A network with multiple Telnet ports.

15 pts.
Tags:
FTP
FTP permission
Network administration
Telnet
As a network security beginner I stumbled across a question which I think I have the answer to but I need an expert to give me better insight. The question is..what if an administrator was to(stupidly) change the port number of a FTP port from 21 to 23. As you know 23 happens to be the port number for Telnet. My question is..would this only make FTP impossible or would this also make Telnetting impossible since there would now be two ports with the number 23 (the original Telnet port and the port which was changed from 21 to 23), which in my opinion would inhibit the data transfer process.

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  • carlosdl
    "what if an administrator was to(stupidly) change the port number of a FTP port from 21 to 23" Change it where ? A computer or some other equipment ? Typically, you would not be able to make two different services/programs listen on the same port simultaneously.
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  • TomLiotta
    A proxy function could run on 23 and manage connections, but that's a pretty unlikely scenario. Practically speaking, Carlosdl is right -- the two services wouldn't peacefully coexist. Whichever service managed to bind() first should succeed. The other service should fail with EADDRINUSE or other error when it tried to start. They could each have their separate time ranges, though. Telnet could be started from, say, 7:00 AM to 6:00 PM; and FTP might be allowed from 7:00 PM to 6:00 AM (leaving plenty cut-over time at each end.) The question implies that the telnet service is left at port 23. It could be changed to port 21 or any unused port, and all could be well -- assuming that the server supports remote clients querying for service names and that the clients are intelligent enough to go check. Changing ports isn't unheard of. But changing to a different 'well known' port is usually... stupid. Tom
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  • Onesandzerosrs5
    Thanks for the prompt response guys. Lets say he changed it on a computer...which would mean the comp would have two Telnet ports open. Is it possible to have two ports on a device(a computer) with the same number?
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  • carlosdl
    Onesandzeros, I think your last question has already been answered.
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  • carlosdl
    Just a small clarification: Programs don't "create" ports. They just use them. For a specific IP address, there is only one port 23. You could have two programs trying to use it, which is a different thing, but as explained above, one of them would fail to bind() (the function used to associate a local address with a socket) if the other one is already using it.
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  • TomLiotta
    Is it possible to have two ports on a device(a computer) with the same number? It's possible to have two connections to the same port. That's what "server" programs allow. A server program (e.g., a telnet server program) can accept and manage many connections from remote telnet clients at one time. All of the clients will connect to the same port on the server. The server side of sockets programming uses a few additional sockets APIs to enable multiple connections. But there's just one port for a given number. Tom
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  • James Murray
    I've always assumed that if it were possible to have multiple connections to the same port, we wouldn't need so many ports. We could assign every connection to the same port. Therefore in my mind each connection requires a different port to be successful. My assumption about which assignment would be successful would be dependent on the system the ports were connected on and how "stupidly" the administrator was I appreciated everyone's comments, thanks.
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  • Pjb0222
    You can run multiple instances of a program and bind each instance to different ports. This assumes the application will allow multiple instances.
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