I have a server and a few clients, software-wise, and I want the client to figure out the IP address of the server without actually referring to a specific database.
I had a misconception about UDP up until now that UDP only used the Port as a common identifier between servers and clients, but I was wrong apparently.
Requirement:
I want my server to broadcast a message to ALL devices on the network, once every 5 seconds or so.
That message will contain the IP of the server.
Software using such technique already:
A photo transfering app for iOS, called "Photo Transfer App" is really easy to use because it basically tells you to open the app on your Mac and on your iPhone, and then you press "Discover", and BAM! your devices have found each other.
But how?
How can my requirement be achieved?
EDIT
I googled a bit. Could server host on 255.255.255.255and clients connect to that address solve my problem?
According to this Wikipedia article, 255.255.255.255 is used for broadcasting.
Can someone confirm?
If you will use broadcast address (255.255.255.255) or network broadcast (like 192.168.1.255 for network 192.168.1.xxx or in the prefix notation 192.168.1.0/24). You will get essential limitation: your broadcast will NEVER leave that network. So all host inside the LAN will see your broadcast message but nobody else.
Multicast as mentioned by null is better because it can leave your network and pass through the IP routers. But this is feasible only by special network settings on that routers (and require multicast routing capability on that routers).
Another technology without such limitation can be use is dynamic DNS (server will register itself via DNS).
Multicast would be better than broadcast. See also: Bonjour, multicast DNS & SSDP.
More information on Bonjour is available here.
Related
My friend wanted to connect to my computer using Remote Desktop Connection. But the problem is I am confused what my Ip address is.
My computer is connected to the internet via router via broadband internet network. My ip address is dynamic.
Here, my main purpose is not only the remote connection but also learning how dynamic ip connect to another pc.
I searched for ip address on Google. They show me an ip address. But I think it is not mine, it's related with the router or broadband network. I also find a WAN ip (it is different from that i found on google) on router settings. It did't work.
I used Team Viewer. It worked perfectly. But I want to do that manually because I am going to make a multiplayer game on GM8.
It will helpful if someone explain about ip and port forwarding.
Teamviewer is a great tool, but uses different techniques than what you plan to do. Teamviewer always uses an outgoing connection and use a mediator on the Internet to connect you and the other PC.
You should ask your Internet provider if he technically enables you to be reachable from the outside Internet. Often this is not possible at all, even if you configure your router the correct way.
When you ask this you can ask him if you have a static IP.
It seems you are not aware of basics of IP networking, so I'd strongly advise against trying this on your router as wrong settings would render it useless. But here's for your information how port forwarding and IP Address and dynamic DNS can be used to solve your problem.
Basically your ISP is likely to give you a router having an IP address. If this IP address is a global IP address, it is possible to connect to this IP from outside. How do you find out whether your IP address is global? Look for your WAN IP address setting. If it is in 10.x.x.x or 192.168.x.x range, it's unlikely to be global and in that case it might not be possible to connect to your computer from outside - without help of a third server (some kind of a registration server, where you connect and register your application). The Registration server would determine your globally visible IP address and then convey it to another Application who is interested in connecting to it. This is somewhat complicated to make it work (but if you intend to make a game - this is something you'd have to do regardless). This is mostly how software like TeamViewer would work.
If you have a global IP address - it means it can technically be reached from anywhere in the world. In that case you could use port forwarding to make things work for you. Port forwarding works basically as follows - You expose a certain port (on TCP) to external world - say 8000 and then you make a setting like following on your router.
<TCP>-<RouterIP>-8000 --> <TCP>-<Your LAN IP><Your application Port>
(You can find you lan ip using ipconfig on windows or ifconfig on Linux).
Now all connections coming to port 8000 would be directed to your application. You might want to do it on UDP as well and the protocol above would change. That is how you 'open' a few ports to be accessible from outside, configure them on your router and then run corresponding applications on your network.
There's another thing called dynamic DNS, where the IP address you use if it is dynamic (and global) can be registered with a Dynamic DNS server so that you don't have to know and remember the current WAN IP Address. But that can be for later.
Hope that helps.
I'm trying to develop an applicaton for p2p communication between two android devices. In order to punch a hole through my NAT(s), I'd need to know my external ip address and port.
To that end, I've developed a java server on GAE to report my "remote" ip address and port. The problem is that on GAE I can get my ip address, but not my port. Without it, I'm unable to successfully punch the hole.
So, my question is what's the best, free method to find out my external IP address and port?
That's a question that has no answer with TCP.
Here's the problem: your "port" is not a fixed value. You don't have "an" external port. You typically get one dynamically assigned for each outbound connection.
As answers you should see from the test sites posted in another answer clearly indicate, it's a moving target (though it may stay stationary for a short time due to the browser using HTTP/1.1 keepalives and actually reusing the same connection, not just the same port)... but if you hit the site repeatedly, you'll see it either drift around randomly, or increment. Trying it from two different web browsers on the same machine, you'd never see the same port number -- the port corresponds to the specific source connection, not the machine sourcing the connection.
Sometimes, you may find that it's the same port number as the port your machine's stack opened for the outbound connection, but even when it is, it doesn't matter, because no traffic should be able to return to your machine on that port unless it is from the IP address and port of the machine to which you made the outbound connection. Any decent network address translating device would never accept traffic from another source IP address and/or port, other than the one you addressed in the outbound connection.
There is no standard, simple, predictable, reliable, or consistent way to punch a hole in TCP NAT and then exploit that hole for a peer-to-per connection. To the extent that such things are possible in a given NAT implementation, that is an implementation that is shoddy, broken, defective, and insecure.
