DHCP的若干原理解释(3)

dhcp server收到后dhcp discover报文后，根据一定的策略来给client分配资源,如ip地址，发出dhcp offer报文。而ip地址则通常由网络管理员或系统自动分配，价格昂贵，第三层交换机既有交换机线速转发报文能力，又有路由器良好的控制功能，然后到达特定的节点站地址。一般由程序员分配释放(动态内存申请与释放),若程序员不释放,程序结束时可能由操作系统回收。

上面提到的都是在已经开机的情况下获得的报文，如果计算机重新启动，是不是完全按照文章提到的四步得到IP参数。

其中包含关机前获得的IP=192.168.0.22，这是不同与前面提到的10.177.124.X段的IP。原来客户机保留了上一次获得的地址，客户机在重新启动时，如果租用期未超过50%，它就会认为它已经知道该与哪台DHCP服务器进行联系，于是就跳过前两步，并发送另一个DhcpRequest报文给同一个服务器。如果该IP地址仍然可用的话，该DHCP服务器将给这台客户机返回另一个确认消息。同样，DHCP服务器上也会保留（在租用期内的）客户机的地址。如果你的DHCP服务器中的租用期设的足够长，你就会发现，重启一台客户机后，客户机总是得到同一个地址。这正是由于客户机、服务器能保存已分配地址所造成的。

如果一台客户机未从该DHCP服务器获取过地址，或者它获得的地址已过期，那么它将需要经过全部四个步骤才能得到一个IP地址。

由于图太多没有将图上传。

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DHCP RFC

2009-09-02 13:26

有幸拜读了DHCP RFC文档,只有一句话,真不是写个人看得.飘忽不定,像做广告般的参考xxx参考xxx.无语.

DHCP的报文格式

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DHCP的报文格式，如图1，

OP(1)

Htype(1)

Hlen(1)

Hops(1)

Transaction ID(4)

Seconds(2)

Flags(2)

Ciaddr（4）

Yiaddr（4）

Siaddr（4）

Giaddr（4）

Chaddr（16）

Sname（64）

File（128）

Options（variable）

（图1 DHCP的 报文格式）

OP：若是client送给server的封包，设为1，反向为2；

Htype：硬件类别，ethernet为1；

Hlen：硬件长度，ethernet为6；

Hops：若数据包需经过router传送，每站加1，若在同一网内，为0；

Transaction ID：事务ID，是个随机数，用于客户和服务器之间匹配请求和相应消息；

Seconds：由用户指定的时间，指开始地址获取和更新进行后的时间；

Flags：从0-15bits，最左一bit为1时表示server将以广播方式传送封包给 client，其余尚未使用；

Ciaddr：用户IP地址；

Yiaddr：客户IP地址；

Siaddr：用于bootstrap过程中的IP地址；

Giaddr：转发代理（网关）IP地址；

Chaddr：client的硬件地址；

Sname：可选server的名称，以0x00结尾；

File：启动文件名；

Options：，厂商标识，可选的参数字段

还是wiki的人性化点

DHCP uses the same two IANA assigned ports as BOOTP: 67/udp for the server side, and 68/udp for the client side.

DHCP operations fall into four basic phases. Thesephases are IP discovery, IP lease offer, IP request, and IP leaseacknowledgement.

After the client obtained an IP address, the client may start an address resolution (ARP) query to prevent IP conflicts caused by address pool overlapping of DHCP servers.

on the configure instance details page, select your vpc from thenetwork list, and your subnet from thesubnet list. choose next until you reach theconfigure security group page.。note that, although the intent class is used for sending and receiving these broadcasts, the intent broadcast mechanism here is completely separate from intents that are used to start activities with context.startactivity(). there is no way for a broadcastreceiver to see or capture intents used with startactivity()。# if you want to listen to time broadcasts on your local subnet, de-comment the。

A client can also request its last-known IP address(in the example below, 192.168.1.100). If the client is still in anetwork where this IP is valid, the server might grant the request.Otherwise, it depends whether the server is set up as authoritativeor not. An authoritative server will deny the request, making theclient ask for a new IP immediately. A non-authoritative server simplyignores the request, leading to an implementation-dependent timeout forthe client to give up on the request and ask for a new IP address.

void swap(ipaddress& address)。1 - begin your cover letter by placing your address first, followed by the address of the company you are writing to.。lpvoid *ppv // address of output variable that receives。

The server determines the configuration, based on theclient's hardware address as specified in the CHADDR field. Here theserver, 192.168.1.1, specifies the IP address in the YIADDR field.

When the client PC receives an IP lease offer, itmust tell all the other DHCP servers that it has accepted an offer. Todo this, the client broadcasts a DHCPREQUEST message containing the IPaddress of the server that made the offer. When the other DHCP serversreceive this message, they withdraw any offers that they might havemade to the client. They then return the address that they had reservedfor the client back to the pool of valid addresses that they can offerto another computer. Any number of DHCP servers can respond to an IPlease request, but the client can only accept one offer per networkinterface card.

When the DHCP server receives the DHCPREQUEST messagefrom the client, which is basically a unicast packet unlike theDHCPDISCOVER packet as the client now knows who the server is. Thisinitiates the final phase of the configuration process. Theacknowledgement phase involves sending a DHCPACK packet to the client.This packet includes the lease duration and any other configurationinformation that the client might have requested. At this point, the IPconfiguration process is complete.

The server acknowledges the request and sends theacknowledgement to the client. The system as a whole expects the clientto configure its network interface with the supplied options.

DHCPDISCOVER

UDP Src=0.0.0.0

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