I am writing you because of a new problem I need to solve, and I have now been banging my head against a wall for too long now.
Basically, I need to create an application that can take care of the following:
A user starts an app, which sends a broadcast to the subnet, and recieves a response of all servers there with their IP (and some additional info). The user can then select what server he wants to connect to.
Making it work is simple enough, with identifying the subnet, and broadcasting with UDP, and then having a different server application recieving it and sending back a response . The problem lies with these restrictions, that I need to take into consideration:
There will most likely also be clients on the server machines in the network, meaning that we can assume that the application is present on all machines. Every machine needs to have the listener running, and every machine can launch the GUI for selecting a server.
I am only allowed to add one exception to the firewall - an exception that handles both sending out the broadcasts, recieving broadcasts, sending answers and recieving answers.
I should also only be adding one Windows Service
on a server machine, the listener should run as a windows service, so the user won't notice it. Nor will the user notice, that the response is sent back to the client.
On the client machine, the user can start an application, which will notify the application to emmit the broadcast, and will get all the server responses, so the user can choose one to connect to.
Besides from the application that the user launches in order to select a server, there should be no interaction with the user whatsoever. Not even a popup, requesting the user to allow traffic trough the firewall - it should all be automatically
It needs to work on and in between Win XP, Win Vista and Win 7.
I don't know if I am putting too many constrains on myself, but I really hope that I can make the application with these requirements.
I have a few ideas - I just need to figure out how to do it:
Should i make everything into one application, that I add to the firewall exception list, so it will take care of the traffic on both the server and the client machines?
Should I add a custom exception to the firewall, allowing UDP traffic on a specific port, and then have all traffic flow trough that?
Is there a third and better option for managing that?
It is OK to have the service running on both client and server machines. But can it take care of everything for me - like it handling both the broadcast send/recieve and answer send/recieve? And is there any way to extract the information about servers on the network from a service?
I know it is a lot, but I really hope that you will be able to help me out.
let me know if I wasn't clear enough, or if you need further explanations.
I am coding in C# .Net, and I can utilize all I want from the .Net framework. As soon as I have this functionality implemented
All the best
/Sagi
The kind of peer-to-peer networking problems become simple to the point of being trivial if you designate one machine as the master server. It should have a well-known name that all sub-servers can connect to so they can publish (and withdraw) their availability. A client can then send a query request to the same server and get a list of known servers in return.
This can also solve your firewall problem, the master server could be listening on port 80.
Look into the System.Net.PeerToPeer namespace for a p2p solution supported by the framework.
Maybe a UPnP server and client may be a solution to your problem?
Related
Background
I have multiple servers that I currently connect to remotely to run a number of different commands/scripts to obtain information about the servers and/or applications running on the servers.
I'd like to automate running the commands/scripts (or the code contained in the scripts converted to C#/.NET) and have the server send alerts/notifications/messages to a client (basically a Windows Form) running on multiple workstations, but need some guidance.
For reference, I have limited experience creating Windows Services, but feel fairly confident in being able to create them on the server to handle to command/script automation, which I'm assuming would be the best way to go about handling the command/script automation on the server (since the commands/scripts would need to be run all the time or at set intervals).
Question
How can I connect multiple servers to multiple clients so that the server sends alerts/notifications/messages to the client when a command/script or even an event occurs on the server?
For instance, if an application on the server has a built-in command that can be run to determine the status of the application (up, down, limbo, etc.), I would like the Windows Form on the client to receive an alert from the server when the command returns "down" or "limbo" when it is run, presumably from a Windows Service. The alerts would be displayed on the Windows Form that would be setup basically as a dashboard for the servers that the client can connect to.
An even better outcome would be that the client runs as a background application and a notification appears similar to how Microsoft Outlook displays a notification when new email messages arrive (although these notifications would likely require user interaction to close instead of fading out like the Outlook notifications).
I would also like for the client to use a configuration file that has the connection information for the servers in it so that the servers being used can be changed quickly new servers are added or existing servers are decommissioned.
