Linux无法解析域名的背后原因
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生产环境：
操作系统：CentOS release 6.8 (Final)
问题：无法解析域名，无法ping ip,公网不畅通。
分析：由电信专线换成联通专线，很有可能是这种原因造成的，但是IP和DNS都换成了联通啊。
解决问题过程：
[[email protected] ~]#wget https://cn.wordpress.org/wordpress-4.8.1-zh_CN.tar.gz
--2017-09-0410:46:26-- https://cn.wordpress.org/wordpress-4.8.1-zh_CN.tar.gz
正在解析主机 cn.wordpress.org... 失败：域名解析暂时失败。
wget: 无法解析主机地址 “cn.wordpress.org”
[[email protected] ~]#ping www.hao123.com #无法解析
ping: unknown host www.hao123.com
[[email protected] ~]#cat /etc/resolv.conf
nameserver202.101.224.68 #这个是电信的DNS，要改吗？
nameserver 8.8.8.8
nameserver 8.8.4.4
[[email protected] ~]#cat /etc/resolv.conf
nameserver220.248.192.12 #修改成联通的DNS
nameserver 8.8.8.8
nameserver 8.8.4.4
[[email protected] ~]#ping www.hao123.com
ping: unknown host www.hao123.com #还是无法解析
[[email protected] ~]# netstat –rn #查看路由信息，发现联通网关不存在
Kernel IP routingtable
Destination Gateway Genmask Flags MSS Window irtt Iface
113.195.210.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
117.40.239.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
0.0.0.0 117.40.239.1 0.0.0.0 UG 0 0 0 eth0
[[email protected] ~]#route add default gw 113.195.210.1 #添加联通网关
[[email protected] ~]#netstat -rn
Kernel IP routingtable
Destination Gateway Genmask Flags MSS Window irtt Iface
113.195.210.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
117.40.239.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
0.0.0.0 113.195.210.1 0.0.0.0 UG 0 0 0 eth0
0.0.0.0 117.40.239.1 0.0.0.0 UG 0 0 0 eth0
[[email protected] ~]#ping www.hao123.com #可以解析了,但延时严重哦
PING hao123.n.shifen.com(112.80.248.211) 56(84) bytes of data.
64 bytes from112.80.248.211: icmp_seq=1 ttl=55 time=31.0 ms
64 bytes from112.80.248.211: icmp_seq=2 ttl=55 time=33.4 ms
64 bytes from 112.80.248.211:icmp_seq=3 ttl=55 time=31.0 ms
64 bytes from112.80.248.211: icmp_seq=4 ttl=55 time=31.1 ms
64 bytes from112.80.248.211: icmp_seq=5 ttl=55 time=31.1 ms
[[email protected] ~]#wget https://cn.wordpress.org/wordpress-4.8.1-zh_CN.tar.gz
--2017-09-0411:11:47-- https://cn.wordpress.org/wordpress-4.8.1-zh_CN.tar.gz
正在解析主机 cn.wordpress.org... 66.155.40.250, 66.155.40.249
正在连接 cn.wordpress.org|66.155.40.250|:443... 已连接。
已发出 HTTP 请求，正在等待回应... 200 OK
长度：8641990 (8.2M) [application/octet-stream]
正在保存至: “wordpress-4.8.1-zh_CN.tar.gz”
8% [===> ]716,386 399K/s ^Z
[8]+ Stopped wgethttps://cn.wordpress.org/wordpress-4.8.1-zh_CN.tar.gz
小结：最终原因是在设置网关时少写了一个字母“T”，同时把备份的"ifcfg-eth0"相关文件删除，重新启动网络服务，会把相对应的电信IP和网关自动删除掉，从而避免冲突。
[[email protected] network-scripts]#vi /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=static
IPADDR=113.195.*.*
NETMASK=255.255.255.0
GATEWAY=113.195.*.* #少写了一个字母”T”，导致找不到网关
DNS=220.248.192.12 #联通DNS
HWADDR=00:21:97:68:F2:70
TYPE=Ethernet
UUID=480c82ca-d673-466d-902a-9f31eaeb897c
ONBOOT=yes
NM_CONTROLLED=no
到这里把所有上述问题解决掉了。
顺便测试一下联通网络：
[[email protected] network-scripts]# speedtest-cli --share
Retrieving speedtest.net configuration...
Retrieving speedtest.net server list...
Testing from China Unicom (113.195.210.147)...
Selecting best server based on latency...
Hosted by ChinaTelecom.Hunan (Changsha) [107.66 km]: 34.473 ms
Testing download speed........................................
Download: 79.57 Mbit/s #下载速度
Testing upload speed..................................................
Upload: 48.33 Mbit/s #上传速度
Share results: https://www.speedtest.net/result/6594856504.png #测试结果
[[email protected] network-scripts]# speedtest-cli
Retrieving speedtest.net configuration...
Retrieving speedtest.net server list...
Testing from China Unicom (113.195.210.147)...
Selecting best server based on latency...
Hosted by ChinaTelecom.Hunan (Changsha) [107.66 km]: 36.537 ms
Testing download speed........................................
Download: 49.98 Mbit/s #下载速度
Testing upload speed..................................................
Upload: 45.33 Mbit/s #上传速度
[[email protected] network-scripts]# speedtest-cli
Retrieving speedtest.net configuration...
Retrieving speedtest.net server list...
Testing from China Unicom (113.195.210.147)...
Selecting best server based on latency...
Hosted by ChinaTelecom.Hunan (Changsha) [107.66 km]: 45.695 ms
Testing download speed........................................
Download: 81.35 Mbit/s
Testing upload speed..................................................
Upload: 22.21 Mbit/s
本文出自 “sky9890” 博客，请务必保留此出处http://sky9896.blog.51cto.com/2330653/1962474
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