udhcp源码详解（六）--script

首先得讲明udhcp客户端设置IP的机制。

在udhcp中，客户端通过和主机的多次报文交互协商而获得可用的IP地址。在获得IP地址之后，客户端使用execle函数调用shell脚本完成客户端ip,netmak,dns,gateway等等参数的设定。而shell脚本的调用在script.h和script.c中实现。

以下为script.c源码中的主要实现函数：

/* get a rough idea of how long an option will be (rounding up...) */
static int max_option_length[] = {
	[OPTION_IP] =		sizeof("255.255.255.255 "),
	[OPTION_IP_PAIR] =	sizeof("255.255.255.255 ") * 2,
	[OPTION_STRING] =	1,
	[OPTION_BOOLEAN] =	sizeof("yes "),
	[OPTION_U8] =		sizeof("255 "),
	[OPTION_U16] =		sizeof("65535 "),
	[OPTION_S16] =		sizeof("-32768 "),
	[OPTION_U32] =		sizeof("4294967295 "),
	[OPTION_S32] =		sizeof("-2147483684 "),
};

本函数为option字段占用的最大字节数。如OPTION_BOOLEAN变量里，true 对应yes。False对应no，那么这种类型的变量中对应的最大字节占用就为sizeof（yes）。

static int upper_length(int length, struct dhcp_option *option)
{
	return max_option_length[option->flags & TYPE_MASK] *
	       (length / option_lengths[option->flags & TYPE_MASK]);
}

转化为shell脚本参数格式所占字节大小的上限。

/* Fill dest with the text of option 'option'. */
static void fill_options(char *dest, unsigned char *option, struct dhcp_option *type_p)
{
	int type, optlen;
	u_int16_t val_u16;
	int16_t val_s16;
	u_int32_t val_u32;
	int32_t val_s32;
	int len = option[OPT_LEN - 2];//报文中该option的长度

	dest += sprintf(dest, "%s=", type_p->name);

	type = type_p->flags & TYPE_MASK;
	optlen = option_lengths[type];
	for(;;) {
		switch (type) {
		case OPTION_IP_PAIR:
			dest += sprintip(dest, "", option);
			*(dest++) = '/';
			option += 4;
			optlen = 4;
		case OPTION_IP:	/* Works regardless of host byte order. */
			dest += sprintip(dest, "", option);
 			break;
		case OPTION_BOOLEAN:
			dest += sprintf(dest, *option ? "yes " : "no ");
			break;
		case OPTION_U8:
			dest += sprintf(dest, "%u ", *option);
			break;
		case OPTION_U16:
			memcpy(&val_u16, option, 2);
			dest += sprintf(dest, "%u ", ntohs(val_u16));
			break;
		case OPTION_S16:
			memcpy(&val_s16, option, 2);
			dest += sprintf(dest, "%d ", ntohs(val_s16));
			break;
		case OPTION_U32:
			memcpy(&val_u32, option, 4);
			dest += sprintf(dest, "%lu ", (unsigned long) ntohl(val_u32));
			break;
		case OPTION_S32:
			memcpy(&val_s32, option, 4);
			dest += sprintf(dest, "%ld ", (long) ntohl(val_s32));
			break;
		case OPTION_STRING:
			memcpy(dest, option, len);
			dest[len] = '\0';
			return;	 /* Short circuit this case */
		}
		option += optlen;
		len -= optlen;
		if (len <= 0) break;
	}
}

//读取options，并输出至dest所指内存。
例如 router输出格式为
router=192.168.2.3

/* put all the paramaters into an environment */
static char **fill_envp(struct dhcpMessage *packet)
{
	int num_options = 0;
	int i, j;
	char **envp;
	unsigned char *temp;
	char over = 0;

	if (packet == NULL)
		num_options = 0;
	else {
		for (i = 0; options[i].code; i++)
			if (get_option(packet, options[i].code))
				num_options++;
		if (packet->siaddr) num_options++;
		if ((temp = get_option(packet, DHCP_OPTION_OVER)))
			over = *temp;
		if (!(over & FILE_FIELD) && packet->file[0]) num_options++;
		if (!(over & SNAME_FIELD) && packet->sname[0]) num_options++;		
	}
	
	envp = xmalloc((num_options + 5) * sizeof(char *));
	envp[0] = xmalloc(sizeof("interface=") + strlen(client_config.interface));
	sprintf(envp[0], "interface=%s", client_config.interface);
	envp[1] = find_env("PATH", "PATH=/bin:/usr/bin:/sbin:/usr/sbin");
	envp[2] = find_env("HOME", "HOME=/");//读取本地环境变量

