I'm not sure I've phrased the question correctly but I'll explain a bit more.
I have a server running on a virtual machine on PC1. I can access this through a particular IP address on the same computer.
Now on a different PC2, when I try the same IP address I see the content served by PC1's server. Both computers are on the same network.
I don't really understand how that IP address is serving the same info on PC2. I'm not sure of the mechanics in the background either and it would really help if someone could explain what's happening here.
If you are using a wifi network then it is because of it you can access the content of the server you have created.
You can see the architecture as that of the internet but on a small scale. Your IP addresses are stores in the wifi (routing tables) so that it can send packets accordingly (See hoping and packet transmission). To be precise, each and every individual system maintains a routing table in it. Thus, in order to fetch a particular site, a system sees its routing table. If the particular IP address is present, the router returns it and the system shows that page. On the contrary, if the IP address is not present the router asks the nearby systems/servers for that particular IP address and the phenomenon continues till the IP address is found.
So, when you search for a local server via system B, whose data is in system A, then the router requests all its child systems to search for the particular IP address in their routing tables, and thus you can access the local server via systems connected on the same network.
To add furthermore, since the local servers are known to be locally operated, the router just sends the seek requests to its child systems only and not globally.
I have tried to keep it as simple as possible supposing that you have not learned about computer networks yet.
Related
We have two identical servers A and B in our office. These two servers are synced together in all aspect. That's if some changes take place in one server then it will take effect in another server. This has been done to minimize the downtime. Now server A has a public IP address (X.X.X.X) form one ISP. Server B has a public IP address(XX.XX.XX.XX) from a different ISP. Now for some reason IP address X.X.X.X goes down. Now How can we automatically forward traffic to another IP address so website will not go down?
You're basically asking about high availability, you'd have a third server (called load balancer) in front which would sent traffic to either server based on their status. Have a look at a simple setup in here.
can you use DNS-level fail over using https://docs.aws.amazon.com/Route53/latest/DeveloperGuide/dns-failover.html
If I open a socket between two computers on the WAN, while at least one of them has a dynamic ip-address, and this address changes even though the socket hasn't been explicitly closed : Do have a warranty that this socket will remain open at least until my program really asks to close it?
Regards
As soon as the IP address of one of the two computers changes then the computer that held on to the original address, (not knowing the new address) would no longer be able to communicate with the other. There is no automatic process of informing the connected clients that a new IP address has been assigned, you could conceivably create one but you would have to communicate the new address over the old IP address before you changed it. Also the other computer wouldn't immediately know that addresses had changed at all, you would have to wait until it timed out. The other big challenge is the likelihood that the computer wouldn't even know that it's own IP address had changed because on a WAN it might be exposed to the internet via a NATed addresses.
I'm on the client side. There're multiple network interfaces. How can I let different processes use different network interfaces to communicate? Since I want to connect to the same server, routing seems not working here. Also, connect() doesn't have arguments to specify local address or interface as bind() does.
If your goal is to increase bandwidth to the server by using multiple network interfaces in parallel, then that's probably not something you can (or should) do at the application level. Instead, you should study up on Link Aggregation and then configure your computer and networking stack to use that. Once that is working properly, you will get the parallelization-speedup you want automatically, without the client application having to do anything special to enable it.
"The bind() system call is frequently misunderstood. It is used to
bind to a particular IP address. Only packets destined to that IP
address will be received, and any transmitted packets will carry that
IP address as their source. bind() does not control anything about the
routing of transmitted packets. So for example, if you bound to the IP
address of eth0 but you send a packet to a destination where the
kernel's best route goes out eth1, it will happily send the packet out
eth1 with the source IP address of eth0. This is perfectly valid for
TCP/IP, where packets can traverse unrelated networks on their way to
the destination."
More info e.g. here.
That's why you probably misunderstand bind() call.
The appropriate way to bind to physical topology (to some specific interface) is to use SO_BINDTODEVICE socket option. This is done by setsockopt() call.
Source Policy Routing might be helpful.
Try the following steps:
Use iptables to give packets from different process with different marks.
Use iproute2 to route packets with different marks to different table.
In different table, set the default route to different uplink.
The whole process require certain amount of understanding about linux networking.
Here is an example shows how to route all traffic for a user through one specific uplink: http://www.niftiestsoftware.com/2011/08/28/making-all-network-traffic-for-a-linux-user-use-a-specific-network-interface/
You could try follow similar approach by running different process with different user and route traffic from one user to one uplink.
Also you could let processes communicate with the server with different port and mark the traffic by port.
I've read tons of questions about this all over the web, and can't seem to find a solid answer. If I have an iPhone that's running on cellular data and another iOS device on wifi (in two separate locations), is it possible for them to send data to each other directly without sending it first to a web server, then retrieving it? Are the only options sending and receiving from a server/Apple's iCloud? What if I knew the devices' ip addresses? Note that the iPhone has WiFi disabled.
