I'm writting a java application, and I need to quickly discover any other running clients on any wired or wireless local networks in order to establish a TCP connection.
what's the best way of doing this? Are there libraries or code snippets that would do this?
Multicast UDP is a good way of doing this. It's used in a couple of technologies that support automatic discovery of networked devices over local IP networks (UPnP and ZeroConf).
Multicast UDP is not TCP, but it is still based on IP and, so, uses the same addressing mechanism i.e. IP addresses. Quite often it is compared to radio broadcasting i.e. a multicast sender only needs to send 1 message (i.e. it is like a broadcast) but only clients that are "tuned-in" to the multicast channel will receive it.
You can do a quick search on google or wikipedia for these as a starter, but the basic idea is as follows:
when a client starts, it sends out a multicast UDP "hello" message to some pre-specified multicast address and port (e.g. UPnP uses 239.255.255.250:1900)
existing clients are listening for incoming multicast "hello" messages on the specified address and port - when a client receives one, it sends a response to the sender
the client sending the "hello" message receives a response from each existing client on the network and is now aware of the IP address of each client
If you are looking for libraries to use, UPnP libraries can tend to be a bit heavyweight and a lot of folk generally don't like them, so ZeroConf might be a little more suitable. I don't know of any java implementations of such things but I'm sure you can find some with a little digging around.
A network scan can be very long, even longer on wireless networks. If you need them quickly thru Java you may implement a "meeting point" server on your network. This server listen to a predefined port, clients register on the server on startup and the server can distribute information about the clients on request.
HTH.
I guess you need to do a scan on your application's port on all IPs in your subnet.
Just what are the available IPs - or what is your subnet for that matter?
I'm afraid determining that could turn out to be impossible as the network is designed to be transparent to your application.
So, i'd use brute force: pick your IP and change the last byte. Might be too much, might be not enough though.
Or you send a broadcast (which usually would be targeted at x.x.x.255) and see who answers.
See Datagram Broadcasting and Multicasts. But i think that's not TCP/IP anymore.
There is a JGroups toolkit for reliable multicast communications. It allows automatic discovery of additional clients using Multicast techniques as described in other answers.
It also provides communication APIs on top of multicast sockets.
It is used in a number of projects such as JBoss, Tomcat and more to provide an infrastructure for distributed cache. See more here.
Related
I am building an IoT solution where I will have multiple devices connected to my local network that I want to connect to a hub that controls my solution.
Once I connect a new device I want to connect it with the hub and give it a name to identify it. I would like to implement the detection in an automatic way (so I don´t have to enter IP addresses manually). As an example when a Chromecast is present in a network I can see it in my streaming applications in my phone. So I want to do something similar to connect the hub with the devices.
My ideas so far is that it is two ways of doing this:
The hub scans the network for new devices (either periodically or when I say there is a new device present).
The devices scan the network to find the hub once connected.
Is any of these approaches to prefer over the other and in that case why?
When doing a scan, in whatever direction I choose, what is the most effective way to do the scan? I am doing an implementation using Java and what I have so far is this:
int timeout = 100;
for (int i = 1; i < 255; i++)
{
String host = subnet + "." + i;
if (InetAddress.getByName(host).isReachable(timeout))
{
String hostname = InetAddress.getByName(host).getHostName();
String canonicalHostName = InetAddress.getByName(host).getCanonicalHostName();
System.out.println(host + " is reachable. Hostname: " + hostname + ", CanonicalHostName: " + canonicalHostName);
}
}
What I have seen here is that the hostname returned is for most stuff in my network just the IP address, and not the name that I see as the hostname in my router. I thought I could use the hostname as an identifyer to detect specific devices and understand what they where - but with this small poc that does not seem to work. So how can I identify the devices in a good way?
Is there any library/solution for Java (or Javascript) and ESP8266 to do this already? (Feels like a common problem if implementing "smart home" devices).
There is no single way that we discover devices on a local area network.
Rather than scanning a network, devices generally either use a multicast or broadcast protocol to announce their presence, or they rendezvous at a (usually external) preconfigured server.
