Practical experience: Writing and debugging VHost/Virtio drivers in QEMU

In this issue, we will share with you the causes of common network problems.

Whether you are a user, IT manager, contractor or integrator, the five most common network problems we encounter are: illegal DHCP server access, improper DHCP service configuration, ARP attack, broadcast storm, and DNS server abnormality. So how do you troubleshoot and solve them?

1. Illegal DHCP server
(1) Problem phenomenon
The terminal cannot access the Internet after obtaining the IP address and other network parameters. After checking the network parameters obtained by the terminal, it is found that it is not the correct address assigned by the DHCP server. For example, the normal gateway IP is 192.168.96.1, but the gateway obtained by the terminal is 192.168.1.1. An example of how to check the network parameters is as follows:


(2) Cause of the problem

Some devices are not properly configured: Some devices also have the DHCP server enabled by default, but are not configured and directly connected to the internal network;
Illegal device access: For example, some customers directly and illegally connect some network devices to the network.
(3) Troubleshooting ideas and solutions


2. Improper DHCP server configuration
(1) Symptom
The terminal cannot obtain an IP address when connected to the network or cannot connect to the network after obtaining the IP address set by the server.

(2) Cause of the problem

Insufficient IP addresses in the address pool or too long an address lease: resulting in insufficient IP addresses to be allocated when the device accesses the network;
Wrong gateway or DNS settings: resulting in the terminal obtaining the wrong gateway and DNS and being unable to connect to the network.
(3) Troubleshooting ideas and solutions
(4) Improper DHCP server configuration - correct configuration method


3. ARP attack
(1) Symptom
The terminal is disconnected or unstable. Check the ARP table of the computer or gateway and find that the IP and MAC mapping table is abnormal.
(2) Cause

Illegal device attack: Illegal devices intercept the target host's Internet data through ARP attacks;
IP address conflict: Illegal devices mistakenly use the same IP address as existing normal devices.
(3) Troubleshooting ideas and solutions

4. Broadcast storm
(1) Problem phenomenon

All devices in the local area network cannot connect to the network or the network is very slow, and all indicator lights on the switch flash quickly, especially for wireless networks.

A common phenomenon that triggers a radio storm is a loop in the network.

(2) Causes of the problem

Network loop: A loop exists in the local network, causing broadcast data to be sent in a loop;
Terminal virus infection: The terminal is infected with a virus and sends a large amount of broadcast data to the local network;
Unreasonable subnet planning: The subnet is too large, and the broadcast data accumulates. This situation generally has no obvious impact on the wired network.
(3) Troubleshooting ideas and solutions


(4) Broadcasting Storm - Inspection Method

(5) Broadcasting Storm - Solution

Reasonable planning of subnets: Local area networks need to be planned reasonably. For example, the number of hosts in a local area network should not be too many. Generally, subnets can be planned based on location (such as different buildings) and business (such as different departments or purposes).

Set network isolation: Within the same subnet, terminals can be isolated from each other through switch port isolation and other functions, reducing the risk of internal mutual influence.

Enable spanning tree/loop monitoring: Switches that support spanning tree and loop monitoring can automatically detect loop problems in the network after setting, automatically disconnect faulty nodes, and maintain stability and availability.

Set broadcast rate suppression: Managed switches all support broadcast packet rate suppression. Through settings, only packets with a specified flow size are allowed to pass, reducing broadcast packet traffic.

5. DNS server abnormality
(1) Symptoms

All devices in the local network cannot open web pages, or some applications and websites cannot be accessed, but QQ can be logged in normally.