[NETFILTER]: Add SIP connection tracking helper

Add SIP connection tracking helper. Originally written by
Christian Hentschel <chentschel@arnet.com.ar>, some cleanup, minor
fixes and bidirectional SIP support added by myself.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

5 files changed, 784 insertions, 0 deletions

diff --git a/include/linux/netfilter_ipv4/ip_conntrack_sip.h b/include/linux/netfilter_ipv4/ip_conntrack_sip.h
new file mode 100644
index 000000000000..913dad66c0fb
--- /dev/null
+++ b/include/linux/netfilter_ipv4/ip_conntrack_sip.h
@@ -0,0 +1,44 @@
+#ifndef __IP_CONNTRACK_SIP_H__
+#define __IP_CONNTRACK_SIP_H__
+#ifdef __KERNEL__
+
+#define SIP_PORT        5060
+#define SIP_TIMEOUT     3600
+
+#define POS_VIA         0
+#define POS_CONTACT     1
+#define POS_CONTENT     2
+#define POS_MEDIA       3
+#define POS_OWNER       4
+#define POS_CONNECTION  5
+#define POS_REQ_HEADER  6
+#define POS_SDP_HEADER  7
+
+struct sip_header_nfo {
+        const char      *lname;
+        const char      *sname;
+        const char      *ln_str;
+        size_t          lnlen;
+        size_t          snlen;
+        size_t          ln_strlen;
+        int             (*match_len)(const char *, const char *, int *);
+};
+
+extern unsigned int (*ip_nat_sip_hook)(struct sk_buff **pskb,
+                                       enum ip_conntrack_info ctinfo,
+                                       struct ip_conntrack *ct,
+                                       const char **dptr);
+extern unsigned int (*ip_nat_sdp_hook)(struct sk_buff **pskb,
+                                       enum ip_conntrack_info ctinfo,
+                                       struct ip_conntrack_expect *exp,
+                                       const char *dptr);
+
+extern int ct_sip_get_info(const char *dptr, size_t dlen,
+                           unsigned int *matchoff,
+                           unsigned int *matchlen,
+                           struct sip_header_nfo *hnfo);
+extern int ct_sip_lnlen(const char *line, const char *limit);
+extern const char *ct_sip_search(const char *needle, const char *haystack,
+                                 size_t needle_len, size_t haystack_len);
+#endif /* __KERNEL__ */
+#endif /* __IP_CONNTRACK_SIP_H__ */
diff --git a/net/ipv4/netfilter/Kconfig b/net/ipv4/netfilter/Kconfig
index f86aeda3a5a0..ff4b118f14a9 100644
--- a/net/ipv4/netfilter/Kconfig
+++ b/net/ipv4/netfilter/Kconfig
@@ -191,6 +191,18 @@ config IP_NF_H323
           If you want to compile it as a module, say 'M' here and read
           Documentation/modules.txt.  If unsure, say 'N'.
 
+config IP_NF_SIP
+        tristate "SIP protocol support (EXPERIMENTAL)"
+        depends on IP_NF_CONNTRACK && EXPERIMENTAL
+        help
+          SIP is an application-layer control protocol that can establish,
+          modify, and terminate multimedia sessions (conferences) such as
+          Internet telephony calls. With the ip_conntrack_sip and
+          the ip_nat_sip modules you can support the protocol on a connection
+          tracking/NATing firewall.
+
+          To compile it as a module, choose M here.  If unsure, say Y.
