To begin with, why I've written a Telnet client is not important. It's my job and I have to do it. So, on to my question...
My Telnet client is simple and allows for both interactive communication (used by a human) as well as automated communication (used by a program). However, in order for my client to know when the server is done sending data so it can give control back to the user, I've had to hard code all of the possible command prompts for a given server into the client application. For instance, I create a list of prompts and then pass this list to the client so it can watch for these in the data stream from the server. When it sees that a data stream ends with one, it then gives control back to the user, who will enter a response or a command and send it. The client then watches again for any of these prompts and so on. The list could look like the following for a Telnet server running on CentOS:
private static void SetTerminalPrompts()
{
prompts.Add("login: ");
prompts.Add("Password: ");
prompts.Add("]$ ");
prompts.Add("]# ");
}
This would be used like:
response = telnetClient.ReceiveUntilPrompt(prompts);
And public string ReceiveUntilPrompt(List<string> prompts) simply reads the data stream watching for a prompt from the list.
The problem is watching for these prompts. In a general case, the Telnet client will not know the server to which it is connecting. And it seems silly to attempt to create a complete list of possible prompts by guessing all servers to which the client may ever connect (even though I've found several examples and posts that say just that).
What I would like to know is how do the Telnet clients that come with Windows or Linux know when to give control back to the user? How do they know when the Telnet server is done sending data?
Related
I'm totally confused and stunned about my problem. I wrote a multi client server using sockets.
I created a first GUI that receives clients and display them in a gridview, in the click gridview event it opens a new GUI with the socket between the clicked client and the server. So everything about the connection between server and the clients go well and fast
But my problems are :
the send information between them like sending full process it sometimes shown full and it sometimes less result .
send files management for example when the server request full folders/files in a directory sometimes it shows them all and sometimes less result .
But the command such open a window open an url, send a message , commands like those works so perfectly and instantly .
I am so confused right now what's the problem
Notice 1 : I used the extern IP for connection between server/client.
Notice 2 : internet connection is perfect (definitely not slow)
I tried to use different buffer sizes, but what I'm really confused about is that sometimes the result comes full with a specific buffer size and sometimes not with the same buffer size .
Thank you for your time !
I would also recommend you use wireshark so that you are able to get the full information and transactions that are really going on in the network.
This will help you to rule out where the issue is coming from.
To determine the packet size see the image
Here I am not talking about how the TCP handshaking protocol works as I assume it is working. However do note that in the transmission if you see something like RST then maybe somebody issued a reset command implying that some error occurred in the transmission. Such as something due to checksum for example. Although it is generally a problem for someone working directly on TCP/IP protocol
This should help you confirm that you sent and recieved the correct length.
I am building a c#/wpf project.
It's architecture is this:
A console application which will be on a virtual machine (or my home computer) that will be the server side.
A wpf application that will be the client app.
Now my problem is this - I want the server to be able to send changes to the clients. If for example I have a change for client ABC, I want the server to know how to call a service on the clients computer.
The problem is, that I don't know how the server will call the clients.
A small example in case I didn't explain it well:
The server is on computer 1, and there are two clients, on computers 2 and 3.
Client 2 has a Toyota car and client 3 has a BMW car.
The server on computer 1 wants to tell client 2 that it has a new car, an Avenger.
How do I keep track and call services on the clients?
I thought of saving their ip address (from calling ipconfig from the cmd) in the DB - but isn't that based on the WI-FI/network they are connected to?
Thanks for any help!
You could try implementing SignalR. It is a great library that uses web sockets to push data to clients.
Edit:
SignalR can help you solve your problem by allowing you to set up Hubs on your console app (server) that WPF application (clients) can connect to. When the clients start up you will register them with a specified Hub. When something changes on the server, you can push from the server Hub to the client. The client will receive the information from the server and allow you to handle it as you see fit.
Rough mockup of some code:
namepsace Server{}
public class YourHub : Hub {
public void SomeHubMethod(string userName) {
//clientMethodToCall is a method in the WPF application that
//will be called. Client needs to be registered to hub first.
