iptables-saveのコードリーディング

2017-03-03

はじめに

iptables-save でルールを出力する部分のコードリーディングのメモです。

`iptables_save_main` 関数からの流れ

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/* Format:
 * :Chain name POLICY packets bytes
 * rule
 */
int
iptables_save_main(int argc, char *argv[])
{
	const char *tablename = NULL;
	int c;

	iptables_globals.program_name = "iptables-save";
	c = xtables_init_all(&iptables_globals, NFPROTO_IPV4);
	if (c < 0) {
		fprintf(stderr, "%s/%s Failed to initialize xtables\n",
				iptables_globals.program_name,
				iptables_globals.program_version);
		exit(1);
	}
#if defined(ALL_INCLUSIVE) || defined(NO_SHARED_LIBS)
	init_extensions();
	init_extensions4();
#endif

	while ((c = getopt_long(argc, argv, "bcdt:", options, NULL)) != -1) {
		switch (c) {
		case 'c':
			show_counters = 1;
			break;

		case 't':
			/* Select specific table. */
			tablename = optarg;
			break;
		case 'M':
			xtables_modprobe_program = optarg;
			break;
		case 'd':
			do_output(tablename);
			exit(0);
		}
	}

	if (optind < argc) {
		fprintf(stderr, "Unknown arguments found on commandline\n");
		exit(1);
	}

	return !do_output(tablename);
}

-t オプションを指定しない場合は tablename = NULL で do_output 関数を呼び出します。

iptables/ip6tables-save.c#L25

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static int show_counters = 0;

iptables/iptables-save.c#L59-#L118

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static int do_output(const char *tablename)
{
	struct xtc_handle *h;
	const char *chain = NULL;

	if (!tablename)
		return for_each_table(&do_output);

	h = iptc_init(tablename);
	if (h == NULL) {
		xtables_load_ko(xtables_modprobe_program, false);
		h = iptc_init(tablename);
	}
	if (!h)
		xtables_error(OTHER_PROBLEM, "Cannot initialize: %s\n",
			   iptc_strerror(errno));

	time_t now = time(NULL);

	printf("# Generated by iptables-save v%s on %s",
	       IPTABLES_VERSION, ctime(&now));
	printf("*%s\n", tablename);

	/* Dump out chain names first,
	 * thereby preventing dependency conflicts */
	for (chain = iptc_first_chain(h);
	     chain;
	     chain = iptc_next_chain(h)) {

		printf(":%s ", chain);
		if (iptc_builtin(chain, h)) {
			struct xt_counters count;
			printf("%s ",
			       iptc_get_policy(chain, &count, h));
			printf("[%llu:%llu]\n", (unsigned long long)count.pcnt, (unsigned long long)count.bcnt);
		} else {
			printf("- [0:0]\n");
		}
	}

	for (chain = iptc_first_chain(h);
	     chain;
	     chain = iptc_next_chain(h)) {
		const struct ipt_entry *e;

		/* Dump out rules */
		e = iptc_first_rule(chain, h);
		while(e) {
			print_rule4(e, h, chain, show_counters);
			e = iptc_next_rule(e, h);
		}
	}

	now = time(NULL);
	printf("COMMIT\n");
	printf("# Completed on %s", ctime(&now));
	iptc_free(h);

	return 1;
}

tablename が NULL で呼び出された場合 for_each_table 関数で各テーブルごとに do_output 関数を呼び出します。

iptables/iptables-save.c#L34-#L56

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/* Debugging prototype. */
static int for_each_table(int (*func)(const char *tablename))
{
	int ret = 1;
	FILE *procfile = NULL;
	char tablename[XT_TABLE_MAXNAMELEN+1];

	procfile = fopen("/proc/net/ip_tables_names", "re");
	if (!procfile)
		return ret;