Research (so far)
I've read about WCF and duplex contracts, and how WCF can be hosted in Windows Services. From what I've read, this seems promising. However, I'm not quite sure how I would set this up so that the client can connect to a WCF service on multiple servers.
One thing that I'm concerned about with WCF is that in all of the WCF examples (which implement a calculator-type service) I've seen the client has to initiate the communication with the server in order to receive a message through a callback. In the calculator service examples, the client sends numbers to the service and the result is provided in the callback. I've also seen an asynchronous example, but in that example the client initiated a single, long running request and the callback returned a single response when it was finished processing.
And, just so I'm clear about bindings in WCF, it is possible to create and use bindings for multiple servers using a configuration file without having to use SvcUtil.exe to generate the code, correct? The reason I ask is because the servers that will be configured will likely be change for different users, so the client needs to be flexible when connecting to the services.
I've just now started looking at Sockets, but I'm not familiar enough with them to know if this would be the better option to achieve my objective.
Summary
I'm just looking for guidance, so if you can help direct me to some resources that will help me achieve my objective, I would appreciate it. I've searched extensively, but the majority of my searching either doesn't apply to my scenario, it is limited to a single server/client interaction, or it is limited to a single server with multiple clients.
Since I'm not sure what direction to go in, I don't have any code examples, although I have implemented the examples in the following Microsoft article: Windows Communication Foundation - Getting Started Tutorial
So you want to build a system of
multiple servers which execute commands on the computer they are running on
multiple clients which will receive the status of the commands executed on server or such information from the server
This would be my advice
Servers can be implemented as windows service. You will be able to administrate them easily this way using the services console or the scm. Checkout this link for a creating a simple C# service How do you write and use a Windows Service in C#?
Also, you can set the service to run as an in-built service user with different levels of permissions in addition to regular user accounts.
I have not used WCF, but usually clients connect to the server; this is a pretty common model, and hence all samples are such. Initiating connection from server is not a big deal (at least in a socket program), but just a bad model. You have to ask yourself, if no client is connected to your servers, how can they relay a status to the end user. You have to think clearly about the communication model. I would suggest a central repository of messages. It can be a file on a shared file system or a database or any such entity which can act as a data repository. This way all servers can convey there messages without caring if a client is connected or not. You can use Sockets to achieve what you want to do. Check the asychronous socket server sample from MSDN to understand how to do it.
Making the client run in the background and just have a notification area icon is also easy in c#. You can use NotifyIcon Class for that. This CodeProject article (Formless System Tray Application) demonstrates its usage. To show notification a la outlook style, you can refer to the following post: How to create form popup from from system tray on windows application (not web) with c#. Look at not only the accepted answer but other answers too; there are lot of useful links in it.
So far we have windows service talking over sockets, storing messages in a central repository and capable of handling multiple clients with toast style pops for client side notification.
You need a far richer client side GUI so the end users can take actions on the messages sent from the server. You can maintain a list of servers in app.config for the client that the client connects on startup. You should to provide a GUI for users to manage all servers and their connections.
Lat but not least, by building such a client server model, you are effectively building a security loophole in your systems. You should implement a good authorization mechanism. Checkout the following post: Authenticate user in WinForms (Nothing to do with ASP.Net)
EDIT:
You can also implement your server to accept "custom command" when you implement it as a service. This way, your client server communication will be standardized by using ServiceController to pass the command. This post might help: How to send a custom command to a .NET windows Service from .NET code?.
Don't get confused in the "command" terminology here. ServiceController issues standard commands to a service for start, stop, pause, resume and restart the service. These are the same items you see on the context menu when you right click a service in the services.msc snap-in. The same way a service can respond to custom commands. In your case the custom command maybe a request to execute a process.
Note that some mechanisms I have described are geared towards an intranet setup while others scale fine on both intranet and internet
I have a WPF applications that runs on 2'000 PCs (xp, vista, win 7, win 8),
and the only thing it does is to show new notifications taken from a main server.
At the moment everything works trough pooling and it is hard for the server to handle that load.
