// SPDX-License-Identifier: GPL-2.0-only
/* 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>
 */

#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_core.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>

#define NAT_HELPER_NAME "sip"

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Christian Hentschel <chentschel@arnet.com.ar>");
MODULE_DESCRIPTION("SIP NAT helper");
MODULE_ALIAS_NF_NAT_HELPER(NAT_HELPER_NAME);

static struct nf_conntrack_nat_helper nat_helper_sip =
        NF_CT_NAT_HELPER_INIT(NAT_HELPER_NAME);

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_ensure_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_conn_help *help = nfct_help(ct->master);
        struct nf_conntrack_expect *pair_exp;
        int range_set_for_snat = 0;
        struct nf_nat_range2 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);

        /* Do media streams SRC manip according with the parameters
         * found in the paired expectation.
         */
        if (exp->class != SIP_EXPECT_SIGNALLING) {
                spin_lock_bh(&nf_conntrack_expect_lock);
                hlist_for_each_entry(pair_exp, &help->expectations, lnode) {
                        if (pair_exp->tuple.src.l3num == nf_ct_l3num(ct) &&
                            pair_exp->tuple.dst.protonum == ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.dst.protonum &&
                            nf_inet_addr_cmp(&ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.src.u3, &pair_exp->saved_addr) &&
                            ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.src.u.all == pair_exp->saved_proto.all) {
                                range.flags = (NF_NAT_RANGE_MAP_IPS | NF_NAT_RANGE_PROTO_SPECIFIED);
                                range.min_proto.all = range.max_proto.all = pair_exp->tuple.dst.u.all;
                                range.min_addr = range.max_addr = pair_exp->tuple.dst.u3;
                                range_set_for_snat = 1;
                                break;
                        }
                }
                spin_unlock_bh(&nf_conntrack_expect_lock);
        }

        /* When no paired expectation has been found, change src to
         * where master sends to, but only if the connection actually came
         * from the same source.
         */
        if (!range_set_for_snat &&
            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;
                range_set_for_snat = 1;
        }

        /* Perform SRC manip. */
        if (range_set_for_snat)
                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, NF_CT_EXP_F_SKIP_MASTER);
                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,
                                           NF_CT_EXP_F_SKIP_MASTER);
                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,
                                           NF_CT_EXP_F_SKIP_MASTER);
                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)
{
        nf_nat_helper_unregister(&nat_helper_sip);
        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);
        nf_nat_helper_register(&nat_helper_sip);
        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);