	if (packet == NULL) {
		envp[3] = NULL;
		return envp;
	}

	envp[3] = xmalloc(sizeof("ip=255.255.255.255"));//按最大的点分十进制格式申请空间
	sprintip(envp[3], "ip=", (unsigned char *) &packet->yiaddr);
	for (i = 0, j = 4; options[i].code; i++) {//读取
		if ((temp = get_option(packet, options[i].code))) {
			envp[j] = xmalloc(upper_length(temp[OPT_LEN - 2], &options[i]) + strlen(options[i].name) + 2);
			fill_options(envp[j], temp, &options[i]);
			j++;
		}
	}
	if (packet->siaddr) {
		envp[j] = xmalloc(sizeof("siaddr=255.255.255.255"));
		sprintip(envp[j++], "siaddr=", (unsigned char *) &packet->siaddr);
	}
	if (!(over & FILE_FIELD) && packet->file[0]) {
		/* watch out for invalid packets */
		packet->file[sizeof(packet->file) - 1] = '\0';
		envp[j] = xmalloc(sizeof("boot_file=") + strlen(packet->file));
		sprintf(envp[j++], "boot_file=%s", packet->file);
	}
	if (!(over & SNAME_FIELD) && packet->sname[0]) {
		/* watch out for invalid packets */
		packet->sname[sizeof(packet->sname) - 1] = '\0';
		envp[j] = xmalloc(sizeof("sname=") + strlen(packet->sname));
		sprintf(envp[j++], "sname=%s", packet->sname);
	}	
	envp[j] = NULL;
	return envp;
}

按指定的格式填充环境变量

/* Call a script with a par file and env vars */
void run_script(struct dhcpMessage *packet, const char *name)
{
	int pid;
	char **envp;

	if (client_config.script == NULL)
		return;

	/* call script */
	pid = fork();
	if (pid) {
		waitpid(pid, NULL, 0);
		return;
	} else if (pid == 0) {
		envp = fill_envp(packet);//从数据报文和主机中获取相关信息
		
		/* close fd's? */
		
		/* exec script */
		DEBUG(LOG_INFO, "execle'ing %s", client_config.script);
		execle(client_config.script, client_config.script,
		       name, NULL, envp);
		LOG(LOG_ERR, "script %s failed: %s",
		    client_config.script, strerror(errno));
		exit(1);
	}			
}

execle(client_config.script,client_config.script,name, NULL, envp);

函数在执行时envp所存环境变量能够被shell script所直接使用。这种用法不大常见，具体execle函数用法，请google之。

 

以下为运行至execle函数时，envp的一个示例：

interface=eth0

PATH= …(过长不写明)…

HOME=/root

ip=192.168.0.24

subnet=255.255.255.0

router=192.168.10.2

dns= 129.219.13.81

domain=local

wins=192.168.10.10

leases=8640000

dhcptype=5

serverid=192.168.253.23

 

由上面可以看出这些参数和udhcpd服务端配置文件所指定的参数一致。

 

 

值得注意的是options.c文件的全局dhcp_option变量并没有将所有的option包括进去，从这点上讲如果你是要扩展某个option字段，则必需在options按指定的格式中加入相关字段，同时如果该字段不属于option_lengths中指定类型中任何一种，那么你也必需在其指定该类型字段所占的字节数。当然script.c文件中的max_option_length字段也要添加相应信息。

sample.script脚本内容如下：

#!/bin/sh

# Currently, we only dispatch according tocommand.  However, a more

# elaborate system might dispatch bycommand and interface or do some

# common initialization first, especiallyif more dhcp event notifications

# are added.

 

exec /usr/share/udhcpc/sample.$1

该脚本就一句话，就是执行/usr/share/udhcpc/sample.$1脚本。其中的$1为调用执行sample.script说指定的参数。

例如：

execle(client_config.script, client_config.script,

                          name, NULL, envp);

中，client_config.script为/usr/…/sample.script，name为”bound”，那么在sample.script被执行时exec /usr/share/udhcpc/sample.$1语句中的$1就可以被替换为”bound”，亦即执行的是

exec /usr/share/udhcpc/sample.bound。

这里$1可以为deconfig nak renew bound四个值。

 

在sample.bound脚本中，出项了broadcast，subnet，ip，router等等变量这些变量若在envp中被指定了，那么该变量就存在一个有意义的值。

更多信息请参见：

1、  dhcp基本原理介绍：

H3C的dhcp技术白皮书

http://www.h3c.com.cn/Products___Technology/Technology/IPv4_IPv6_series/Other_technology/Technology_book/200802/333443_30003_0.htm

dhcp的rfc2131文档

http://www.ietf.org/rfc/rfc2131.txt

2、  dhcp报文中的options字段信息(可选变长选项字段)介绍：

http://www.networksorcery.com/enp/protocol/bootp/options.htm

rfc2132

http://www.ietf.org/rfc/rfc2132.txt

3、  execle指定环境变量并调用执行shell脚本介绍：

APUE 8.10

http://blog.csdn.net/baidu20008/article/details/9764765

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