1 /* 2 * This is a module which is used for setting the MSS option in TCP packets. 3 * 4 * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca> 5 * Copyright (C) 2007 Patrick McHardy <kaber@trash.net> 6 * 7 * This program is free software; you can redistribute it and/or modify 8 * it under the terms of the GNU General Public License version 2 as 9 * published by the Free Software Foundation. 10 */ 11 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt 12 #include <linux/module.h> 13 #include <linux/skbuff.h> 14 #include <linux/ip.h> 15 #include <linux/gfp.h> 16 #include <linux/ipv6.h> 17 #include <linux/tcp.h> 18 #include <net/dst.h> 19 #include <net/flow.h> 20 #include <net/ipv6.h> 21 #include <net/route.h> 22 #include <net/tcp.h> 23 24 #include <linux/netfilter_ipv4/ip_tables.h> 25 #include <linux/netfilter_ipv6/ip6_tables.h> 26 #include <linux/netfilter/x_tables.h> 27 #include <linux/netfilter/xt_tcpudp.h> 28 #include <linux/netfilter/xt_TCPMSS.h> 29 30 MODULE_LICENSE("GPL"); 31 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>"); 32 MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment"); 33 MODULE_ALIAS("ipt_TCPMSS"); 34 MODULE_ALIAS("ip6t_TCPMSS"); 35 36 static inline unsigned int 37 optlen(const u_int8_t *opt, unsigned int offset) 38 { 39 /* Beware zero-length options: make finite progress */ 40 if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0) 41 return 1; 42 else 43 return opt[offset+1]; 44 } 45 46 static u_int32_t tcpmss_reverse_mtu(struct net *net, 47 const struct sk_buff *skb, 48 unsigned int family) 49 { 50 struct flowi fl; 51 const struct nf_afinfo *ai; 52 struct rtable *rt = NULL; 53 u_int32_t mtu = ~0U; 54 55 if (family == PF_INET) { 56 struct flowi4 *fl4 = &fl.u.ip4; 57 memset(fl4, 0, sizeof(*fl4)); 58 fl4->daddr = ip_hdr(skb)->saddr; 59 } else { 60 struct flowi6 *fl6 = &fl.u.ip6; 61 62 memset(fl6, 0, sizeof(*fl6)); 63 fl6->daddr = ipv6_hdr(skb)->saddr; 64 } 65 ai = nf_get_afinfo(family); 66 if (ai != NULL) 67 ai->route(net, (struct dst_entry **)&rt, &fl, false); 68 69 if (rt != NULL) { 70 mtu = dst_mtu(&rt->dst); 71 dst_release(&rt->dst); 72 } 73 return mtu; 74 } 75 76 static int 77 tcpmss_mangle_packet(struct sk_buff *skb, 78 const struct xt_action_param *par, 79 unsigned int family, 80 unsigned int tcphoff, 81 unsigned int minlen) 82 { 83 const struct xt_tcpmss_info *info = par->targinfo; 84 struct tcphdr *tcph; 85 int len, tcp_hdrlen; 86 unsigned int i; 87 __be16 oldval; 88 u16 newmss; 89 u8 *opt; 90 91 /* This is a fragment, no TCP header is available */ 92 if (par->fragoff != 0) 93 return 0; 94 95 if (!skb_make_writable(skb, skb->len)) 96 return -1; 97 98 len = skb->len - tcphoff; 99 if (len < (int)sizeof(struct tcphdr)) 100 return -1; 101 102 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff); 103 tcp_hdrlen = tcph->doff * 4; 104 105 if (len < tcp_hdrlen || tcp_hdrlen < sizeof(struct tcphdr)) 106 return -1; 107 108 if (info->mss == XT_TCPMSS_CLAMP_PMTU) { 109 struct net *net = xt_net(par); 110 unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family); 111 unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu); 112 113 if (min_mtu <= minlen) { 114 net_err_ratelimited("unknown or invalid path-MTU (%u)\n", 115 min_mtu); 116 return -1; 117 } 118 newmss = min_mtu - minlen; 119 } else 120 newmss = info->mss; 121 122 opt = (u_int8_t *)tcph; 123 for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) { 124 if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) { 125 u_int16_t oldmss; 126 127 oldmss = (opt[i+2] << 8) | opt[i+3]; 128 129 /* Never increase MSS, even when setting it, as 130 * doing so results in problems for hosts that rely 131 * on MSS being set correctly. 