// SPDX-License-Identifier: GPL-2.0-or-later

  /* Amanda extension for TCP NAT alteration.
   * (C) 2002 by Brian J. Murrell <netfilter@interlinx.bc.ca>
   * based on a copy of HW's ip_nat_irc.c as well as other modules

   * (C) 2006-2012 Patrick McHardy <kaber@trash.net>

   */
  
  #include <linux/kernel.h>
  #include <linux/module.h>
  #include <linux/skbuff.h>
  #include <linux/udp.h>

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

  #include <net/netfilter/nf_nat_helper.h>

  #include <linux/netfilter/nf_conntrack_amanda.h>

  #define NAT_HELPER_NAME "amanda"

  MODULE_AUTHOR("Brian J. Murrell <netfilter@interlinx.bc.ca>");
  MODULE_DESCRIPTION("Amanda NAT helper");
  MODULE_LICENSE("GPL");

  MODULE_ALIAS_NF_NAT_HELPER(NAT_HELPER_NAME);
  
  static struct nf_conntrack_nat_helper nat_helper_amanda =
  	NF_CT_NAT_HELPER_INIT(NAT_HELPER_NAME);

  static unsigned int help(struct sk_buff *skb,

  			 enum ip_conntrack_info ctinfo,

  			 unsigned int protoff,

  			 unsigned int matchoff,
  			 unsigned int matchlen,
  			 struct nf_conntrack_expect *exp)
  {
  	char buffer[sizeof("65535")];
  	u_int16_t port;

  
  	/* Connection comes from client. */
  	exp->saved_proto.tcp.port = exp->tuple.dst.u.tcp.port;
  	exp->dir = IP_CT_DIR_ORIGINAL;
  
  	/* When you see the packet, we need to NAT it the same as the
  	 * this one (ie. same IP: it will be TCP and master is UDP). */
  	exp->expectfn = nf_nat_follow_master;
  
  	/* Try to get same port: if not, try to change it. */
  	for (port = ntohs(exp->saved_proto.tcp.port); port != 0; port++) {

  		int res;

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

  		res = nf_ct_expect_related(exp, 0);

  		if (res == 0)

  			break;

  		else if (res != -EBUSY) {

  			port = 0;

  			break;

  		}

  	}

  	if (port == 0) {
  		nf_ct_helper_log(skb, exp->master, "all ports in use");

  		return NF_DROP;

  	}

  
  	sprintf(buffer, "%u", port);

  	if (!nf_nat_mangle_udp_packet(skb, exp->master, ctinfo,
  				      protoff, matchoff, matchlen,
  				      buffer, strlen(buffer))) {

  		nf_ct_helper_log(skb, exp->master, "cannot mangle packet");

  		nf_ct_unexpect_related(exp);

  		return NF_DROP;

  	}

  	return NF_ACCEPT;

  }
  
  static void __exit nf_nat_amanda_fini(void)
  {

  	nf_nat_helper_unregister(&nat_helper_amanda);

  	RCU_INIT_POINTER(nf_nat_amanda_hook, NULL);

  	synchronize_rcu();
  }
  
  static int __init nf_nat_amanda_init(void)
  {

  	BUG_ON(nf_nat_amanda_hook != NULL);

  	nf_nat_helper_register(&nat_helper_amanda);

  	RCU_INIT_POINTER(nf_nat_amanda_hook, help);

  	return 0;
  }
  
  module_init(nf_nat_amanda_init);
  module_exit(nf_nat_amanda_fini);