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.
Related
I am trying to understand how exactly routing works:
if 2 computers are on the same network
if they are on different networks.
More specifically I am trying to understand this: Routing
I am also trying to understand the difference between
IP Address,
Net Address,
Mac Address.
From what I understand:
1) IP Address: is used when computers communicate on the internet only.
2) Net Address: is a local version of the IP address and each device on the network has a unique net address. It's used when devices on the same network want to communicate with each other.
3) Mac Address: is a globally unique address and no other computer in the world has the same Mac address. In reality this is not true because it can be changed. It's used when ???
When a computer wants to communicate with another on the same network, they use net address, right? If the computers are on different networks what exactly happens?
Question: Can someone please fix my mistakes if any and explain what I am missing?
Thank you very much.
There are many network types, but since the most used ones are Ethernet and IP networks (and you seem to be asking about them), I will answer shortly based on them.
IP addresses are always used. They may not be used for deciding who gets the packets directly, but they are the basis even in local networks, since it is an IP network. There can also be other network types that have their own mechanisms, but they are not that common.
In local Ethernet the machines ask via ARP protocol "who has this IP address?" and get a reply with a MAC address. After that they send and receive packets based on that MAC address. The packets still have the IP address information, otherwise the receiving machine wouldn't know what is the destination. Do note that the receiving machine might be a firewall or other middleware device, not the actual computer that has the address. Also a single machine and network card may have several IP addresses set up for it.
In IP networks the IP address is used for routing. All routing devices have a routing table that will tell where the packets should go. If it's a simple device, it usually has a local network and everything else goes via a default gw, which will know better what to do with the packets.
A home router will just push them to the operator, there another router will know what addresses go to their networks, others are pushed forward via another connection, until a bigger place is reached where there are inter-operator connections and they choose again the correct route. And then it goes to smaller and smaller pipes the other way around.
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.
I'm trying to get Age of Empires II (AoE2) to work on my LAN. AoE2 is notorious for it's connectivity problems on modern systems, probably because it used a now deprecated network framework called DirectPlay (in DX9) and the code probably wasn't robust back in the day either.
When I host a LAN game on a computer (win7) for AoE2, Wireshark shows my computer sending a couple packets via SSDP protocol to the multicast address 239.255.255.250. This actually goes to my router (for forwarding I assume) and my router returns a packet using ICMP protocol that says "Destination unreachable (Port unreachable)". Because nothing is forwarded to the other computers on the network, they can't see the game that the host has created.
I think I need to get the application/windows7 to send the packet as something like a broadcast, or I need to get the router to broadcast packets going to that multicast address. Does anyone have thoughts or suggestions on how to do this?
My router/gateway is running DD-WRT firmware v24-sp2.
My first guess is you're using wifi, by default most systems disable multicast on wifi because it can have a detrimental effect on the time slicing that wifi uses. however for just a couple machines it shouldn't be an issue.
here's how to disable multicasting but it should point you in the right direction for enabling it: ddwrt multicast
Secondly make sure they are all in the same VLAN a VLAN is defined as a "broadcast domain" meaning machines on separate VLANs will NEVER get broadcast or multicast from other VLANs without some trickery.
Lastly make sure you've enabled multicasting between LAN ports I believe the option is "multicast forward"
Edit: Just a few things to add to the list in case others have this issue. Broadcasting doesn't exist in ipv6, also a machine running ipv6 MAY NOT see broadcasts from a machine on ipv4 and a machine on ipv4 WILL NOT see multicasts to an ipv6 multi-cast address.
Have you tried LogMeIn Hamachi?
Is not a LAN client itself but it creates a fake Online-LAN and gives you a working IP that will allow you to play with who have it.
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.
Im not too sure why this wont work. My application works fine if the client and server are ran on the same PC hence the 127.0.0.1 but it wont connect to my other laptop using IP 82.41.108.125 which is the IP of that device.
Any reason why this is happening?
Your firewall could quite possibly be blocking the port assuming all of your coding is working fine - try opening up the port number you're running the application on (in the client/server comptuers' firewall options).
Some additional information that should shed some light on things
Networked devices use ARP (Address Resolution Protocol) and RARP (Reverse Address Resolution Protocol) to map IPs to link layer (MAC) addresses and back. Your network interface card (the thing that plugs into your ethernet cable) will have a unique MAC address on both computers. Each PC has an IP configured for it which is used by higher level protocols (those in the Network Layer).
Computer A will know computer B's IP address (and it will be different from computer A's) if your program works correctly. When computer A goes to send to comptuer B, assuming they're directly connected and not going through a router or something in between, computer A's link layer will need to translate that IP for comptuer B into a MAC address it can use. It does this by sending out a broadcast to all network PCs on the same sub network asking "Hey, is this your IP!?" essentially. The one that has an IP matching the broadcasted one yes, "Yes, it's mine - and here's my MAC address so you can talk directly to me."
So, if two computers have the same IP this all goes to hell :) don't do it - give them unique IPs and make sure that comptuer A transmits to the same UDP port and IP that computer B is listening on as well. communications work in {IP, Port} pairs - its like a telephone number and area code.
Also...
As said by EJP - UDP is a connectionless protocol - computer A just sends to computer B and hopes that computer B is listening correctly. If computer B wasn't listening or was but wasn't in the right state to process the data, the data will simply be dropped and lost. Computer A will not know that this happened. If you want reliable communications where it will keep trying and you will have assurance that compuer B received computer A's data then use TCP instead - it does a 3 way handshake to establish a connection and uses acknowledgements to ensure data is retransmitted when it doesn't reach the other end.
It could be that you're listening on localhost (127.0.0.1) and don't accept other traffic. Use a wildcard address.
Check on the server with netstat -npu.
If that's not the case then it's probably firewall issue, verify with tcpdump or wireshark.
Check firewall on your laptop... Most of the time that acts as spoilsport.