I am trying to connect robot controler CODESYS program with OPC Expert through OPC UA server and that's listening 4840 port and on OPC Expert I'm getting like opc.tcp://hp-nb:4840/uadiscovery. I also installed freeopcua in my machine and that's listening 4840 port as well and on OPC Expert it's like opc.tcp//0.0.0.0:4840. On OPC Expert I need to connect with CODESYS application. I can connect with CODESYS OPC server but I'm not able to connect my application from CODESYS controler. Is it happening because of two servers are listening on same port? if not then how can I resolve my issue? Attached an image of my issue enter image description here
Two programs or processes running on the same computer cannot listen on the same port.
It would be possible only if that computer had two network interface cards with two different IPs and you could choose through which interface each process connect, a thing that is not usually possible to do
UPDATE
CODESYS Soft PLC emulator, which is usually installed with CODESYS, also includes an OPC Server listening on port 4840, if you stop the Soft PLC right clicking the icon on the windows notification area probably it will close the listening port
In theory, two OPC UA servers could share the same port (on the same network interface) if they coordinate together. For opc.tcp, there is no standard way for them to do it. Therefore, two servers from the same vendor that are written in such special way could theoretically share the port. In reality, however, OPC UA servers using "opc.tcp" cannot share the same port, because they do not know how to coordinate the access and route the data.
For other protocols the situation is different. E.g. for OPC UA over HTTPS it is easier to have servers listening on the same port - mainly because the operating system already contains a piece that can listen on the port "for them", and route the requests based on the URL.
In your situation, however, it is not just that two server cannot share the same port. There is also a problem with the fact that one of the servers returns "0.0.0.0" as its IP address in the connection point description, which is invalid anyhow.
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I'm working on a simple Swift app where one user can find other machines on a local network which are running an instance of my app and then send data to that machine using TCP sockets.
My question is how to find IP addresses of devices on same network which are running an instance of my app (cross-platform)?
I was thinking about listing all devices on local network and then
checking whether they have opened specific port (the port my app is
using)?
I also found that Apple provides service called Bonjour which could make my process discoverable. I'm not sure if this solution is good for cross-platform communication.
Apart of Mac-related stuff and high-level solutions (I believe keywords "network service discovery" will bring you to them), there are a couple of things that will work for a local network:
I Have a server that clients should report to. Some short hello-like UDP message and a timeout mechanism will be sufficient to keep a list of available clients in the network.
II Use IP or UDP multicast groups to notify others that a client has just connected to the network. Send a message to a multicast group and listen to this group to build a list of clients.
However, broadcasts and multicasts won't be transmitted through a router. So if your network is large enough only neighbouring clients will hear your notification. In order to overcome it,
III DHCP servers can be configured to provide custom data to clients via unassigned DHCP options. Large networks have usually such server. You probably can use it to send out a list of clients, but I'm not sure about this.
i'm planning on implementing web server hosted by Raspberry Pi at home, while also having a Dedicated Server with public IP.
The main problem is that my provider charge a lot for a static ip, so i simply dont want to pay.
And here is the question:
Is it possible to achieve what i've mentioned on given diagram (if yes, then maybe some hints?)
Will RPi be accessible through local network while connected to VPN
Thank you for any help!
1.
That is basically possible, it is called a Reverse-Proxy (See wiki for a brief description). The exact implementation depends on the web server you use.
Your dedicated Server will then accept client connections, get the content over the VPN from your Raspberry and serve it to the client like it's his.
2.
Your Pi will still be accessible from the local network while being in the VPN, since it should use a virtual adapter for the connection if its a client on its own. Otherwise, if the router acts as a gateway to the VPN it will do the routing and again your Pi will be fine.
For example, when you make an ssh connection, you are connected to port 22. What happens then? On a very high level brief overview, I know that if port 22 is open on the other end and if you can authenticate to it as a certain user, then you get a shell on that machine.
But I don't understand how ports tie into this model of services and connections to different services from remote machines? Why is there a need for so many specific ports running specific services? And what exactly happens when you try to connect to a port?
I hope this question isn't too confusing due to my naive understanding. Thanks.
Imagine your server as a house with 65536 doors. If you want to visit family "HTTP", you go to door 80. If you were to visit family "SMTP", you would visit door no. 25.
Technically, a port is just one of multiple possible endpoints for outgoing/incomming connections. Many of the port numbers are assigned to certain services by convention.
Opening/establishing a connection means (when the transport protocol is TCP, which are most of the “classical” services like HTTP, SMTP, etc.) that you are performing a TCP handshake. With UDP (used for things like streaming and VoIP), there's no handshake.