Clients.User(userName).clientMethodToCall("This is a test.");
//One issue you may face is mapping client connections.
//There are a couple different ways/methodologies to do this.
//Just figure what will work best for you.
}
}
}
namespace Client{
public class HubService{
public IHubProxy CreateHubProxy(){
var hubConnection = new HubConnection("http://serverAddress:serverPort/");
IHubProxy yourHubProxy = hubConnection.CreateHubProxy("YourHub");
return yourHubProxy;
}
}
}
Then in your WPF window:
var hubService = new HubService();
var yourHubProxy = hubService.CreateHubProxy();
yourHubProxy.Start().Wait();
yourHubProxy.On("clientMethodToCall", () => DoSometingWithServerData());
You need to create some kind of subscription model for the clients to the server to handle a Publish-Subscribe channel (see http://www.enterpriseintegrationpatterns.com/patterns/messaging/PublishSubscribeChannel.html). The basic architecture is this:
Client sends a request to the messaging channel to register itself as a subscriber to a certain kind of message/event/etc.
Server sends messages to the channel to be delivered to subscribers to that message.
There are many ways to handle this. You could use some of the Azure services (like Event hub, or Topic) if you don't want to reinvent the wheel here. You could also have your server application track all of these things (updates to IP addresses, updates to subscription interest, making sure that messages don't get sent more than once; taking care of message durability [making sure messages get delivered even if the client is offline when the message gets created]).
In general, whatever solution you choose is plagued with a common problem - clients hide behind firewalls and have dynamic IP addresses. This makes it difficult (I've heard of technologies claiming to overcome this but haven't seen any in action) for a server to push to a client.
In reality, the client talks and the server listens and response. However, you can use this approach to simulate a push by;
1. polling (the client periodically asks for information)
2. long polling (the client asks for information and the server holds onto the request until information arrives or a timeout occurs)
3. sockets (the client requests server connection that is used for bi-directional communication for a period of time).
Knowing those terms, your next choice is to write your own or use a third-party service (azure, amazon, other) to deliver messages for you. I personally like long polling because it is easy to implement. In my application, I have the following setup.
A web API server on Azure with and endpoint that listens for message requests
A simple loop inside the server code that checks the database for new messages every 100ms.
A client that calls the API, handling the response.
As mentioned, there are many ways to do this. In your particular case, one way would be as follows.
Client A calls server API to listen for message
Server holds onto call, waiting for new message entry in database
Client B calls server API to post new message
Server saves message to database
Server instance from step 2 sees new message
Server returns message to Client A.
Also, the message doesn't have to be stored in a database - it just depends on your needs.
Sounds like you want to track users à la https://www.simple-talk.com/dotnet/asp.net/tracking-online-users-with-signalr/ , but in a desktop app in the sense of http://www.codeproject.com/Articles/804770/Implementing-SignalR-in-Desktop-Applications or damienbod.wordpress.com/2013/11/20/signalr-a-complete-wpf-client-using-mvvm/ .
i have developed a server application with c# and a client application with flash action script 3.0. Flash socket asking for a policy file when called from a browser with a message
<policy-file-request/>
everything is normal so far. My server is waiting for this message and sending to client a policy file string which is like this:
public const String POLICY_FILE = "<?xml version=\"1.0\"?>\n" +
"<!DOCTYPE cross-domain-policy SYSTEM \"http://www.adobe.com/xml/dtds/cross-domain-policy.dtd\">\n" +
"<cross-domain-policy>" +
"<allow-access-from domain=\"*\" to-ports=\"*\" />" +
"</cross-domain-policy>\u0000";
this string is being sent this way:
if (message.Contains("policy-file-request"))
{
client.Send(Encoding.ASCII.GetBytes(Statics.POLICY_FILE));
return;
}
I'm pretty sure that this was working but i really don't know what happened and started not working. When flash client receives this message from server, connection was succesfull and everything was going how it had to go. But now the flash client waits 20 seconds (timeout of flash socket) and throws security exception
[SecurityErrorEvent type="securityError" bubbles=false cancelable=false eventPhase=2 text="Error #2048"]
I'm stuck and can't move forward. I'm listening to port 963, server machine fully qualified name is "mypc.domain.local" which can be accessible across my network. there is also an IIS running on this machine and the flash application is hosted here.
http://mypc.domain.local:90/page.html
this is the way, i call my flash application and
mypc.domain.local:963
is the address of server running. i am also working on this machine. i tried calling the page http://localhost:90/page.html or http://127.0.0.1:90/page.html and also tried the connection to server as localhost:963 or 127.0.0.1:963. same result on every combination.
