CODEWITHSUNDEEP
node.jstcp
cool
cool

Reputation: 3505

Nodejs TCP connection client port assignment

I created tcp connection between client and server using nodejs (net module). Server is listening on already predefined port and client is connecting to that port.

As far as i understand port for client is dynamically assigned by node? Is that correct?

  1. What kind of algorithm node is using to assign "random" port for the client? How this works, is this determined by node or by OS?
  2. Is it possible to define static port which client is going to use? Is it possible to define range of ports for the client to use?

NOTE: I think i found discussion/question with similar subject on stackoverflow before, but i cannot find it anymore. I would apprecaite if you can share any reliable resources regarding this subject.

Upvotes: 4

Views: 3188

Answers (2)

user3356715
user3356715

Reputation: 191

The selected answer is provides a lot of info, but does not deal with the underlying problem. Node does not appear to allow https.request to specify a port for the client. There exist localAddress and localPort options, but they appear to be broken.

I've opened a new question on this issue. Hopefully someone will answer with something other than "just don't do that."

Is there a way to set the source port for a node js https request?

Upvotes: 0

jfriend00
jfriend00

Reputation: 707736

The source port number is usually pretty much irrelevant to your programming unless you have a router or firewall that is somehow restrictive in that regard. It is merely used by the underlying TCP infrastructure to keep track of different TCP connections.

From this article:

A TCP/IP connection is identified by a four element tuple: {source IP, source port, destination IP, destination port}. To establish a TCP/IP connection only a destination IP and port number are needed, the operating system automatically selects source IP and port.

The above referenced article describes how Linux selects the source port number.

As to your particular questions:

What kind of algorithm node is using to assign "random" port for the client? How this works, is this determined by node or by OS?

It is determined by the OS. That source port number is selected by the originating host at the TCP level before the connection is even made to node.js.

Some other reference articles:

Does the TCP source port have to be unique per host?

how can an application use port 80/HTTP without conflicting with browsers?

Note: there is no security reason I'm aware of for a firewall to limit the source port number or block certain source port numbers. They are a TCP bookkeeping number only, not related at all to security or the type of service being used. Note, this is different than the destination port which is usually correlated directly with the type of service being used (e.g. 80 is HTTP, 25 is SMTP, 143 is IMAP, etc... When you make a TCP connection to a different host, you specify the host address and the destination port number. You don't specify the source port number.

Upvotes: 2

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