computer science

Portmap

Portmap 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 Portmap rather than just read about it. In short: The port mapper (rpc.portmap or just portmap, or rpcbind) is an Open Network Computing Remote Procedure Call (ONC RPC) service that runs on network nodes that provide other ONC RPC services. Version 2 of the port mapper protocol maps ONC RPC program number/version number pairs to the network port number for that version of that program.

Key takeaways

Portmap 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 Portmap to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Portmap from memory before moving on to harder problems.

Reference excerpt

The port mapper (rpc.portmap or just portmap, or rpcbind) is an Open Network Computing Remote Procedure Call (ONC RPC) service that runs on network nodes that provide other ONC RPC services. Version 2 of the port mapper protocol maps ONC RPC program number/version number pairs to the network port number for that version of that program. When an ONC RPC server is started, it will tell the port mapper, for each particular program number/version number pair it implements for a particular transport protocol (TCP or UDP), what port number it is using for that particular program number/version number pair on that transport protocol. Clients wishing to make an ONC RPC call to a particular version of a particular ONC RPC service must first contact the port mapper on the server machine to determine the actual TCP or UDP port to use. Versions 3 and 4 of the protocol, called the rpcbind protocol, map a program number/version number pair, and an indicator that specifies a transport protocol, to a transport-layer endpoint address for that program number/version number pair on that transport protocol. The port mapper service always uses TCP or UDP port 111; a fixed port is required for it, as a client would not be able to get the port number for the port mapper service from the port mapper itself. The port mapper must be started before any other RPC servers are started. The port mapper service first appeared in SunOS 2.0.

Example portmap instance This shows the different programs and their versions, and which ports they use. For example, it shows that NFS is running, both version 2 and 3, and can be reached at TCP port 2049 or UDP port 2049, depending on what transport protocol the client wants to use, and that the mount protocol, both version 1 and 2, is running, and can be reached at UDP port 644 or TCP port 645, depending on what transport protocol the client wants to use.

Security concerns The port mapper service was discovered to be used in Distributed Denial of Service (DDoS) attacks and Distributed Reflective Denial of Service (DRDoS) attacks in 2015. By using a spoofed port mapper request, an attacker can amplify the effects on a target because a portmap query will return many times more data than in the original request.

References

External links portmap(8) – Linux Administration and Privileged Commands Manual rpcbind(8) – Solaris 11.4 System Administration Commands Reference Manual RFC 1833 - the specification of the ONC RPC port mapping protocol.

Worked examples

Example 1 — a first encounter with Portmap

Start with the simplest possible case. Write down what Portmap 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 Portmap 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 Portmap 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 Portmap

In research: Portmap 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 Portmap 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: Portmap is common in secondary-school and first-year university syllabi. It links to neighbouring topics Internet Standards, Internet protocols, Remote procedure call, so understanding it makes those chapters shorter.
In everyday life: Look for Portmap 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 Portmap in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Portmap 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 Portmap out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Portmap in simple terms?

Why does Portmap 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 Portmap?

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 Portmap.