CentOS6下DHCP服务（二）简单配置案例及故障排查

1、预分配网络参数如下：
linux服务器：eth0 IP为192.168.8.250  做为局域网DHCP服务器
局域网网段设置为192.168.8.0/24；
内部计算机的router为192.168.8.254；
DNS主机为联通的202.102.224.68和202.102.227.68
每个用户默认租约为3天(3*24*60*60=259200秒），最长为6天
要分配的IP范围只有192.168.8.101至192.168.8.199，其他IP则保留

2、根据网络参数，编辑DHCP服务配置文件/etc/dhcp/dhcpd.conf如下：

#整体环境设置

ddns-update-style  none;
ignore client-update ;
default-lease-time  259200;
max-lease-time     518400;
option routers  192.168.8.254;
option domain-name    "centos.me"
option  domain-name-servers  202.102.224.68,202.102.227.68;

#IP分配

subnet  192.168.8.0 netmask 255.255.255.0 {
    range  192.168.8.101   192.168.8.199;
    }

3、dhcp服务器的启动与观察

3.1 启动前要注意
    1、linux服务器的网络环境已经设置好
    2、防火墙规则已经处理好

如果以上都设置好了那么现在开始启动dhcp
[root@NMS dhcp]# /etc/init.d/dhcpd start
正在启动 dhcpd：                                           [失败]

很不幸!失败了~那么为什么会失败呢？我们来查看下启动日志/var/log/messages
这里面记录了dhcp的启动过程，都发生了什么！

[root@NMS dhcp]# tail  -n 30 /var/log/messages
Aug 31 12:25:00 NMS rsyslogd: [origin software="rsyslogd" swVersion="5.8.10" x-pid="1214" x-info="http://www.rsyslog.com"] rsyslogd was HUPed
Aug 31 15:40:48 NMS dhcpd: Internet Systems Consortium DHCP Server 4.1.1-P1
Aug 31 15:40:48 NMS dhcpd: Copyright 2004-2010 Internet Systems Consortium.
Aug 31 15:40:48 NMS dhcpd: All rights reserved.
Aug 31 15:40:48 NMS dhcpd: For info, please visit https://www.isc.org/software/dhcp/
Aug 31 15:40:48 NMS dhcpd: /etc/dhcp/dhcpd.conf line 4: expecting allow/deny key
Aug 31 15:40:48 NMS dhcpd: ignore client-update
Aug 31 15:40:48 NMS dhcpd:         ^
Aug 31 15:40:48 NMS dhcpd: /etc/dhcp/dhcpd.conf line 4: expecting a parameter or declaration
Aug 31 15:40:48 NMS dhcpd: ignore client-update ;
Aug 31 15:40:48 NMS dhcpd:                       ^
Aug 31 15:40:48 NMS dhcpd: /etc/dhcp/dhcpd.conf line 9: semicolon expected.
Aug 31 15:40:48 NMS dhcpd: option
Aug 31 15:40:48 NMS dhcpd:  ^
Aug 31 15:40:48 NMS dhcpd: Configuration file errors encountered -- exiting
Aug 31 15:40:48 NMS dhcpd:
Aug 31 15:40:48 NMS dhcpd: This version of ISC DHCP is based on the release available
Aug 31 15:40:48 NMS dhcpd: on ftp.isc.org.  Features have been added and other changes
Aug 31 15:40:48 NMS dhcpd: have been made to the base software release in order to make
Aug 31 15:40:48 NMS dhcpd: it work better with this distribution.
Aug 31 15:40:48 NMS dhcpd:
Aug 31 15:40:48 NMS dhcpd: Please report for this software via the CentOS Bugs Database:
Aug 31 15:40:48 NMS dhcpd:     http://bugs.centos.org/
Aug 31 15:40:48 NMS dhcpd:
Aug 31 15:40:48 NMS dhcpd: exiting.

