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