1 /* Kernel module to match TCP MSS values. */ 2 3 /* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca> 4 * Portions (C) 2005 by Harald Welte <laforge@netfilter.org> 5 * 6 * This program is free software; you can redistribute it and/or modify 7 * it under the terms of the GNU General Public License version 2 as 8 * published by the Free Software Foundation. 9 */ 10 11 #include <linux/module.h> 12 #include <linux/skbuff.h> 13 #include <net/tcp.h> 14 15 #include <linux/netfilter/xt_tcpmss.h> 16 #include <linux/netfilter/x_tables.h> 17 18 #include <linux/netfilter_ipv4/ip_tables.h> 19 #include <linux/netfilter_ipv6/ip6_tables.h> 20 21 MODULE_LICENSE("GPL"); 22 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>"); 23 MODULE_DESCRIPTION("Xtables: TCP MSS match"); 24 MODULE_ALIAS("ipt_tcpmss"); 25 MODULE_ALIAS("ip6t_tcpmss"); 26 27 static bool 28 tcpmss_mt(const struct sk_buff *skb, struct xt_action_param *par) 29 { 30 const struct xt_tcpmss_match_info *info = par->matchinfo; 31 const struct tcphdr *th; 32 struct tcphdr _tcph; 33 /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */ 34 const u_int8_t *op; 35 u8 _opt[15 * 4 - sizeof(_tcph)]; 36 unsigned int i, optlen; 37 38 /* If we don't have the whole header, drop packet. */ 39 th = skb_header_pointer(skb, par->thoff, sizeof(_tcph), &_tcph); 40 if (th == NULL) 41 goto dropit; 42 43 /* Malformed. */ 44 if (th->doff*4 < sizeof(*th)) 45 goto dropit; 46 47 optlen = th->doff*4 - sizeof(*th); 48 if (!optlen) 49 goto out; 50 51 /* Truncated options. */ 52 op = skb_header_pointer(skb, par->thoff + sizeof(*th), optlen, _opt); 53 if (op == NULL) 54 goto dropit; 55 56 for (i = 0; i < optlen; ) { 57 if (op[i] == TCPOPT_MSS 58 && (optlen - i) >= TCPOLEN_MSS 59 && op[i+1] == TCPOLEN_MSS) { 60 u_int16_t mssval; 61 62 mssval = (op[i+2] << 8) | op[i+3]; 63 64 return (mssval >= info->mss_min && 65 mssval <= info->mss_max) ^ info->invert; 66 } 67 if (op[i] < 2) 68 i++; 69 else 70 i += op[i+1] ? : 1; 71 } 72 out: 73 return info->invert; 74 75 dropit: 76 par->hotdrop = true; 77 return false; 78 } 79 80 static struct xt_match tcpmss_mt_reg[] __read_mostly = { 81 { 82 .name = "tcpmss", 83 .family = NFPROTO_IPV4, 84 .match = tcpmss_mt, 85 .matchsize = sizeof(struct xt_tcpmss_match_info), 86 .proto = IPPROTO_TCP, 87 .me = THIS_MODULE, 88 }, 89 { 90 .name = "tcpmss", 91 .family = NFPROTO_IPV6, 92 .match = tcpmss_mt, 93 .matchsize = sizeof(struct xt_tcpmss_match_info), 94 .proto = IPPROTO_TCP, 95 .me = THIS_MODULE, 96 }, 97 }; 98 99 static int __init tcpmss_mt_init(void) 100 { 101 return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg)); 102 } 103 104 static void __exit tcpmss_mt_exit(void) 105 { 106 xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg)); 107 } 108 109 module_init(tcpmss_mt_init); 110 module_exit(tcpmss_mt_exit); 111