+
 config IP_NF_QUEUE
         tristate "IP Userspace queueing via NETLINK (OBSOLETE)"
         help
@@ -503,6 +515,12 @@ config IP_NF_NAT_H323
         default IP_NF_NAT if IP_NF_H323=y
         default m if IP_NF_H323=m
 
+config IP_NF_NAT_SIP
+        tristate
+        depends on IP_NF_IPTABLES!=n && IP_NF_CONNTRACK!=n && IP_NF_NAT!=n
+        default IP_NF_NAT if IP_NF_SIP=y
+        default m if IP_NF_SIP=m
+
 # mangle + specific targets
 config IP_NF_MANGLE
         tristate "Packet mangling"
diff --git a/net/ipv4/netfilter/Makefile b/net/ipv4/netfilter/Makefile
index 461cb1eb5de7..3ded4a3af59c 100644
--- a/net/ipv4/netfilter/Makefile
+++ b/net/ipv4/netfilter/Makefile
@@ -31,6 +31,7 @@ obj-$(CONFIG_IP_NF_AMANDA) += ip_conntrack_amanda.o
 obj-$(CONFIG_IP_NF_TFTP) += ip_conntrack_tftp.o
 obj-$(CONFIG_IP_NF_FTP) += ip_conntrack_ftp.o
 obj-$(CONFIG_IP_NF_IRC) += ip_conntrack_irc.o
+obj-$(CONFIG_IP_NF_SIP) += ip_conntrack_sip.o
 obj-$(CONFIG_IP_NF_NETBIOS_NS) += ip_conntrack_netbios_ns.o
 
 # NAT helpers 
@@ -40,6 +41,7 @@ obj-$(CONFIG_IP_NF_NAT_AMANDA) += ip_nat_amanda.o
 obj-$(CONFIG_IP_NF_NAT_TFTP) += ip_nat_tftp.o
 obj-$(CONFIG_IP_NF_NAT_FTP) += ip_nat_ftp.o
 obj-$(CONFIG_IP_NF_NAT_IRC) += ip_nat_irc.o
+obj-$(CONFIG_IP_NF_NAT_SIP) += ip_nat_sip.o
 
 # generic IP tables 
 obj-$(CONFIG_IP_NF_IPTABLES) += ip_tables.o
diff --git a/net/ipv4/netfilter/ip_conntrack_sip.c b/net/ipv4/netfilter/ip_conntrack_sip.c
new file mode 100644
index 000000000000..fc87ce0da40d
--- /dev/null
+++ b/net/ipv4/netfilter/ip_conntrack_sip.c
@@ -0,0 +1,471 @@
+/* SIP extension for IP connection tracking.
+ *
+ * (C) 2005 by Christian Hentschel <chentschel@arnet.com.ar>
+ * based on RR's ip_conntrack_ftp.c and other modules.
+ *
+ * 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/config.h>
+#include <linux/module.h>
+#include <linux/ctype.h>
+#include <linux/skbuff.h>
+#include <linux/in.h>
+#include <linux/ip.h>
+#include <linux/udp.h>
+
+#include <linux/netfilter.h>
+#include <linux/netfilter_ipv4.h>
+#include <linux/netfilter_ipv4/ip_conntrack_helper.h>
+#include <linux/netfilter_ipv4/ip_conntrack_sip.h>
+
+#if 0
+#define DEBUGP printk
+#else
+#define DEBUGP(format, args...)
+#endif
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Christian Hentschel <chentschel@arnet.com.ar>");
+MODULE_DESCRIPTION("SIP connection tracking helper");
+
+#define MAX_PORTS       8
+static unsigned short ports[MAX_PORTS];
+static int ports_c;
+module_param_array(ports, ushort, &ports_c, 0400);
+MODULE_PARM_DESC(ports, "port numbers of sip servers");
+
+static unsigned int sip_timeout = SIP_TIMEOUT;
+module_param(sip_timeout, uint, 0600);
+MODULE_PARM_DESC(sip_timeout, "timeout for the master SIP session");
+
+unsigned int (*ip_nat_sip_hook)(struct sk_buff **pskb,
+                                enum ip_conntrack_info ctinfo,
+                                struct ip_conntrack *ct,
+                                const char **dptr);
+EXPORT_SYMBOL_GPL(ip_nat_sip_hook);
+
+unsigned int (*ip_nat_sdp_hook)(struct sk_buff **pskb,
+                                enum ip_conntrack_info ctinfo,
+                                struct ip_conntrack_expect *exp,
+                                const char *dptr);
+EXPORT_SYMBOL_GPL(ip_nat_sdp_hook);
+
+int ct_sip_get_info(const char *dptr, size_t dlen,
+                                unsigned int *matchoff,
+                                unsigned int *matchlen,
+                                struct sip_header_nfo *hnfo);
+EXPORT_SYMBOL_GPL(ct_sip_get_info);
+
+
+static int digits_len(const char *dptr, const char *limit, int *shift);
+static int epaddr_len(const char *dptr, const char *limit, int *shift);
