I need to create a program that will communicate with other programs on the same computer via UDP sockets. It will read commands from stdin, and some of this commands will make it to send/receive packets without halting execution. I've read some information out there, but since I'm not familiar with socket programming and need to get this done quickly, I have the following questions:
I need to get a random unused port for the program to listen to, and reserve it so other programs can communicate with this and also the port isn't reserved by another program. I also need to store the port number on a variable for future usage.
Since the communication is across processes on the same machine, I'm wondering if I can use PF_LOCAL.
Also a code sample of the setup of such socket would be welcome, as well as an example of sending/receiving character strings.
Call bind() specifying port 0. That will allow the OS to pick an unused port. You can then use getsockname() to retreive the chosen port.
Answer by Remy Lebeau is good if you need a temporary port. It is not so good if you need a persistent reserved port because other software also uses the same method to get a port (including OS TCP stack that needs a new temporary port for each connection).
So the following might happen:
You call bind with 0 and getsockname() to get a port;
then save it into config (or into several configs) for future uses;
software that needs this port starts and binds the port.
Then you need to e.g. restart the software:
software stops and unbinds the port: the port can now be returned by bind(0) and getsockname() again;
e.g. TCP stack needs a port and binds your port;
software can't start because port is already bound.
So for "future uses" you need a port that is not in ephemeral port range (that's the range from which bind(host, 0) returns a port).
My solution for this issue is the port-for command-line utility.
If it being a random port is actually important, you should do something like:
srand(time(NULL));
rand() % NUM_PORTS; // NUM_PORTS isn't a system #define
Then specify that port in bind. If it fails, pick a new one (no need to re-seed the random generator. If the random port isn't important, look at Remy Lebeau's answer.
Related
NiFi 1.5:
i want to use the port in HandleHTTPRequest. how to decide the port can be used in HandleHTTPRequest? is there any range of port can be used here.
i already saw the nifi configuration, but could not find the ports that can be used here.
please guide me. thanks.
Any port that the OS user running NiFi can access can be used for HandleHttpRequest, but only one can be used at a time (not a range). For example, sometimes the "NiFi user" (meaning the OS user that executed the nifi.sh command to start it) isn't allowed to use system ports (<1024) so you'd need to choose one above that number. Also you need to choose a port that is not already in use by another process.
The reason a port range is not available is because HandleHttpRequest is meant to model a well-known URL as a web service; if the ports could change, the clients would all have to try the same range.
I am setting up port scanner for remote server in my application using Go. I am using DialTimeout function in Go net package to check whether a remote host port is opened or not. The result is fine with success case. But, if i/o timeout happens, I need to identify whether
The port is closed (No service is running) or
Port is blocked (Firewall filtered) or
Due to internet connectivity down in local system where the application is running.
Have tried nmap cli command, I can able to differentiate those failure 3 cases exactly.
nmap command tried: nmap -sA -p port_number host_ip
I found a Go 3rd party libray to use nmap.
But, I don't want to use nmap in my application. Are there any other alternatives in Go to exactly differentiate those 3 cases?
In the simple world
Lets assume you want to scan a Linux system.
If you get an ICMP message type 3 code 3, the firewall explicitly told you:
Hi, I am the firewall of your target host. The host is running. I hereby inform you that you (potentially amongst others) can not access this port. So now that you know you should quit your connection attempts. However, I won't tell you wether it is because there is no service running behind it (in which case my response is simply a courtesy) or because I was told to deny you access. Goodbye!
The port is closed if you do not get above answer and can not make a connection. I hence strongly advice to use context.WithTimeout to make a connection.
In the real world
However, this only applies if the admin of the target host did not change the ICMP message type to respond with - or chose just to drop any packets coming from sources which are not allowed to access the respective service. In the latter case, there is no way for you to know wether the port is closed or filtered.
All of the above only applies if we are talking of an iptables based firewall on the target system with default settings.
Now assume something which is by far more likely: A border firewall plus a local firewall. The border firewall might send other ICMP messages (or, again, simply drop your packages). Those rules apply additionally to the rules of the local firewall. So it is a misconception that you are actually scanning a host. It is more accurate to say that you scan the services reachable via a specific IP.
EDIT
Why would one send an ICMP message explicitly rejecting connection attempts?
