udhcpd为局域网提供自动获取IP地址和DNS功能
创建好AP以后，STA终端可以连接到AP。此时STA想要连接到局域网就需要有IP地址，如果想连接到外网就需要网关。如果想连接到域名表示的外网，就需要dns服务器。
udhcpd是dhcp服务器，动态的提供IP地址，网关和DNS。所以一般AP端通过hostapod建立了无线的物理连接之后还需要提供DHCP服务功能，这样其他STA就可以获取IP地址等配置，只有获取到IP地址，才是真正意义上组成了局域网。
udhcpd(/etc/udhcpd.conf)关键配置项：
在我的系统中AP是通过wlan0模拟出来的。所以udhcp需要为wlan0提供dhcp服务，需要如下配置：
# The interface that udhcpd will use
interface wlan0 根据实际情况，指定udchpd提供IP地址的范围:
# The start and end of the IP lease block
start 192.168.1.2
end 192.168.1.254 指定租期文件存储位置，在启动udhcpd程序之前，需要先创建好这个文件(一般用touch创建一个空文件)，否则程序会报错：
# The location of the leases file
lease_file /var/udhcpd.leasesudhcp指定子网掩码和网关
option subnet 255.255.255.0
opt router 192.168.1.1udhcp指定dns服务器地址(可以存在多个)，一般是上一级网络的dns地址，比如AP共享的是eth0的网络,则可以把eth0的dns填写到这里，还可以添加常用的公共dns服务器，比如阿里的，或者114的。
opt dns 123.150.150.150 114.114.114.114udhcpd配置模板
# Sample udhcpd configuration file (/etc/udhcpd.conf)
# Values shown are defaults
# The start and end of the IP lease block
start 192.168.0.20
end 192.168.0.254
# The interface that udhcpd will use
interface eth0
# The maximum number of leases (includes addresses reserved
# by OFFER's, DECLINE's, and ARP conflicts). Will be corrected
# if it's bigger than IP lease block, but it ok to make it
# smaller than lease block.
#max_leases 254
# The amount of time that an IP will be reserved (leased to nobody)
# if a DHCP decline message is received (seconds)
#decline_time 3600
# The amount of time that an IP will be reserved
# if an ARP conflict occurs (seconds)
#conflict_time 3600
# How long an offered address is reserved (seconds)
#offer_time 60
# If client asks for lease below this value, it will be rounded up
# to this value (seconds)
#min_lease 60
# The location of the pid file
#pidfile /var/run/udhcpd.pid
# The location of the leases file
#lease_file /var/lib/misc/udhcpd.leases
# The time period at which udhcpd will write out leases file.
# If this is 0, udhcpd will never automatically write leases file.
# Specified in seconds.
#auto_time 7200
# Every time udhcpd writes a leases file, the below script will be called
#notify_file # default: no script
#notify_file dumpleases # useful for debugging
# The following are BOOTP specific options
# next server to use in bootstrap
#siaddr 192.168.0.22 # default: 0.0.0.0 (none)
# tftp server name
#sname zorak # default: none
# tftp file to download (e.g. kernel image)
#boot_file /var/nfs_root # default: none
# NOTE: "boot_file FILE" and "opt bootfile FILE" are conceptually the same,
# but "boot_file" goes into BOOTP-defined fixed-size field in the packet,
# whereas "opt bootfile" goes into DHCP option 0x43.
# Same for "sname HOST" and "opt tftp HOST".
# Static leases map
#static_lease 00:60:08:11:CE:4E 192.168.0.54
#static_lease 00:60:08:11:CE:3E 192.168.0.44 optional_hostname
# The remainder of options are DHCP options and can be specified with the
# keyword 'opt' or 'option'. If an option can take multiple items, such
# as the dns option, they can be listed on the same line, or multiple
# lines.
# Examples:
opt dns 192.168.10.2 192.168.10.10
option subnet 255.255.255.0
opt router 192.168.10.2
opt wins 192.168.10.10
option dns 129.219.13.81 # appended to above DNS servers for a total of 3
option domain local
option lease 864000 # default: 10 days
option msstaticroutes 10.0.0.0/8 10.127.0.1 # single static route
option staticroutes 10.0.0.0/8 10.127.0.1, 10.11.12.0/24 10.11.12.1
# Arbitrary option in hex or string form:
option 0x08 01020304 # option 8: "cookie server IP addr: 1.2.3.4"
option 14 "dumpfile"
# Currently supported options [hex option value] (for more info, see options.c):
#opt lease NUM # [0x33]
#opt subnet IP # [0x01]
#opt broadcast IP # [0x1c]
#opt router IP_LIST # [0x03]
#opt ipttl NUM # [0x17]
#opt mtu NUM # [0x1a]
#opt hostname STRING # [0x0c] client's hostname
#opt domain STRING # [0x0f] client's domain suffix
#opt search STRING_LIST # [0x77] search domains
#opt nisdomain STRING # [0x28]
#opt timezone NUM # [0x02] (localtime - UTC_time) in seconds. signed
#opt tftp STRING # [0x42] tftp server name
#opt bootfile STRING # [0x43] tftp file to download (e.g. kernel image)
#opt bootsize NUM # [0x0d] size of that file
#opt rootpath STRING # [0x11] (NFS) path to mount as root fs
#opt wpad STRING # [0xfc] Web Proxy Auto Discovery Protocol
#opt serverid IP # [0x36] default: server's IP
#opt message STRING # [0x38] error message (udhcpd sends it on success too)
#opt vlanid NUM # [0x84] 802.1P VLAN ID
#opt vlanpriority NUM # [0x85] 802.1Q VLAN priority
# RFC 5071: PXELINUX Options
#opt 0xd0 F100747E # [0xd0] magic
#opt pxeconffile STRING # [0xd1]
#opt pxepathprefix STRING # [0xd2]
#opt reboottime NUM # [0xd3] bootstrap timeout
# Options specifying server(s)
#opt dns IP_LIST # [0x06]
#opt wins IP_LIST # [0x2c]
#opt nissrv IP_LIST # [0x29]
#opt ntpsrv IP_LIST # [0x2a]
#opt lprsrv IP_LIST # [0x09]
#opt swapsrv IP # [0x10]
# Options specifying routes
#opt routes IP_PAIR_LIST # [0x21]
#opt staticroutes STATIC_ROUTES # [0x79] RFC 3442 classless static route option
#opt msstaticroutes STATIC_ROUTES # [0xf9] same, using MS option number
# Obsolete options, no longer supported
#opt logsrv IP_LIST # [0x07] 704/UDP log server (not syslog!)
#opt namesrv IP_LIST # [0x05] IEN 116 name server, obsolete (August 1979!!!)
#opt cookiesrv IP_LIST # [0x08] RFC 865 "quote of the day" server, rarely (never?) used
#opt timesrv IP_LIST # [0x04] RFC 868 time server, rarely (never?) used
# TODO: in development
#opt userclass STRING # [0x4d] RFC 3004. set of LASCII strings. "I am a printer" etc
#opt sipsrv STRING LIST # [0x78] RFC 3361. flag byte, then: 0: domain names, 1: IP addrs
#opt ip6rd .... # [0xd4] IPv6 rapid deployment