I have a software solution which contains a C++ library. The library deals with connectivity related things. But I am stuck on a point. I need to deal with all the connected network interface. To optimize the software solution a bit I have decided I will only work with the interfaces which are connected to the internet. That's why I need to determine whether the interface is connected to internet or not. I have searched on the web for simple solutions that will work on all the platforms. But I haven't found so. Now I am seeking help to find a comprehensive solution that will work on all platform such as iOS, Windows, Android, Linux, and MAC. Any kind of suggestion and advice will be appreciated in this regard.
This was done by the suggestion given by Remy Lebeau. I am checking TCP connection with google, amazon and twitter. Firstly I am resolving the address and collecting only the IPv4 addresses. After that I am trying to connect with all of these addresses sequentially. If any of them succeeds then I am saying it is connected with internet and returning from the function without trying any other addresses from the collected list.
If TCP connection with all the addresses fails, then I am saying that internet connection is unavailable.
This solution works for me in almost all cases.
I am trying to create an application that can run as a service and another program to communicate with it (client)
i can find plenty of sample code about services but nothing about programs that interact with services
can someone please link some examples?
Sounds like you could and maybe should use TCP sockets. Or at least that would be the easiest and most straight forward.
Alternatively since it sounds like both client and service are on the same computer you could use some form of IPC (interprocess communication). So depending on your OS you may decide to use IPC instead.
If a program is running on a Linux machine, is there a way for that program to scan for ports that are allowed through the firewall? for example, if a programmer wants to make a chat system, but the program needs to know what ports aren't being refused access to incoming connections by a user's firewall, is there a way to check for this in your code? A program may not fail to bind a socket to a port even if the firewall is blocking that same port from incoming connections. Is there a way to check for open firewall ports?
Sidenote: This is purely for educational purposes and free of bad intentions, to be clear I am writing a chat system, and during testing, I was unable to connect desktop->laptop until I manually opened a port via allowing it through my firewall. This seemed a bit off to me, and unlike something that a programmer's code should require a user to do. Not to mention I don't want to leave the few people using this code at risk (if leaving a port permanently open does so). So It seems like I would be better suited finding a way to utilize ports that are already open to incoming TCP connections.
sidesidenote: all clients are running fedora
You have stumbled onto the second biggest issue governing the creation of new Internet applications nowadays. The first biggest of course is NAT, which is a strongly related issue (and hopefully going away eventually because of IPv6).
And there is no easy answer. One good answer is UPnP, but that's not an easy answer, and by no means universal. My network doesn't run it.
Another answer is to somehow tunnel everything you do over https (or http if you must). But that's a huge pain for something like a chat application.
Let's say I have a server, and two clients connected to it. (via TCP, but it doesn't matter)
My goal is to allow a direct connection between those two clients. This is to allow direct voice contact between two players, for example, or any other client plugin they may have installed which don't need server interaction (like playing some kind of random game between the two). The server can be there to help setting up the connection.
From duskwuff's answer, I got several leads:
http://en.wikipedia.org/wiki/STUN which describes an algorithm to do that, and
http://en.wikipedia.org/wiki/UDP_hole_punching
From those, I got more leads:
http://www.h-online.com/security/features/How-Skype-Co-get-round-firewalls-747197.html
http://nutss.gforge.cis.cornell.edu/stunt.php -- A possible STUN implementation with TCP
With time, I could surely work out something for my program. For now I'm using C++ and TCP (Qt Sockets or Boost sockets), but if needed I don't mind doing UDP in C and wrapping it.
The bounty is there for any programmer having experience with those in C and C++ that may give tips to make this easier, by linking to example programs, updated libraries, or any other useful information. A documented, flexible & working C++ TCP implementation would be the best but I'll take what I get!
Punching TCP holes in NAT is sometimes/often possible (it depends of the NAT behavior). This is not a simple subject to learn, but read the corresponding chapter about NAT traversal from Practical JXTA II (available online on Scribd) to understand the nature of the issues to solve.
Then, read this. It comes from the guy who wrote that: http://nutss.gforge.cis.cornell.edu/stunt.php (one of the links in your question).
I am not a C/C++ specialist, but the issues to solve are not language specific. As long as you have access to TCP from your code base, that's enough. Keep in mind that implementing UDP traversal is easier than TCP.
Hope these tips help.
P.S.: I am not aware of a C/C++ implementation of the solution. The code mentioned in Cornell's link is NOT operational as confirmed by the author. I tried to resuscitate it myself, but he let me know it was completely tweaked for research purposes and far from production ready.
I'm not aware of any way to reliably punch through firewalls for TCP, but there's a similar method for UDP traffic that's pretty well documented:
http://en.wikipedia.org/wiki/STUN
http://en.wikipedia.org/wiki/UDP_hole_punching
A few links to projects that might be of interest or helpful:
http://sourceforge.net/projects/stun/
http://udt.sourceforge.net/
http://www.telehash.org/
You're looking for rendezvous server for NAT hole punching: the server that is publicly accessible (not behind NAT/firewall or they are properly configured) to help computers behind NAT/firewall to establish peer-to-peer connection.
UDP is more popular in NAT punching because provides much better results than TCP. Clear and informative description of UDP NAT hole punching can be found here.
If you need reliable communication, you can use reliable protocols over UDP:
SCTP (libraries) - standardized one, or
one of many custom protocols, e.g. RakNet (I used this library, it's quite mature and feature-rich and has NAT punching implementation), Enet or many others (Q8)
Ephemeral ports won't magically eliminate the need to relay through the server, because they are only valid during the life of the session opened through a well known service port. Basically ephemeral ports depend on a server session.
You will need to use the server to relay communications between both clients, that is act as a proxy server. One option would be to setup a SSH tunnel through a SSH proxy server, with the added benefit of security.
Still this doesn't guarantee that the firewall won't block the connection. That depends on the firewall type and configuration. Most residential routers that act as firewalls, by default block all incoming connections. This is normally fine because most of the time the computers behind the firewall act only as clients, which initiate the connections to the outside. And this setup varies, because some restrict initiating connections only to well known service ports like HTTP, HTTPS, FTP, SFTP, SSH, etc., and if your proxy server uses a non-well-known-service port then the connection will be blocked.
But firewalls can be setup to block outgoing traffic also, this is most common in corporate networks, which don't even allow direct connections to web servers and route everything through proxy servers, in order to control resource usage.
You can also research on the use of UPnP to open ports dynamically.
For a while, I've been interested in creating a proof-of-concept chat program using C++. I have given the idea a lot of thought and even wrote down the beginnings of how I would design the system, but I have hit a barrier in my thinking when it comes to the implementation.
I want to know what an implementation of a peer-to-peer chat client with a server to route connections would look like in C++.
The server would be used as a central registry of the peers, but not used as the primary connection. The server would not interact with the clients in any way except to assign connections between peers to achieve an optimal path between peers. In a first version, it would merely be a directory to which all clients connect, and the clients can then use the directory to connect to the other clients available for chat. (I hope that explains it a bit more). :)
You should look at the XMPP stuff. It is all about routing and co-ordinating messaging. It uses de-centralization and a peer-to-peer like architecture.
There are also plenty of open source implementations. For example,
Jabber.org
I cannot really think at something better than the chat example in
the Boost.Asio documentation. Search for the examples documentation in Boost.Asio.