nf_nat_helper.c

  1// SPDX-License-Identifier: GPL-2.0-only
  2/* nf_nat_helper.c - generic support functions for NAT helpers
  3 *
  4 * (C) 2000-2002 Harald Welte <laforge@netfilter.org>
  5 * (C) 2003-2006 Netfilter Core Team <coreteam@netfilter.org>
  6 * (C) 2007-2012 Patrick McHardy <kaber@trash.net>
  7 */
  8#include <linux/module.h>
  9#include <linux/gfp.h>
 10#include <linux/types.h>
 11#include <linux/skbuff.h>
 12#include <linux/tcp.h>
 13#include <linux/udp.h>
 14#include <net/tcp.h>
 15
 16#include <net/netfilter/nf_conntrack.h>
 17#include <net/netfilter/nf_conntrack_helper.h>
 18#include <net/netfilter/nf_conntrack_ecache.h>
 19#include <net/netfilter/nf_conntrack_expect.h>
 20#include <net/netfilter/nf_conntrack_seqadj.h>
 21#include <net/netfilter/nf_nat.h>
 22#include <net/netfilter/nf_nat_helper.h>
 23
 24/* Frobs data inside this packet, which is linear. */
 25static void mangle_contents(struct sk_buff *skb,
 26			    unsigned int dataoff,
 27			    unsigned int match_offset,
 28			    unsigned int match_len,
 29			    const char *rep_buffer,
 30			    unsigned int rep_len)
 31{
 32	unsigned char *data;
 33
 34	SKB_LINEAR_ASSERT(skb);
 35	data = skb_network_header(skb) + dataoff;
 36
 37	/* move post-replacement */
 38	memmove(data + match_offset + rep_len,
 39		data + match_offset + match_len,
 40		skb_tail_pointer(skb) - (skb_network_header(skb) + dataoff +
 41			     match_offset + match_len));
 42
 43	/* insert data from buffer */
 44	memcpy(data + match_offset, rep_buffer, rep_len);
 45
 46	/* update skb info */
 47	if (rep_len > match_len) {
 48		pr_debug("nf_nat_mangle_packet: Extending packet by "
 49			 "%u from %u bytes\n", rep_len - match_len, skb->len);
 50		skb_put(skb, rep_len - match_len);
 51	} else {
 52		pr_debug("nf_nat_mangle_packet: Shrinking packet from "
 53			 "%u from %u bytes\n", match_len - rep_len, skb->len);
 54		__skb_trim(skb, skb->len + rep_len - match_len);
 55	}
 56
 57	if (nf_ct_l3num((struct nf_conn *)skb_nfct(skb)) == NFPROTO_IPV4) {
 58		/* fix IP hdr checksum information */
 59		ip_hdr(skb)->tot_len = htons(skb->len);
 60		ip_send_check(ip_hdr(skb));
 61	} else
 62		ipv6_hdr(skb)->payload_len =
 63			htons(skb->len - sizeof(struct ipv6hdr));
 64}
 65
 66/* Unusual, but possible case. */
 67static bool enlarge_skb(struct sk_buff *skb, unsigned int extra)
 68{
 69	if (skb->len + extra > 65535)
 70		return false;
 71
 72	if (pskb_expand_head(skb, 0, extra - skb_tailroom(skb), GFP_ATOMIC))
 73		return false;
 74
 75	return true;
 76}
 77
 78/* Generic function for mangling variable-length address changes inside
 79 * NATed TCP connections (like the PORT XXX,XXX,XXX,XXX,XXX,XXX
 80 * command in FTP).
 81 *
 82 * Takes care about all the nasty sequence number changes, checksumming,
 83 * skb enlargement, ...
 84 *
 85 * */
 86bool __nf_nat_mangle_tcp_packet(struct sk_buff *skb,
 87				struct nf_conn *ct,
 88				enum ip_conntrack_info ctinfo,
 89				unsigned int protoff,
 90				unsigned int match_offset,
 91				unsigned int match_len,
 92				const char *rep_buffer,
 93				unsigned int rep_len, bool adjust)
 94{
 95	struct tcphdr *tcph;
 96	int oldlen, datalen;
 97
 98	if (skb_ensure_writable(skb, skb->len))
 99		return false;
100
101	if (rep_len > match_len &&
102	    rep_len - match_len > skb_tailroom(skb) &&
103	    !enlarge_skb(skb, rep_len - match_len))
104		return false;
105
106	tcph = (void *)skb->data + protoff;
107
108	oldlen = skb->len - protoff;
109	mangle_contents(skb, protoff + tcph->doff*4,
110			match_offset, match_len, rep_buffer, rep_len);
111
112	datalen = skb->len - protoff;
113
114	nf_nat_csum_recalc(skb, nf_ct_l3num(ct), IPPROTO_TCP,
115			   tcph, &tcph->check, datalen, oldlen);
116
117	if (adjust && rep_len != match_len)
118		nf_ct_seqadj_set(ct, ctinfo, tcph->seq,
119				 (int)rep_len - (int)match_len);
120
121	return true;
122}
123EXPORT_SYMBOL(__nf_nat_mangle_tcp_packet);
124
125/* Generic function for mangling variable-length address changes inside
126 * NATed UDP connections (like the CONNECT DATA XXXXX MESG XXXXX INDEX XXXXX
127 * command in the Amanda protocol)
128 *
129 * Takes care about all the nasty sequence number changes, checksumming,
130 * skb enlargement, ...
