computer science

Netstat

Netstat is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Netstat rather than just read about it. In short: In computing, netstat is a command-line network utility that displays open network sockets, routing tables, and a number of network interface (network interface controller or software-defined network interface) and network protocol statistics. It is available on Unix, Plan 9, Inferno, and Unix-like operating systems including macOS, Linux, Solaris and BSD.

Key takeaways

Netstat belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Netstat to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Netstat from memory before moving on to harder problems.

Reference excerpt

In computing, netstat is a command-line network utility that displays open network sockets, routing tables, and a number of network interface (network interface controller or software-defined network interface) and network protocol statistics. It is available on Unix, Plan 9, Inferno, and Unix-like operating systems including macOS, Linux, Solaris and BSD. It is also available on IBM OS/2 and on Microsoft Windows NT-based operating systems including Windows XP, Windows Vista, Windows 7, Windows 8 and Windows 10. It is used for finding problems in the network and to determine the amount of traffic on the network as a performance measurement. On Linux this program is mostly obsolete, although still included in many distributions. On Linux, netstat (part of "net-tools") is superseded by ss (part of iproute2). The replacement for netstat -r is ip route, the replacement for netstat -i is ip -s link, and the replacement for netstat -g is ip maddr, all of which are recommended instead.

Statistics provided Netstat provides statistics for the following:

Proto – The name of the protocol (TCP, UDP, etc.). On some systems, the protocol name may be followed by "4" or "6", depending on whether the protocol is running over IPv4 or IPv6. Local Address – The IP address of the local computer and the port number being used. The name of the local computer that corresponds to the IP address and the name of the port is shown unless the -n parameter is specified. An asterisk (*) is shown for the host if the server is listening on all interfaces. If the port is not yet established, the port number is shown as an asterisk. Foreign Address – The IP address and port number of the remote computer to which the socket is connected. The names that corresponds to the IP address and the port are shown unless the -n parameter is specified. If the port is not yet established, the port number is shown as an asterisk (*). State – Indicates the state of a TCP connection. The possible states are as follows: CLOSE_WAIT, CLOSED, ESTABLISHED, FIN_WAIT_1, FIN_WAIT_2, LAST_ACK, LISTEN, SYN_RECEIVED, SYN_SEND, and TIME_WAIT. For more information about the states of a TCP connection, see RFC 793.

Parameters Parameters used with this command must be prefixed with a hyphen (-) rather than a slash (/). Some parameters are not supported on all platforms.

Examples

Cross platform On macOS, BSD systems, Linux distributions, and Microsoft Windows: To display the statistics for only the TCP or UDP protocols, type one of the following commands:

netstat -sp tcp netstat -sp udp

Unix-like On Unix-like systems: To display all ports open by a process with id pid:

netstat -aop | grep "pid" To continuously display open TCP and UDP connections numerically and also which program is using them on Linux:

netstat -nutpacw

Windows On Microsoft Windows: To display active TCP connections and the process IDs every 5 seconds, type the following command (works on NT based systems only, or Windows 2000 with hotfix):

netstat -o 5 To display active TCP connections and the process IDs using numerical form, type the following command (works on NT based systems only, or Windows 2000 with hotfix):

netstat -no

*nix

Wildcards Netstat uses an asterisk * as a wildcard which means "any". An example would be Example output:

Under "Local Address" *, in *:smtp, means the process is listening on all of the network interfaces the machine has for the port mapped as smtp (see /etc/services for service resolution). This can also be shown as 0.0.0.0. The first *, in *:*, means connections can come from any IP address, and the second *, in *:*, means the connection can originate from any port on the remote machine.

Caveats Some versions of netstat lack explicit field delimiters in their printf-generated output, leading to numeric fields running together and thus corrupting the output data.

Platform specific remarks

Under Linux, raw data can often be obtained from the /proc/net/dev to work around the printf output corruption arising in netstat's network interface statistics summary, netstat -i, until such time as the problem is corrected. On the Windows platform, netstat information can be retrieved by calling the GetTcpTable and GetUdpTable functions in the IP Helper API, or IPHLPAPI.DLL. Information returned includes local and remote IP addresses, local and remote ports, and (for GetTcpTable) TCP status codes. In addition to the command-line netstat.exe tool that ships with Windows, GUI-based netstat programs are available. On the Windows platform, this command is available only if the Internet Protocol (TCP/IP) protocol is installed as a component in the properties of a network adapter in Network Connections. On the Windows platform running Remote Desktop Services (formerly Terminal Services) it will only show connections for the current user, not for the whole computer. On macOS, the /System/Library/CoreServices/Applications folder (or /Applications/Utilities in OS X Mountain Lion and earlier) contains a network GUI utility called Network Utility, the Netstat tab of which runs the netstat command and displays its output in the tab.

See also ss, a Linux utility to investigate sockets from iproute2 meant to replace netstat lsof -i bmon

References

Further reading Dyson, Peter (1995). Mastering OS/2 Warp. Sybex. ISBN 978-0782116632. Stanek, William R. (2008). Windows Command-Line Administrator's Pocket Consultant, 2nd Edition. Microsoft Press. ISBN 978-0735622623.

External links

netstat(8) – Linux Programmer's Manual – Administration and Privileged Commands from Manned.org netstat(1) – FreeBSD General Commands Manual netstat(8) – Solaris 11.4 System Administration Commands Reference Manual netstat(1) – Inferno General commands Manual Microsoft Learn: Netstat – documentation for the Windows netstat.exe command-line program net-tools project page on SourceForge Netstat Command Archived 2022-01-15 at the Wayback Machine: WindowsCMD.com Archived 2022-01-11 at the Wayback Machine

Illustrations

Worked examples

Example 1 — a first encounter with Netstat

Start with the simplest possible case. Write down what Netstat claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Netstat before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Netstat ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Netstat

In research: Netstat appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Netstat in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Netstat is common in secondary-school and first-year university syllabi. It links to neighbouring topics Inferno (operating system) commands, OS/2 commands, Plan 9 commands, so understanding it makes those chapters shorter.
In everyday life: Look for Netstat outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Netstat in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Netstat means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Netstat out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Netstat in simple terms?

Why does Netstat matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Netstat?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Netstat.