].tuple.src.u3) &&
  		    !nf_inet_addr_cmp(&addr, &ct->tuplehash[!dir].tuple.dst.u3)) {
  			buflen = sip_sprintf_addr(ct, buffer,
  					&ct->tuplehash[!dir].tuple.dst.u3,
  					true);

  			if (!mangle_packet(skb, protoff, dataoff, dptr, datalen,

  					   poff, plen, buffer, buflen)) {
  				nf_ct_helper_log(skb, ct, "cannot mangle maddr");

  				return NF_DROP;

  			}

  		}
  
  		/* The received= parameter (RFC 2361) contains the address
  		 * from which the server received the request. */
  		if (ct_sip_parse_address_param(ct, *dptr, matchend, *datalen,
  					       "received=", &poff, &plen,

  					       &addr, false) > 0 &&

  		    nf_inet_addr_cmp(&addr, &ct->tuplehash[dir].tuple.dst.u3) &&
  		    !nf_inet_addr_cmp(&addr, &ct->tuplehash[!dir].tuple.src.u3)) {
  			buflen = sip_sprintf_addr(ct, buffer,
  					&ct->tuplehash[!dir].tuple.src.u3,
  					false);

  			if (!mangle_packet(skb, protoff, dataoff, dptr, datalen,

  					   poff, plen, buffer, buflen)) {

  				nf_ct_helper_log(skb, ct, "cannot mangle received");

  				return NF_DROP;

  			}

  		}
  
  		/* The rport= parameter (RFC 3581) contains the port number
  		 * from which the server received the request. */
  		if (ct_sip_parse_numerical_param(ct, *dptr, matchend, *datalen,
  						 "rport=", &poff, &plen,
  						 &n) > 0 &&
  		    htons(n) == ct->tuplehash[dir].tuple.dst.u.udp.port &&
  		    htons(n) != ct->tuplehash[!dir].tuple.src.u.udp.port) {
  			__be16 p = ct->tuplehash[!dir].tuple.src.u.udp.port;
  			buflen = sprintf(buffer, "%u", ntohs(p));

  			if (!mangle_packet(skb, protoff, dataoff, dptr, datalen,

  					   poff, plen, buffer, buflen)) {
  				nf_ct_helper_log(skb, ct, "cannot mangle rport");

  				return NF_DROP;

  			}

  		}

  	}

  next:

  	/* Translate Contact headers */

  	coff = 0;

  	in_header = 0;

  	while (ct_sip_parse_header_uri(ct, *dptr, &coff, *datalen,

  				       SIP_HDR_CONTACT, &in_header,
  				       &matchoff, &matchlen,
  				       &addr, &port) > 0) {

  		if (!map_addr(skb, protoff, dataoff, dptr, datalen,
  			      matchoff, matchlen,

  			      &addr, port)) {
  			nf_ct_helper_log(skb, ct, "cannot mangle contact");

  			return NF_DROP;

  		}

  	}

  	if (!map_sip_addr(skb, protoff, dataoff, dptr, datalen, SIP_HDR_FROM) ||

  	    !map_sip_addr(skb, protoff, dataoff, dptr, datalen, SIP_HDR_TO)) {
  		nf_ct_helper_log(skb, ct, "cannot mangle SIP from/to");

  		return NF_DROP;

  	}

  	/* Mangle destination port for Cisco phones, then fix up checksums */
  	if (dir == IP_CT_DIR_REPLY && ct_sip_info->forced_dport) {
  		struct udphdr *uh;

  		if (!skb_make_writable(skb, skb->len)) {
  			nf_ct_helper_log(skb, ct, "cannot mangle packet");

  			return NF_DROP;

  		}

  
  		uh = (void *)skb->data + protoff;
  		uh->dest = ct_sip_info->forced_dport;
  
  		if (!nf_nat_mangle_udp_packet(skb, ct, ctinfo, protoff,

  					      0, 0, NULL, 0)) {
  			nf_ct_helper_log(skb, ct, "cannot mangle packet");

  			return NF_DROP;

  		}

  	}

  	return NF_ACCEPT;
  }

  static void nf_nat_sip_seq_adjust(struct sk_buff *skb, unsigned int protoff,
  				  s16 off)

  {
  	enum ip_conntrack_info ctinfo;
  	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);
  	const struct tcphdr *th;
  
  	if (nf_ct_protonum(ct) != IPPROTO_TCP || off == 0)
  		return;

  	th = (struct tcphdr *)(skb->data + protoff);

  	nf_ct_seqadj_set(ct, ctinfo, th->seq, off);

