The top tool provides a dynamic, real-time view of the processes in a running system. It can display a variety of information, including a system summary and the tasks currently being managed by the Linux kernel. It also has a limited ability to manipulate processes. Both its operation and the information it displays are highly configurable, and any configuration details can be made to persist across restarts. By default, the processes shown are ordered by the percentage of CPU usage, giving an easy view into the processes that are consuming the most resources.
For detailed information about using top , refer to its man page: man top. The ps tool takes a snapshot of a select group of active processes. By default this group is limited to processes owned by the current user and associated with the same terminal.
This interface provides help and auto-completion via the Tab key, which guides you through the many options available. In the editor, you can define specific parameters with the set command. Use the Tab key auto-completion to see all available options:. When you finish your changes, save them to the connection and quit to complete the operation:. Network Manager and nmcli are a complete solution for network configuration. If you have never used it before, start with some of the basic commands and gradually move to more complex settings.
Consult the man pages and examples to execute specific tasks. Next, let's review some commands that help you with network troubleshooting, starting with name resolution. When managing connectivity issues, DNS name resolution is often a source of headaches. The nslookup utility helps you check and troubleshoot DNS name resolution.
Install it with dnf :. To quickly check the name resolution for a particular host, use nslookup with the hostname as an argument. The command uses the default DNS configuration for the name resolution:.
If the name resolution fails, you can use an alternative name server for the resolution by providing its address as the third argument:. This information is useful for troubleshooting as it helps to identify whether the issue is local due to a specific server, or something more extensive. The nslookup utility is a useful tool to help troubleshoot network issues related to DNS name resolution.
Next, let's take a look at troubleshooting local network sockets. Another common task when troubleshooting network connectivity issues is determining whether a connection is established or a particular service is available on a server. The ss command, short for socket statistics , is a convenient tool that displays network socket information.
It's the modern replacement for netstat that provides similar functionality, but includes a few extra features. Since the default command displays all established network connections, this list can be extensive on a busy machine, with over a thousand entries. To help you with troubleshooting, ss provides several filtering options.
You can also filter the list by source or destination hostname or IP address. For example, here is a list of established connections for destination IP address These commands are useful to troubleshoot network connectivity in general or between specific hosts in the network.
Another practical application for ss is to verify whether network services are listening on the local machine with the correct address and port. To verify listening sockets, use the option -l :.
You can use the same filtering options described above to filter specific socket types. For example, to list all listening TCP sockets enter:. Instead of displaying the service name, like ssh or hostmon , ss can display port numbers by using the -n option:. Finally, another useful option is -p to display process related information such as user name and process ID PID. Some services may require elevated privileges via sudo or as root to list process information:.
The ss tool is an essential command in the sysadmin toolbox for network troubleshooting tasks. Next, let's take a look at tracepath to trace network connectivity between hosts. The tracepath command is a network troubleshooting tool that displays the network connectivity path between the local host and a remote host, identifying all routers used to route the traffic between them. In case you're unable to connect to a network service in a remote host, tracepath helps you determine where the issue is.
The main difference is that tracepath uses random UDP ports instead of the ICMP protocol for the trace, thus not requiring elevated privileges to run.
MTR Matt's traceroute is a program with a command-line interface that serves as a network diagnostic and troubleshooting tool. This command combines the functionality of the ping and traceroute commands. Just like a traceroute , the mtr command will show the route from a computer to a specified host.
With the mtr command, you will get more information about the route and be able to see problematic devices along the way. If you see a sudden increase in response time or packet loss, then obviously, there is a bad link somewhere. Show numeric IP addresses if you use -g , you will get IP addresses numbers instead of hostnames :. You can install tcpdump with the command below:. Before starting any capture, you need to know which interfaces tcpdump can use.
You will need to use sudo or have root access in this case. If you want to capture traffic on eth0 , you can initiate that with tcpdump -i eth0 sample output:. You can filter out traffic coming from a specific host. For example, to find traffic coming from and going to 8. The netstat tool for printing network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.
This utility is part of the net-tool package, as is ifconfig. In the new iproute2 package, the ss tool is used to achieve the same objectives. Use the nslookup utility to query Internet name servers interactively. To check the use of a specific DNS server in this case, query using the specific nameserver ns1. Checking DNS A records to see the IPs of a domain is a common practice, but sometimes you need to verify if an IP address is related to a specific domain.
For that purpose, you need a reverse DNS lookup. The receipt of corresponding Echo Reply messages is displayed, along with round-trip times.
Simple ping commands take only one parameter: The hostname or the host IP address that you want to verify. A simple ping example is just like below:. Otherwise, it will ping until you stop it. After every ping command, it will display a summary report with the following information:. Ping uses a numerical TTL value to attempt to reach a given host computer via the route path. This is also known as the hop limit.
Normally, when you run a simple ping command without passing any additional parameters, Linux will ping that host for an infinite amount of time.
If you want to ping a host ten times, use the following command:. On systems with multiple interfaces, you can specify the interface for the ping command to use. If the system has both eth0 and eth1 and I want to ping using the eth0 :.
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