Visit Google homepage to display this page, which cannot be displayed

When we visit Google When the home page shows that the page cannot be displayed , What happened?

First, we enter www.goole.com

Our host will send a ARP Request message , A broadcast packet , Send a message to all hosts in this network segment , I want to know the gateway address that can take us to the Internet , Gateway sees ip The address is its own , give ARP reply

Then our application layer DNS start-up , Configured for us DNS The server sends a message DNS Query packets , and DNS The default server is outside the gateway by default , That is, we must successfully configure the gateway address before we can correctly receive DNS Respond to , To get the corresponding Google domain name IP Address , To establish a connection between two hosts . Under normal circumstances , Our host knows the default from the network configuration already configured in our host DNS The server , Send them DNS request , And it is recursive query , That is, if there is no , Instead of the client, send DNS request , Finally, we take a DNS Respond to the packet to obtain the relevant domain name IP Address .

Be careful :
1. If we were local hosts There are relevant domain name mappings in the file , We will not initiate DNS analysis
2. If the browser DNS There is relevant information in the cache , We will not initiate DNS analysis

We know Google ip After the address , We will establish TCP Shake hands three times to connect and then send HTTP request , Finally HTTP Reply and TCP Four handshakes end the connection .

Cause of error :
If our gateway address is configured incorrectly, our DNS The request message cannot reach through the router at all Internet, That is, we take a series of DNS request , End the conversation , What passed to our website is , Web page not accessible .

Redirection error :
Input www.google.com Then our host sent it directly ARP request : The corresponding is who has This ip Address , obtain ARP Respond to : This ip Address of the Mac Address .
The gateway address is not requested , The next one is not initiated by the external server DNS Request packet , It is TCP Of SYN paragraph , What you get is sent back by the corresponding intranet address RST,ACK Data packets , Is the packet that ends the connection .

Explain us hosts There is corresponding address mapping in the file , This leads us to think Google In the local network segment , Got it ARP After responding, we try to 80 Port initiation TCP Connect , Because the system is not configured to Web The server , Therefore, it cannot accept this connection request , In response RST Data packets . Then our host sends TCP spurious retransmission,TCP False retransmission , Receive RST Respond to .

From the above analysis, we can know , Redirection causes us to locate other designated hosts when we visit the web page , Specified page , adopt Wireshark Through packet capturing analysis, we can know , Missing the key DNS Requests and responses .

To sum up , If our host ARP The cache table contains the corresponding gateway router Mac-IP cache , Then... Can be missing in the flow ARP Requests and responses .
without DNS Requests and responses , It's probably ours DNS Caching or hosts There are... In the file IP- Domain mapping .

By analyzing the data packets, we can know that the server configuration is wrong , Or our host configuration is wrong , You can also know whether DNS Server error , Or the router goes wrong .