Some devices use mDNS - based loosely on the DNS protocol, they multicast packets to advertise their presence on the network. Apple uses mDNS in their products. It's poorly supported on Android, and requires additional software on Windows. mDNS names are normally in the .local domain.
Some devices use UPNP and SSDP - Belkin's Wemo line of products does this. UPNP and SSDP are overly complicated protocols based on XML and SOAP, which are poor choices for devices with limited RAM and processing power like the ESP8266 and ESP32.
Some devices just roll their own protocol. Haiku's "Big Ass Fan" line does this - they broadcast UDP packets with a homegrown protocol that at least initially was vulnerable to a variety of problems. I don't recommend going this route unless you really know what you're doing. Other, established protocols have already had a chance to have the bugs ironed out. Unless you're experienced in protocol design you're more likely to reinvent problems other protocols have had than a wonderful shiny new discoverability protocol.
These kinds of devices will either periodically broadcast or multicast a packet announcing themselves, or the thing you're calling a "hub" would broadcast or multicast a request and the devices would respond to that request.
Not all devices present an interface for direct control over the LAN they're connected to. Some simply rendezvous with a remote server. In this case you discover them by asking the server to enumerate them, and you control them via that server. Google's Nest products work this way - initial provisioning is done via Bluetooth; after that apps communicate with the devices through remote servers.
Devices rendezvousing in this manner are generally preconfigured with the name of the rendezvous server, but they might also be configured with the name of the server during network provisioning (generally an app communicates with them to share wifi credentials; it might also share information about a rendezvous server as well).
We generally do not scan for names in IP address blocks or actively probe for new devices unless we're debugging a network or doing some kind of security sweep.
The process of scanning an IP address block that you described is problematic and unreliable. The only reason it works is that some routers pick up a device's name from the device's DHCP request (or the router may be configured to know the device's name). The router also handles DNS for the devices on the network, generally by forwarding them to the ISP's DNS servers or to DNS servers that the network's owner configured it to use. It intercepts DNS requests for devices whose names it knows and replies to them itself, instead of forwarding them to an external DNS server.
You also have to know about the network configuration to do this properly. Is the network only a /24? What if it's /22? or /16? Are you prepared to scan 2^24 IP addresses if the network is configured as a /8?
Though the router may intercept requests for names and return addresses, it won't necessarily intercept names for addresses and return names.
Scanning also generates an unnecessary network traffic. While your one "hub" scanning may not seem like much, if you have multiple scanners from different makers running it doesn't scale well.
If your "hub" bypasses the router for DNS requests then it also will not be able to resolve names provided by the router.
Also not all routers do this. It's not part of the Internet architecture, it's a convenience feature that some routers provide. You cannot depend on it working.
You could also attempt an active scan of the network, trying to communicate with each IP address on it. We do this for network debugging, but running this continually to detect new devices would be a hostile way to interact with the network.
Network infrastructure devices - switches and routers - of course maintain lists of all active devices on the network. You can access these lists using SNMP (Simple Network Management Protocol), but few consumer switches and routers support SNMP. And allowing access to your network infrastructure to a random piece of software is a network security nightmare.
Your best bet is a simple multicast/broadcast protocol like mDNS. Don't scan, announce or request. It's responsive, doesn't place a burden on the network, doesn't depend on peculiarities of routers, and won't make network administrators hate you.
Long time ago I implemented a similar solution to discover clients connecting to the network. My strategy consisted in taking advantage of the DHCP configuration.
If your devices must ask IP addresses from a Linux/Unix DHCP server under your control, you could configure it to let you know when a device connects or disconnects from the network.
We were using Linux DHCP server, and in the dhcpd.conf there is a section about events. Basically it says that when the DHCP commits to a particular IP address lease for a client it raises an event, and when that happens, it can run a listener you could have defined for that event. In the listener we can ask the DHCP to execute a command, e.g. write the client information (e.g. MAC address) and its IP address lease to a named pipe, and your Java application could just be reading from this queue. (See also dhcp-eval for further details). A similar event can be run when a client releases an IP lease or when a lease expires.
