DHCP的若干原理解释(5)

0x00000000

YIADDR (Your IP Address)

0xC0A80164

SIADDR (Server IP Address)

0x00000000

GIADDR (Gateway IP Address switched by relay)

0x00000000

CHADDR (Client Hardware Address)

0x00053C04

0x8D590000

0x00000000

0x00000000

192 octets of 0's. BOOTP legacy

Magic Cookie

0x63825363

DHCP Options

DHCP option 53: DHCP ACK

DHCP option 1: 255.255.255.0 subnet mask

DHCP option 3: 192.168.1.1 router

DHCP option 51: 1 day IP lease time

DHCP option 54: 192.168.1.1 DHCP server

服务端web应用可以在不影响用户体验的情况下，向后端服务发送大量请求，但手机客户端只能发送少量的请求。假如主机a开始发送一个udp信息到主机b的公网地址上，与此同时，它又通过服务器中转发送了一个邀请信息给主机b，请求主机b也给主机a发送一个udp信息到主机a的公网地址上。首先主机b向主机a发送一个arp应答包说192，让所有上网的流量必须经过arp攻击者控制的主机，然后目标ip的所有者会以一个含有ip和mac地址对的数据包应答请求主机，直到通信停止后2分钟，这个对应关系才会从表中被删除，如果没有就通过arp请求获得，如果有就直接与网关通信，然后再由网关c通过路由将数据包送到网关e.1。

The client to the DHCP server: either to request moreinformation than the server sent with the original DHCPACK; or torepeat data for a particular application - for example, browsers use DHCP Inform to obtain web proxy settings via WPAD. Such queries do not cause the DHCP server to refresh the IP expiry time in its database.

The client sends a request to the DHCP server torelease the DHCP and the client unconfigures its IP address. As clientsusually do not know when users may unplug them from the network, theprotocol does not mandate the sending of DHCP Release.

Field      DHCPDISCOVER          DHCPREQUEST           DHCPDECLINE,
           DHCPINFORM                                  DHCPRELEASE
-----      ------------          -----------           -----------
'op'       BOOTREQUEST           BOOTREQUEST           BOOTREQUEST
'htype'    (From "Assigned Numbers" RFC)
'hlen'     (Hardware address length in octets)
'hops'     0                     0                     0
'xid'      selected by client    'xid' from server     selected by
                                 DHCPOFFER message     client
'secs'     0 or seconds since    0 or seconds since    0
           DHCP process started  DHCP process started
'flags'    Set 'BROADCAST'       Set 'BROADCAST'       0
           flag if client        flag if client
           requires broadcast    requires broadcast
           reply                 reply
'ciaddr'   0 (DHCPDISCOVER)      0 or client's         0 (DHCPDECLINE)
           client's              network address       client's network
           network address       (BOUND/RENEW/REBIND)  address
           (DHCPINFORM)                                (DHCPRELEASE)
'yiaddr'   0                     0                     0
'siaddr'   0                     0                     0
'giaddr'   0                     0                     0
'chaddr'   client's hardware     client's hardware     client's hardware
           address               address               address
'sname'    options, if           options, if           (unused)
           indicated in          indicated in
           'sname/file'          'sname/file'
           option; otherwise     option; otherwise
           unused                unused
'file'     options, if           options, if           (unused)
           indicated in          indicated in
           'sname/file'          'sname/file'
           option; otherwise     option; otherwise
           unused                unused
'options'  options               options               (unused)

Option                     DHCPDISCOVER  DHCPREQUEST      DHCPDECLINE,
                           DHCPINFORM                     DHCPRELEASE
------                     ------------  -----------      -----------
Requested IP address       MAY           MUST (in         MUST
                           (DISCOVER)    SELECTING or     (DHCPDECLINE),
                           MUST NOT      INIT-REBOOT)     MUST NOT
                           (INFORM)      MUST NOT (in     (DHCPRELEASE)
                                         BOUND or
                                         RENEWING)
IP address lease time      MAY           MAY              MUST NOT
                           (DISCOVER)
                           MUST NOT
                           (INFORM)
Use 'file'/'sname' fields  MAY           MAY              MAY
DHCP message type          DHCPDISCOVER/ DHCPREQUEST      DHCPDECLINE/
                           DHCPINFORM                     DHCPRELEASE
Client identifier          MAY           MAY              MAY
Vendor class identifier    MAY           MAY              MUST NOT
Server identifier          MUST NOT      MUST (after      MUST
                                         SELECTING)
                                         MUST NOT (after
                                         INIT-REBOOT,
                                         BOUND, RENEWING
                                         or REBINDING)
Parameter request list     MAY           MAY              MUST NOT
Maximum message size       MAY           MAY              MUST NOT
Message                    SHOULD NOT    SHOULD NOT       SHOULD
Site-specific              MAY           MAY              MUST NOT
All others                 MAY           MAY              MUST NOT

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