Problem solving process of no internet access

Colleagues give feedback in the group , Administrative Area 192.168.207.0/24 The network segment has some pc Unable to access the Internet , Let the coordination solve .

Possible reasons for not being able to access the Internet ：

1、 Unable to get gateway mac Address , Layer 2 cannot be packaged , Most of them occur when the broadcast domain is not connected to the wrong broadcast domain or the gateway is caused by a loop arp The response could not be sent .

2、dns fault ,dns The address becomes the wrong address , Resolve the wrong address or the domain name cannot be resolved . There is impersonation in the environment dns The equipment .

3、ip Conflict , Lead to ip become 169.254.xxx.xxx, Unable to get gateway mac Address , As a result, the assembly of the package cannot be completed .

4、pc The network card is set to automatically obtain ip,dhcp Failed to get address , No default route was generated .

5、 A device pretends to be a gateway ip, This causes the package to be sent to an error mac Address .

6、 Own equipment problems , The network protocol is not in effect ,ping127.0.0.1 no .

Asked about the situation , The feedback system tray network logo turns yellow exclamation point , I suspect there is ip Conflict , Let's see ip Whether the address becomes 169.254.xx.xx, Screenshot feedback is fixed ip, Did not become 169.254.xx.xx, But it will recover .

Busy with other things , Let's keep watching , Pay attention to whether the network cable is unplugged or ip Address becomes 169.254.xxx.xxx Display of ？

There was feedback from others , can ping Through gateway , But no internet access , Let me judge whether dns error ？

There have been... Before , There are devices in the environment that pretend to be ipv6 Of dns About the server , Let's refer to the positioning solution in the blog .

Reference resources https://blog.csdn.net/wj31932/article/details/114985121?spm=1001.2014.3001.5502

feedback  

Let me grab my bag ,cmd Next ipconfig/flushdns,ping baidu.com

Grab a screenshot ：

Look at the screenshot , Find out dns And visit a ip There was no response , Let me send you my bag , feedback pc unable ping through 192.168.205.1, Unable to access the Internet , Can't send out .

It is suspected that a device in the environment pretends to be a gateway , Let's view the gateway 192.168.207.1 Of mac Address

  The gateway is a layer 3 switch , Each network segment 192.168.205.1,192.168.207.1,MAC The address is

c8-50-e9-67-fa-0c

The feedback is 192.168.207.1 Of mac yes 00-0e-5e-00-00-01

  Other feedback is this

Confirm that a device in the environment impersonates a gateway ip Address , Cause packets to be sent to the gateway , Wrong place

Let's execute mac Address static binding arp  -s  192.168.207.1  c8-50-e9-67-fa-0c

Some people give feedback , You can go online , Some feedback has not been implemented yet .

I wanted to bind it , It seems that we can find out where this device is , Close the corresponding port of the switch to locate .

 

Give feedback when you have any questions ok 了

Based on past experience ,shutdown Port , The murderer will turn himself in .

For a while , Someone jumped out and said , His test environment doesn't work , Let go .

Explain why , Let's ip Changed .

After correction , Found this mac Of ip Be modified ：

Problem cause analysis ：

There are and in the broadcast domain 192.168.207.1 Conflicting ip, Some devices cause arp request , Be wrong mac Address substitution , The package was sent to an error mac, It leads to the inability to access the Internet .