Here's an example
subnet 192.168.1.0 netmask 255.255.255.0 {
option routers 192.168.1.2;
on commit {
set clip = binary-to-ascii(10, 8, ".", leased-address);
set clhw = binary-to-ascii(16, 8, ":", substring(hardware, 1, 6));
execute("/usr/local/sbin/dhcpevent", "commit", clip, clhw, host-decl-name);
}
...
If you can control your DHCP config there is so much you can extract from it.
I have implemented a simple P2P multicast network where each peer connected to the network sends data to every other peer. In order to make it possible, I made use of Java MulticastSocket Class, so every peer joins a multicast group defined by an available for multicast IP address, and a port number.
I got some issues while trying to run the program using EC2 services, where each generated instance represents a peer of the network, because the peers are not able to communicate in so far as exchanged messages do not reach the receivers.
Hence, basically my question is: is there a way to make EC2 instances communicate to each other using Java MulticastSocket? Can anyone help?
Natively, you can't.
Q. Does Amazon VPC support multicast or broadcast?
No.
https://aws.amazon.com/vpc/faqs/
Same thing for EC2 Classic (though there is hardly any reason you'd ever want to use that, if your account even allows it).
VPC looks like Ethernet, but it isn't. Put a packet sniffer on and try a ping. Watch the ARP traffic on both sides and you'll see something enlightening -- the source machine arps for the target and gets a response, but you'll see neither of these packets on the target machine. The ARP response comes from the network infrastructure itself.
There's a workhackaround, if you're feeling crafty: you can build an overlay mesh network that transports multicast over unicast.
See https://aws.amazon.com/articles/6234671078671125
We are working on a Android project with the below requirements.
The application should be able to send data to all the devices which are running our application which exists in the WiFi LAN.
Some payloads are expected to be of size >= 5MB.
The data shouldn't be lost and if lost the client should know the failure.
All the devices should be able to communicate with all other. There will be no message targeted to a specific device instead all the messages should be reached all the devices in the N/W.
No internet hence no remote server.
Study we have done:-
UDP Broadcasting - UDP doesn't guarantee the message delivery but this is a prime requirement in our case. Hence not an option.
TCP - TCP guarantees the message delivery but requires the receiver IP address to be known before hand and in our case we need to send the message to all the devices inside the LAN. Hence not a straight option.
Solutions we are looking into:-
A Hybrid approach - Name one of the devices in the N/W as Server. Post all the messages to a local Server. The Server keeps a open socket to all the devices(which have our application) & when there is a message from a device then it routes the message to all the devices. The disadvantages of this approach are,
Server having multiple sockets open each per device. But in our case we are expecting devices <=5 in LAN.
Server discovery using continuous UDP broadcast.
We want to have all the data in all the devices. So if we newly introduce any device into the LAN then that device needs to get all the data from the server.
So my question, have you any time worked on these kind of hybrid approaches? Or can you suggest any other approaches?
Your hybrid approach is the way to go.
Cleanly split your problem into parts and solve them independently:
Discovery: Devices need to be able to discover the server, if there is any.
Select server: Decide which of your devices assumes the server role.
Server implementation: The server distributes all data to all devices and sends notifications as necessary. Push or pull with notifications does not matter.
Client implementation: Clients only talk to the server. The device which contains the server should also contain a normal client, potentially passing data to the server directly, but using the same abstract protocol.
You could use mDNS (aka Bonjour or zeroconf) for the discovery, but I would not even recommend that. It often createsmore problems than it solves, and it does not solve your 'I need one server' problem. I would suggest you handcraft a simple UDP broadcast protocol for the discovery, which already tells you who the server is, if there is any.
Select server: One approach is to use network meta data which you have anyway, for example 'use the device with the highest IP address'. This often works better than fancy arbitration algorithms. Once you established a server new devices would use this, rather than switching the server role.