+static int skp_digits_len(const char *dptr, const char *limit, int *shift);
+static int skp_epaddr_len(const char *dptr, const char *limit, int *shift);
+
+struct sip_header_nfo ct_sip_hdrs[] = {
+        {       /* Via header */
+                .lname          = "Via:",
+                .lnlen          = sizeof("Via:") - 1,
+                .sname          = "\r\nv:",
+                .snlen          = sizeof("\r\nv:") - 1, /* rfc3261 "\r\n" */
+                .ln_str         = "UDP ",
+                .ln_strlen      = sizeof("UDP ") - 1,
+                .match_len      = epaddr_len,
+        },
+        {       /* Contact header */
+                .lname          = "Contact:",
+                .lnlen          = sizeof("Contact:") - 1,
+                .sname          = "\r\nm:",
+                .snlen          = sizeof("\r\nm:") - 1,
+                .ln_str         = "sip:",
+                .ln_strlen      = sizeof("sip:") - 1,
+                .match_len      = skp_epaddr_len
+        },
+        {       /* Content length header */
+                .lname          = "Content-Length:",
+                .lnlen          = sizeof("Content-Length:") - 1,
+                .sname          = "\r\nl:",
+                .snlen          = sizeof("\r\nl:") - 1,
+                .ln_str         = ":",
+                .ln_strlen      = sizeof(":") - 1,
+                .match_len      = skp_digits_len
+        },
+        {       /* SDP media info */
+                .lname          = "\nm=",
+                .lnlen          = sizeof("\nm=") - 1,
+                .sname          = "\rm=",
+                .snlen          = sizeof("\rm=") - 1,
+                .ln_str         = "audio ",
+                .ln_strlen      = sizeof("audio ") - 1,
+                .match_len      = digits_len
+        },
+        {       /* SDP owner address*/
+                .lname          = "\no=",
+                .lnlen          = sizeof("\no=") - 1,
+                .sname          = "\ro=",
+                .snlen          = sizeof("\ro=") - 1,
+                .ln_str         = "IN IP4 ",
+                .ln_strlen      = sizeof("IN IP4 ") - 1,
+                .match_len      = epaddr_len
+        },
+        {       /* SDP connection info */
+                .lname          = "\nc=",
+                .lnlen          = sizeof("\nc=") - 1,
+                .sname          = "\rc=",
+                .snlen          = sizeof("\rc=") - 1,
+                .ln_str         = "IN IP4 ",
+                .ln_strlen      = sizeof("IN IP4 ") - 1,
+                .match_len      = epaddr_len
+        },
+        {       /* Requests headers */
+                .lname          = "sip:",
+                .lnlen          = sizeof("sip:") - 1,
+                .sname          = "sip:",
+                .snlen          = sizeof("sip:") - 1, /* yes, i know.. ;) */
+                .ln_str         = "@",
+                .ln_strlen      = sizeof("@") - 1,
+                .match_len      = epaddr_len
+        },
+        {       /* SDP version header */
+                .lname          = "\nv=",
+                .lnlen          = sizeof("\nv=") - 1,
+                .sname          = "\rv=",
+                .snlen          = sizeof("\rv=") - 1,
+                .ln_str         = "=",
+                .ln_strlen      = sizeof("=") - 1,
+                .match_len      = digits_len
+        }
+};
+EXPORT_SYMBOL_GPL(ct_sip_hdrs);
+
+/* get line lenght until first CR or LF seen. */
+int ct_sip_lnlen(const char *line, const char *limit)
+{
+        const char *k = line;
+
+        while ((line <= limit) && (*line == '\r' || *line == '\n'))