由日志我们看出来配置文件错误造成dhcp启动失败，“Configuration file errors encountered -- exiting”
那么我来看配置文件，根据提示找出其中的错误；

[root@NMS dhcp]# cat  -n  /etc/dhcp/dhcpd.conf
     1    #整体环境设置
     2    
     3    ddns-update-style  none;
     4    ignore client-update ;
     5    default-lease-time  259200;
     6    max-lease-time     518400;
     7    option routers  192.168.8.254;
     8    option domain-name    "centos.me"
     9    option  domain-name-servers  202.102.224.68,202.102.227.68;
    10    
    11    #IP分配
    12    
    13    subnet  192.168.8.0 netmask 255.255.255.0 {
    14        range  192.168.8.101   192.168.8.199;
    15        }
    16        
第一个错误信息表示配置文件第4行有错误，是关键字错误！仔细看是update少了个s应该是updates
Aug 31 15:40:48 NMS dhcpd: /etc/dhcp/dhcpd.conf line 4: expecting allow/deny key
Aug 31 15:40:48 NMS dhcpd: ignore client-update
第二个错误也是在第4行，根据指数符号（^）标注的位置，原来配置项结束后要紧接";"，我不小多了个空格，所以报错
Aug 31 15:40:48 NMS dhcpd: /etc/dhcp/dhcpd.conf line 4: expecting a parameter or declaration
Aug 31 15:40:48 NMS dhcpd: ignore client-update ;
第三个错误指出错误在第9行，可以看出我们在最后没有加";"所以报错，semicolon是分号的意思；
Aug 31 15:40:48 NMS dhcpd: /etc/dhcp/dhcpd.conf line 9: semicolon expected.
Aug 31 15:40:48 NMS dhcpd: option

按照日志信息更改后的配置文件如下：
[root@NMS etc]# cat /etc/dhcp/dhcpd.conf  -n
     1    #整体环境设置
     2    
     3    ddns-update-style  none;
     4    ignore client-updates;
     5    default-lease-time  259200;
     6    max-lease-time     518400;
     7    option routers  192.168.8.254;
     8    option domain-name    "centos.me";
     9    option  domain-name-servers  202.102.224.68,202.102.227.68;
    10    
    11    #IP分配
    12    
    13    subnet  192.168.8.0 netmask 255.255.255.0 {
    14        range  192.168.8.101   192.168.8.199;
    15        }
    16        
 
接下来再次启动DHCP，

[root@NMS etc]# /etc/init.d/dhcpd start
正在启动 dhcpd：                                           [确定]

可以看到这次能启动了。

DHCP使用的端口是port 67,查看端口是否监听；
[root@NMS etc]# netstat -tlunp |grep dhcp
udp        0      0 0.0.0.0:67                  0.0.0.0:*                               17017/dhcpd

再次查看日志文件输出信息

[root@NMS etc]# tail -n  10 /var/log/messages
Aug 31 16:09:16 NMS dhcpd: Internet Systems Consortium DHCP Server 4.1.1-P1
Aug 31 16:09:16 NMS dhcpd: Copyright 2004-2010 Internet Systems Consortium.
Aug 31 16:09:16 NMS dhcpd: All rights reserved.
Aug 31 16:09:16 NMS dhcpd: For info, please visit https://www.isc.org/software/dhcp/
Aug 31 16:09:16 NMS dhcpd: Not searching LDAP since ldap-server, ldap-port and ldap-base-dn were not specified in the config file
Aug 31 16:09:16 NMS dhcpd: Wrote 0 leases to leases file.
Aug 31 16:09:16 NMS dhcpd: Listening on LPF/eth0/00:0c:29:23:fe:20/192.168.8.0/24
Aug 31 16:09:16 NMS dhcpd: Sending on   LPF/eth0/00:0c:29:23:fe:20/192.168.8.0/24
Aug 31 16:09:16 NMS dhcpd: Sending on   Socket/fallback/fallback-net

看到这些信息说明成功了！没有报错！

4、服务器端已经准备好，万事俱备只欠东风了就等给客户端服务了！接下来看下客户端的设置
4.1客户端是linux

客户端环境
[root@nmserver-7 ~]# uname -a
Linux nmserver-7.test.com 3.10.0-514.el7.centos.plus.i686 #1 SMP Wed Jan 25 12:55:04 UTC 2017 i686 i686 i386 GNU/Linux
[root@nmserver-7 ~]# cat /etc/redhat-release
CentOS release 7.3.1611 (AltArch)

在客户端修改网卡配置文件，让主机自动取得网络参数；
[root@nmserver-7 ~]# vi /etc/sysconfig/network-scripts/ifcfg-ens33

