1 // SPDX-License-Identifier: GPL-2.0-only 2 /* Kernel module to match TCP MSS values. */ 3 4 /* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca> 5 * Portions (C) 2005 by Harald Welte <laforge@netfilter.org> 6 */ 7 8 #include <linux/module.h> 9 #include <linux/skbuff.h> 10 #include <net/tcp.h> 11 12 #include <linux/netfilter/xt_tcpmss.h> 13 #include <linux/netfilter/x_tables.h> 14 15 #include <linux/netfilter_ipv4/ip_tables.h> 16 #include <linux/netfilter_ipv6/ip6_tables.h> 17 18 MODULE_LICENSE("GPL"); 19 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>"); 20 MODULE_DESCRIPTION("Xtables: TCP MSS match"); 21 MODULE_ALIAS("ipt_tcpmss"); 22 MODULE_ALIAS("ip6t_tcpmss"); 23 24 static bool 25 tcpmss_mt(const struct sk_buff *skb, struct xt_action_param *par) 26 { 27 const struct xt_tcpmss_match_info *info = par->matchinfo; 28 const struct tcphdr *th; 29 struct tcphdr _tcph; 30 /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */ 31 const u_int8_t *op; 32 u8 _opt[15 * 4 - sizeof(_tcph)]; 33 unsigned int i, optlen; 34 35 /* If we don't have the whole header, drop packet. */ 36 th = skb_header_pointer(skb, par->thoff, sizeof(_tcph), &_tcph); 37 if (th == NULL) 38 goto dropit; 39 40 /* Malformed. */ 41 if (th->doff*4 < sizeof(*th)) 42 goto dropit; 43 44 optlen = th->doff*4 - sizeof(*th); 45 if (!optlen) 46 goto out; 47 48 /* Truncated options. */ 49 op = skb_header_pointer(skb, par->thoff + sizeof(*th), optlen, _opt); 50 if (op == NULL) 51 goto dropit; 52 53 for (i = 0; i < optlen; ) { 54 if (op[i] == TCPOPT_MSS 55 && (optlen - i) >= TCPOLEN_MSS 56 && op[i+1] == TCPOLEN_MSS) { 57 u_int16_t mssval; 58 59 mssval = (op[i+2] << 8) | op[i+3]; 60 61 return (mssval >= info->mss_min && 62 mssval <= info->mss_max) ^ info->invert; 63 } 64 if (op[i] < 2) 65 i++; 66 else 67 i += op[i+1] ? : 1; 68 } 69 out: 70 return info->invert; 71 72 dropit: 73 par->hotdrop = true; 74 return false; 75 } 76 77 static struct xt_match tcpmss_mt_reg[] __read_mostly = { 78 { 79 .name = "tcpmss", 80 .family = NFPROTO_IPV4, 81 .match = tcpmss_mt, 82 .matchsize = sizeof(struct xt_tcpmss_match_info), 83 .proto = IPPROTO_TCP, 84 .me = THIS_MODULE, 85 }, 86 { 87 .name = "tcpmss", 88 .family = NFPROTO_IPV6, 89 .match = tcpmss_mt, 90 .matchsize = sizeof(struct xt_tcpmss_match_info), 91 .proto = IPPROTO_TCP, 92 .me = THIS_MODULE, 93 }, 94 }; 95 96 static int __init tcpmss_mt_init(void) 97 { 98 return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg)); 99 } 100 101 static void __exit tcpmss_mt_exit(void) 102 { 103 xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg)); 104 } 105 106 module_init(tcpmss_mt_init); 107 module_exit(tcpmss_mt_exit); 108