+                line++;
+
+        while (line <= limit) {
+                if (*line == '\r' || *line == '\n')
+                        break;
+                line++;
+        }
+        return line - k;
+}
+EXPORT_SYMBOL_GPL(ct_sip_lnlen);
+
+/* Linear string search, case sensitive. */
+const char *ct_sip_search(const char *needle, const char *haystack,
+                          size_t needle_len, size_t haystack_len)
+{
+        const char *limit = haystack + (haystack_len - needle_len);
+
+        while (haystack <= limit) {
+                if (memcmp(haystack, needle, needle_len) == 0)
+                        return haystack;
+                haystack++;
+        }
+        return NULL;
+}
+EXPORT_SYMBOL_GPL(ct_sip_search);
+
+static int digits_len(const char *dptr, const char *limit, int *shift)
+{
+        int len = 0;
+        while (dptr <= limit && isdigit(*dptr)) {
+                dptr++;
+                len++;
+        }
+        return len;
+}
+
+/* get digits lenght, skiping blank spaces. */
+static int skp_digits_len(const char *dptr, const char *limit, int *shift)
+{
+        for (; dptr <= limit && *dptr == ' '; dptr++)
+                (*shift)++;
+
+        return digits_len(dptr, limit, shift);
+}
+
+/* Simple ipaddr parser.. */
+static int parse_ipaddr(const char *cp, const char **endp,
+                        u_int32_t *ipaddr, const char *limit)
+{
+        unsigned long int val;
+        int i, digit = 0;
+
+        for (i = 0, *ipaddr = 0; cp <= limit && i < 4; i++) {
+                digit = 0;
+                if (!isdigit(*cp))
+                        break;
+
+                val = simple_strtoul(cp, (char **)&cp, 10);
+                if (val > 0xFF)
+                        return -1;
+
+                ((u_int8_t *)ipaddr)[i] = val;
+                digit = 1;
+
+                if (*cp != '.')
+                        break;
+                cp++;
+        }
+        if (!digit)
+                return -1;
+
+        if (endp)
+                *endp = cp;
+
+        return 0;
+}
+
+/* skip ip address. returns it lenght. */
+static int epaddr_len(const char *dptr, const char *limit, int *shift)
+{
+        const char *aux = dptr;
+        u_int32_t ip;
+
+        if (parse_ipaddr(dptr, &dptr, &ip, limit) < 0) {
+                DEBUGP("ip: %s parse failed.!\n", dptr);
+                return 0;
+        }
+
+        /* Port number */
+        if (*dptr == ':') {
+                dptr++;
+                dptr += digits_len(dptr, limit, shift);
+        }
+        return dptr - aux;
+}
+
+/* get address length, skiping user info. */
+static int skp_epaddr_len(const char *dptr, const char *limit, int *shift)
+{
+        int s = *shift;
+
+        for (; dptr <= limit && *dptr != '@'; dptr++)
+                (*shift)++;
+
+        if (*dptr == '@') {
+                dptr++;
+                (*shift)++;
+        } else
+                *shift = s;
+
+        return epaddr_len(dptr, limit, shift);
+}
+
+/* Returns 0 if not found, -1 error parsing. */
+int ct_sip_get_info(const char *dptr, size_t dlen,
+                    unsigned int *matchoff,
+                    unsigned int *matchlen,
+                    struct sip_header_nfo *hnfo)
+{
+        const char *limit, *aux, *k = dptr;
+        int shift = 0;
+
+        limit = dptr + (dlen - hnfo->lnlen);
+
+        while (dptr <= limit) {
+                if ((strncmp(dptr, hnfo->lname, hnfo->lnlen) != 0) &&
+                    (strncmp(dptr, hnfo->sname, hnfo->snlen) != 0)) {
+                        dptr++;
+                        continue;
+                }