	while (fgets(tablename, sizeof(tablename), procfile)) {
		if (tablename[strlen(tablename) - 1] != '\n')
			xtables_error(OTHER_PROBLEM,
				   "Badly formed tablename `%s'\n",
				   tablename);
		tablename[strlen(tablename) - 1] = '\0';
		ret &= func(tablename);
	}

	fclose(procfile);
	return ret;
}

/proc/net/ip_tables_names の出力結果の各行がテーブル名になっていて、各テーブルごとに引数 func で指定された関数を実行します。

iptables/iptables.c#L1069-#L1148

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/* We want this to be readable, so only print out neccessary fields.
 * Because that's the kind of world I want to live in.  */
void print_rule4(const struct ipt_entry *e,
		struct xtc_handle *h, const char *chain, int counters)
{
	const struct xt_entry_target *t;
	const char *target_name;

	/* print counters for iptables-save */
	if (counters > 0)
		printf("[%llu:%llu] ", (unsigned long long)e->counters.pcnt, (unsigned long long)e->counters.bcnt);

	/* print chain name */
	printf("-A %s", chain);

	/* Print IP part. */
	print_ip("-s", e->ip.src.s_addr,e->ip.smsk.s_addr,
			e->ip.invflags & IPT_INV_SRCIP);

	print_ip("-d", e->ip.dst.s_addr, e->ip.dmsk.s_addr,
			e->ip.invflags & IPT_INV_DSTIP);

	print_iface('i', e->ip.iniface, e->ip.iniface_mask,
		    e->ip.invflags & IPT_INV_VIA_IN);

	print_iface('o', e->ip.outiface, e->ip.outiface_mask,
		    e->ip.invflags & IPT_INV_VIA_OUT);

	print_proto(e->ip.proto, e->ip.invflags & XT_INV_PROTO);

	if (e->ip.flags & IPT_F_FRAG)
		printf("%s -f",
		       e->ip.invflags & IPT_INV_FRAG ? " !" : "");

	/* Print matchinfo part */
	if (e->target_offset) {
		IPT_MATCH_ITERATE(e, print_match_save, &e->ip);
	}

	/* print counters for iptables -R */
	if (counters < 0)
		printf(" -c %llu %llu", (unsigned long long)e->counters.pcnt, (unsigned long long)e->counters.bcnt);

	/* Print target name and targinfo part */
	target_name = iptc_get_target(e, h);
	t = ipt_get_target((struct ipt_entry *)e);
	if (t->u.user.name[0]) {
		const struct xtables_target *target =
			xtables_find_target(t->u.user.name, XTF_TRY_LOAD);

		if (!target) {
			fprintf(stderr, "Can't find library for target `%s'\n",
				t->u.user.name);
			exit(1);
		}

		printf(" -j %s", target->alias ? target->alias(t) : target_name);
		if (target->save)
			target->save(&e->ip, t);
		else {
			/* If the target size is greater than xt_entry_target
			 * there is something to be saved, we just don't know
			 * how to print it */
			if (t->u.target_size !=
			    sizeof(struct xt_entry_target)) {
				fprintf(stderr, "Target `%s' is missing "
						"save function\n",
					t->u.user.name);
				exit(1);
			}
		}
	} else if (target_name && (*target_name != '\0'))
#ifdef IPT_F_GOTO
		printf(" -%c %s", e->ip.flags & IPT_F_GOTO ? 'g' : 'j', target_name);
#else
		printf(" -j %s", target_name);
#endif