132 */ 133 if (oldmss <= newmss) 134 return 0; 135 136 opt[i+2] = (newmss & 0xff00) >> 8; 137 opt[i+3] = newmss & 0x00ff; 138 139 inet_proto_csum_replace2(&tcph->check, skb, 140 htons(oldmss), htons(newmss), 141 false); 142 return 0; 143 } 144 } 145 146 /* There is data after the header so the option can't be added 147 * without moving it, and doing so may make the SYN packet 148 * itself too large. Accept the packet unmodified instead. 149 */ 150 if (len > tcp_hdrlen) 151 return 0; 152 153 /* tcph->doff has 4 bits, do not wrap it to 0 */ 154 if (tcp_hdrlen >= 15 * 4) 155 return 0; 156 157 /* 158 * MSS Option not found ?! add it.. 159 */ 160 if (skb_tailroom(skb) < TCPOLEN_MSS) { 161 if (pskb_expand_head(skb, 0, 162 TCPOLEN_MSS - skb_tailroom(skb), 163 GFP_ATOMIC)) 164 return -1; 165 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff); 166 } 167 168 skb_put(skb, TCPOLEN_MSS); 169 170 /* 171 * IPv4: RFC 1122 states "If an MSS option is not received at 172 * connection setup, TCP MUST assume a default send MSS of 536". 173 * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum 174 * length IPv6 header of 60, ergo the default MSS value is 1220 175 * Since no MSS was provided, we must use the default values 176 */ 177 if (xt_family(par) == NFPROTO_IPV4) 178 newmss = min(newmss, (u16)536); 179 else 180 newmss = min(newmss, (u16)1220); 181 182 opt = (u_int8_t *)tcph + sizeof(struct tcphdr); 183 memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr)); 184 185 inet_proto_csum_replace2(&tcph->check, skb, 186 htons(len), htons(len + TCPOLEN_MSS), true); 187 opt[0] = TCPOPT_MSS; 188 opt[1] = TCPOLEN_MSS; 189 opt[2] = (newmss & 0xff00) >> 8; 190 opt[3] = newmss & 0x00ff; 191 192 inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), false); 193 194 oldval = ((__be16 *)tcph)[6]; 195 tcph->doff += TCPOLEN_MSS/4; 196 inet_proto_csum_replace2(&tcph->check, skb, 197 oldval, ((__be16 *)tcph)[6], false); 198 return TCPOLEN_MSS; 199 } 200 201 static unsigned int 202 tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par) 203 { 204 struct iphdr *iph = ip_hdr(skb); 205 __be16 newlen; 206 int ret; 207 208 ret = tcpmss_mangle_packet(skb, par, 209 PF_INET, 210 iph->ihl * 4, 211 sizeof(*iph) + sizeof(struct tcphdr)); 212 if (ret < 0) 213 return NF_DROP; 214 if (ret > 0) { 215 iph = ip_hdr(skb); 216 newlen = htons(ntohs(iph->tot_len) + ret); 217 csum_replace2(&iph->check, iph->tot_len, newlen); 218 iph->tot_len = newlen; 219 } 220 return XT_CONTINUE; 221 } 222 223 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES) 224 static unsigned int 225 tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par) 226 { 227 struct ipv6hdr *ipv6h = ipv6_hdr(skb); 228 u8 nexthdr; 229 __be16 frag_off, oldlen, newlen; 230 int tcphoff; 231 int ret; 232 233 nexthdr = ipv6h->nexthdr; 234 tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off); 