+                aux = ct_sip_search(hnfo->ln_str, dptr, hnfo->ln_strlen,
+                                    ct_sip_lnlen(dptr, limit));
+                if (!aux) {
+                        DEBUGP("'%s' not found in '%s'.\n", hnfo->ln_str,
+                               hnfo->lname);
+                        return -1;
+                }
+                aux += hnfo->ln_strlen;
+
+                *matchlen = hnfo->match_len(aux, limit, &shift);
+                if (!*matchlen)
+                        return -1;
+
+                *matchoff = (aux - k) + shift;
+
+                DEBUGP("%s match succeeded! - len: %u\n", hnfo->lname,
+                       *matchlen);
+                return 1;
+        }
+        DEBUGP("%s header not found.\n", hnfo->lname);
+        return 0;
+}
+
+static int set_expected_rtp(struct sk_buff **pskb,
+                            struct ip_conntrack *ct,
+                            enum ip_conntrack_info ctinfo,
+                            u_int32_t ipaddr, u_int16_t port,
+                            const char *dptr)
+{
+        struct ip_conntrack_expect *exp;
+        enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
+        int ret;
+
+        exp = ip_conntrack_expect_alloc(ct);
+        if (exp == NULL)
+                return NF_DROP;
+
+        exp->tuple.src.ip = ct->tuplehash[!dir].tuple.src.ip;
+        exp->tuple.src.u.udp.port = 0;
+        exp->tuple.dst.ip = ipaddr;
+        exp->tuple.dst.u.udp.port = htons(port);
+        exp->tuple.dst.protonum = IPPROTO_UDP;
+
+        exp->mask.src.ip = 0xFFFFFFFF;
+        exp->mask.src.u.udp.port = 0;
+        exp->mask.dst.ip = 0xFFFFFFFF;
+        exp->mask.dst.u.udp.port = 0xFFFF;
+        exp->mask.dst.protonum = 0xFF;
+
+        exp->expectfn = NULL;
+        exp->flags = 0;
+
+        if (ip_nat_sdp_hook)
+                ret = ip_nat_sdp_hook(pskb, ctinfo, exp, dptr);
+        else {
+                if (ip_conntrack_expect_related(exp) != 0)
+                        ret = NF_DROP;
+                else
+                        ret = NF_ACCEPT;
+        }
+        ip_conntrack_expect_put(exp);
+
+        return ret;
+}
+
+static int sip_help(struct sk_buff **pskb,
+                    struct ip_conntrack *ct,
+                    enum ip_conntrack_info ctinfo)
+{
+        unsigned int dataoff, datalen;
+        const char *dptr;
+        int ret = NF_ACCEPT;
+        int matchoff, matchlen;
+        u_int32_t ipaddr;
+        u_int16_t port;
+
+        /* No Data ? */
+        dataoff = (*pskb)->nh.iph->ihl*4 + sizeof(struct udphdr);
+        if (dataoff >= (*pskb)->len) {
+                DEBUGP("skb->len = %u\n", (*pskb)->len);
+                return NF_ACCEPT;
+        }
+
+        ip_ct_refresh(ct, *pskb, sip_timeout * HZ);
+
+        if (!skb_is_nonlinear(*pskb))
+                dptr = (*pskb)->data + dataoff;
+        else {
+                DEBUGP("Copy of skbuff not supported yet.\n");
+                goto out;
+        }
+
+        if (ip_nat_sip_hook) {
+                if (!ip_nat_sip_hook(pskb, ctinfo, ct, &dptr)) {
+                        ret = NF_DROP;
+                        goto out;
+                }
+        }
+
+        /* After this point NAT, could have mangled skb, so
+           we need to recalculate payload lenght. */
+        datalen = (*pskb)->len - dataoff;
+
+        if (datalen < (sizeof("SIP/2.0 200") - 1))
+                goto out;
+
+        /* RTP info only in some SDP pkts */
+        if (memcmp(dptr, "INVITE", sizeof("INVITE") - 1) != 0 &&
+            memcmp(dptr, "SIP/2.0 200", sizeof("SIP/2.0 200") - 1) != 0) {
+                goto out;
+        }
