I have been working with Raw Berkeley Sockets in C, which allow the programmer to access Layer 2 (IP Headers) of the Internet Protocol Suite. I would like to know how the raw sockets handle transport on Layer 1 (Link Layer), specifically with regard to wireless local area networks.
For example, how does the socket know which network interface (eth0 or wlan0) to send the packet to?
If I am behind a wireless router, should I still fill the IP Source Address of my IP Header with my LAN address (192.168.1.1), or with the internet address (74.125.226.68) of my router?
If I do fill the IP Source address with my LAN address (192.168.1.1), does the IP Header get changed when it goes through the router, or does it stay that way, so that the receiving end of the packet attempts to send the response to a local host on its network?
I would appreciate any insights in this area.
For example, how does the socket know which network interface (eth0 or wlan0) to send the packet to?
You can use the socket option SO_BINDTODEVICE to bind your socket to a specific interface. Otherwise, the operating system chooses one.
If I am behind a wireless router, should I still fill the IP Source Address of my IP Header with my LAN address (192.168.1.1), or with the internet address (74.125.226.68) of my router?
You should use your LAN address. The router is responsible to take care of NAT (network address translation) if necessary.
If I do fill the IP Source address with my LAN address (192.168.1.1), does the IP Header get changed when it goes through the router, or does it stay that way, so that the receiving end of the packet attempts to send the response to a local host on its network?
If NAT is enabled at the router, the source address is changed to reflect the router's external address. Otherwise, your packages are likely to be dropped by routers or firewalls, because your source address is invalid in their context.
Related
I'm recoding a simple ftp server and I'm stuck of the implemetation of the PASV command.
In fact, when the client send a PASV command, I have to create a new server socket and send back the infos (address and port) to the client so that he can connect to it to create the data connection.
What I do step by step is:
Create a server socket with INADDR_ANY and random port (port 0)
listen() with this socket
getsockname() this socket to get it's infos
send back the infos to the client on the command connection
the accept() is done later, when the user need to use the data con
(Is that the right way to do it ?)
The thing is, the server socket that I create on the server side is binded with INADDR_ANY so getsockname() on it always return 0.0.0.0 (cause it is binded to virtually all the ips off the system).
Is that case, what address should I send back to the client and how to find it ?
How real ftp servers handle this ?
Thanks :)
What I do step by step is:
Create a server socket with INADDR_ANY and random port (port 0)
listen() with this socket
getsockname() this socket to get it's infos
send back the infos to the client on the command connection
the accept() is done later, when the user need to use the data con
(Is that the right way to do it ?)
In general, yes, however steps #1 and #3 tend to be more complicated than that.
For one thing, getsockname() cannot get the true IP until the socket is actually connected to someone (which, in this case, means you would have to call getsockname() on the socket returned by accept(), not on the socket that you call accept() on). So, when binding to INADDR_ANY, you should just report the IP of the interface that the command socket is connected to. In which case, it is better to just bind the listening socket to only that interface directly and not to INAADDR_ANY at all.
For another thing, even if the server machine only has 1 interface installed, if the server is behind a NAT router, and the client is outside the NAT, then you would have to report the router's public IP instead of your server's listening IP. You would have to know the router's public IP ahead of time, either by storing it in your app's configuration, or by dynamically querying the router itself via uPNP, or by querying an outside service like http://iplookup.flashfxp.com/. Unless the router is FTP-aware (some are) and is smart enough to replace the reported IP for you when passing through the router, in which case you do have to report your listening IP instead.
You should call getsockname() on the command socket. That gives you the IP address the client used to connect to you. It can be different for each client on a multi-homed host. If you're behind a NAT device you should use its public IP address, which you will have to obtain via configuration.
Please Clarify my doubt in ARP. In the below Topology,.
Looking for better understanding, how ARP works when Hosts are connected between default Gateways .
Host A(10.10.10.1)-----Switch-----(10.10.10.2)Router(20.20.20.2)--------HostB
When a Host want to communicate to Host B , does hostA send an ARP request for Destination IP (20.20.20.1) or to Default Gateway (10.10.10.2) .
If it send ARP for HOST B mean, Host A identifies Host B is in different network, it will send the Packet only to the Configured Default Gateway , so will it request for Default Gateway Mac address
Host B mac will not be learnt by Host A by any cost, So does Host A will generate ARP request for each time?