There are various reasons to come to that decision. There is a good answer on serverfault.com
I have an application that I'm currently testing that listens for requests over the network on port 1025. I chose this port arbitrarily and it is easy to change, my concern is that my application will collide with other system processes listening on that port. My two primary question are as follows:
Are there common system processes that use port 1025 that may conflict with my application?
Assuming the answer to 1) is yes, what is another less commonly used port or range of ports my application can safely utilize?
Any port in the 49152 – 65535 range cannot be registered by any company, and therefore you're more likely to not run into any problems with port conflicts.
A list of registered ports, etc. can be found at this wikipedia link here.
If you don't like Wikipedia, check this link out, from the Internet Assigned Numbers Authority (IANA).
That'll do in most cases. If you need to run more than one instance (a listener) of your application on the same IP address, for example, the port collision still occurs. To overcome this case, you may choose to bind your socket to port 0 (= any port) to have system assign the port number for you. Then you can use getsockname() (BSD socket API), or equivalents, to get the actual port number the system has assigned for the socket. Downside of it is obviously that the port number will change every time. There needs to be a way to tell (either manually, programmatically or systematically) its listening port to connectors. In fact, many VoIP/peer-to-peer applications work this way, with a help of 'rendezvous' server.
Is it possible to listen on a port and run a command when a user attempts to connect to that port? Ideal application is for a server that should only be run when someone is actually using it. Windows or Linux solutions work.
linux/unix:
man nc
NAME
nc - TCP/IP swiss army knife
some options that you may be interested
-l listen mode, for inbound connects
-p port local port number (port numbers can be individual or ranges: lo-hi [inclusive])
-e prog specify program to exec after connect (use with caution)
i think nc is also available under windows platform.
One solution could be via inetd or xinetd, specify the port number and a program to run, for you probably a shell script.
I am note sure what is the exact scope of a question but if bound to the programming-level, you could write your server in a way that when nobody is using it, no resources apart from the listening part are allocated. I would call it lazy initialization. When someone connects simply initialize the whole logic of your program. When all connections are gone, deinitialize everything.
Preliminary story
There is this program which uses the Parallel Port to synchronize with other hardware. It will set the Parallel Port output to a specified (byte) value. This works without problems when using the built-in Parallel Port of a PC. The target platforms are Windows XP to 7, all worked fine so far. Source code is in Delphi, accessible and can be modified.
How it works
In Delphi I can use the io.dll to set the value of the Parallel Port, but there are also other solutions available, like inpout32.dll or port.dll. I call something like PortOut, specify a port number and the byte value and the port is set.
What I now want to do - and where I need help
Now the change: this needs to work on a machine which has no Parallel Port built-in (not even on the mainboard). There are several options available:
use a USB to Parallel Port adapter to add a LPT port to the PC
use a PCI card which adds a LPT port to the PC
use a PCI Express card which adds a LPT port to the PC
I am currently heading for and concentrating on the easiest and cheapest possibility: a USB to Parallel Port adapter.
Main question
There seem to be differences between Parallel Port adapters which are made to connect just a printer and other adapters which seem to be more powerful. Is there really a difference? Or can I just use one of these 5$ printer-adapters, plug in my own hardware and access the port from Delphi code? Or do I need a special adapter? Has anyone experience with this? There is a related question here, but the different adapter types (if existent) are not mentioned there. This page suggests that there are indeed differences:
Contrary to all other USB parallel ports which can connect to printers only, this makes connection to most hardware.
I hope there exists a solution via USB because for this you don't have to open the PC, which means the adapter can be added on demand.
Sub-question
Do you have experience with PCI (Express) solution? I have to use one if the USB approach is not successful.
Since I've been wrestling with this very thing recently here's what I've discovered; If you mean by using IO port addressing (indicated by your reference to inpout32.dll), no. Unless your USB-parallel port driver supports full port emulation or virtualization, which most do not, this is generally not possible. If you need to directly access the port to do normal "bit-twiddling", you should get a separate Parallel port PCI-card. Most of them present themselves as normal IO at the standard address(es). I am presuming you're not planning on using the parallel port to actually communicate with a printer, right?
What is interesting is that USB-Serial adapters are much easier to use since they appear as simple virtual devices where you can merely "open" them using a simple stream; TFileStream.Create("COM1", fmOpenRead) or Windows.CreateFile("COM2", ...);
Here is some devices that purport to do full emulation of a parallel port through USB:
https://www-user.tu-chemnitz.de/~ygu/bastelecke/PC/USB2LPT/index.en.htm