+        /* Get ip and port address from SDP packet. */
+        if (ct_sip_get_info(dptr, datalen, &matchoff, &matchlen,
+                            &ct_sip_hdrs[POS_CONNECTION]) > 0) {
+
+                /* We'll drop only if there are parse problems. */
+                if (parse_ipaddr(dptr + matchoff, NULL, &ipaddr,
+                                 dptr + datalen) < 0) {
+                        ret = NF_DROP;
+                        goto out;
+                }
+                if (ct_sip_get_info(dptr, datalen, &matchoff, &matchlen,
+                                    &ct_sip_hdrs[POS_MEDIA]) > 0) {
+
+                        port = simple_strtoul(dptr + matchoff, NULL, 10);
+                        if (port < 1024) {
+                                ret = NF_DROP;
+                                goto out;
+                        }
+                        ret = set_expected_rtp(pskb, ct, ctinfo,
+                                               ipaddr, port, dptr);
+                }
+        }
+out:
+        return ret;
+}
+
+static struct ip_conntrack_helper sip[MAX_PORTS];
+static char sip_names[MAX_PORTS][10];
+
+static void fini(void)
+{
+        int i;
+        for (i = 0; i < ports_c; i++) {
+                DEBUGP("unregistering helper for port %d\n", ports[i]);
+                ip_conntrack_helper_unregister(&sip[i]);
+        }
+}
+
+static int __init init(void)
+{
+        int i, ret;
+        char *tmpname;
+
+        if (ports_c == 0)
+                ports[ports_c++] = SIP_PORT;
+
+        for (i = 0; i < ports_c; i++) {
+                /* Create helper structure */
+                memset(&sip[i], 0, sizeof(struct ip_conntrack_helper));
+
+                sip[i].tuple.dst.protonum = IPPROTO_UDP;
+                sip[i].tuple.src.u.udp.port = htons(ports[i]);
+                sip[i].mask.src.u.udp.port = 0xFFFF;
+                sip[i].mask.dst.protonum = 0xFF;
+                sip[i].max_expected = 1;
+                sip[i].timeout = 3 * 60; /* 3 minutes */
+                sip[i].me = THIS_MODULE;
+                sip[i].help = sip_help;
+
+                tmpname = &sip_names[i][0];
+                if (ports[i] == SIP_PORT)
+                        sprintf(tmpname, "sip");
+                else
+                        sprintf(tmpname, "sip-%d", i);
+                sip[i].name = tmpname;
+
+                DEBUGP("port #%d: %d\n", i, ports[i]);
+
+                ret = ip_conntrack_helper_register(&sip[i]);
+                if (ret) {
+                        printk("ERROR registering helper for port %d\n",
+                                ports[i]);
+                        fini();
+                        return ret;
+                }
+        }
+        return 0;
+}
+
+module_init(init);
+module_exit(fini);
diff --git a/net/ipv4/netfilter/ip_nat_sip.c b/net/ipv4/netfilter/ip_nat_sip.c
new file mode 100644
index 000000000000..6ffba63adca2
--- /dev/null
+++ b/net/ipv4/netfilter/ip_nat_sip.c
@@ -0,0 +1,249 @@
+/* SIP extension for UDP NAT alteration.
+ *
+ * (C) 2005 by Christian Hentschel <chentschel@arnet.com.ar>
+ * based on RR's ip_nat_ftp.c and other modules.
+ *
+ * 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/ip.h>
+#include <linux/udp.h>
+
+#include <linux/netfilter_ipv4.h>
+#include <linux/netfilter_ipv4/ip_nat.h>
+#include <linux/netfilter_ipv4/ip_nat_helper.h>
+#include <linux/netfilter_ipv4/ip_conntrack_helper.h>
+#include <linux/netfilter_ipv4/ip_conntrack_sip.h>
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Christian Hentschel <chentschel@arnet.com.ar>");
+MODULE_DESCRIPTION("SIP NAT helper");
+
+#if 0
+#define DEBUGP printk
+#else
+#define DEBUGP(format, args...)
