I need to launch several instances of a game in the same computer using Linux. The game is created launching a server at a specified port number (using a command line command with --port NUMBER as an argument) and then each of the players attachs itself to that port to play the game (in a similar fashion).
I need to launch hundreds of games in parallel through a little C program that uses the stdlib.h system library function (system(const char *command)) to launch the game server and the game players.
What I need to know is: how can I check from withing that C program that a port is available before launching the game server on that port number? It´s important to note that my program itself does not bind to any port, it just launchs (with system) the program that will actually try to connect to that port.
You have to use bind() directly, and if it doesn't succeed you can try another port.
Checking if a port is free and then binding is not possible and would be a race condition: You just checked, that a port was free, but someone already used it.
Reading /proc/net/tcp can help you though, but the race condition still applies.
The simplest way is normally trying to open a port and then handle the error as a already used port. A nice example can be found here
Three simple steps:
Try to open socket on the port you desire
If it works return true and close the socket
If not, return false
In the example given, they do it remotely, but you easily can change that.
Related
I have a question linked to Linux and serial port.
I want to be able to receive and send messages to a dedicated serial port and to redirect it to another port (/dev/tty).
For the first part, I’m able to dialog with my hardware equipment without any problem, but I’m just wondering if it’s possible to intercept and redirect message coming from a serial port #1 to another port #2.
To give more context, I had used a GPS Antenna and NTP open source software for years.
Since 2018, the new GPS antenna protocol has modified the order of bytes in the message used by NTP to steer and now it’s not working anymore.
So my idea is to put a simple C program (middleware) which fixes this byte ordering; but I’m wondering if I have to build a kernel-specific module or if it can be done in another way. The NTP software uses the symbolic link to dialog.
Thanks for your help.
You can probably use a simple redirect, look here:
Pipe One Serial Port to Another in Linux
If the ports are in different rates you can use stty or perhaps screen to adjust: https://unix.stackexchange.com/a/117064
If you need it to be in c program to manipulate it you can use the following: https://stackoverflow.com/a/6947758/8901188
Using c it will need to run in an infinite loop so it can constantly read, manipulate and write the data.
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'm trying to find a way to determine how long a connection as been established to any IP and any port and I want to use C.
So far, I can get the remote IP and port as well as the local IP and port and the program name and its PID by forking and calling netstat and extracting data from it byte-by-byte.
However, I prefer if theres a standard API thats included with linux that I can directly use in my program so I don't always have to call external programs to see if a connection is established and for how long.
Is there a way I can do this without the need for calling external programs and extracting output from them?
What I want to do later in my program is kill a process if the established connection time is way too long (I'm trying to defeat the slowloris DOS attack).
On a Linux machine, you have a daemon that listens on TCP port A. However, it is usually stopped because it is rarely used and takes away a large amount of system resources. Instead, I want to do something like this:
Code an application that listens on port B and does the following as soon as a connection is established: If the daemon is stopped, start it and wait until it listens on port A. Now the difficult part: Connect the client to the daemon in a completely transparent way, i.e. without the client having to reconnect on port A. Also, but this is irrelevant for this question, the application will shut down the daemon when there are no connections for a certain amount of time.
Of course, I could have my application connect to the daemon and pipe all communication. I do not want that. I want some way to forward the established connection to the daemon and then get rid of the connected socket, while the client is now happily connected with the daemon. In some way, I want to give the daemon's process my already connected socket. Is there any way to do something like this?
I'm running Debian, if that's important. I would want to code the application in C/C++, and it's okay to have OS-specific solutions (i.e. use syscalls). Forgive me though, I am not much of a Linux coder, so I am not very familiar with Linux system programming. If there is some obvious way to do it, I simply didn't know.
Of course, I am open for any kind of suggestion.
This problem has a pre-existing standard solution, generically known as inetd. It has been around for a long time, first in Unix systems and then Linux.
The more modern implementation is xinetd
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