	printf("\n");
}

ルールの出力は以下のようになっています。

1082行目: -A チェイン名 を出力します。
1088行目: -s ソースアドレス を出力します。ただし場合によっては出力しません（下記 print_ip 関数参照）。
1088行目: -d デスティネーションアドレス を出力します。ただし場合によっては出力しません（下記 print_ip 関数参照）。
1091行目: 入力インターフェースの情報を出力します。ただし場合によっては出力しません（下記 print_iface 関数参照）。
1094行目: 出力インターフェースの情報を出力します。ただし場合によっては出力しません（下記 print_iface 関数参照）。
1097行目: -p オプションでのプロトコルの指定を出力します。ただし場合によっては出力しません（下記 print_proto 関数参照）。
1099行目: ルールがフラグメントルールの場合は -f オプションを出力します。
1104行目: e->target_offset が 0 以外の場合は print_match_save 関数を IPT_MATCH_ITERATE マクロでループして -m オプションを出力します。
1109行目: counters が負の場合は -c オプションでカウンタの数値を出力します。
1115～1145行目: いろいろ分岐はありますが場合によって -j か -g オプションでターゲット名と場合によってその後に targinfo のオプションを出力します。

include/linux/netfilter_ipv4/ip_tables.h#L217-#L222

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/* Helper functions */
static __inline__ struct xt_entry_target *
ipt_get_target(struct ipt_entry *e)
{
	return (void *)e + e->target_offset;
}

include/linux/netfilter/x_tables.h#L33-#L54

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struct xt_entry_target {
	union {
		struct {
			__u16 target_size;

			/* Used by userspace */
			char name[XT_EXTENSION_MAXNAMELEN];
			__u8 revision;
		} user;
		struct {
			__u16 target_size;

			/* Used inside the kernel */
			struct xt_target *target;
		} kernel;

		/* Total length */
		__u16 target_size;
	} u;

	unsigned char data[0];
};

include/xtables.h#L283-#L358

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struct xtables_target
{
	/*
	 * ABI/API version this module requires. Must be first member,
	 * as the rest of this struct may be subject to ABI changes.
	 */
	const char *version;

	struct xtables_target *next;

	const char *name;

	/* Real target behind this, if any. */
	const char *real_name;

	/* Revision of target (0 by default). */
	u_int8_t revision;

	/* Extension flags */
	u_int8_t ext_flags;

	u_int16_t family;

	/* Size of target data. */
	size_t size;

	/* Size of target data relevant for userspace comparison purposes */
	size_t userspacesize;

	/* Function which prints out usage message. */
	void (*help)(void);

	/* Initialize the target. */
	void (*init)(struct xt_entry_target *t);

	/* Function which parses command options; returns true if it
           ate an option */
	/* entry is struct ipt_entry for example */
	int (*parse)(int c, char **argv, int invert, unsigned int *flags,
		     const void *entry,
		     struct xt_entry_target **targetinfo);

	/* Final check; exit if not ok. */
	void (*final_check)(unsigned int flags);

	/* Prints out the target iff non-NULL: put space at end */
	void (*print)(const void *ip,
		      const struct xt_entry_target *target, int numeric);

	/* Saves the targinfo in parsable form to stdout. */
	void (*save)(const void *ip,
		     const struct xt_entry_target *target);

	/* Print target name or alias */
	const char *(*alias)(const struct xt_entry_target *target);

	/* Pointer to list of extra command-line options */
	const struct option *extra_opts;

	/* New parser */
	void (*x6_parse)(struct xt_option_call *);
	void (*x6_fcheck)(struct xt_fcheck_call *);
	const struct xt_option_entry *x6_options;

	size_t udata_size;

	/* Ignore these men behind the curtain: */
	void *udata;
	unsigned int option_offset;
	struct xt_entry_target *t;
	unsigned int tflags;
	unsigned int used;
	unsigned int loaded; /* simulate loading so options are merged properly */
};

include/linux/netfilter_ipv4/ip_tables.h#L99-#L121

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/* This structure defines each of the firewall rules.  Consists of 3
   parts which are 1) general IP header stuff 2) match specific
   stuff 3) the target to perform if the rule matches */
struct ipt_entry {
	struct ipt_ip ip;

	/* Mark with fields that we care about. */
	unsigned int nfcache;

	/* Size of ipt_entry + matches */
	u_int16_t target_offset;
	/* Size of ipt_entry + matches + target */
	u_int16_t next_offset;

	/* Back pointer */
	unsigned int comefrom;