131 *
132 * XXX - This function could be merged with nf_nat_mangle_tcp_packet which
133 *       should be fairly easy to do.
134 */
135bool
136nf_nat_mangle_udp_packet(struct sk_buff *skb,
137			 struct nf_conn *ct,
138			 enum ip_conntrack_info ctinfo,
139			 unsigned int protoff,
140			 unsigned int match_offset,
141			 unsigned int match_len,
142			 const char *rep_buffer,
143			 unsigned int rep_len)
144{
145	struct udphdr *udph;
146	int datalen, oldlen;
147
148	if (skb_ensure_writable(skb, skb->len))
149		return false;
150
151	if (rep_len > match_len &&
152	    rep_len - match_len > skb_tailroom(skb) &&
153	    !enlarge_skb(skb, rep_len - match_len))
154		return false;
155
156	udph = (void *)skb->data + protoff;
157
158	oldlen = skb->len - protoff;
159	mangle_contents(skb, protoff + sizeof(*udph),
160			match_offset, match_len, rep_buffer, rep_len);
161
162	/* update the length of the UDP packet */
163	datalen = skb->len - protoff;
164	udph->len = htons(datalen);
165
166	/* fix udp checksum if udp checksum was previously calculated */
167	if (!udph->check && skb->ip_summed != CHECKSUM_PARTIAL)
168		return true;
169
170	nf_nat_csum_recalc(skb, nf_ct_l3num(ct), IPPROTO_UDP,
171			   udph, &udph->check, datalen, oldlen);
172
173	return true;
174}
175EXPORT_SYMBOL(nf_nat_mangle_udp_packet);
176
177/* Setup NAT on this expected conntrack so it follows master. */
178/* If we fail to get a free NAT slot, we'll get dropped on confirm */
179void nf_nat_follow_master(struct nf_conn *ct,
180			  struct nf_conntrack_expect *exp)
181{
182	struct nf_nat_range2 range;
183
184	/* This must be a fresh one. */
185	BUG_ON(ct->status & IPS_NAT_DONE_MASK);
186
187	/* Change src to where master sends to */
188	range.flags = NF_NAT_RANGE_MAP_IPS;
189	range.min_addr = range.max_addr
190		= ct->master->tuplehash[!exp->dir].tuple.dst.u3;
191	nf_nat_setup_info(ct, &range, NF_NAT_MANIP_SRC);
192
193	/* For DST manip, map port here to where it's expected. */
194	range.flags = (NF_NAT_RANGE_MAP_IPS | NF_NAT_RANGE_PROTO_SPECIFIED);
195	range.min_proto = range.max_proto = exp->saved_proto;
196	range.min_addr = range.max_addr
197		= ct->master->tuplehash[!exp->dir].tuple.src.u3;
198	nf_nat_setup_info(ct, &range, NF_NAT_MANIP_DST);
199}
200EXPORT_SYMBOL(nf_nat_follow_master);
201
202u16 nf_nat_exp_find_port(struct nf_conntrack_expect *exp, u16 port)
203{
204	static const unsigned int max_attempts = 128;
205	int range, attempts_left;
206	u16 min = port;
207
208	range = USHRT_MAX - port;
209	attempts_left = range;
210
211	if (attempts_left > max_attempts)
212		attempts_left = max_attempts;
213
214	/* Try to get same port: if not, try to change it. */
215	for (;;) {
216		int res;
217
218		exp->tuple.dst.u.tcp.port = htons(port);
219		res = nf_ct_expect_related(exp, 0);
220		if (res == 0)
221			return port;
222
223		if (res != -EBUSY || (--attempts_left < 0))
224			break;
225
226		port = min + get_random_u32_below(range);
227	}
228
229	return 0;
230}
231EXPORT_SYMBOL_GPL(nf_nat_exp_find_port);