I need to detect the presence/absence of internet connection. More precisely, let us suppose that the application is broken up into 2 parts - A and B.
A is responsible for checking whether or not the system is connected to the internet. If it finds that there is no connection, it starts up part B. And as soon as it detects that there is a network connection, it kills B and continues its own work.
What would be the best way to do the A part of the application? Continual pings sounds hideous. There has to be a better way of doing this (preferably in C).
With sufficient privilege you can test the various network interfaces and examine their state. This would tell you if any of the interfaces was connected to a network and operating. However, this won't tell you if the connection is actually usable, i.e., connected to the internet (or your local net if that's all you need). I don't know of anyway to do that short of actually using it.
Using ICMP (ping) can be useful at a low level, but presumably what you need is a connection to an actual endpoint via TCP/IP to do real work. I would say that you should change the design of your application so that B is responsible for indicating when it is unable to continue due to the absence of resources that it relies on -- network or otherwise. A and B should communicate so that A is aware of the situation and is able to either kill B or respond to B terminating itself and thus continuing its work.
A lot of companies have measures in place to prevent outgoing ICMP requests, TCP connections to ports other than 80/443 for example, or even to prevent you from reaching the internet directly by (transparently) proxying your traffic.
Under an internet connection I would understand any way to contact the outside, be it UDP, TCP or ICMP. Depending on what your application needs to contact the internet for, I would suggest to check over the same protocol, as that is the only thing that matters to your app.
If your application uses HTTP to communicate to an external source, try to connect to a few sites you would suspect to not be blacklisted and that have a reliable uptime. Like google.com, microsoft.com, apple.com, and so on...
Edit:
I am unsure what the specifics are, so let me give you an example with a hypothetical situation.
Application A collects data on the system it is running on and forwards it to a Web Service listening on yourserverhost.yourcompany.com:80
Application B would basically take over the job of the Web Service when it is down and log everything so no data is lost.
When all is well, App A will be sending the data to your web service
Once this connection drops, you immediatly launch App B (the obvious remark here would be, why not keep App B running as a failsafe)
App A connects to App B and forwards what it had been buffering
App A continues to try to reestablish the connection to your Web Service and once it is back up will request App B to stop
If the problem you are facing is nothing like this, please provide a more concrete description of what App A and App B are supposed to be doing. I will be more than happy to help.
In your code, you have to check whether the internet connection exists by using a socket to open a connection to a website.
Firstrun: Ask user to input the network parameters, like proxy settings. Save this info.
Next runs: Use these settings to check for the Internet connection. You may simply do a DNS search.
If results are negative, ask user to check settings.
Check whether the cable is connected , if so ping your internet connection to any host as google.com.
ping google.com
Related
I am trying to find a way to secure our robot against unwanted Choregraphe connections. We are required to work on a University-wide network, and we need a way to stop people from connecting who may have obtained the robot's IP address at some stage without our knowledge.
As there is no access to the root user account on the Pepper, I cannot simply lock down access using iptables, so I thought I might try looking at a way to forcibly close connections from ALChoregraphe when it registers on the robot.
However, running the command:
qicli info ALChoregraphe
I can see that the only method available is requestDisconnection. There is no way to close the connection forcibly.
I have tried using ALServiceManager to stop the service, but it apparently only knows about services that are installed as packages.
So far the only solution I have is to change the color of the eye LEDs to indicate that a connection has been established, and reset them when a disconnect is received.
Aside from moving the robot to its own network, do you have any suggestions on how I could go about handling this?
Thanks!
At the moment, there is no other way to prevent connections to the robots. All you can do is to make sure that unwanted clients cannot access the network of your robot.
In Choregraphe 2.4 and later, you can kick the existing Choregraphe after 30 seconds. If anyway it fails, you should unregister the services ALChoregraphe and ALChoregrapheRecorder using qicli call ServiceDirectory.unregisterService <serviceID> where serviceID is the number facing the services when listed with qicli info.
What I'm doing
I'm implementing a websocket server on a stellaris board as the title says. At the moment I'm able to establish connection to the client and send a few frames.
The way I'm implementing the websocket
The way I'm developing it is something like a master slave communication. Whenever the client sends a string, the server decodes it and then answers. At the moment I'm simply responding to a character 'e', which is designed to be just a counter. The thing is that I implemented the websocket on the client side to send 'e' whenever it receives a message and then displays the message on the page.
The problem
The problem is that it does about 15 transactions and then I can see the communication being re-transmitted from and to the stellaris board and then the communication closes. After the connection closes I noticed that that I can't access any other page on the board. It simply doesn't respond anymore.
My assumptions of what may be causing it
This lead me to believe that the transactions are being too fast and there may be an implementation bug, lwIP bug or hardware bug (I'm using the enet_io example as base).
