Currently, I'm working on TCP client/server implementation in C. In that, I found that I can give any random number as a port number for my PC. Is it correct procedure? Or is there any standard port number for my PC?
I don't know the standard, but I say it's not. At least, I don't like to do it like that.
You can check occupied ports by parsing the outputs of programs like netstat and avoid using those. You can also use the method that attempts connecting on one port, and upon failure, tries another port. Unless you're really really unlucky, you should get a valid port on second try.
You should use ports within the ranges of 49152–65535. Ports below 49152 are reserved/registered.
Basically, you can use any port (given sufficient access rights). But server and client have to agree on the port, and it should not be already used by another application.
Hence, many ports are already reserved for special applications. 80 is for HTTP, 22 is for SSH and so on. The file /etc/services gives more detailed information.
Port numbers 0-1023 are called Well Known Ports, numbers 1024-49151 are called Registered Ports (not all of them are, but you get the idea).
If your question is whether you can give any port number to have your server listening to,
then you are thinking wrong, TCP/IP port numbers below 1024 are special in that normal users are not allowed to run servers on them, you can use non-privileged ports(ports > 1024). just make sure that any other application is not already using that port (above 1024) using netstat
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Please forgive me for being naive on the subject, however, I do not understand ports and how they work in the slightest. I am trying to make a program where two computers could communicate given their IP addresses and I am using TCP protocol. I don't, however, know what port(s) I would be able to use for this application, because when I look up TCP ports I get a list of ports each with their own function. Any help is useful.
P.S. I am using C to create my program
The short answer is you can choose any port you like - although the safe range is generally considered to be between 1024 and 65535. The only problem that you will encounter is when some other program installed on the device is already listening on that port. Unfortunately, there is no port that is guaranteed to be available to listen on.
One possible solution to this is to have a primary listening port and a fallback secondary port. You can then first try to connect on the primary port and, if a satisfactory response is not received, try to connect on the secondary port. However, even this is not infallible, as there is a chance that the secondary post could also be in use.
The easiest approach is to try to create your listener on the port that you have chosen, and if the port fails to create, let the user know that some other application is preventing execution of your application.
I am using 80(http) and 443(https) default ports for my webserver.
What ther ports other than this can i use for my webserver.
I need this basically to start my webserver using non-default ports.
Any. Look at these for the ones to not use:
http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers
You can use whatever ports you want to use, provided no other server on your box is also trying to use it.
This is, of course, subject to any OS-specific issues like needing to run with elevated privileges for binding to ports below 1024.
The IANA (naming authority) and ICANN (assigned names and numbers) is responsible for assigning ports to specific applications but there's nothing requiring you to follow those "rules" at all.
If you use (for example) port 23 for your HTTP server, that will work. It's likely to confuse any telnet programs attempting to connect to that box but, as stated, the box is under your jurisdiction, not that of the IANA. Provided your browsers hook up to the specific port 23, they'll work just fine.
By way of example, many IBM mainframe systems will use port 23 for their 3270-protocol terminal programs and bump "real" telnet up to port 1023.
And, in any case, why should you not use a port because it's "allocated" to the Quake game server, or Dropbox, or Symantec bloatware? :-)
I'm trying to create a proxy server, and am having trouble decide on a trivial thing -- the port number on which it will listen. Is there some kind of convention on which port a proxy server should run on? Or should it just be greater than 1024?
Normally, you just pick a port and make it changeable for the user (or yourself). Simply, if it is already used on the system you run your proxy on, use a different one.
Many free proxy servers even listen (e.g. this) to default ports, like 8080. As long as the port is not in use on your system, it does not matter, as the real port is negotiated after a connection attempt.
For your interest, a list of registered ports can be found on the IANA (Internet Assigned Numbers Authority), which manages stuff like that. That is only a hint. I normaly use a random port like 8888 or so, even if someone somewhere already uses that.
I'm writing a program in C in which the server listens on a well known port, waits for client to connect, and then creates a random port for the client to use and send this port number back to the client. My main difficulty is how to create a "random" port. Should I just be using srand and create a random 4 digit port is the usable range? Or is there a better way to do this? I know that if I use port 0 a port will be chosen for me but the problem here is the fact that I don't think i can "get/see" the actual value of the port so that I can send this port number back to the client.
Thanks...
Binding port 0 is the solution. It gives you an arbitrary port, not a random port, but this is what many applications do (e.g. FTP etc).
After binding, you can use getsockname to figure out which port you got.
What you do is bind() with port set to 0. The system will assign one. Then use getsockname() to discover what port the system assigned. Send that back to the client. That way there is no race condition and you follow any system rules for port assignment.
A random 4-digit port checked to make sure it's not in use is OK for that purpose..
Technically speaking, it sounds like you're trying to implement this for added security (some kind of primitive port knocking routine)? It might be worth mentioning that this approach is generally not considered too secure. It also imposes some artificial constraints on how many clients you can serve at a time and actually adds unnecessary load on the server. Why not just listen on the single well-known port for all clients?
I'm guessing TCP considering your description of listening and automatic port assignment by the OS. In this case, you don't need to worry about it. Once you accept the TCP connection, the OS on both sides takes care of all that you're trying to do and you're left with a working connection, ready for use. Unless you have a particular reason for doing any of this yourself, it's already done for you.
I am trying to write a basic C client/server program using sockets in Unix. I am logging on my school's Unix server from my home computer. I am logging on twice, once to simulate the server and the other to simulate the client. Do I use AF_INET or AF_UNIX? Whose IP address should I use, the one from my home computer or my school? If I use my school's IP address, how do I find out what it is? How do I find out what port number I should use? Does the port number for my client and server have to be the same?
Usually you'll want to use AF_INET — then you'll be able to communicate between more than one computer later.
Use 127.0.0.1; that means "this computer".
Make up a port number and use it for both. Usually you'll want to pick something between 1024 and 65536, exclusive.
Since the programs are running on the schools computer you should use that IP address.
You can use the command /usr/sbin/ifconfig -a to find it (it's the inet address). However, if both logins are at the same physical computer (for example at my school that isn't always the case) you can just use 127.0.0.1.
You can use any port number you want, but choose a high (like four digits) since the lower ones are default for some services.
The port numbers for the client and server do not have to be the same, but it might be easier for you to remember if they are.
See here for a lot of examples. There is some stuff you have to understand:
TCP, UDP and the difference between them
What is a socket, types of sockets (stream, datagram, sequential packets)
Socket API's - BSD, POSIX, WinSock (if you're planning to program for Windows)