	/* Packet and byte counters. */
	struct xt_counters counters;

	/* The matches (if any), then the target. */
	unsigned char elems[0];
};

target->save の例。拡張ごとに struct xtables_target の配列が定義されています。

extensions/libxt_MARK.c#L248-#L262

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static struct xtables_target mark_tg_reg[] = {
	{
		.family        = NFPROTO_UNSPEC,
		.name          = "MARK",
		.version       = XTABLES_VERSION,
		.revision      = 0,
		.size          = XT_ALIGN(sizeof(struct xt_mark_target_info)),
		.userspacesize = XT_ALIGN(sizeof(struct xt_mark_target_info)),
		.help          = MARK_help,
		.print         = MARK_print_v0,
		.save          = MARK_save_v0,
		.x6_parse      = MARK_parse_v0,
		.x6_fcheck     = MARK_check,
		.x6_options    = MARK_opts,
	},

extensions/libxt_MARK.c#L176-#L183

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static void MARK_save_v0(const void *ip, const struct xt_entry_target *target)
{
	const struct xt_mark_target_info *markinfo =
		(const struct xt_mark_target_info *)target->data;

	printf(" --set-mark");
	print_mark(markinfo->mark);
}

この拡張の場合は --set-mark というオプションが出力されるようです。

include/linux/netfilter_ipv4/ip_tables.h#L66-#L82

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/* Yes, Virginia, you have to zero the padding. */
struct ipt_ip {
	/* Source and destination IP addr */
	struct in_addr src, dst;
	/* Mask for src and dest IP addr */
	struct in_addr smsk, dmsk;
	char iniface[IFNAMSIZ], outiface[IFNAMSIZ];
	unsigned char iniface_mask[IFNAMSIZ], outiface_mask[IFNAMSIZ];

	/* Protocol, 0 = ANY */
	u_int16_t proto;

	/* Flags word */
	u_int8_t flags;
	/* Inverse flags */
	u_int8_t invflags;
};

include/linux/netfilter_ipv4/ip_tables.h#L83-#L87

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    /* Values for "flag" field in struct ipt_ip (general ip structure). */
    #define IPT_F_FRAG		0x01	/* Set if rule is a fragment rule */
    #define IPT_F_GOTO		0x02	/* Set if jump is a goto */
    #define IPT_F_MASK		0x03	/* All possible flag bits mask. */

`iptables/iptables.c#L1039-#L1067 <https://git.netfilter.org/iptables/tree/iptables/iptables.c?id=482c6d3731e2681cb4baae835c294840300197e6#n1039>`_

.. code-block:: c
    :linenos: table
    :linenostart: 1039

    /* print a given ip including mask if neccessary */
    static void print_ip(const char *prefix, uint32_t ip,
    		     uint32_t mask, int invert)
    {
    	uint32_t bits, hmask = ntohl(mask);
    	int i;

    	if (!mask && !ip && !invert)
    		return;

    	printf("%s %s %u.%u.%u.%u",
    		invert ? " !" : "",
    		prefix,
    		IP_PARTS(ip));

    	if (mask == 0xFFFFFFFFU) {
    		printf("/32");
    		return;
    	}

    	i    = 32;
    	bits = 0xFFFFFFFEU;
    	while (--i >= 0 && hmask != bits)
    		bits <<= 1;
    	if (i >= 0)
    		printf("/%u", i);
    	else
    		printf("/%u.%u.%u.%u", IP_PARTS(mask));
    }

`iptables/iptables.c#L989-#L1013 <https://git.netfilter.org/iptables/tree/iptables/iptables.c?id=482c6d3731e2681cb4baae835c294840300197e6#n989>`_