What is wrong here? what could have been changed causing my working code broke down?
Thanks.
It's hard to tell without more code, but based on what you've shown, it appears that when that request comes in, you respond with the contents of the policy file, which isn't an actual valid HTTP response. My guess for the 20 second timeout would be that it's still waiting for the HTTP headers.
If possible, try to use the HTTP classes already in the BCL instead of doing http 'manually', but if you have to do the socket stuff yourself, then use something like Fiddler during debugging since it's great for identifying violations of the HTTP protocol.
Okay I know there is lots of info out there on this and I promise you I have read it all and tried umpteen different methods to get this working!!
I have a socket server program which runs on a laptop. I then have up to 50 laptops connected wirelessly via the same LAN to the server. The client laptops all connect to the server (using Socket.ConnectAsync) and the server uses async methods as well to send and receive data. The server shows a list of connected client laptops to the user and this list seems to be accurate and picks up whenever a client disconnects and connects. However, the client laptops never seem to detect when connection to the server has been lost under certain circumstances (ie if server program crashes, if server laptop goes in to standby mode etc.) I have got a timer on the client laptops which polls the connection every 5 seconds as follows:
bool SocketConnected(Socket s)
{
bool part1 = s.Poll(0, SelectMode.SelectWrite);
bool part2 = (s.Available == 0);
if (!part1 && part2)
{
return false;
}
else
{
return true;
}
}
I have tried using all selectmodes (SelectWrite,SelectRead,SelectError) and have tried using different time out values. I have tried checking s.Connected value after these operations and have tried all manners of other methods to determine the connection state and nothing seems to produce reliable results!! I think I can achieve the result I desire by sending dummy information every 5 seconds and checking s.Connected after doing so, however I don't really want to do this as each laptop is already sending lots of data to the server as it is. Any help at all is massively appreciated! Thanks
The only reliable way to check if a connection is alive is to send something to the other end and see if it arrives. You can do this either manually by sending and receiving a "ping" value from time to time, or automatically by enabling the KeepAlive socket option.
The MSDN documentation for Socket.Poll is very explicit about the exact situations (server crashes, standby) you mentioned:
This method cannot detect certain kinds of connection problems, such
as a broken network cable, or that the remote host was shut down
ungracefully. You must attempt to send or receive data to detect these
kinds of errors.
I have 2 sets of code for you to look at, both are available at PasteBin here:
First is my c# Socket server: http://pastebin.com/wvT4f19m
Second is my code within my AS3 application: http://pastebin.com/bKvabFSP
In the code, what I am trying to do is a simple Send/Receive to see what happens. If I open my application in 2 instances the c# socket server registers that they exist and all is fine!. If I close one of my instances, the c# server still thinks that the user exists and the socket isn't closed.
My code is based off the example at : http://msdn.microsoft.com/en-us/library/fx6588te.aspx
In the MS example, the following lines are added to the SendCallBack() function:
handler.Shutdown(SocketShutdown.Both);
handler.Close();
These definately close the sockets, something I do not want to happen.
I am new at socket programming and it has taken me a fair amount of time to play with the MS example to get it working roughly how I need it. The only problem is the acknowledgement of user disconnects so that I can remove the user from the Clients list that I have set up in the server. Also, when disconnects are acknowledged, I can inform other clients.
thanks all!
Upon each attempt to send data to the user, I do a quick check for a successful transmission/Poll the user and if it fails, the user is removed from my server.
Closing the socket won't affect your listener, it will only affect the current connection. Why do you say this is not what you want?
It sounds like this is exactly what you want.