Is it possible to run a TCP Ping in Silverlight? I know raw sockets are not allowed, which is a big limit. But can you achieve something similar to speedtest.net which does a ping utilizing Flash?
It depends on what you're wanting to do. If you want to do a "real" ping, i.e., using ICMP, no you can't. But if you just want to know whether you're connected to the Internet in some fashion, or if some server is online, you could use the WebClient classes to see if a given web server responds, or if you control the server, you could use the Socket classes to try to open up a connection to a Silverlight-accessible port.
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I need to write a proxy server in C language on Linux (Ubuntu 20.04). The purpose of this proxy server is as follows. There're illogical governmental barriers in accessing the free internet. Some are:
Name resolution: I ping telegram.org and many other sites which the government doesn't want me to access. I ask 8.8.8.8 to resolve the name, but they response of behalf of the server that the IP may be resolved to 10.10.34.35!
Let's concentrate on this one, because when this is solved many other problems will be solved too. For this, I need to setup such a configuration:
A server outside of my country is required. I prepared it. It's a VPS. Let's call it RS (Remote Server).
A local proxy server is required. Let's call it PS. PS runs on the local machine (client) and knows RS's IP. I need it to gather all requests going to be sent through the only NIC available on client, process them, scramble them, and send them to RS in a way to be hidden from the government.
The server-side program should be running on RS on a specific port to get the packet, unscramble it, and send it to the internet on behalf of the client. After receiving the response from the internet, it should send it back to the client via the PS.
PS will deliver the response to the client application which originates the request. Of course this happens after it will unscramble and will find the original response from the internet.
This is the design and some parts is remained gloomy for me. Since I'm not an expert in network programming context, I'm going to ask my questions in the parts I'm getting into trouble or are not clear for me.
Now, I'm in part 2. See whether I'm right. There're two types of sockets, a RAW socket and a stream socket. A RAW socket is opened this way:
socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
And a stream socket is opened this way:
socket(AF_INET, SOCK_STREAM, 0);
For RAW sockets, we use sockaddr_ll and for stream sockets we use sockaddr_in. May I use stream sockets between client applications and PS? I think not, because I need the whole RAW packet. I should know the protocol and maybe some other info of the packet, because the whole packet should be retrieved transparently in RS. For example, I should know whether it has been a ping packet (ICMP) or a web request (TCP). For this, I need to have packet header in PS. So I can't use a stream socket, because it doesn't contain the packet header. But until now, I've used RAW sockets for interfaces and have not written a proxy server to receive RAW packets. Is it possible? In another words, I've the following questions to go to next step:
Can a RAW socket be bound to localhost:port instead of an interface so that it may receive all low-level packets containing packet headers (RAW packets)?
I may define a proxy server for browser. But can I put the whole system behind the proxy server so that packets of other apps like PING may route automatically via it?
Do I really need RAW sockets in PS? Can't I change the design to suffice the data I got from the packets payload?
Maybe I'm wrong in some of the concepts and will appreciate your guidance.
Thank you
Can a RAW socket be bound to localhost:port instead of an interface so that it may receive all low-level packets containing packet headers (RAW packets)?
No, it doesn't make sense. Raw packets don't have port numbers so how would it know which socket to go to?
It looks like you are trying to write a VPN. You can do this on Linux by creating a fake network interface called a "tun interface". You create a tun interface, and whenever Linux tries to send a packet through the interface, instead of going to a network cable, it goes to your program! Then you can do whatever you like with the packet. Of course, it works both ways - you can send packets from your program back to Linux through the tun interface, and Linux will act like they just arrived on a network cable.
Then, you can set up your routing table so that all traffic goes to the tun interface, except for traffic to the VPN server ("RS"), which goes to your real ethernet/wifi interface. Otherwise you'd have an endless loop where your VPN program PS tried to send packets to RS but they just went back to PS.
There's already a question How exactly does a remote program like team viewer work which gives a basic description, but I'm interested in how the comms works once the client has registered with the server. If the client is behind a NAT then it won't have its own IP address so how can the server (or another client) send a message to it? Or does the client just keep polling the server to see if its got any requests?
Are there any open source equivalents of LogMeIn or TeamViewer?
The simplest and most reliable way (although not always the most efficient) is to have each client make an outgoing TCP connection to a well-known server somewhere and keep that connection open. As long as the TCP connection is open, data can pass over that TCP connection in either direction at any time. It appears that both LogMeIn and TeamViewer use this method, at least as a fall-back. The main drawbacks for this technique are that all data has to pass through a TeamViewer/LogMeIn company server (which can become a bottleneck), and that TCP doesn't handle dropped packets very well -- it will stall and wait for the dropped packets to be resent, rather than giving up on them and sending newer data instead.
The other technique that they can sometimes use (in order to get better performance) is UDP hole-punching. That technique relies on the fact that many firewalls will accept incoming UDP packets from remote hosts that the firewalled-host has recently sent an outgoing UDP packet to. Given that, the TeamViewer/LogMeIn company's server can tell both clients to send an outgoing packet to the IP address of the other client's firewall, and after that (hopefully) each firewall will accept UDP packets from the other client's Internet-facing IP address. This doesn't always work, though, since different firewalls work in different ways and may not include the aforementioned UDP-allowing logic.
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'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.
Let me explain my scenario before asking the question.
I am in creation phase of 17 different multiplayer games that can be played online, directly from browser.
To accomplish this, I have choosed Silverlight.
Communication will be done using Sockets.
Image 17 different type of games like Chess, Backgammon, Pool and hundred of online users communicating between client app and server app using Sockets binded to the same PORT number.
Wouldn't be faster (for my server) if every different type of game will use another PORT number ? Chess will use 4502, Backgammon will use 4503, Pool 4504.
Will this make a difference ? Or should I use the same PORT number 4502 for all games with no fear that something bad can happen ?
A socket that has been established as a server can accept connection requests from multiple clients. The original server socket does not become part of the connection. The accept method makes a new socket which participates in the connection and returns this socket. The server's original socket remains available for listening for further connection requests.
So it has no advantage to use different server ports. After all web servers get all their requests on port 80 and handle this very well.
As far as speed of processing on your server goes it will most likely make very little difference whether you receive all your communications on one socket or 17. The one socket approach would be a tiny bit faster since your server application will probably have fewer threads to switch between. However there will be other things that will have a higher overhead such as actually processing the game moves or authorising client requests etc.
As for the question of whether to use one or multiple sockets the bigest thing you should think about is deployment constraints. The TCP port numbers that Silverlight is allowed to use a non-standard (i.e not 80 or 443) and if there is a firewall or proxy between your client and server you may be better sticking to a single port to make the access control list on the firewall simpler.