The Linux system itself is designed to have powerful network and communication functions, supporting TCP/IP, ATM, IPX/SPX, X.2.5, Frame-relay, AppleTalk, and other network communication protocols, and integrating the implementation of these protocols into the Linux kernel, providing efficient network communication processing capabilities

　　Viewing and testing network configuration is the first step in managing Linux network services

　　The network interface card (NIC) of the host is usually called ‘network interface’

　　View information of all active network interfaces

　　When ifconfig command is used without any options or parameters, it will display the information of the network interfaces that are currently enabled (active) in the host

　　View information of all network interfaces

　　View the information of the specified network interface

　　When you only need to view the information of one network interface, you can use the name of the network interface as a parameter for the ifconfig command, regardless of whether the network interface is active or not

　　View the current hostname

　　Temporarily modify the current hostname

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　　Executing the route command directly can display the routing table information in the current host

　　The routing table in the Linux system determines the destination of data sent from this host to other hosts and networks, which is a key piece of information for troubleshooting network issues

　　Route -n displays the result using IP addresses

　　netstat command

　　Checking the network connection status, routing table, interface statistics, and other information is an effective tool for understanding network status and troubleshooting network service failures

　　
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　　ss is the abbreviation for Socket Statistics. The ss command is used to display information about active sockets. It can display similar content to netstat. However, the advantage of ss is that it can display more detailed information about TCP and connection states, and it is faster and more efficient than netstat.

　　Syntax format: ss [options]

　　In the early operation and maintenance work, checking the number of server connections generally used the netstat command. In fact, there is a command that is more efficient than netstat, that is the ss (Socket Statistics) command!

　　The ss command can be used to obtain socket statistics, and it can display similar content to netstat.

　　The advantage of ss is that it can display more detailed information about TCP and connection status, and it is faster and more efficient than netstat. The reasons are as follows:

　　1) When the number of socket connections on the server becomes very large, whether using the netstat command or directly cat /proc/net/tcp, the execution speed will be very slow. You may not have a personal feeling, but believe me, when the number of connections maintained by the server reaches tens of thousands, using netstat is a waste of life, while using ss is a time-saver.

　　The secret of ss’s speed lies in its use of tcp_diag in the TCP protocol stack. tcp_diag is a module for analysis and statistics that can obtain first-hand information from the Linux kernel, which ensures the quick and efficient operation of ss. Of course, if tcp_diag is not available in your system, ss can still run normally, but the efficiency will be slightly slower (but still faster than netstat).

　　Almost all Linux systems will include the netstat command by default, but not all systems will include the ss command by default.

　　The netstat command is a member of the net-tools toolset, which is generally pre-installed in Linux systems; the ss command is a member of the iproute toolset;

　　net-tools is a set of standard Unix network tools used for configuring network interfaces, setting routing table information, managing ARP tables, displaying and statistics of various network information, etc., but unfortunately, this tool has not been updated or maintained since 2001.

　　iproute is a set of tools that can support IPv4/IPv6 networks and is used for managing TCP/UDP/IP networks

　　If there is no ss command, you can install it as follows:

　　yum install iproute iproute-doc

　　2.1: Use the ping command to test network connectivity

　　Test network connectivity

　　Basic command format

　　ping [options] target host

　　‘Press Ctrl C to terminate the ping test’

　　‘Feedback: Target host is unreachable may mean that the target address does not exist or the host is already closed’

　　‘Feedback: Network is unreachable indicates that there are no available routing records, such as gateways, and it is impossible to reach the network where the target host is located’

　　‘Feedback: Request timeout indicates that the data has reached the other side but cannot return’

　　Test the network nodes between the current host and the target host

　　For nodes that cannot respond, the connection status will be displayed as *

　　The traceroute command is more accurate in locating the fault points (break points) of network connections than the ping command, and its execution speed is therefore slower than that of the ping command.