.. code-block:: c
    :linenos: table
    :linenostart: 989

    /* This assumes that mask is contiguous, and byte-bounded. */
    static void
    print_iface(char letter, const char *iface, const unsigned char *mask,
    	    int invert)
    {
    	unsigned int i;

    	if (mask[0] == 0)
    		return;

    	printf("%s -%c ", invert ? " !" : "", letter);

    	for (i = 0; i < IFNAMSIZ; i++) {
    		if (mask[i] != 0) {
    			if (iface[i] != '\0')
    				printf("%c", iface[i]);
    		} else {
    			/* we can access iface[i-1] here, because
    			 * a few lines above we make sure that mask[0] != 0 */
    			if (iface[i-1] != '\0')
    				printf("+");
    			break;
    		}
    	}
    }

`iptables/iptables.c#L958-#L979 <https://git.netfilter.org/iptables/tree/iptables/iptables.c?id=482c6d3731e2681cb4baae835c294840300197e6#n958>`_

.. code-block:: c
    :linenos: table
    :linenostart: 958

    static void print_proto(uint16_t proto, int invert)
    {
    	if (proto) {
    		unsigned int i;
    		const char *invertstr = invert ? " !" : "";

    		const struct protoent *pent = getprotobynumber(proto);
    		if (pent) {
    			printf("%s -p %s", invertstr, pent->p_name);
    			return;
    		}

    		for (i = 0; xtables_chain_protos[i].name != NULL; ++i)
    			if (xtables_chain_protos[i].num == proto) {
    				printf("%s -p %s",
    				       invertstr, xtables_chain_protos[i].name);
    				return;
    			}

    		printf("%s -p %u", invertstr, proto);
    	}
    }

`include/linux/netfilter_ipv4/ip_tables.h#L58-#L60 <https://git.netfilter.org/iptables/tree/include/linux/netfilter_ipv4/ip_tables.h?id=482c6d3731e2681cb4baae835c294840300197e6#n58>`_

.. code-block:: c
    :linenos: table
    :linenostart: 58

    /* fn returns 0 to continue iteration */
    #define IPT_MATCH_ITERATE(e, fn, args...) \
    	XT_MATCH_ITERATE(struct ipt_entry, e, fn, ## args)

`include/linux/netfilter/x_tables.h#L126-#L143 <https://git.netfilter.org/iptables/tree/include/linux/netfilter/x_tables.h?id=482c6d3731e2681cb4baae835c294840300197e6#n126>`_

.. code-block:: c
    :linenos: table
    :linenostart: 126

    /* fn returns 0 to continue iteration */
    #define XT_MATCH_ITERATE(type, e, fn, args...)			\
    ({								\
    	unsigned int __i;					\
    	int __ret = 0;						\
    	struct xt_entry_match *__m;				\
    								\
    	for (__i = sizeof(type);				\
    	     __i < (e)->target_offset;				\
    	     __i += __m->u.match_size) {			\
    		__m = (void *)e + __i;				\
    								\
    		__ret = fn(__m , ## args);			\
    		if (__ret != 0)					\
    			break;					\
    	}							\
    	__ret;							\
    })

`iptables/iptables.c#L1015-#L1037 <https://git.netfilter.org/iptables/tree/iptables/iptables.c?id=482c6d3731e2681cb4baae835c294840300197e6#n1015>`_

.. code-block:: c
    :linenos: table
    :linenostart: 1015

    static int print_match_save(const struct xt_entry_match *e,
    			const struct ipt_ip *ip)
    {
    	const struct xtables_match *match =
    		xtables_find_match(e->u.user.name, XTF_TRY_LOAD, NULL);

    	if (match) {
    		printf(" -m %s",
    			match->alias ? match->alias(e) : e->u.user.name);

    		/* some matches don't provide a save function */
    		if (match->save)
    			match->save(ip, e);
    	} else {
    		if (e->u.match_size) {
    			fprintf(stderr,
    				"Can't find library for match `%s'\n",
    				e->u.user.name);
    			exit(1);
    		}
    	}
    	return 0;
    }

iptables-saveのコードリーディング

はじめに

iptables_save_main 関数からの流れ

`iptables_save_main` 関数からの流れ