Since a MAC address is only useful for a layer-2 domain which is bounded by a layer-3 device (router), a host will not ARP for a MAC address on a different subnet unless it has a misconfigured mask.
The host will ARP for the address of its configured gateway so that it can send the frame to the gateway, and the gateway will remove the layer-2 frame and try to forward the layer-3 packet to the port toward the end host. The router will then encapsulate the layer-3 packet with a new layer-2 frame (possibly using ARP if the nest hop isn't in its ARP cache) for the next link.
I need to communicate Arduino module with my Phone through ESP8266 wifi module
They all link to the local LAN network and didn't know the IP address of each other, so I plan to send UDP broadcast message from my phone, but ESP8266 module seems doesn't receive the message
The ESP8266 library I use is from here:
https://github.com/itead/ITEADLIB_Arduino_WeeESP8266
and use the function registerUDP() and recv()
Can somebody help me?
I don't have enough reputation to leave a comment so I will make an answer.
I have a feeling that your registerUDP() need to specify an IP address and I guess you are supplying your DHCP assigned IP address. Where as the UDP broadcast is sent using a multicast or broadcast IP.
You can modify the initiator code to send UDP packet to all IP within the same subnet (e.g. 192.168.1.1 to 192.168.1.254) and see if it works.
My question is, how to connect to socket on romote mechine?
I can only connect sockets on same network..
I wrote a simple code (in c), that simulate a server (open socket and listen for client). in Mac.
I'm trying to connect this socket as a client from iPhone (with simple objectiv-c code).
If my internet on both, server and client, is on the same network (WiFi) and in client I trying to connect to 192.168.1.x, it's working.
But when, in client, I'm trying to connect via external IP (with the same port) connection is failed.
I never did this bofore. Maybe I miss somthing.. I've tried to turn my FireWall off. It did not help.
Thanks.
Edit: If it's not clear.. my Mac is connected by router.
In a setting like this, the "external IP" would typically be the IP of the router. In all likelihood you'll need to configure the router to forward the relevant port to the internal IP address.
It could also be the case that for the port forwarding to work, the request has to come in on the external (WAN) interface. This depends on how the router is configured. If that's the case, you'll need to make sure that you're accessing the external IP via the cellular network and not the Wi-Fi connection on your iPhone.
If you're connecting to your server via a local IP address (i.e., you're connecting to another machine on the same local router via a delegated DHCP address), then your issue is on the network, not the IP-stack of the local machine.
You'll need to look into your router settings ... many routers will block a number of services, especially those on custom ports, in order to prevent malicious attacks from sources external to the local network.
I'm new to this forum (at least at asking questions), so please be patient. I'm trying to write simple client-server applications to communicate using datagram (UDP) sockets, with C and Windows.
The scenario is very simple, I want to be able to send/receive data from a machine in some local network (behind a GATEWAY/NAT) to some host having public IP. I can send the data, but it seems impossible to receive any data back, because the remote server can't see the client's local IP.
I don't want the user to perform manual port forwarding on the GATEWAY, and I also don't want to use UPnP because of security issues (though it also should be enabled by the user).
I think it is possible because applications like Skype or Browsers can do it, but how ?
Are there some automatically forwarded ports or things like that ? I'm exhausted of searching the web ... PLEASE HELP !
For UDP connections, most home NAT gateways will automatically create a temporary reverse mapping for each outbound packet.
Consider a scenario where your client is running on the internal machine with IP address 192.168.4.5, sending a UDP packet from port 65000 to the external address 64.34.119.12:6789, via a gateway with external IP address 192.0.43.10. When the gateway sees your internal client send a UDP packet from 192.168.4.5:65000 to the external address, it will NAT it to an external address and port, like 192.0.43.10:5500. Your server will see a packet with a source address of 192.0.43.10:5500 and destination address 64.34.119.12:6789. The router also sets up a reverse mapping, so that if it sees a packet arrive on the external interface with a source of 64.34.119.12:6789 and a destination of 192.0.43.10:5500, it will redirect it back to 192.168.4.5:65000. This mapping typically times out after a short while.
This means that in simple cases, all you need to do is:
On the client, use the same port to send to the server and listen for responses;
On the server, respond to the client at the address and port that the client's packet was receieved from, using the same server port that recieved the packet;
Have the client send the initial packet in the conversation;
Don't leave the "connection" idle for more than a few minutes at a time.