1/* 2 * Copyright (c) 2013 Patrick McHardy <kaber@trash.net> 3 * 4 * This program is free software; you can redistribute it and/or modify 5 * it under the terms of the GNU General Public License version 2 as 6 * published by the Free Software Foundation. 7 */ 8 9#include <linux/module.h> 10#include <linux/skbuff.h> 11#include <net/tcp.h> 12 13#include <linux/netfilter_ipv4/ip_tables.h> 14#include <linux/netfilter/x_tables.h> 15#include <linux/netfilter/xt_SYNPROXY.h> 16#include <net/netfilter/nf_conntrack.h> 17#include <net/netfilter/nf_conntrack_seqadj.h> 18#include <net/netfilter/nf_conntrack_synproxy.h> 19 20static struct iphdr * 21synproxy_build_ip(struct sk_buff *skb, u32 saddr, u32 daddr) 22{ 23 struct iphdr *iph; 24 25 skb_reset_network_header(skb); 26 iph = (struct iphdr *)skb_put(skb, sizeof(*iph)); 27 iph->version = 4; 28 iph->ihl = sizeof(*iph) / 4; 29 iph->tos = 0; 30 iph->id = 0; 31 iph->frag_off = htons(IP_DF); 32 iph->ttl = sysctl_ip_default_ttl; 33 iph->protocol = IPPROTO_TCP; 34 iph->check = 0; 35 iph->saddr = saddr; 36 iph->daddr = daddr; 37 38 return iph; 39} 40 41static void 42synproxy_send_tcp(const struct sk_buff *skb, struct sk_buff *nskb, 43 struct nf_conntrack *nfct, enum ip_conntrack_info ctinfo, 44 struct iphdr *niph, struct tcphdr *nth, 45 unsigned int tcp_hdr_size) 46{ 47 nth->check = ~tcp_v4_check(tcp_hdr_size, niph->saddr, niph->daddr, 0); 48 nskb->ip_summed = CHECKSUM_PARTIAL; 49 nskb->csum_start = (unsigned char *)nth - nskb->head; 50 nskb->csum_offset = offsetof(struct tcphdr, check); 51 52 skb_dst_set_noref(nskb, skb_dst(skb)); 53 nskb->protocol = htons(ETH_P_IP); 54 if (ip_route_me_harder(nskb, RTN_UNSPEC)) 55 goto free_nskb; 56 57 if (nfct) { 58 nskb->nfct = nfct; 59 nskb->nfctinfo = ctinfo; 60 nf_conntrack_get(nfct); 61 } 62 63 ip_local_out(nskb); 64 return; 65 66free_nskb: 67 kfree_skb(nskb); 68} 69 70static void 71synproxy_send_client_synack(const struct sk_buff *skb, const struct tcphdr *th, 72 const struct synproxy_options *opts) 73{ 74 struct sk_buff *nskb; 75 struct iphdr *iph, *niph; 76 struct tcphdr *nth; 77 unsigned int tcp_hdr_size; 78 u16 mss = opts->mss; 79 80 iph = ip_hdr(skb); 81 82 tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts); 83 nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER, 84 GFP_ATOMIC); 85 if (nskb == NULL) 86 return; 87 skb_reserve(nskb, MAX_TCP_HEADER); 88 89 niph = synproxy_build_ip(nskb, iph->daddr, iph->saddr); 90 91 skb_reset_transport_header(nskb); 92 nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size); 93 nth->source = th->dest; 94 nth->dest = th->source; 95 nth->seq = htonl(__cookie_v4_init_sequence(iph, th, &mss)); 96 nth->ack_seq = htonl(ntohl(th->seq) + 1); 97 tcp_flag_word(nth) = TCP_FLAG_SYN | TCP_FLAG_ACK; 98 if (opts->options & XT_SYNPROXY_OPT_ECN) 99 tcp_flag_word(nth) |= TCP_FLAG_ECE; 100 nth->doff = tcp_hdr_size / 4; 101 nth->window = 0; 102 nth->check = 0; 103 nth->urg_ptr = 0; 104 105 synproxy_build_options(nth, opts); 106 107 synproxy_send_tcp(skb, nskb, skb->nfct, IP_CT_ESTABLISHED_REPLY, 108 niph, nth, tcp_hdr_size); 109} 110 111static void 112synproxy_send_server_syn(const struct synproxy_net *snet, 113 const struct sk_buff *skb, const struct tcphdr *th, 114 const struct synproxy_options *opts, u32 recv_seq) 115{ 116 struct sk_buff *nskb; 117 struct iphdr *iph, *niph; 118 struct tcphdr *nth; 119 unsigned int tcp_hdr_size; 120 121 iph = ip_hdr(skb); 122 123 tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts); 124 nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER, 125 GFP_ATOMIC); 126 if (nskb == NULL) 127 return; 128 skb_reserve(nskb, MAX_TCP_HEADER); 129 130 niph = synproxy_build_ip(nskb, iph->saddr, iph->daddr); 131 132 skb_reset_transport_header(nskb); 133 nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size); 134 nth->source = th->source; 135 nth->dest = th->dest; 136 nth->seq = htonl(recv_seq - 1); 137 /* ack_seq is used to relay our ISN to the synproxy hook to initialize 138 * sequence number translation once a connection tracking entry exists. 139 */ 140 nth->ack_seq = htonl(ntohl(th->ack_seq) - 1); 141 tcp_flag_word(nth) = TCP_FLAG_SYN; 142 if (opts->options & XT_SYNPROXY_OPT_ECN) 143 tcp_flag_word(nth) |= TCP_FLAG_ECE | TCP_FLAG_CWR; 144 nth->doff = tcp_hdr_size / 4; 145 nth->window = th->window; 146 nth->check = 0; 147 nth->urg_ptr = 0; 148 149 synproxy_build_options(nth, opts); 150 151 synproxy_send_tcp(skb, nskb, &snet->tmpl->ct_general, IP_CT_NEW, 152 niph, nth, tcp_hdr_size); 153} 154 155static void 156synproxy_send_server_ack(const struct synproxy_net *snet, 157 const struct ip_ct_tcp *state, 158 const struct sk_buff *skb, const struct tcphdr *th, 159 const struct synproxy_options *opts) 160{ 161 struct sk_buff *nskb; 162 struct iphdr *iph, *niph; 163 struct tcphdr *nth; 164 unsigned int tcp_hdr_size; 165 166 iph = ip_hdr(skb); 167 168 tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts); 169 nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER, 170 GFP_ATOMIC); 171 if (nskb == NULL) 172 return; 173 skb_reserve(nskb, MAX_TCP_HEADER); 174 175 niph = synproxy_build_ip(nskb, iph->daddr, iph->saddr); 176 177 skb_reset_transport_header(nskb); 178 nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size); 179 nth->source = th->dest; 180 nth->dest = th->source; 181 nth->seq = htonl(ntohl(th->ack_seq)); 182 nth->ack_seq = htonl(ntohl(th->seq) + 1); 183 tcp_flag_word(nth) = TCP_FLAG_ACK; 184 nth->doff = tcp_hdr_size / 4; 185 nth->window = htons(state->seen[IP_CT_DIR_ORIGINAL].td_maxwin); 186 nth->check = 0; 187 nth->urg_ptr = 0; 188 189 synproxy_build_options(nth, opts); 190 191 synproxy_send_tcp(skb, nskb, NULL, 0, niph, nth, tcp_hdr_size); 192} 193 194static void 195synproxy_send_client_ack(const struct synproxy_net *snet, 196 const struct sk_buff *skb, const struct tcphdr *th, 197 const struct synproxy_options *opts) 198{ 199 struct sk_buff *nskb; 200 struct iphdr *iph, *niph; 201 struct tcphdr *nth; 202 unsigned int tcp_hdr_size; 203 204 iph = ip_hdr(skb); 205 206 tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts); 207 nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER, 208 GFP_ATOMIC); 209 if (nskb == NULL) 210 return; 211 skb_reserve(nskb, MAX_TCP_HEADER); 212 213 niph = synproxy_build_ip(nskb, iph->saddr, iph->daddr); 214 215 skb_reset_transport_header(nskb); 216 nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size); 217 nth->source = th->source; 218 nth->dest = th->dest; 219 nth->seq = htonl(ntohl(th->seq) + 1); 220 nth->ack_seq = th->ack_seq; 221 tcp_flag_word(nth) = TCP_FLAG_ACK; 222 nth->doff = tcp_hdr_size / 4; 223 nth->window = ntohs(htons(th->window) >> opts->wscale); 224 nth->check = 0; 225 nth->urg_ptr = 0; 226 227 synproxy_build_options(nth, opts); 228 229 synproxy_send_tcp(skb, nskb, NULL, 0, niph, nth, tcp_hdr_size); 230} 231 232static bool 233synproxy_recv_client_ack(const struct synproxy_net *snet, 234 const struct sk_buff *skb, const struct tcphdr *th, 235 struct synproxy_options *opts, u32 recv_seq) 236{ 237 int mss; 238 239 mss = __cookie_v4_check(ip_hdr(skb), th, ntohl(th->ack_seq) - 1); 240 if (mss == 0) { 241 this_cpu_inc(snet->stats->cookie_invalid); 242 return false; 243 } 244 245 this_cpu_inc(snet->stats->cookie_valid); 246 opts->mss = mss; 247 opts->options |= XT_SYNPROXY_OPT_MSS; 248 249 if (opts->options & XT_SYNPROXY_OPT_TIMESTAMP) 250 synproxy_check_timestamp_cookie(opts); 251 252 synproxy_send_server_syn(snet, skb, th, opts, recv_seq); 253 return true; 254} 255 256static unsigned int 257synproxy_tg4(struct sk_buff *skb, const struct xt_action_param *par) 258{ 259 const struct xt_synproxy_info *info = par->targinfo; 260 struct synproxy_net *snet = synproxy_pernet(dev_net(par->in)); 261 struct synproxy_options opts = {}; 262 struct tcphdr *th, _th; 263 264 if (nf_ip_checksum(skb, par->hooknum, par->thoff, IPPROTO_TCP)) 265 return NF_DROP; 266 267 th = skb_header_pointer(skb, par->thoff, sizeof(_th), &_th); 268 if (th == NULL) 269 return NF_DROP; 270 271 if (!synproxy_parse_options(skb, par->thoff, th, &opts)) 272 return NF_DROP; 273 274 if (th->syn && !(th->ack || th->fin || th->rst)) { 275 /* Initial SYN from client */ 276 this_cpu_inc(snet->stats->syn_received); 277 278 if (th->ece && th->cwr) 279 opts.options |= XT_SYNPROXY_OPT_ECN; 280 281 opts.options &= info->options; 282 if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP) 283 synproxy_init_timestamp_cookie(info, &opts); 284 else 285 opts.options &= ~(XT_SYNPROXY_OPT_WSCALE | 286 XT_SYNPROXY_OPT_SACK_PERM | 287 XT_SYNPROXY_OPT_ECN); 288 289 synproxy_send_client_synack(skb, th, &opts); 290 return NF_DROP; 291 292 } else if (th->ack && !(th->fin || th->rst || th->syn)) { 293 /* ACK from client */ 294 synproxy_recv_client_ack(snet, skb, th, &opts, ntohl(th->seq)); 295 return NF_DROP; 296 } 297 298 return XT_CONTINUE; 299} 300 301static unsigned int ipv4_synproxy_hook(const struct nf_hook_ops *ops, 302 struct sk_buff *skb, 303 const struct net_device *in, 304 const struct net_device *out, 305 int (*okfn)(struct sk_buff *)) 306{ 307 struct synproxy_net *snet = synproxy_pernet(dev_net(in ? : out)); 308 enum ip_conntrack_info ctinfo; 309 struct nf_conn *ct; 310 struct nf_conn_synproxy *synproxy; 311 struct synproxy_options opts = {}; 312 const struct ip_ct_tcp *state; 313 struct tcphdr *th, _th; 314 unsigned int thoff; 315 316 ct = nf_ct_get(skb, &ctinfo); 317 if (ct == NULL) 318 return NF_ACCEPT; 319 320 synproxy = nfct_synproxy(ct); 321 if (synproxy == NULL) 322 return NF_ACCEPT; 323 324 if (nf_is_loopback_packet(skb)) 325 return NF_ACCEPT; 326 327 thoff = ip_hdrlen(skb); 328 th = skb_header_pointer(skb, thoff, sizeof(_th), &_th); 329 if (th == NULL) 330 return NF_DROP; 331 332 state = &ct->proto.tcp; 333 switch (state->state) { 334 case TCP_CONNTRACK_CLOSE: 335 if (th->rst && !test_bit(IPS_SEEN_REPLY_BIT, &ct->status)) { 336 nf_ct_seqadj_init(ct, ctinfo, synproxy->isn - 337 ntohl(th->seq) + 1); 338 break; 339 } 340 341 if (!th->syn || th->ack || 342 CTINFO2DIR(ctinfo) != IP_CT_DIR_ORIGINAL) 343 break; 344 345 /* Reopened connection - reset the sequence number and timestamp 346 * adjustments, they will get initialized once the connection is 347 * reestablished. 