+#endif
+
+extern struct sip_header_nfo ct_sip_hdrs[];
+
+static unsigned int mangle_sip_packet(struct sk_buff **pskb,
+                                      enum ip_conntrack_info ctinfo,
+                                      struct ip_conntrack *ct,
+                                      const char **dptr, size_t dlen,
+                                      char *buffer, int bufflen,
+                                      struct sip_header_nfo *hnfo)
+{
+        unsigned int matchlen, matchoff;
+
+        if (ct_sip_get_info(*dptr, dlen, &matchoff, &matchlen, hnfo) <= 0)
+                return 0;
+
+        if (!ip_nat_mangle_udp_packet(pskb, ct, ctinfo,
+                                      matchoff, matchlen, buffer, bufflen))
+                return 0;
+
+        /* We need to reload this. Thanks Patrick. */
+        *dptr = (*pskb)->data + (*pskb)->nh.iph->ihl*4 + sizeof(struct udphdr);
+        return 1;
+}
+
+static unsigned int ip_nat_sip(struct sk_buff **pskb,
+                               enum ip_conntrack_info ctinfo,
+                               struct ip_conntrack *ct,
+                               const char **dptr)
+{
+        enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
+        char buffer[sizeof("nnn.nnn.nnn.nnn:nnnnn")];
+        unsigned int bufflen, dataoff;
+        u_int32_t ip;
+        u_int16_t port;
+
+        dataoff = (*pskb)->nh.iph->ihl*4 + sizeof(struct udphdr);
+
+        ip   = ct->tuplehash[!dir].tuple.dst.ip;
+        port = ct->tuplehash[!dir].tuple.dst.u.udp.port;
+        bufflen = sprintf(buffer, "%u.%u.%u.%u:%u", NIPQUAD(ip), ntohs(port));
+
+        /* short packet ? */
+        if (((*pskb)->len - dataoff) < (sizeof("SIP/2.0") - 1))
+                return 0;
+
+        /* Basic rules: requests and responses. */
+        if (memcmp(*dptr, "SIP/2.0", sizeof("SIP/2.0") - 1) == 0) {
+                const char *aux;
+
+                if ((ctinfo) < IP_CT_IS_REPLY) {
+                        mangle_sip_packet(pskb, ctinfo, ct, dptr,
+                                          (*pskb)->len - dataoff,
+                                          buffer, bufflen,
+                                          &ct_sip_hdrs[POS_CONTACT]);
+                        return 1;
+                }
+
+                if (!mangle_sip_packet(pskb, ctinfo, ct, dptr,
+                                       (*pskb)->len - dataoff,
+                                       buffer, bufflen, &ct_sip_hdrs[POS_VIA]))
+                        return 0;
+
+                /* This search should ignore case, but later.. */
+                aux = ct_sip_search("CSeq:", *dptr, sizeof("CSeq:") - 1,
+                                    (*pskb)->len - dataoff);
+                if (!aux)
+                        return 0;
+
+                if (!ct_sip_search("REGISTER", aux, sizeof("REGISTER"),
+                    ct_sip_lnlen(aux, *dptr + (*pskb)->len - dataoff)))
+                        return 1;
+
+                return mangle_sip_packet(pskb, ctinfo, ct, dptr,
+                                         (*pskb)->len - dataoff,
+                                         buffer, bufflen,
+                                         &ct_sip_hdrs[POS_CONTACT]);
+        }
+        if ((ctinfo) < IP_CT_IS_REPLY) {
+                if (!mangle_sip_packet(pskb, ctinfo, ct, dptr,
+                                       (*pskb)->len - dataoff,
+                                       buffer, bufflen, &ct_sip_hdrs[POS_VIA]))
+                        return 0;
+
+                /* Mangle Contact if exists only. - watch udp_nat_mangle()! */
+                mangle_sip_packet(pskb, ctinfo, ct, dptr, (*pskb)->len - dataoff,
+                                  buffer, bufflen, &ct_sip_hdrs[POS_CONTACT]);
+                return 1;
+        }
+        /* This mangle requests headers. */
+        return mangle_sip_packet(pskb, ctinfo, ct, dptr,
+                                 ct_sip_lnlen(*dptr,
+                                              *dptr + (*pskb)->len - dataoff),
+                                 buffer, bufflen, &ct_sip_hdrs[POS_REQ_HEADER]);
+}
+
+static int mangle_content_len(struct sk_buff **pskb,
+                              enum ip_conntrack_info ctinfo,
+                              struct ip_conntrack *ct,
+                              const char *dptr)