TYPE=Ethernet
DEVICE=ens33
ONBOOT=yes
BOOTPROTO=dhcp

~                                                                                                                                                
                                                                                                                                            
~                                                                                                                                                
"/etc/sysconfig/network-scripts/ifcfg-ens33" 5L, 54C

注意：重点是将BOOTPROTO由原来的静态“static“，改为”dhcp“就可通过dhcp服务器来获取网络参数了！

修改配置文件后重启客户端网络
[root@nmserver-7 ~]# systemctl restart network

查看IP，可以看到客户端已经获取到的IP是192.168.8.101/24
[root@nmserver-7 ~]# ifconfig
bridge0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether b6:83:d4:66:3a:4c  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

ens33: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.8.101  netmask 255.255.255.0  broadcast 192.168.8.255
        inet6 fe80::20c:29ff:fe56:bccf  prefixlen 64  scopeid 0x20<link>
        ether 00:0c:29:56:bc:cf  txqueuelen 1000  (Ethernet)
        RX packets 29323  bytes 2119863 (2.0 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 29052  bytes 2256533 (2.1 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1  (Local Loopback)
        RX packets 2212  bytes 150063 (146.5 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 2212  bytes 150063 (146.5 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

查看一下客户端路由
[root@nmserver-7 ~]# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.8.254   0.0.0.0         UG    100    0        0 ens33
192.168.8.0     0.0.0.0         255.255.255.0   U     100    0        0 ens33

查看客户端DHCP连接状态，
[root@nmserver-7 ~]# netstat -tunlp |grep dhc
udp        0      0 0.0.0.0:68              0.0.0.0:*                           4195/dhclient       
udp        0      0 0.0.0.0:22424           0.0.0.0:*                           4195/dhclient       
udp6       0      0 :::56121                :::*                                4195/dhclient       
查看客户端DNS设置
[root@nmserver-7 dhclient]# cat /etc/resolv.conf
# Generated by NetworkManager
search centos.me test.com
nameserver 202.102.224.68
nameserver 202.102.227.68

通过以上信息可以看到客户端获取的网络参数与之前DHCP服务器上设置的一致！！

那么在客户端网络重启的过程中，DHCP服务器都做了什么呢？我们来看下DHCP服务器的日志信息，可以清楚看到
DHCP协议工作的4个过程：

[root@NMS etc]# tail -n  10 /var/log/messages
Aug 31 16:09:16 NMS dhcpd: Listening on LPF/eth0/00:0c:29:23:fe:20/192.168.8.0/24
Aug 31 16:09:16 NMS dhcpd: Sending on   LPF/eth0/00:0c:29:23:fe:20/192.168.8.0/24
Aug 31 16:09:16 NMS dhcpd: Sending on   Socket/fallback/fallback-net
Aug 31 16:52:21 NMS dhcpd: DHCPDISCOVER from 00:0c:29:56:bc:cf via eth0                             #IP租用请求
Aug 31 16:52:22 NMS dhcpd: DHCPOFFER on 192.168.8.101 to 00:0c:29:56:bc:cf (nmserver-7) via eth0　　#IP租用提供
Aug 31 16:52:22 NMS dhcpd: DHCPREQUEST for 192.168.8.101 (192.168.8.6) from 00:0c:29:56:bc:cf (nmserver-7) via eth0 #IP租用选择
Aug 31 16:52:22 NMS dhcpd: DHCPACK on 192.168.8.101 to 00:0c:29:56:bc:cf (nmserver-7) via eth0  #IP租用确认

查看DHCP服务器与客户端的租约信息，

[root@NMS etc]# cat /var/lib/dhcpd/dhcpd.leases
# The format of this file is documented in the dhcpd.leases(5) manual page.
# This lease file was written by isc-dhcp-4.1.1-P1

lease 192.168.8.101 {
  starts 4 2017/08/31 09:22:42;
  ends 0 2017/09/03 09:22:42;
  cltt 4 2017/08/31 09:22:42;
  binding state active;
  next binding state free;
  hardware ethernet 00:0c:29:56:bc:cf;
  client-hostname "nmserver-7";
}
server-duid "\000\001\000\001!:\200\254\000\014)#\376 ";

从这个文件里面我们就知道有多少个客户端已经向DHCP申请IP了。目前只有一个客户端：
客户端主机名：nmserver-7
MAC地址：00:0c:29:56:bc:cf
客户端IP：192.168.8.101