348 */ 349 nf_ct_seqadj_init(ct, ctinfo, 0); 350 synproxy->tsoff = 0; 351 this_cpu_inc(snet->stats->conn_reopened); 352 353 /* fall through */ 354 case TCP_CONNTRACK_SYN_SENT: 355 if (!synproxy_parse_options(skb, thoff, th, &opts)) 356 return NF_DROP; 357 358 if (!th->syn && th->ack && 359 CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL) { 360 /* Keep-Alives are sent with SEG.SEQ = SND.NXT-1, 361 * therefore we need to add 1 to make the SYN sequence 362 * number match the one of first SYN. 363 */ 364 if (synproxy_recv_client_ack(snet, skb, th, &opts, 365 ntohl(th->seq) + 1)) 366 this_cpu_inc(snet->stats->cookie_retrans); 367 368 return NF_DROP; 369 } 370 371 synproxy->isn = ntohl(th->ack_seq); 372 if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP) 373 synproxy->its = opts.tsecr; 374 break; 375 case TCP_CONNTRACK_SYN_RECV: 376 if (!th->syn || !th->ack) 377 break; 378 379 if (!synproxy_parse_options(skb, thoff, th, &opts)) 380 return NF_DROP; 381 382 if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP) 383 synproxy->tsoff = opts.tsval - synproxy->its; 384 385 opts.options &= ~(XT_SYNPROXY_OPT_MSS | 386 XT_SYNPROXY_OPT_WSCALE | 387 XT_SYNPROXY_OPT_SACK_PERM); 388 389 swap(opts.tsval, opts.tsecr); 390 synproxy_send_server_ack(snet, state, skb, th, &opts); 391 392 nf_ct_seqadj_init(ct, ctinfo, synproxy->isn - ntohl(th->seq)); 393 394 swap(opts.tsval, opts.tsecr); 395 synproxy_send_client_ack(snet, skb, th, &opts); 396 397 consume_skb(skb); 398 return NF_STOLEN; 399 default: 400 break; 401 } 402 403 synproxy_tstamp_adjust(skb, thoff, th, ct, ctinfo, synproxy); 404 return NF_ACCEPT; 405} 406 407static int synproxy_tg4_check(const struct xt_tgchk_param *par) 408{ 409 const struct ipt_entry *e = par->entryinfo; 410 411 if (e->ip.proto != IPPROTO_TCP || 412 e->ip.invflags & XT_INV_PROTO) 413 return -EINVAL; 414 415 return nf_ct_l3proto_try_module_get(par->family); 416} 417 418static void synproxy_tg4_destroy(const struct xt_tgdtor_param *par) 419{ 420 nf_ct_l3proto_module_put(par->family); 421} 422 423static struct xt_target synproxy_tg4_reg __read_mostly = { 424 .name = "SYNPROXY", 425 .family = NFPROTO_IPV4, 426 .hooks = (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD), 427 .target = synproxy_tg4, 428 .targetsize = sizeof(struct xt_synproxy_info), 429 .checkentry = synproxy_tg4_check, 430 .destroy = synproxy_tg4_destroy, 431 .me = THIS_MODULE, 432}; 433 434static struct nf_hook_ops ipv4_synproxy_ops[] __read_mostly = { 435 { 436 .hook = ipv4_synproxy_hook, 437 .owner = THIS_MODULE, 438 .pf = NFPROTO_IPV4, 439 .hooknum = NF_INET_LOCAL_IN, 440 .priority = NF_IP_PRI_CONNTRACK_CONFIRM - 1, 441 }, 442 { 443 .hook = ipv4_synproxy_hook, 444 .owner = THIS_MODULE, 445 .pf = NFPROTO_IPV4, 446 .hooknum = NF_INET_POST_ROUTING, 447 .priority = NF_IP_PRI_CONNTRACK_CONFIRM - 1, 448 }, 449}; 450 451static int __init synproxy_tg4_init(void) 452{ 453 int err; 454 455 err = nf_register_hooks(ipv4_synproxy_ops, 456 ARRAY_SIZE(ipv4_synproxy_ops)); 457 if (err < 0) 458 goto err1; 459 460 err = xt_register_target(&synproxy_tg4_reg); 461 if (err < 0) 462 goto err2; 463 464 return 0; 465 466err2: 467 nf_unregister_hooks(ipv4_synproxy_ops, ARRAY_SIZE(ipv4_synproxy_ops)); 468err1: 469 return err; 470} 471 472static void __exit synproxy_tg4_exit(void) 473{ 474 xt_unregister_target(&synproxy_tg4_reg); 475 nf_unregister_hooks(ipv4_synproxy_ops, ARRAY_SIZE(ipv4_synproxy_ops)); 476} 477 478module_init(synproxy_tg4_init); 479module_exit(synproxy_tg4_exit); 480 481MODULE_LICENSE("GPL"); 482MODULE_AUTHOR("Patrick McHardy <kaber@trash.net>"); 483