Unless you want to understand the deeper voodoo of IP networks, you could just say, that's about it. Nothing overly special.
TCP-IP ports on your machine are essentially a mechanism to get messages to the right endpoints.
Each of the possible 65536 ports (16 total bits) fall under certain categories as designated by the Internet Assigned Numbers Authority (IANA).
But I don't understand how ports tie into this model of services and
connections to different services from remote machines? Why is there a
need for so many specific ports running specific services?
...
And what exactly happens when you try to connect to a port?
Think of it this way: How many applications on your computer communicate with other machines? Web browser, e-mail client, SSH client, online games, etc. Not to mention all of the stuff running under the hood.
Now think: how many physical ports do you have on your machine? Most desktop machines have one. Occasionally two or three. If a single application had to take complete control over your network interface nothing else would be able to use it! So TCP ports are a way of turning 1 connection into 65536 connections.
For example, when you make an ssh connection, you are connected to
port 22. What happens then?
Think of it like sending a package. Your SSH client in front of you needs to send information to a process running on the other machine. So you supply the destination address in the form of "user#[ip or hostname]" (so that it knows which machine on the network to send it to), and "port 22" (so it gets to the right application running on the machine). Your application then packs up a TCP parcel and stamps a destination and a return address and sends it to the network.
The network finds the destination computer and delivers the package. So now it's at the right machine, but it still needs to get to the right application. What do you think would happen if your SSH packet got delivered to an e-mail client? That's what the port number is for. It effectively tells your computer's local TCP mailman where to make the final delivery. Then the application does whatever it needs to with the data (such as verify authentication) and sends a response packet using your machine's return address. The back and forth continues as long as the connection is active.
Hope that helps. :)
The port is meant to allow applications on TCP/IP to exchange data. Each machine on the internet has one single address which is its IP. The port allows different applications on one machine to send and receive data with multiple servers on the network/internet. Common application like ftp and http servers communicate on default ports like 21 and 80 unless network administrators change those default ports for security reasons
I want to develop an iPhone app with a simple IM feature. I am thinking about setting up an HTTP server on an iPhone. If the iPhone is using wifi and is behind a firewall, how can I make sure that other iPhone clients can connect to it?
It's not the firewall that will disturb the connection as much it is the NAT.
When you are connected through wireless router to connect the internet you are surfing via NAT. it means you dont really have an extenral IP but once you initiate connection the router will map your intenral IP to one of his externatl ports and for certain time window he will pass connections to you if he will get it to the right port.
That being said, there is no actual way of setting a server behind a NAT unless you can configure port forwarding in the router and internal static IP.
Hope i was clear enough, good luck
I do not really think that you need to get an HTPP server up and running on iPhone to make an application that can send and receive messages (IM). The idea of making one iPhone user to directly connect to one another does not seem right to me since the users will need to know IP addresses of one another to do that.
Interconnectivity between different users of the chat can be solved by making your application communicate via a dedicated TCP port. It is generally advisable to choose ports with a number higher than 1024 since those below are generally found on the list of so-called well-known ports and are used for Web (like port 80), FTP (port 21), SSH (22), DNS (53), etc., it will be the responsibility of the user to make sure the port used by your application is open on the firewall. In order to solve this problem you can actually use port 80 for communication if you find that the port you have selected is blocked. You can do this because you know that this port will not be blocked in most cases. Indeed Yahoo Messenger is reported to use this technique when the firewall blocks the port it uses for communication.
The port should be used by your application to connect to the Web-server that will actually store user credentials, perform authentication, message transmission, etc., and the server should reside on capable hardware to be able to support large number of simultaneous connections. I can suggest using either a VPS (like the one provided by Linode) or a cloud (like Amazon EC2, Google Application Engine, Rackspace).
I have two servers, written in Java, that communicate through sockets and TCP. The servers are both on the same Linux machine. If the servers were on different machines then data would have to go through network adapters and network cables, but since the servers are on the same machine how does the OS actually move data efficiently form one server to the other.
Generally, the messages will be sent over the loopback interface:
In TCP/IP a loopback device is a virtual network interface implemented in software only and not connected to any hardware, but which is fully integrated into the computer system's internal network infrastructure. Any traffic that a computer program sends to the loopback interface is immediately received on the same interface.
However, you can manage to configure your situation so that, for example, the server is referred to via an external IP address so that messages actually go out over the network before being routed back to the same machine.
If two programs are using TCP/IP to communicate on the same machine, they are probably connected through the loopback interface