一、DHCP的工作流程
(1) DHCP伺服器發現階段。DHCP客戶機向廣播地址255.255.255.255發送DHCPDiscover消息。
(2) DHCP伺服器響應階段。DHCP伺服器從尚未出租的IP地址池中選擇一個分配給DHCP客戶機,向DHCP客戶機發送一個包含出租的IP地址和其他設置的DHCPoffer提供信息。
(3) IP地址選擇階段。DHCP客戶機選擇某個DHCP伺服器提供的IP地址,以廣播方式回答一個DHCPrequest請求信息.
(4) IP地址確認階段。伺服器回應包含它所提供的IP地址和其他設置的DHCPack確認信息。然後客戶機將其與TCP/IP協議與網卡綁定。
(5) 客戶機重新登錄。再次發送前次分配的IP地址的DHCPRequest請求請求信息,DHCP伺服器檢查是否可以繼續讓客戶機使用,然後發回DHCPack確認信息或者DHCPnck否認信息。如果IP已不能用,客戶機需從(1)開始重新申請。
(6) 更新租約。IP地址租借期限到了,如果DHCP客戶機要延長其租約,必須更新。DHCP客戶機啟動時和IP租約期限過一半時,都會向伺服器發送更新期IP租約的信息。
二、DHCP伺服器配置文件
路徑在/etc/dhcp/dhcpd.conf。下面是一個配置文件的例子:
- # dhcpd.conf
- #
- # Sample configuration file for ISC dhcpd
- #
-
- # option definitions common to all supported networks...
- option domain-name "example.org";
- option domain-name-servers ns1.example.org, ns2.example.org;
-
- default-lease-time 600;
- max-lease-time 7200;
-
- # Use this to enble / disable dynamic dns updates globally.
- #ddns-update-style none;
-
- # If this DHCP server is the official DHCP server for the local
- # network, the authoritative directive should be uncommented.
- #authoritative;
-
- # Use this to send dhcp log messages to a different log file (you also
- # have to hack syslog.conf to complete the redirection).
- log-facility local7;
-
- # No service will be given on this subnet, but declaring it helps the
- # DHCP server to understand the network topology.
-
- subnet 10.152.187.0 netmask 255.255.255.0 {
- }
-
- # This is a very basic subnet declaration.
-
- subnet 10.254.239.0 netmask 255.255.255.224 {
- range 10.254.239.10 10.254.239.20;
- option routers rtr-239-0-1.example.org, rtr-239-0-2.example.org;
- }
-
- # This declaration allows BOOTP clients to get dynamic addresses,
- # which we don't really recommend.
-
- subnet 10.254.239.32 netmask 255.255.255.224 {
- range dynamic-bootp 10.254.239.40 10.254.239.60;
- option broadcast-address 10.254.239.31;
- option routers rtr-239-32-1.example.org;
- }
-
- # A slightly different configuration for an internal subnet.
- subnet 10.5.5.0 netmask 255.255.255.224 {
- range 10.5.5.26 10.5.5.30;
- option domain-name-servers ns1.internal.example.org;
- option domain-name "internal.example.org";
- option routers 10.5.5.1;
- option broadcast-address 10.5.5.31;
- default-lease-time 600;
- max-lease-time 7200;
- }
-
- # Hosts which require special configuration options can be listed in
- # host statements. If no address is specified, the address will be
- # allocated dynamically (if possible), but the host-specific information
- # will still come from the host declaration.
-
- host passacaglia {
- hardware ethernet 0:0:c0:5d:bd:95;
- filename "vmunix.passacaglia";
- server-name "toccata.fugue.com";
- }
-
- # Fixed IP addresses can also be specified for hosts. These addresses
- # should not also be listed as being available for dynamic assignment.
- # Hosts for which fixed IP addresses have been specified can boot using
- # BOOTP or DHCP. Hosts for which no fixed address is specified can only
- # be booted with DHCP, unless there is an address range on the subnet
- # to which a BOOTP client is connected which has the dynamic-bootp flag
- # set.
- host fantasia {
- hardware ethernet 08:00:07:26:c0:a5;
- fixed-address fantasia.fugue.com;
- }
-
- # You can declare a class of clients and then do address allocation
- # based on that. The example below shows a case where all clients
- # in a certain class get addresses on the 10.17.224/24 subnet, and all
- # other clients get addresses on the 10.0.29/24 subnet.