  }

  /* Handles expected signalling connections and media streams */

  static void nf_nat_sip_expected(struct nf_conn *ct,

  				struct nf_conntrack_expect *exp)
  {

  	struct nf_nat_range range;

  
  	/* This must be a fresh one. */
  	BUG_ON(ct->status & IPS_NAT_DONE_MASK);
  
  	/* For DST manip, map port here to where it's expected. */

  	range.flags = (NF_NAT_RANGE_MAP_IPS | NF_NAT_RANGE_PROTO_SPECIFIED);

  	range.min_proto = range.max_proto = exp->saved_proto;
  	range.min_addr = range.max_addr = exp->saved_addr;

  	nf_nat_setup_info(ct, &range, NF_NAT_MANIP_DST);

  
  	/* Change src to where master sends to, but only if the connection
  	 * actually came from the same source. */

  	if (nf_inet_addr_cmp(&ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.src.u3,
  			     &ct->master->tuplehash[exp->dir].tuple.src.u3)) {

  		range.flags = NF_NAT_RANGE_MAP_IPS;

  		range.min_addr = range.max_addr
  			= ct->master->tuplehash[!exp->dir].tuple.dst.u3;

  		nf_nat_setup_info(ct, &range, NF_NAT_MANIP_SRC);

  	}
  }

  static unsigned int nf_nat_sip_expect(struct sk_buff *skb, unsigned int protoff,

  				      unsigned int dataoff,

  				      const char **dptr, unsigned int *datalen,
  				      struct nf_conntrack_expect *exp,
  				      unsigned int matchoff,
  				      unsigned int matchlen)
  {
  	enum ip_conntrack_info ctinfo;
  	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);
  	enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);

  	struct nf_ct_sip_master *ct_sip_info = nfct_help_data(ct);

  	union nf_inet_addr newaddr;

  	u_int16_t port;

  	__be16 srcport;

  	char buffer[INET6_ADDRSTRLEN + sizeof("[]:nnnnn")];

  	unsigned int buflen;

  
  	/* Connection will come from reply */

  	if (nf_inet_addr_cmp(&ct->tuplehash[dir].tuple.src.u3,
  			     &ct->tuplehash[!dir].tuple.dst.u3))
  		newaddr = exp->tuple.dst.u3;

  	else

  		newaddr = ct->tuplehash[!dir].tuple.dst.u3;

  
  	/* If the signalling port matches the connection's source port in the
  	 * original direction, try to use the destination port in the opposite
  	 * direction. */

  	srcport = ct_sip_info->forced_dport ? :
  		  ct->tuplehash[dir].tuple.src.u.udp.port;
  	if (exp->tuple.dst.u.udp.port == srcport)

  		port = ntohs(ct->tuplehash[!dir].tuple.dst.u.udp.port);
  	else
  		port = ntohs(exp->tuple.dst.u.udp.port);

  	exp->saved_addr = exp->tuple.dst.u3;

  	exp->tuple.dst.u3 = newaddr;

  	exp->saved_proto.udp.port = exp->tuple.dst.u.udp.port;
  	exp->dir = !dir;

  	exp->expectfn = nf_nat_sip_expected;

  
  	for (; port != 0; port++) {

  		int ret;

  		exp->tuple.dst.u.udp.port = htons(port);

  		ret = nf_ct_expect_related(exp);
  		if (ret == 0)
  			break;
  		else if (ret != -EBUSY) {
  			port = 0;

  			break;

  		}

  	}

  	if (port == 0) {
  		nf_ct_helper_log(skb, ct, "all ports in use for SIP");

  		return NF_DROP;

  	}

  	if (!nf_inet_addr_cmp(&exp->tuple.dst.u3, &exp->saved_addr) ||

  	    exp->tuple.dst.u.udp.port != exp->saved_proto.udp.port) {

  		buflen = sip_sprintf_addr_port(ct, buffer, &newaddr, port);

  		if (!mangle_packet(skb, protoff, dataoff, dptr, datalen,

  				   matchoff, matchlen, buffer, buflen)) {
  			nf_ct_helper_log(skb, ct, "cannot mangle packet");

  			goto err;

  		}

  	}
  	return NF_ACCEPT;
  
  err:
  	nf_ct_unexpect_related(exp);
  	return NF_DROP;
  }

  static int mangle_content_len(struct sk_buff *skb, unsigned int protoff,
  			      unsigned int dataoff,

  			      const char **dptr, unsigned int *datalen)

  {

  	enum ip_conntrack_info ctinfo;
  	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);

  	unsigned int matchoff, matchlen;
  	char buffer[sizeof("65536")];
  	int buflen, c_len;