See also: https://www.rfc-editor.org/rfc/rfc5128
Sounds like your app could use a STUN server to get its external address.
Good Day,
I basically have a laptop connected to the internet with a fixed IP Address. I need to have a way for a server possibly though a PHP script, send data/string to the laptop and it is able to receive it.
Now, if I was within a local network, i could do that. But what if I wanted to send from an external network? Do devices have a particular address that I can access from any internet connection?
Do devices have a particular address that I can access from any
internet connection?
No, it does not, unless you do use external IP for your laptop (doubtfully).
The easiest and fastest solution I could think of is to do the other way round: open TCP or UDP socket on your server and use laptop to connect and request data from the server. Of course, it is not suitable for all scenarios, but in many cases it works. Write more information (what is the purpose of this? What are the requirements and limitations?) so more specific answer may be provided.
OK, to make things easier to explain, let's say:
- Your router's public IP address is 10.10.10.10
- Your laptop's private IP address is 192.168.0.1
- You want to communicate via port 80 (since you mentioned PHP)
What you need to do is configure your router so that it forwards packets destined to 10.10.10.10:80 to 192.168.0.1:80. This is the simplest form of NAT.
Then from anywhere else with an Internet connection, you can send packets to your laptop by sending packets to 10.10.10.10:80.
I am developing a networked application that runs on a few different computers on a LAN. One of the core needs is for the app to maintain a list of peers on the LAN with which it has communicated in the past, so that it can restore previous sessions. The naive solution would be to just remember the IP and store it in a table, but what happens when the IP of a peer changes?
Instead, I thought I'd store the hostname of the peers so even if the IP changes they will still be reachable via their hostname. (I know hostnames can change as well but that is good enough).
So my question is what exactly is needed to make hostname resolution work on a LAN with mixed Windows/Mac/Linux clients?
Without the use of a central authority the only reliable way to achieve this is through the use of zerconfiguration name resolution. This means that without a multicast router you will only be able to dynamically resolve peers on the same subnet as the resolving host. You could use something like bonjour for mac, netbios or ssdp for windows or avahi for linux but you can't assume that these are enabled. I may be overlooking some more popular protocols that perform this function well but I would personally throw together a quick udp broadcast name resolution protocol for your application. Take a look at these for some more ideas:
Zeroconf Name resolution
Universal local network name resolution method without DNS?
http://en.wikipedia.org/wiki/Zero_configuration_networking#Name_resolution
http://en.wikipedia.org/wiki/Broadcast_address#IP_networking
I would pick a specific udp port to listen on (lets say 12000) and then when you're ready to resolve hosts send a "hello" udp packet out to 255.255.255.255 on port 12000 and all of the other hosts on your network running your app should reply with a packet containing their hostname, possibly other information.
we have little network devices which are shipped with IP address 0.0.0.1 to ensure that they never collide with any other device in their new environment (thus none of the 10.x.x.x, 172.16.x.x or 192.168.x.x ranges) until configuration. DHCP is no solution since there might be no DHCP server in the field.
The devices would listen to UDP broadcasts and answer with broadcasts until they are given their new IP address this way.
This worked fine with Windows XP - but sucks with Windows 7: the config program does not receive the answer packets from the devices which still have 0.0.0.1. Wireshark sees the packets, then they are dumped by the system.
Question: Is there any reason (RFC?) that actually prohibits using this address in a local environment? Or is it just MS that was overcautious? Where can I read why they treat this address "invalid"? Which ranges are really "invalid" now, too?
Any idea of a workaround on the PC side (Win 7)?
I know that it is not recommended to use 0.xxx addresses for work places, but for this very reason - having a not-used address - it works perfectly.
Edit: there is a device out there called "Netburner" which might have faced the similar issue, according to their forum. See: http://forum.embeddedethernet.com/viewtopic.php?f=5&t=612&p=2198 Does - by coincidence - anybody know some background information?
It sounds as if your configuration application is listening for broadcast packets on all network interfaces and expecting to receive packets from foreign subnets.
That should not work - the OS should only pass-on broadcast packets from the subnets each network interface is on, not from all subnets on the same physical (e.g. Ethernet) segment. I am reasonably certain that doing otherwise is broken behaviour WRT the IP protocol.
The are two ways to deal with this:
Make sure that your network interface has an IP address in the target subnet. You can have more than one IP addresses for each network card, so that should not interfere with normal network operations.
Configure or modify you application to use raw sockets, like Wireshark. Keep in mind, however, that this overrides all normal checks and balances and should be avoided, since it can cause behaviour that is almost impossible to diagnose - which is why it is frowned upon by meny network administrators.
Can you you add new routing table entries to Windows machines easily? Windows has to know which interface to use when routing a broadcast packet to the 0.0.0.x network.
The Unix machines I'm familiar with have a routing table that maps network/netmask entries to either gateways or interfaces (if the network is a local network). The local network (192.168.0.0/16 for my home network) gets sent to interface eth0. Everything else 0.0.0.0/0 gets sent to a specific gateway machine 192.168.0.1.
If my machine sent a UDP broadcast message to network 0.0.0.0/24 (in other words, UDP broadcast sent to 0.0.0.255, then my machine would forward the packet to the gateway machine (which it can look up via arp). The switches in the middle wouldn't propagate the packet to other network devices, because the MAC address is set.
If my machine had another routing entry for 0.0.0.0/24 to the local interface, then my machine would send the packet on the wire using an ethernet broadcast group, and the switches would forward the packet to all connections. (Yay! Just like hubs in the 90s! :)
So I figure you need to add a routing entry for 0.0.0.0/24 to your client machines, so that they can properly address the broadcast packet.