-
- class "foo" {
- match if substring (option vendor-class-identifier, 0, 4) = "SUNW";
- }
-
- shared-network 224-29 {
- subnet 10.17.224.0 netmask 255.255.255.0 {
- option routers rtr-224.example.org;
- }
- subnet 10.0.29.0 netmask 255.255.255.0 {
- option routers rtr-29.example.org;
- }
- pool {
- allow members of "foo";
- range 10.17.224.10 10.17.224.250;
- }
- pool {
- deny members of "foo";
- range 10.0.29.10 10.0.29.230;
- }
- }
DHCP配置文件通常包括3部分:declarations, parameters, options。
declarations:描述網路的布局;描述客戶;提供客戶的地址。
parameters:表明如何執行任務,是否要執行任務,或將哪些網路配置選項發送給客戶。
option:配置DHCP可選參數。
常用聲明:
shared-network 告知DHCP伺服器是否為一些子網路分享相同網路
subnet 描述一個IP是否屬於子網
range 提供動態分配IP地址的起始和結束範圍
host 為特定的主機提供網路參數
group 為一組參數提供聲明
常用參數:
ddns-update-style 配置DHCP-DNS互動更新模式
default-lease-time 指定默認租賃時間的長度,單位為秒
max-lease-time 指定最大租賃時間長度,單位為秒
hardware 指定網卡介面類型和MAC地址
server-name 告知DHCP客戶伺服器名稱
fixed-address 為客戶端指定一個固定的IP地址
常用選項:
domain-name 為客戶端指定域名
domain-name-servers為客戶端指明DNS伺服器IP地址
host-name 為客戶端指明主機名稱
routers 為客戶端指明默認網關
broadcast-address 為客戶端設定廣播地址
subnet-mask 為客戶端設定子網掩碼
ntp-server 為客戶端設定格林威治時間的偏移時間,單位為秒
下面是一個DHCPv6的配置文件的例子:
- # Server configuration file example for DHCPv6
- # From the file used for TAHI tests.
-
- # IPv6 address valid lifetime
- # (at the end the address is no longer usable by the client)
- # (set to 30 days, the usual IPv6 default)
- default-lease-time 2592000;
-
- # IPv6 address preferred lifetime
- # (at the end the address is deprecated, i.e., the client should use
- # other addresses for new connections)
- # (set to 7 days, the usual IPv6 default)
- preferred-lifetime 604800;
-
- # T1, the delay before Renew
- # (default is 1/2 preferred lifetime)
- # (set to 1 hour)
- option dhcp-renewal-time 3600;
-
- # T2, the delay before Rebind (if Renews failed)
- # (default is 3/4 preferred lifetime)
- # (set to 2 hours)
- option dhcp-rebinding-time 7200;
-
- # Enable RFC 5007 support (same than for DHCPv4)
- allow leasequery;
-
- # Global definitions for name server address(es) and domain search list
- option dhcp6.name-servers 3ffe:501:ffff:100:200:ff:fe00:3f3e;
- option dhcp6.domain-search "test.example.com","example.com";
-
- # Set preference to 255 (maximum) in order to avoid waiting for
- # additional servers when there is only one
- ##option dhcp6.preference 255;
-
- # Server side command to enable rapid-commit (2 packet exchange)
- ##option dhcp6.rapid-commit;
-
- # The delay before information-request refresh
- # (minimum is 10 minutes, maximum one day, default is to not refresh)
- # (set to 6 hours)
- option dhcp6.info-refresh-time 21600;
-
- # The path of the lease file
- dhcpv6-lease-file-name "/usr/local/var/db/dhcpd6.leases";
-
- # Static definition (must be global)
- host myclient {
- # The entry is looked up by this
- host-identifier option
- dhcp6.client-id 00:01:00:01:00:04:93:e0:00:00:00:00:a2:a2;
-
- # A fixed address
- fixed-address6 3ffe:501:ffff:100::1234;
-
- # A fixed prefix
- fixed-prefix6 3ffe:501:ffff:101::/64;
-
- # Override of the global definitions,
- # works only when a resource (address or prefix) is assigned
- option dhcp6.name-servers 3ffe:501:ffff:100:200:ff:fe00:4f4e;
-
- # For debug (to see when the entry statements are executed)
- # (log "sol" when a matching Solicitation is received)
- ##if packet(0,1) = 1 { log(debug,"sol"); }
- }
-
- # The subnet where the server is attached
- # (i.e., the server has an address in this subnet)
- subnet6 3ffe:501:ffff:100::/64 {
- # Two addresses available to clients
- # (the third client should get NoAddrsAvail)
- range6 3ffe:501:ffff:100::10 3ffe:501:ffff:100::11;
-
- # Use the whole /64 prefix for temporary addresses
- # (i.e., direct application of RFC 4941)
- range6 3ffe:501:ffff:100:: temporary;
-
- # Some /64 prefixes available for Prefix Delegation (RFC 3633)
- prefix6 3ffe:501:ffff:100:: 3ffe:501:ffff:111:: /64;
- }
-
- # A second subnet behind a relay agent
- subnet6 3ffe:501:ffff:101::/64 {
- range6 3ffe:501:ffff:101::10 3ffe:501:ffff:101::11;
-
- # Override of the global definitions,
- # works only when a resource (address or prefix) is assigned
- option dhcp6.name-servers 3ffe:501:ffff:101:200:ff:fe00:3f3e;
-
- }
-
- # A third subnet behind a relay agent chain
- subnet6 3ffe:501:ffff:102::/64 {
- range6 3ffe:501:ffff:102::10 3ffe:501:ffff:102::11;
- }