  	/* Get actual SDP length */
  	if (ct_sip_get_sdp_header(ct, *dptr, 0, *datalen,
  				  SDP_HDR_VERSION, SDP_HDR_UNSPEC,
  				  &matchoff, &matchlen) <= 0)
  		return 0;
  	c_len = *datalen - matchoff + strlen("v=");
  
  	/* Now, update SDP length */

  	if (ct_sip_get_header(ct, *dptr, 0, *datalen, SIP_HDR_CONTENT_LENGTH,
  			      &matchoff, &matchlen) <= 0)

  		return 0;

  	buflen = sprintf(buffer, "%u", c_len);

  	return mangle_packet(skb, protoff, dataoff, dptr, datalen,
  			     matchoff, matchlen, buffer, buflen);

  }

  static int mangle_sdp_packet(struct sk_buff *skb, unsigned int protoff,
  			     unsigned int dataoff,

  			     const char **dptr, unsigned int *datalen,
  			     unsigned int sdpoff,

  			     enum sdp_header_types type,
  			     enum sdp_header_types term,
  			     char *buffer, int buflen)

  {

  	enum ip_conntrack_info ctinfo;
  	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);

  	unsigned int matchlen, matchoff;

  	if (ct_sip_get_sdp_header(ct, *dptr, sdpoff, *datalen, type, term,

  				  &matchoff, &matchlen) <= 0)

  		return -ENOENT;

  	return mangle_packet(skb, protoff, dataoff, dptr, datalen,
  			     matchoff, matchlen, buffer, buflen) ? 0 : -EINVAL;

  }

  static unsigned int nf_nat_sdp_addr(struct sk_buff *skb, unsigned int protoff,

  				    unsigned int dataoff,

  				    const char **dptr, unsigned int *datalen,
  				    unsigned int sdpoff,

  				    enum sdp_header_types type,
  				    enum sdp_header_types term,
  				    const union nf_inet_addr *addr)

  {

  	enum ip_conntrack_info ctinfo;
  	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);
  	char buffer[INET6_ADDRSTRLEN];

  	unsigned int buflen;

  	buflen = sip_sprintf_addr(ct, buffer, addr, false);

  	if (mangle_sdp_packet(skb, protoff, dataoff, dptr, datalen,
  			      sdpoff, type, term, buffer, buflen))

  		return 0;

  	return mangle_content_len(skb, protoff, dataoff, dptr, datalen);

  }

  static unsigned int nf_nat_sdp_port(struct sk_buff *skb, unsigned int protoff,

  				    unsigned int dataoff,

  				    const char **dptr, unsigned int *datalen,

  				    unsigned int matchoff,
  				    unsigned int matchlen,
  				    u_int16_t port)
  {
  	char buffer[sizeof("nnnnn")];
  	unsigned int buflen;
  
  	buflen = sprintf(buffer, "%u", port);

  	if (!mangle_packet(skb, protoff, dataoff, dptr, datalen,
  			   matchoff, matchlen, buffer, buflen))

  		return 0;

  	return mangle_content_len(skb, protoff, dataoff, dptr, datalen);

  }

  static unsigned int nf_nat_sdp_session(struct sk_buff *skb, unsigned int protoff,

  				       unsigned int dataoff,

  				       const char **dptr, unsigned int *datalen,
  				       unsigned int sdpoff,

  				       const union nf_inet_addr *addr)
  {

  	enum ip_conntrack_info ctinfo;
  	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);
  	char buffer[INET6_ADDRSTRLEN];

  	unsigned int buflen;
  
  	/* Mangle session description owner and contact addresses */

  	buflen = sip_sprintf_addr(ct, buffer, addr, false);

  	if (mangle_sdp_packet(skb, protoff, dataoff, dptr, datalen, sdpoff,

  			      SDP_HDR_OWNER, SDP_HDR_MEDIA, buffer, buflen))

  		return 0;

  	switch (mangle_sdp_packet(skb, protoff, dataoff, dptr, datalen, sdpoff,

  				  SDP_HDR_CONNECTION, SDP_HDR_MEDIA,

  				  buffer, buflen)) {
  	case 0:
  	/*
  	 * RFC 2327:
  	 *
  	 * Session description
  	 *
  	 * c=* (connection information - not required if included in all media)
  	 */
  	case -ENOENT:
  		break;
  	default:

  		return 0;

  	}

  	return mangle_content_len(skb, protoff, dataoff, dptr, datalen);

