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