I'm now required to provide the same service to another customer which has 4'000 PC,
so pooling is no longer an option.
The customer wants also to be able to specify which Active Directory group should receive the notification,
and every notification must been received by the client even if offline at the time (it will receive it when back online).
Security is not an issue.
I'm searching for the best architecture suited for the task,
this is what I have considered so far:
1) WCF-MSMQ: discarded, it requires msmq active on every client machine.
2) WCF-nettcpbinding publish-subscribe (https://msdn.microsoft.com/en-us/library/ms752254(v=vs.110).aspx):
It would be perfect but how I'm supposed to handle different groups?
Should I check the group of the user and based on that subscribe it to a different topic?
3) Keep all clients always connected to the server:
on connect the client sends it's Domain Name to the server,
the server associate a connection to a user and keep looping trough all the connections while sending notification when available for that user.
I'm not sure how many connections I'm able to always keep opened.
Should I use WCF or just make my own sockets system? The logic is pretty basic but it needs to be available on multiple windows platforms with different .NET version.
Do I still need the clients to send back an ACK when received a notification? Or I can just trust the TCP ACK?
Any suggestions will be much appriciated.
I want to pass certain parameters to a desktop application remotely via the internet. I don't want my application to contact the server repeatedly, because many such applications can bring the server down easily. Is there a way to initiate the connection from the server? How can I identify the applications, as there will many of them running on many computers somewhere around the globe. I don't know where to start - I'm trying to do this in C# and ASP.NET/PHP on the server-side. Please give some advice.
Is there a way to initiate the connection from the server?
No. Not without having the client contact with the server first, informing it with the IP address, port to use etc... Which the server will need to keep for each client, hoping that they don't change (or get updated when they do change).
Long polling by the client is the right solution for what you are doing, even if you don't want to use it.
There are many different ways you could approach this, just thinking out of the box, both your app and the server could utilize a different mechanism for transferring the settings. I'm not recommending any of these methods, please don't shoot me down, they are all just ideas.
As an example, your server could connect out using FTP and output the updated settings to an FTP server on each PC. You could install something like Filezilla on each machine which runs your app. You'd obviously need to configure port forwarding on the router to allow the server FTP access.
You could use email. Setting up an email account where your server can login to send out the settings. Your app could possibly login to the same email account possibly even a single Gmail account to retrieve the settings.
Another idea would be to use a file sharing service like Dropbox, Google Drive or similar and where the settings could be shared. Obviously this would involve learning any API and I'm not sure if there are any restrictions on this approach.
The last idea and probably my preferred approach would be to host a web service and database on a remote server, both your server and the applications would connect to the same service to transfer the settings. This approach is obviously firewall/router friendly as all the clients connect out to the web service to collect the required data.
Hope this helps?
I am looking forward to make a sort of a chat application using C# and WCF, yet before I start I wanted to clear a few things, so that I don't get carried in the wrong direction. :)
the application should be able to both make event ( send messages ) and listen to events ( receive messages ), therefor if I understand it right, the application should be both the client and the server at once?
what binding should I use? If I understand right, basicHttp binding hard to configure and is used when a WCF app needs to connect to non-WCF app? While for connecting two WCF apps its better to use NetTcpBinding?
how would this applications find each other, considering that they are running on different machines? should there be a central server, to which the app would connect first, saying "I'm user123, my IP is that and that, I'm free for chat" and look for other user IP addresses there? Or is there some other ways for apps to find each other without the central server?
maybe you could direct me to some examples or tutorials on this topic? ( tried googling, no luck ).
Thanks! :)
If your peers (machine that can operate as a client & server) are going to be behind NATs/firewalls then you will have a very very tough time building a chat application using WCF. If the peers will all be on the same network WCF is workable.
To write a chat application from scratch using WCF you will be re-inventing the wheel. Why not employ an existing protocol that's been designed precisely for the purpose such as XMPP. There are XMPP libraries around for .Net. You will need a central server but if you use XMPP you could feasibly piggyback onto one of the many existing free servers.