  }
  
  /* So, this packet has hit the connection tracking matching code.
     Mangle it, and change the expectation to match the new version. */

  static unsigned int nf_nat_sdp_media(struct sk_buff *skb, unsigned int protoff,

  				     unsigned int dataoff,

  				     const char **dptr, unsigned int *datalen,

  				     struct nf_conntrack_expect *rtp_exp,
  				     struct nf_conntrack_expect *rtcp_exp,
  				     unsigned int mediaoff,
  				     unsigned int medialen,
  				     union nf_inet_addr *rtp_addr)

  {

  	enum ip_conntrack_info ctinfo;
  	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);

  	enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);

  	u_int16_t port;

  	/* Connection will come from reply */

  	if (nf_inet_addr_cmp(&ct->tuplehash[dir].tuple.src.u3,
  			     &ct->tuplehash[!dir].tuple.dst.u3))
  		*rtp_addr = rtp_exp->tuple.dst.u3;

  	else

  		*rtp_addr = ct->tuplehash[!dir].tuple.dst.u3;

  	rtp_exp->saved_addr = rtp_exp->tuple.dst.u3;

  	rtp_exp->tuple.dst.u3 = *rtp_addr;

  	rtp_exp->saved_proto.udp.port = rtp_exp->tuple.dst.u.udp.port;
  	rtp_exp->dir = !dir;

  	rtp_exp->expectfn = nf_nat_sip_expected;

  	rtcp_exp->saved_addr = rtcp_exp->tuple.dst.u3;

  	rtcp_exp->tuple.dst.u3 = *rtp_addr;

  	rtcp_exp->saved_proto.udp.port = rtcp_exp->tuple.dst.u.udp.port;
  	rtcp_exp->dir = !dir;

  	rtcp_exp->expectfn = nf_nat_sip_expected;

  
  	/* Try to get same pair of ports: if not, try to change them. */
  	for (port = ntohs(rtp_exp->tuple.dst.u.udp.port);
  	     port != 0; port += 2) {

  		int ret;

  		rtp_exp->tuple.dst.u.udp.port = htons(port);

  		ret = nf_ct_expect_related(rtp_exp);
  		if (ret == -EBUSY)

  			continue;

  		else if (ret < 0) {
  			port = 0;
  			break;
  		}

  		rtcp_exp->tuple.dst.u.udp.port = htons(port + 1);

  		ret = nf_ct_expect_related(rtcp_exp);
  		if (ret == 0)

  			break;

  		else if (ret == -EBUSY) {
  			nf_ct_unexpect_related(rtp_exp);
  			continue;
  		} else if (ret < 0) {

  			nf_ct_unexpect_related(rtp_exp);
  			port = 0;
  			break;
  		}

  	}

  	if (port == 0) {
  		nf_ct_helper_log(skb, ct, "all ports in use for SDP media");

  		goto err1;

  	}

  
  	/* Update media port. */
  	if (rtp_exp->tuple.dst.u.udp.port != rtp_exp->saved_proto.udp.port &&

  	    !nf_nat_sdp_port(skb, protoff, dataoff, dptr, datalen,

  			     mediaoff, medialen, port)) {
  		nf_ct_helper_log(skb, ct, "cannot mangle SDP message");

  		goto err2;

  	}

  	return NF_ACCEPT;

  
  err2:
  	nf_ct_unexpect_related(rtp_exp);
  	nf_ct_unexpect_related(rtcp_exp);
  err1:
  	return NF_DROP;

  }

  static struct nf_ct_helper_expectfn sip_nat = {

  	.name		= "sip",
  	.expectfn	= nf_nat_sip_expected,

  };

  static void __exit nf_nat_sip_fini(void)
  {

  	RCU_INIT_POINTER(nf_nat_sip_hooks, NULL);

  	nf_ct_helper_expectfn_unregister(&sip_nat);

  	synchronize_rcu();
  }

  static const struct nf_nat_sip_hooks sip_hooks = {
  	.msg		= nf_nat_sip,
  	.seq_adjust	= nf_nat_sip_seq_adjust,
  	.expect		= nf_nat_sip_expect,
  	.sdp_addr	= nf_nat_sdp_addr,
  	.sdp_port	= nf_nat_sdp_port,
  	.sdp_session	= nf_nat_sdp_session,
  	.sdp_media	= nf_nat_sdp_media,
  };

  static int __init nf_nat_sip_init(void)
  {

  	BUG_ON(nf_nat_sip_hooks != NULL);
  	RCU_INIT_POINTER(nf_nat_sip_hooks, &sip_hooks);

  	nf_ct_helper_expectfn_register(&sip_nat);

  	return 0;
  }
  
  module_init(nf_nat_sip_init);
  module_exit(nf_nat_sip_fini);