调用client_option_envadd（）函数将值保存到变量中
void client_option_envadd (struct option_cache *oc,
			   struct packet *packet, struct lease *lease,
			   struct client_state *client_state,
			   struct option_state *in_options,
			   struct option_state *cfg_options,
			   struct binding_scope **scope,
			   struct universe *u, void *stuff)
{
	struct envadd_state *es = stuff;
	struct data_string data;
	memset (&data, 0, sizeof data);

	if (evaluate_option_cache (&data, packet, lease, client_state,
				   in_options, cfg_options, scope, oc, MDL)) {
		if (data.len) {
			char name [256];
			if (dhcp_option_ev_name (name, sizeof name,
						 oc->option)) {
				const char *value;
				size_t length;
				value = pretty_print_option(oc->option,  《====对提取字符进行处理
							    data.data,
							    data.len, 0, 0);
				length = strlen(value);

				if (check_option_values(oc->option->universe,
							oc->option->code,
							value, length) == 0) {
					client_envadd(es->client, es->prefix,
						      name, "%s", value);
				} else {
					log_error("suspect value in %s "
						  "option - discarded",
						  name);
				}
				data_string_forget (&data, MDL);
			}
		}
	}
}

static int check_option_values(struct universe *universe,
			       unsigned int opt,
			       const char *ptr,
			       size_t len)
{
	if (ptr == NULL)
		return(-1);

	/* just reject options we want to protect, will be escaped anyway */
	if ((universe == NULL) || (universe == &dhcp_universe)) {
		switch(opt) {
		      case DHO_DOMAIN_NAME:
#ifdef ACCEPT_LIST_IN_DOMAIN_NAME
			      return check_domain_name_list(ptr, len, 0);
#else
			      return check_domain_name(ptr, len, 0);
#endif
		      case DHO_HOST_NAME:
		      case DHO_NIS_DOMAIN:
		      case DHO_NETBIOS_SCOPE:
			return check_domain_name(ptr, len, 0);
			break;
		      case DHO_DOMAIN_SEARCH:
			return check_domain_name_list(ptr, len, 0);
			break;
		      case DHO_ROOT_PATH:
			if (len == 0)
				return(-1);
			for (; (*ptr != 0) && (len-- > 0); ptr++) {
				if(!(isalnum((unsigned char)*ptr) ||
				     *ptr == '#'  || *ptr == '%' ||
				     *ptr == '+'  || *ptr == '-' ||
				     *ptr == '_'  || *ptr == ':' ||
				     *ptr == '.'  || *ptr == ',' ||
				     *ptr == '@'  || *ptr == '~' ||
				     *ptr == '\\' || *ptr == '/' ||
				     *ptr == '['  || *ptr == ']' ||
				     *ptr == '='  || *ptr == ' '))
					return(-1);
			}
			return(0);
			break;
		}
	}

对domain 进行检查，未对wpad 做明显处理
static int check_domain_name(const char *ptr, size_t len, int dots)
{
	const char *p;

	/* not empty or complete length not over 255 characters   */
	if ((len == 0) || (len > 256))
		return(-1);

	/* consists of [[:alnum:]-]+ labels separated by [.]      */
	/* a [_] is against RFC but seems to be "widely used"...  */
	for (p=ptr; (*p != 0) && (len-- > 0); p++) {
		if ((*p == '-') || (*p == '_')) {
			/* not allowed at begin or end of a label */
			if (((p - ptr) == 0) || (len == 0) || (p[1] == '.'))
				return(-1);
		} else if (*p == '.') {
			/* each label has to be 1-63 characters;
			   we allow [.] at the end ('foo.bar.')   */
			size_t d = p - ptr;
			if ((d <= 0) || (d >= 64))
				return(-1);
			ptr = p + 1; /* jump to the next label    */
			if ((dots > 0) && (len > 0))
				dots--;
		} else if (isalnum((unsigned char)*p) == 0) {
			/* also numbers at the begin are fine     */
			return(-1);
		}
	}
	return(dots ? -1 : 0);
}

进行变量保存、
void client_envadd (struct client_state *client,
		    const char *prefix, const char *name, const char *fmt, ...)
{
	char spbuf [1024];
	char *s;
	unsigned len;
	struct string_list *val;
	va_list list;

	va_start (list, fmt);
	len = vsnprintf (spbuf, sizeof spbuf, fmt, list);
	va_end (list);

	val = dmalloc (strlen (prefix) + strlen (name) + 1 /* = */ +
		       len + sizeof *val, MDL);
	if (!val)
		return;
	s = val -> string;
	strcpy (s, prefix);
	strcat (s, name);
	s += strlen (s);
	*s++ = '=';
	if (len >= sizeof spbuf) {
		va_start (list, fmt);
		vsnprintf (s, len + 1, fmt, list);
		va_end (list);
	} else
		strcpy (s, spbuf);
	val -> next = client -> env;
	client -> env = val;
	client -> envc++;
}

通过 启动的/usr/libexec/nm-dhcp-helper 进行数据传递 --》 dbus服务 
NetworkManager -->调用nm-dispatcher --》读取dbus中的WPAD DHCP数据 保存到 环境变量DHCP4_WPAD
启动/etc/NetworkManager/dispatcher.d/11-dhclient 进行处理