You can use netPeerTcpBinding.
See also:
http://blogs.interknowlogy.com/2009/08/05/building-a-really-simple-wcf-p2p-application/
http://msdn.microsoft.com/en-us/library/bb690929(v=vs.90).aspx
http://www.codeproject.com/KB/WCF/Chat_application_using_WC.aspx
There is no right and wrong answer to your question - it all depends on what you require. You then can choose the best architecture for your needs, and then the best technology for that architecture.
There are two things to consider - how will users find people to chat to, and then once they have found someone - how do the applications connect to each other.
For users to find each other, you either need to connect to a central server, which will then show a list of all connected users, or users need to enter the IP address of the person they want to chat to. Note that some firewalls will block incoming connections in this second case, so this may not be feasible in some cases.
Next, once you have found the person you wish to chat to, you need to decide whether messages will route directly to the other user, or if you want to continue to go through a central server.
If you choose to go through a central server, then your application will be functioning as a client, and will therefore pass through firewalls without a problem. However if you connect directly to the other user, both copies of the application will be acting as a client and a server (P2P topology), and therefore firewalls may be an issue. Having said that, you could designate one of the applications to be a server, with the other acting as a client, in which case only the server side needs to worry about the firewall.
Without knowing exactly what you are trying to achieve, it is hard to recommend which architecture will work best for you.
I need to create a system comprising of 2 components:
A single server that process and stores data. It also periodically sends out updates to the agents
Multiple agents that are installed at remote endpoints. These collect data in (often, but not always) long-running operations, and this data needs to get to the server
I'm using C# .NET, and ideally I want to use a standards compliant communications method (i.e. one that could theoritically work with Java too, as we may well also use Java agents in the future). Are there any alternatives to web services? What are my options?
The way I see it I have 3 options using web services, and have made the following observations:
Client pull
No open port required at the agent, as it acts like a client
Would need to poll the server for updates
Server push
Open port at the agent, as it acts like a server
Server must poll agents for results
Hybrid
Open port at the agent, as it acts like both a client and a server
No polling; server pushes out updates when required, client sends results when they are available
The 'hybrid' (where agents are both client and server seems the obvious choice - but this application will typically be installed in enterprise and government environments, and I'm concerned they may have an issue with opening a port at the agent. Am I dwelling too much on this?
Are there any other pros and cons I've missed out?
Our friends at http://www.infrastructures.org swear by pull-based mechanisms: http://www.infrastructures.org/papers/bootstrap/bootstrap.html
A major reason why they prefer client-pull over server-push is that clients may be down, and clients must (in general) apply all the operations pushed by servers. If this criteria isn't important in your case, perhaps their conclusion won't be your conclusion, but I do think it is worth reading the "Push vs Pull" section of their paper to determine for yourself.
I would say that in this day and age you can seriously consider only pull technologies. The problem with push is that clients often are hidden behind Network Address Traversal devices (NAT) like wireless routers, broadband modems or company firewalls and they are, more often than not, unreachable from the server.
Making outbound connections ('phone-home'), specially on well known ports like HTTP/HTTPS can basically be assumed as 'possible' even under most constricted networks.
If you use some kind of messaging server (JMS for Java, not sure for C#) then your messaging server is the only server that needs to open a port and you can have two way communication from your agent to the messaging server and from the server to the messaging server. This would allow you to accomplish the hybrid model without needing to open a port on the agent server.
IMHO, I find your best option is the pull option.. that can satisfy your main system requirements as follow:
The first part: Data needs to get to the server, that's obviously can be done through invoking a web method that send that data as a parameter
2nd part:(Server periodically sends out updates to the agents): You can still do that that thru client (regular) pulls by some sort of a web service method that "asks" for the updates since its last pull (some sort of s time stamp to get the updates it missed)
The hybrid method seems a bit weird to me given that I think of an agent as a part of the system that probably might go "offline" quite often, what will the server then do if that failed? it's usually a tough question/decision, specially if you're not sure if this an intended "going offline" or a system/network failure.. etc