+{
+        unsigned int dataoff, matchoff, matchlen;
+        char buffer[sizeof("65536")];
+        int bufflen;
+
+        dataoff = (*pskb)->nh.iph->ihl*4 + sizeof(struct udphdr);
+
+        /* Get actual SDP lenght */
+        if (ct_sip_get_info(dptr, (*pskb)->len - dataoff, &matchoff,
+                            &matchlen, &ct_sip_hdrs[POS_SDP_HEADER]) > 0) {
+
+                /* since ct_sip_get_info() give us a pointer passing 'v='
+                   we need to add 2 bytes in this count. */
+                int c_len = (*pskb)->len - dataoff - matchoff + 2;
+
+                /* Now, update SDP lenght */
+                if (ct_sip_get_info(dptr, (*pskb)->len - dataoff, &matchoff,
+                                    &matchlen, &ct_sip_hdrs[POS_CONTENT]) > 0) {
+
+                        bufflen = sprintf(buffer, "%u", c_len);
+
+                        return ip_nat_mangle_udp_packet(pskb, ct, ctinfo,
+                                                        matchoff, matchlen,
+                                                        buffer, bufflen);
+                }
+        }
+        return 0;
+}
+
+static unsigned int mangle_sdp(struct sk_buff **pskb,
+                               enum ip_conntrack_info ctinfo,
+                               struct ip_conntrack *ct,
+                               u_int32_t newip, u_int16_t port,
+                               const char *dptr)
+{
+        char buffer[sizeof("nnn.nnn.nnn.nnn")];
+        unsigned int dataoff, bufflen;
+
+        dataoff = (*pskb)->nh.iph->ihl*4 + sizeof(struct udphdr);
+
+        /* Mangle owner and contact info. */
+        bufflen = sprintf(buffer, "%u.%u.%u.%u", NIPQUAD(newip));
+        if (!mangle_sip_packet(pskb, ctinfo, ct, &dptr, (*pskb)->len - dataoff,
+                               buffer, bufflen, &ct_sip_hdrs[POS_OWNER]))
+                return 0;
+
+        if (!mangle_sip_packet(pskb, ctinfo, ct, &dptr, (*pskb)->len - dataoff,
+                               buffer, bufflen, &ct_sip_hdrs[POS_CONNECTION]))
+                return 0;
+
+        /* Mangle media port. */
+        bufflen = sprintf(buffer, "%u", port);
+        if (!mangle_sip_packet(pskb, ctinfo, ct, &dptr, (*pskb)->len - dataoff,
+                               buffer, bufflen, &ct_sip_hdrs[POS_MEDIA]))
+                return 0;
+
+        return mangle_content_len(pskb, ctinfo, ct, dptr);
+}
+
+/* So, this packet has hit the connection tracking matching code.
+   Mangle it, and change the expectation to match the new version. */
+static unsigned int ip_nat_sdp(struct sk_buff **pskb,
+                               enum ip_conntrack_info ctinfo,
+                               struct ip_conntrack_expect *exp,
+                               const char *dptr)
+{
+        struct ip_conntrack *ct = exp->master;
+        enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
+        u_int32_t newip;
+        u_int16_t port;
+
+        DEBUGP("ip_nat_sdp():\n");
+
+        /* Connection will come from reply */
+        newip = ct->tuplehash[!dir].tuple.dst.ip;
+
+        exp->tuple.dst.ip = newip;
+        exp->saved_proto.udp.port = exp->tuple.dst.u.udp.port;
+        exp->dir = !dir;
+
+        /* When you see the packet, we need to NAT it the same as the
+           this one. */
+        exp->expectfn = ip_nat_follow_master;
+
+        /* Try to get same port: if not, try to change it. */
+        for (port = ntohs(exp->saved_proto.udp.port); port != 0; port++) {
+                exp->tuple.dst.u.udp.port = htons(port);
+                if (ip_conntrack_expect_related(exp) == 0)
+                        break;
+        }
+
+        if (port == 0)
+                return NF_DROP;
+
+        if (!mangle_sdp(pskb, ctinfo, ct, newip, port, dptr)) {
+                ip_conntrack_unexpect_related(exp);
+                return NF_DROP;
+        }
+        return NF_ACCEPT;
+}
+
+static void __exit fini(void)
+{
+        ip_nat_sip_hook = NULL;
+        ip_nat_sdp_hook = NULL;
+        /* Make sure noone calls it, meanwhile. */
+        synchronize_net();
+}
+
+static int __init init(void)
+{
+        BUG_ON(ip_nat_sip_hook);
+        BUG_ON(ip_nat_sdp_hook);
+        ip_nat_sip_hook = ip_nat_sip;
+        ip_nat_sdp_hook = ip_nat_sdp;
+        return 0;
+}
+
+module_init(init);
+module_exit(fini);