　　In network testing, use ping first, if a network connection failure is found, use the traceroute command to track and view the fault node

　　Basic command format

　　traceroute is suitable for probing local area networks, and wide area networks may be blocked

　　nslookup command

　　Test DNS domain resolution, resolve the domain name to an IP address

　　Ns = name server

　　DNS port tcp 53 port function connection to DNS server

　　UDP 53 port function resolution

　　Basic command format

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　　The full name of the dig command is ‘domain information groper’, which is a tool for flexibly querying DNS. It prints out the responses from the DNS domain name server, and the dig command is mainly used to query host address information from the DNS domain name server.

　　The dig command is almost the same as the nslookup command, but it is more flexible, easier to use, and has clearer output.

　　Syntax format: dig [parameters] [IP]

　　3.1.1: Temporary configuration: Adjust network parameters using commands

　　Temporary modification, usually used for maintenance

　　Simple, fast, and can directly modify running network parameters

　　Generally, it is only suitable for use during network debugging

　　After the system restarts, the modifications will take effect directly

　　3.1.2: Fixed settings: Modify network parameters through the configuration file

　　Modify the configuration file of various network parameters

　　It is suitable for setting fixed parameters for the server

　　It needs to reload the network service or restart the system for it to take effect

　　Set the IP address and subnet mask of the network interface

　　ifconfig interface name IP address [subnet mask subnet mask]

　　ifconfig network interface IP address[/subnet mask length]

　　Disable or reactivate the network interface

　　ifconfig network interface up

　　ifconfig network interface down

　　For example

　　During the debugging process of the server network, sometimes it is necessary to use a new IP address on the same network card temporarily, but it cannot overwrite the original IP address to make the service program unavailable.

　　At this time, a virtual network interface can be bound to the network card, and then a new IP address can be configured for the virtual interface (equivalent to configuring multiple IP addresses for a network card).

　　Command format for setting up virtual network interfaces

　　ifconfig interface name: serial number IP address

　　[root@localhost ~]# ifconfig ens33:0 192.168.200.10/24

　　The route command can not only be used to view routing table information, but also to add, delete static routing table entries, including setting the default gateway address (the default gateway record is a special static routing entry).

　　The IP address of the default gateway should be in the same subnet as one of the IP addresses of the local interface

　　3.4.1 Add the routing record to the specified subnet

　　3.4.2 Delete the routing record to the specified subnet

　　route del -net subnet address

　　3.4.3 Delete the default gateway record in the routing table

　　Only one default gateway record should be in the routing table of the same host, otherwise it will cause the network understanding of the host to fail

　　4.2.1 Restart network network service

　　/etc/sysconfig/network file, it is in CentOS6 system

　　Save global network settings, mainly including host name informationlottery method and Just click to enter

　　/etc/hostname file, it is in CentOS7 system

　　/etc/resolv.conf file

　　Save the IP address of the DNS server that the local machine needs to use

　　The modifications made to this file take effect immediately

　　In the Linux system, up to 3 different DNS server addresses can be specified (the third and subsequent ones will be ignored), and the first DNS server is used preferentially

　　[root@localhost ~]# vim /etc/resolv.conf

　　search localdomain ‘Default search domain (domain suffix), that is, when accessing the host localhost, it is equivalent to accessing localhost.localdomain’

　　nameserver 100.100.1.30

　　nameserver 100.100.100.30

　　/etc/hosts file

　　Save the mapping records of host name and IP address

　　Generally used to save information of frequently accessed hosts

　　Comparison between hosts file and DNS server

　　By default, the system first looks up the resolution record in the hosts file

　　When accessing an unknown domain, first check if there is a corresponding mapping record in this file, and if not found, go to the DNS server for query

　　The hosts file only takes effect on the current host
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　　Because the hosts file is only stored locally

　　The hosts file can reduce the DNS query process, thereby speeding up access speed

　　Add correct mapping records (frequent websites visited) to the /etc/hosts file, reducing DNS queries and improving internet speed

　　Because CentOS 8 has deprecated network.service, the NetworkManager (NM) command is used to enable network cards. Of course, CentOS 8 can install network.service as a network service, but to adapt to the version, this time we still need to introduce the usage of nmcli commands.

　　Summary of nmcli common commands based on RHEL8/CentOS8