libxt_tcpmss.c revision 8b7c64d6ba156a99008fcd810cba874c73294333 
1/* Shared library add-on to iptables to add tcp MSS matching support. */
2#include <stdio.h>
3#include <netdb.h>
4#include <string.h>
5#include <stdlib.h>
6#include <getopt.h>
7
8#include <xtables.h>
9#include <linux/netfilter/xt_tcpmss.h>
10
11/* Function which prints out usage message. */
12static void tcpmss_help(void)
13{
14	printf(
15"tcpmss match options:\n"
16"[!] --mss value[:value]	Match TCP MSS range.\n"
17"				(only valid for TCP SYN or SYN/ACK packets)\n");
18}
19
20static const struct option tcpmss_opts[] = {
21	{ "mss", 1, NULL, '1' },
22	{ .name = NULL }
23};
24
25static u_int16_t
26parse_tcp_mssvalue(const char *mssvalue)
27{
28	unsigned int mssvaluenum;
29
30	if (string_to_number(mssvalue, 0, 65535, &mssvaluenum) != -1)
31		return (u_int16_t)mssvaluenum;
32
33	exit_error(PARAMETER_PROBLEM,
34		   "Invalid mss `%s' specified", mssvalue);
35}
36
37static void
38parse_tcp_mssvalues(const char *mssvaluestring,
39		    u_int16_t *mss_min, u_int16_t *mss_max)
40{
41	char *buffer;
42	char *cp;
43
44	buffer = strdup(mssvaluestring);
45	if ((cp = strchr(buffer, ':')) == NULL)
46		*mss_min = *mss_max = parse_tcp_mssvalue(buffer);
47	else {
48		*cp = '\0';
49		cp++;
50
51		*mss_min = buffer[0] ? parse_tcp_mssvalue(buffer) : 0;
52		*mss_max = cp[0] ? parse_tcp_mssvalue(cp) : 0xFFFF;
53	}
54	free(buffer);
55}
56
57/* Function which parses command options; returns true if it
58   ate an option */
59static int
60tcpmss_parse(int c, char **argv, int invert, unsigned int *flags,
61             const void *entry, struct xt_entry_match **match)
62{
63	struct xt_tcpmss_match_info *mssinfo =
64		(struct xt_tcpmss_match_info *)(*match)->data;
65
66	switch (c) {
67	case '1':
68		if (*flags)
69			exit_error(PARAMETER_PROBLEM,
70				   "Only one `--mss' allowed");
71		check_inverse(optarg, &invert, &optind, 0);
72		parse_tcp_mssvalues(argv[optind-1],
73				    &mssinfo->mss_min, &mssinfo->mss_max);
74		if (invert)
75			mssinfo->invert = 1;
76		*flags = 1;
77		break;
78	default:
79		return 0;
80	}
81	return 1;
82}
83
84static void
85print_tcpmss(u_int16_t mss_min, u_int16_t mss_max, int invert, int numeric)
86{
87	if (invert)
88		printf("! ");
89
90	if (mss_min == mss_max)
91		printf("%u ", mss_min);
92	else
93		printf("%u:%u ", mss_min, mss_max);
94}
95
96/* Final check; must have specified --mss. */
97static void tcpmss_check(unsigned int flags)
98{
99	if (!flags)
100		exit_error(PARAMETER_PROBLEM,
101			   "tcpmss match: You must specify `--mss'");
102}
103
104/* Prints out the matchinfo. */
105static void
106tcpmss_print(const void *ip, const struct xt_entry_match *match, int numeric)
107{
108	const struct xt_tcpmss_match_info *mssinfo =
109		(const struct xt_tcpmss_match_info *)match->data;
110
111	printf("tcpmss match ");
112	print_tcpmss(mssinfo->mss_min, mssinfo->mss_max,
113		     mssinfo->invert, numeric);
114}
115
116/* Saves the union ipt_matchinfo in parsable form to stdout. */
117static void tcpmss_save(const void *ip, const struct xt_entry_match *match)
118{
119	const struct xt_tcpmss_match_info *mssinfo =
120		(const struct xt_tcpmss_match_info *)match->data;
121
122	printf("--mss ");
123	print_tcpmss(mssinfo->mss_min, mssinfo->mss_max,
124		     mssinfo->invert, 0);
125}
126
127static struct xtables_match tcpmss_match = {
128	.family		= AF_INET,
129	.name		= "tcpmss",
130	.version	= XTABLES_VERSION,
131	.size		= XT_ALIGN(sizeof(struct xt_tcpmss_match_info)),
132	.userspacesize	= XT_ALIGN(sizeof(struct xt_tcpmss_match_info)),
133	.help		= tcpmss_help,
134	.parse		= tcpmss_parse,
135	.final_check	= tcpmss_check,
136	.print		= tcpmss_print,
137	.save		= tcpmss_save,
138	.extra_opts	= tcpmss_opts,
139};
140
141static struct xtables_match tcpmss_match6 = {
142	.family		= AF_INET6,
143	.name		= "tcpmss",
144	.version	= XTABLES_VERSION,
145	.size		= XT_ALIGN(sizeof(struct xt_tcpmss_match_info)),
146	.userspacesize	= XT_ALIGN(sizeof(struct xt_tcpmss_match_info)),
147	.help		= tcpmss_help,
148	.parse		= tcpmss_parse,
149	.final_check	= tcpmss_check,
150	.print		= tcpmss_print,
151	.save		= tcpmss_save,
152	.extra_opts	= tcpmss_opts,
153};
154
155void _init(void)
156{
157	xtables_register_match(&tcpmss_match);
158	xtables_register_match(&tcpmss_match6);
159}
160