235 if (tcphoff < 0) 236 return NF_DROP; 237 ret = tcpmss_mangle_packet(skb, par, 238 PF_INET6, 239 tcphoff, 240 sizeof(*ipv6h) + sizeof(struct tcphdr)); 241 if (ret < 0) 242 return NF_DROP; 243 if (ret > 0) { 244 ipv6h = ipv6_hdr(skb); 245 oldlen = ipv6h->payload_len; 246 newlen = htons(ntohs(oldlen) + ret); 247 if (skb->ip_summed == CHECKSUM_COMPLETE) 248 skb->csum = csum_add(csum_sub(skb->csum, oldlen), 249 newlen); 250 ipv6h->payload_len = newlen; 251 } 252 return XT_CONTINUE; 253 } 254 #endif 255 256 /* Must specify -p tcp --syn */ 257 static inline bool find_syn_match(const struct xt_entry_match *m) 258 { 259 const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data; 260 261 if (strcmp(m->u.kernel.match->name, "tcp") == 0 && 262 tcpinfo->flg_cmp & TCPHDR_SYN && 263 !(tcpinfo->invflags & XT_TCP_INV_FLAGS)) 264 return true; 265 266 return false; 267 } 268 269 static int tcpmss_tg4_check(const struct xt_tgchk_param *par) 270 { 271 const struct xt_tcpmss_info *info = par->targinfo; 272 const struct ipt_entry *e = par->entryinfo; 273 const struct xt_entry_match *ematch; 274 275 if (info->mss == XT_TCPMSS_CLAMP_PMTU && 276 (par->hook_mask & ~((1 << NF_INET_FORWARD) | 277 (1 << NF_INET_LOCAL_OUT) | 278 (1 << NF_INET_POST_ROUTING))) != 0) { 279 pr_info("path-MTU clamping only supported in " 280 "FORWARD, OUTPUT and POSTROUTING hooks\n"); 281 return -EINVAL; 282 } 283 if (par->nft_compat) 284 return 0; 285 286 xt_ematch_foreach(ematch, e) 287 if (find_syn_match(ematch)) 288 return 0; 289 pr_info("Only works on TCP SYN packets\n"); 290 return -EINVAL; 291 } 292 293 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES) 294 static int tcpmss_tg6_check(const struct xt_tgchk_param *par) 295 { 296 const struct xt_tcpmss_info *info = par->targinfo; 297 const struct ip6t_entry *e = par->entryinfo; 298 const struct xt_entry_match *ematch; 299 300 if (info->mss == XT_TCPMSS_CLAMP_PMTU && 301 (par->hook_mask & ~((1 << NF_INET_FORWARD) | 302 (1 << NF_INET_LOCAL_OUT) | 303 (1 << NF_INET_POST_ROUTING))) != 0) { 304 pr_info("path-MTU clamping only supported in " 305 "FORWARD, OUTPUT and POSTROUTING hooks\n"); 306 return -EINVAL; 307 } 308 if (par->nft_compat) 309 return 0; 310 311 xt_ematch_foreach(ematch, e) 312 if (find_syn_match(ematch)) 313 return 0; 314 pr_info("Only works on TCP SYN packets\n"); 315 return -EINVAL; 316 } 317 #endif 318 319 static struct xt_target tcpmss_tg_reg[] __read_mostly = { 320 { 321 .family = NFPROTO_IPV4, 322 .name = "TCPMSS", 323 .checkentry = tcpmss_tg4_check, 324 .target = tcpmss_tg4, 325 .targetsize = sizeof(struct xt_tcpmss_info), 326 .proto = IPPROTO_TCP, 327 .me = THIS_MODULE, 328 }, 329 #if IS_ENABLED(CONFIG_IP6_NF_IPTABLES) 330 { 331 .family = NFPROTO_IPV6, 332 .name = "TCPMSS", 333 .checkentry = tcpmss_tg6_check, 334 .target = tcpmss_tg6, 335 .targetsize = sizeof(struct xt_tcpmss_info), 336 .proto = IPPROTO_TCP, 337 .me = THIS_MODULE, 338 }, 339 #endif 340 }; 341 342 static int __init tcpmss_tg_init(void) 343 { 344 return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg)); 345 } 346 347 static void __exit tcpmss_tg_exit(void) 348 { 349 xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg)); 350 } 351 352 module_init(tcpmss_tg_init); 353 module_exit(tcpmss_tg_exit); 354