Use UDP broadcast for the discovery, with manual heuristic repeats. No fancy logic, just make your protocol resilient against repeated packets and repeat your packets. (Your WLAN router may repeat your packets without your knowledge anyway.)
You may want to use two TCP connections per client, potentially to two different server ports, but that does not matter much: One control connection (always very responsive, no big amounts of data, just a few hundred bytes per message) and one data connection (for the bulk of the data, your > 5 MB chunks). This is so that everything stays responsive.
As I am programming a network chat (java, but should not make a difference for the question), and wanted to use UDP, I ran into the problem of it not working over the internet. After a little research I found out that you have to have port forwarding for the specific port activated. So now it comes to my question:
Does UDP work over the Internet in a not configurable way?
For example, if I would program a whole Network Game would it make sense to use UDP? Or would I require the Player to activate Portforwarding and open the Port etc?
When would it make sense to use UDP then? And why?
I'm actually not understanding the whole point of UDP then.
For my programming point of view I would like to have a way to use it intuitive.
Like creating the DatagramSocket and the DatagramPacket, configure the Packet with the Data and the Destination and send it away over the internet.
As for my Users I don't want them to have to configure any specific things like opening the exact port they want to use etc. I just want them to use the program (server and client) and it should work.
The problem you've run into is not one of UDP vs TCP (although using the unreliable, unordered UDP as the basis of a chat application seems like an odd choice to me).
The problem is that of NAT traversal. In a nutshell, home routers perform a network function called NAT - Network Address Translation. They do it in order to use a single public IP address for all machines inside the NAT (which are given private addresses - usually 10.0.0.0 or 192.168.0.0). The router then switches the source IP address in all packets sent from inside the LAN from the private address to the public one. It uses port numbers to "remember" which machine sent what to what address, in order to perform the backwards translation when the response arrives.
The problem arises when someone wants to initiate a connection to a machine behind a NAT. Without seeing an outgoing connection first, the NAT doesn't know to which internal computer and port it should forward the packet. This is what happens to you.
There are various fixes for this issue, with the simplest one being manual port forwarding (as you've discovered), but it's a well known problem faced by any peer-to-peer application. If you need to contact a machine behind NAT (i.e. contact most home users) and you want your application to work out-of-the box (without your users fiddling with their routers) you need to research NAT traversal techniques, implement them in your application, and hope that the user's home routers support them. It's a huge pain in the neck.
EDITED: with Joachim Pileborg's correct suggestions!
UDP is often a better choice for action-based games, where it's vitally important to have updates to the client or server with the latest data about a player, player input, or the game world.
TCP begins with a 3-way handshake to establish a connection (which takes time). If your game communication protocol is via TCP, all packets in a message have to arrive before the message becomes available. Even a small amount of Internet congestion could cause your game to lag.
TCP is good for communications that MUST arrive in full.
With UDP, the client or server can send the latest player/game state in individual packets that do not depend on arriving in order. If a packet is late, or arrives out of order... it should be ignored.
UDP is good for communications that need to be fast, but losing individual packets is OK.
Both should be available in your Java platform.
Here's some further reading:
http://gafferongames.com/networking-for-game-programmers/udp-vs-tcp/
I'm working on a server-client project. I hosted server on Google app engine so there is no problem with IP there, all the clients can connect to the server easily. Yet the problem occurs when I try to connect to a client, which is quite complex because I don't have static IP for the clients. Can anyone suggest me a good way for server-client coomucication in this case, without requiring that clients must have static IP address?
Thank you very much.
Well, obviously the client should register itself with the server and update it's IP when it changes.
There is, for example, a program which does exactly that and then publishes the IP with a DNS.
But you should be aware that the IPv4 address space is not that big and a lot of internet clients do not own an IP (and work thru the ISP's NAT). If you have clients that do not own an IP then you might want to stick to the usual Pull: the clients should periodically issue a request to the server to check if there are new messages for them. With a Keep-Alive connection and an efficient server implementation the price of such checks might actually be low, although that kind of communication might not work very well with the GAE pricing.