Common network problem location tools that programmers should master

In the process of daily project operation and maintenance, various strange network problems are often encountered. Therefore, troubleshooting network problems has become a necessary skill for a qualified programmer. Here are some commonly used commands for quickly locating network problems in daily work.

ping

This is a small tool that is often used by everyone to check whether two servers can successfully exchange data packets. The ping command sends ICMP messages to the other host. When the ping is successful, it means that the network link between the two hosts is unobstructed. If ping fails, first confirm whether the other party has shut down ping service. If not, you need to check the network link problem between the two parties.

telnet

This command is mainly used to check the network connectivity of the other party's port. If telnet can be connected, it generally proves that TCP three-way handshake has been successfully established, that is, the network layer is unobstructed. If telnet cannot be connected, it is necessary to confirm whether the other party has started the corresponding service port. If it has been started, then it is necessary to check the firewall policies of both parties and other issues.

ifconfig

In addition to checking network card information, the ifconfig command can also modify network configuration. For example, check network card information ifconfig -a ; start the network card with ifconfig eth0 up ; shut down the network card ifconfig eth0 down , etc.

In addition to using ifconfig to view and configure network card information, you can also use ip link show to view network card information, start network card information ip link set down eth0 ; shut down network card ip link set up eth1 and other operations.

Note: Try not to start or shut down the network card when the SSH connection is in progress.

route

route command is generally used to view and configure server routing information. For example, view routing information route -n or route -nee to add or delete routing information route {add | del } -net {NETWORK-ADDRESS} netmask {NETMASK} dev {INTERFACE-NAME} , etc.

Similarly, in addition to using the route command, you can also use ip route show to view routing information; it is equivalent to route -n . Here, it is recommended to use the ip route command.

traceroute

This command is used to view the routing path from the source address to the destination address. For example, traceroute www.baidu.com is used to determine which network devices pass between you and the destination address. In windows , it is tracert

netstat

This command is quite powerful and is also used by programmers more often. This command can display the network status of the server, for example, display all current socket links of the server netstat -a ; display the usage of UDP/TCP ports netstat -apu/netstat -apt ; display network statistics netstat -s ; check the status of port 8080 netstat -anp|grep 8080 .

The command equivalent to netstat is ss command. ss -l lists all the ports opened on the local machine. ss -t/u -a displays all tcp/udp links. To check which process a link belongs to, use ss -tp . To check the local port status, use ss -t src ip:8080 . It is recommended to use ss command here.

tcpdump

Network packet capture tool. When you encounter strange network problems, use tcpdump to capture network packets, and then analyze and locate the problem. Capture packets based on source IP tcpdump -i eth2 src 192.168.199.100 which means only capturing network packets that pass through the eth2 network card and whose source address is 192.168.199.100 . Of course, you can also specify a port to capture packets, such as tcpdump src port 8088 In short, this command is very powerful. I will not introduce it in more detail here. Those who are interested can learn about it by themselves.

nmap

This is a network sniffing and scanning tool, which is more familiar to network security professionals. It is often used to check which ports are open on the other host. For example, nmap -PS 192.168.199.100 can be used to check which ports are open on the other host, and nmap -O 192.168.199.100 can also be used to detect the operating system of the target IP. In short, this command is more commonly used by network security professionals, and less used by programmers. Interested parties can study it in depth. This command is very powerful.

Summarize

The gadgets mentioned above can achieve the same effect through two different commands for locating some problems. For example, ifconfig -a and ip link show ; route -n and ip route show ; and netstat and ss . This is because commands such as ifconfig and netstat come from the net-tools toolkit, while commands such as ip link and ss come from the iproute2 toolkit. net-tools has stopped development and is no longer the default toolkit in many new versions of operating systems. If you want to use it, you need to install it separately. iproute2 is used to replace net-tools and is installed by default in many new versions linux . Moreover, many commands in the iproute2 package are more powerful in terms of function and performance than those provided in net-tools package. I found a picture on the Internet to illustrate the command comparison of the two toolkits.