I'm not looking to put this in the app store, it is for personal use. I know NSNotificationCenter isn't an option.
Using the gamekit framework you can send data between two iOS devices. It is easy to implement. Other than that I don't think there is any other way to send data between two iOS devices.
Actually, it IS possible. You may want to google for something called "UDP hole punching" or "TCP hole punching".
The main approach in short: Assuming you got something like a relay server, that is some server in the internet that is publicly addressable from every private LAN that is connected to the www. No you have your two clients A and B in (different) private LANs, with some Network address translation (NAT) going on, that want to establish a peer to peer connection.
First of all both will tell the server their IP address and the port they have in their own LAN. In the UDP or TCP packet, the server will find the public address and port of the device (or the NAT (router)). So the server knows the private and the public IP address as well as the ports.
If now A wants to communicate with B, it asks the server for help. The server will send a message to B that A wants to communicate with her telling her A's public and private IP and port. A gets back B's public and private information and port.
Now here is where the magic happens. Both clients now send packets out to establish a connection simultaneously to the private and public addresses of the other party and thus punching a whole in their NATs such that incoming connections will not be blocked. Even if one party's connection establishing packets will arrive before this whole is created, the other's packets will get through to such that a connection can be created.
Beware of some NATs that scan the data for IP addresses and translate them as well, but if you encrypt your data or change the appearance of the address (complement, ...) you will be fine.
Now the master question, how can the server communicate with one of the clients without an active connection. Well in this case you can use "connection reversal" and apple's "push notifications". Use the "push notifications" (pn) to tell a client behind a NAT that there is something of interest going on and that it should contact the server. Once it has done that the connection is active and can be used in the previous described fashion.
I hope this helps some people that get to this problem although the post is quite old!
You can only use direct IP address communications if the IP address are publicly reachable IP addresses accessible over the internet, and they are static (enough) so that they are not changing on you regularly as devices get assigned to addresses dynamically. In many (most) cases, that won't be true because your devices will be assigned their IP address dynamically and those addresses are frequently going to be self-assigned IP addresses that aren't publicly addressable.
As others have commented, using Apple-provided mechanisms like iCloud are probably the easiest options. If that's not something you'd like to entertain, there are probably ways to make use of a dynamic DNS service like DynDNS to manage the actual IP addresses of your devices. With something like that you might be able to use a direct IP connection between devices based on a named DNS lookup. You'd probably have to jump through some hoops to make that happen though and I'm not sure you'd want to go to that extent.
I think that Bluetooth would be a good option for you
I have been working on a local LAN service which uses a multicast port to coordinate several machines. Each machine listens on the multicast port for instructions, and when a certain instruction is received, will send messages directly to other machines.
In other words the multicast port is used to coordinate peer-to-peer UDP messaging.
In practice this works quite well but there is a lingering issue related to correctly setting up these peer-to-peer transmissions. Basically, each machine needs to announce on the multicast port its own IP address, so that other machines know where to send messages when they wish to start a P2P transmission.
I realize that in general the idea of identifying the local IP is not necessarily sensible, but I don't see any other way-- the local receiving IP must be announced one way or another. At least I am not working on the internet, so in general I won't need to worry about NATs, just need to identify the local LAN IP. (No more than 1 hop for the multicast packets is allowed.)
I wanted to, if possible, determine the IP passively, i.e., without sending any messages.
I have been using code that calls getifaddrs(), which returns a linked list of NICs on the machine, and I scan this list for non-zero IP addresses and choose the first one.
In general this has worked okay, but we have had issues where for example a machine with both a wired and wifi connection are active, it will identify the wrong one, and the only work-around we found was to turn off the wifi.
Now, I imagine that a more reliable solution would be to send a message to the multicast telling other machines to report back with the source address of the message; that might allow to identify which IP is actually visible to the other machines on the net. Alternatively maybe even just looking at the multicast loopback message would work.
What do you think, are there any passive solutions to identify which address to use? If not, what's the best active solution?
I'm using POSIX socket API from C. Must work on Linux, OS X, Windows. (For Windows I have been using GetAdapterAddresses().)
Your question about how to get the address so you can advertise it right is looking at it from the wrong side. It's a losing proposition to try to guess what your address is. Better for the other side to detect it itself.
When a listening machine receives a message, it is probably doing do using recvfrom(2). The fifth argument is a buffer into which the kernel will store the address of the peer, if the underlying protocol offers it. Since you are using IP/UDP, the buffer should get filled in with a sockaddr_in showing the IP address of the sender.
I'd use the address on the interface I use to send the announcement multicast message -- on the wired interface announce the wired address and on the wireless interface announce the wireless address.
When all the receivers live on the wired side, they will never see the message on the wireless network.
When there is a bridge between the wired and the wireless network, add a second step in discovery for round-trip time estimation, and include a unique host ID in the announcement packet, so multiple routes to the same host can be detected and the best one chosen.
Also, it may be a good idea to add a configuration option to limit the service to certain interfaces.