My assumptions on how to fix it
After seeing this I can imagine that what I need is to control the string being sent to the microcontroller so that it sends once a second, or maybe even less, because at the moment it was doing something like 1000 transactions per second and sometimes more.
The question
So ... after my trials I still have a few questions that need to be answered. Do websockets need this kind of relationship? Where client asks and server serves? Or can I simply stream data from the server to the client as long as the connection is open? Is my supposition that slowing down my rates will work?
Do websockets need this kind of relationship [request-response]? Where client asks and server serves? Or can I simply stream data from the server to the client as long as the connection is open?
The Websocket protocol doesn't require a request-response model (except for the connection establishing handshake).
The server can stream data to the client without worrying about any response or request from the client.
However, it's common practice to get a response or a ping from a client once in a while, just to know they're alive.
This allows the client to renew a connection if a message or ping fails to reach the server - otherwise the client might not notice an abnormally dropped connection (it will just assume no updates are being sent because there's no new data).
It also allows the server to know a connection is still alive even when no information is being exchanged.
Is my supposition that slowing down my rates will work?
I guess this question becomes less relevant due to the first question's answer... however, I should probably note that the web socket client (often a browser) will have limited resources and a different memory management scheme.
Browsers are easy to overwhelm with too much data because they often keep references to all the exchanges since the page was loaded (or refreshed).
This is especially true when logging events to a browser's console.
I'm writing a port scanner in C and i want to detect what service is running on an open port and its version.I've already wrote the scanner code but now i have no idea about how to detect running service.
What can i do?
If you are determined to do it in your own code, you can connect to the port, see if you get any data on it, if nothing then send a few bytes, and check again.
Then match that against expected response.
to get an idea what you are looking for, you can connect manually to the port with telnet and poke at it. In many cases (a web server is an easy example) you must send some correctly formatted data in order to get a usable response.
nmap has done all this and much more (e.g. extensive checks such as looking for byte order and timing of arp traffic)
UPDATE: several people have mentioned well known ports, but that won't help you discover standard services running on nonstandard ports, such as ssh or http servers running on custom ports.
If server sends something first, use that to identify protocol.
If not, send something according to some protocol, such as http, and see what server sends back (valid response or error). You may need to make several attempts with different protocols, and a good order is important to minimize connection count.
Some protocols may be very hard to identify, and it is easy to make custom server with unique protocol you don't know about, or even hide real server under simple fake server of other proto such as http.
If you just want to know what the port usually is, check "well known ports" and official reserved ports.
Also check nmap source code.
I am developing a server application that needs to send a lot of data to the client. However, client can get disconnected at any time and send()/write() on socket will return an error in this case. I would like to check how much data has been actually delivered before a client get disconnected to be able to continue sending data from the place where it left off when the client reconnect.
Is it possible to check it using sockets API?
No, the sockets API does not give you this information. In fact, it is not possible in general to know this. Depending on the particular way in which the connection failed, the TCP stack on one side generally can't know how much data successfully made it to the other side. The only thing it can know is how much data was acknowledged, which is not the same thing. And considering that other things than TCP/IP might have failed (the local OS, the remote OS, the remote process, the remote application logic), the amount of data that has been acknowledged at the TCP level probably doesn't mean much anyway.
You need to use an end-to-end application protocol to have the remote end acknowledge the data it has received and successfully processed (and committed, if applicable).
I've come across an interesting problem. Basically I have 2 mobile phones that are both behind NATs. I want to communicate directly between the 2 devices using UDP.
I know if I initiate a connection from the phones to a server then I can push data back down that connection to the phone (ie send it back from the same port that received the message to the same ip and port that it was received from). So I can easily communicate between the 2 devices by connecting both phones to the server. Then sending data to the server and having it re-routed back to the phones. This bypasses any NAT traversal issues I may come up against.
However I would rather just use the server to point the 2 devices at each other and then let them communicate directly. How would I go about doing this? Is it possible without using something like uPnP?
Any help would be much appreicated!
Edit: I found this document http://www.brynosaurus.com/pub/net/p2pnat/ It looks like hairpin translation is what I'm after but it doesn't look to be widely supported. I wonder how good mobile ISP's support for UPnP is?
What you're looking for is UDP hole punching, see e.g. http://en.wikipedia.org/wiki/UDP_hole_punching
The basic idea is simple, you tell each endpoint the ports to use, and they start sending udp packets. The NAT'ing devices will set up a traversal rule when they see the first outgoing packet, and then the next attempt from the other end will match this traversal rule.
You need a mediator server, so the clients can tell where they are. Then one opens a server by uPNP, and the other connects to it.