| Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame^] | 1 | /* Kernel module to match TCP MSS values. */ |
| 2 | |
| 3 | /* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca> |
| 4 | * |
| 5 | * This program is free software; you can redistribute it and/or modify |
| 6 | * it under the terms of the GNU General Public License version 2 as |
| 7 | * published by the Free Software Foundation. |
| 8 | */ |
| 9 | |
| 10 | #include <linux/module.h> |
| 11 | #include <linux/skbuff.h> |
| 12 | #include <net/tcp.h> |
| 13 | |
| 14 | #include <linux/netfilter_ipv4/ipt_tcpmss.h> |
| 15 | #include <linux/netfilter_ipv4/ip_tables.h> |
| 16 | |
| 17 | #define TH_SYN 0x02 |
| 18 | |
| 19 | MODULE_LICENSE("GPL"); |
| 20 | MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>"); |
| 21 | MODULE_DESCRIPTION("iptables TCP MSS match module"); |
| 22 | |
| 23 | /* Returns 1 if the mss option is set and matched by the range, 0 otherwise */ |
| 24 | static inline int |
| 25 | mssoption_match(u_int16_t min, u_int16_t max, |
| 26 | const struct sk_buff *skb, |
| 27 | int invert, |
| 28 | int *hotdrop) |
| 29 | { |
| 30 | struct tcphdr _tcph, *th; |
| 31 | /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */ |
| 32 | u8 _opt[15 * 4 - sizeof(_tcph)], *op; |
| 33 | unsigned int i, optlen; |
| 34 | |
| 35 | /* If we don't have the whole header, drop packet. */ |
| 36 | th = skb_header_pointer(skb, skb->nh.iph->ihl * 4, |
| 37 | sizeof(_tcph), &_tcph); |
| 38 | if (th == NULL) |
| 39 | goto dropit; |
| 40 | |
| 41 | /* Malformed. */ |
| 42 | if (th->doff*4 < sizeof(*th)) |
| 43 | goto dropit; |
| 44 | |
| 45 | optlen = th->doff*4 - sizeof(*th); |
| 46 | if (!optlen) |
| 47 | goto out; |
| 48 | |
| 49 | /* Truncated options. */ |
| 50 | op = skb_header_pointer(skb, skb->nh.iph->ihl * 4 + sizeof(*th), |
| 51 | optlen, _opt); |
| 52 | if (op == NULL) |
| 53 | goto dropit; |
| 54 | |
| 55 | for (i = 0; i < optlen; ) { |
| 56 | if (op[i] == TCPOPT_MSS |
| 57 | && (optlen - i) >= TCPOLEN_MSS |
| 58 | && op[i+1] == TCPOLEN_MSS) { |
| 59 | u_int16_t mssval; |
| 60 | |
| 61 | mssval = (op[i+2] << 8) | op[i+3]; |
| 62 | |
| 63 | return (mssval >= min && mssval <= max) ^ invert; |
| 64 | } |
| 65 | if (op[i] < 2) i++; |
| 66 | else i += op[i+1]?:1; |
| 67 | } |
| 68 | out: |
| 69 | return invert; |
| 70 | |
| 71 | dropit: |
| 72 | *hotdrop = 1; |
| 73 | return 0; |
| 74 | } |
| 75 | |
| 76 | static int |
| 77 | match(const struct sk_buff *skb, |
| 78 | const struct net_device *in, |
| 79 | const struct net_device *out, |
| 80 | const void *matchinfo, |
| 81 | int offset, |
| 82 | int *hotdrop) |
| 83 | { |
| 84 | const struct ipt_tcpmss_match_info *info = matchinfo; |
| 85 | |
| 86 | return mssoption_match(info->mss_min, info->mss_max, skb, |
| 87 | info->invert, hotdrop); |
| 88 | } |
| 89 | |
| 90 | static int |
| 91 | checkentry(const char *tablename, |
| 92 | const struct ipt_ip *ip, |
| 93 | void *matchinfo, |
| 94 | unsigned int matchsize, |
| 95 | unsigned int hook_mask) |
| 96 | { |
| 97 | if (matchsize != IPT_ALIGN(sizeof(struct ipt_tcpmss_match_info))) |
| 98 | return 0; |
| 99 | |
| 100 | /* Must specify -p tcp */ |
| 101 | if (ip->proto != IPPROTO_TCP || (ip->invflags & IPT_INV_PROTO)) { |
| 102 | printk("tcpmss: Only works on TCP packets\n"); |
| 103 | return 0; |
| 104 | } |
| 105 | |
| 106 | return 1; |
| 107 | } |
| 108 | |
| 109 | static struct ipt_match tcpmss_match = { |
| 110 | .name = "tcpmss", |
| 111 | .match = &match, |
| 112 | .checkentry = &checkentry, |
| 113 | .me = THIS_MODULE, |
| 114 | }; |
| 115 | |
| 116 | static int __init init(void) |
| 117 | { |
| 118 | return ipt_register_match(&tcpmss_match); |
| 119 | } |
| 120 | |
| 121 | static void __exit fini(void) |
| 122 | { |
| 123 | ipt_unregister_match(&tcpmss_match); |
| 124 | } |
| 125 | |
| 126 | module_init(init); |
| 127 | module_exit(fini); |