I'm working on a project that has server and client roles. I would like to have servers and clients automatically detecting each other. At a first glance, zeroconf seems to be the best solution. But it would add a dependency, Bonjour, to the project. I use Qt for the GUI and Qt has native support of broadcast and multicast. So I'm wondering if it's feasible to just use those features to replace zeroconf?
Here is a decent post about how zeroconf works.
I don't think I need the features of obtaining an IP Address and obtaining a Hostname from zeroconf. All I want is let one instance be aware of other instance's existence.
My current plan is combining broadcast and multicast. Each server chooses a unique multicast group and broadcast this group to the others. When a client wants to connect a to specific server, it joins the corresponding group.
Some people mention that it's normal that routers blocking local broadcast. If this is true, my plan would not be feasible.
Is there any standard way to implement this rather than using zeroconf?
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Suppose I have a producer located on one machine and two independent consumers on the other. Consumers share the same IP. I want my data to be sent via WiFi and I want to avoid sending the same packages twice: I mean I want the packages to be replicated as late as possible -- either on router or on destination machine.
I managed to find these options:
1) Just use different ports -- in this case everything will be sent twice, but at least all consumers will get their copy of data.
2) Try to use SO_REUSEPORT -- without multicast this will make only one of two consumers get the packages.
3) Use multicast (with SO_REUSEADDR) -- I guess it is the only solution for my problem, and the packages will be replicated by router, not publisher.
I know, that there are messaging libraries and frameworks, which are supposed to be responsible for marshaling and routing, but I need "pure" UDP solution with maybe some simple logic upon it. I am not sure if I am correct with options that I mentioned, and I don't know whether there exists any more simple and suitable solution.
As people told you in the comments, one popular and efficient solution to you problem (one publisher multiple subscribers over network, aka pubsub) is to send UDP datagrams to a multicast IP address. The listeners must join that IP multicast group in order to tell the intermediate routers to forward those datagrams.
You can code it up yourself, there are plenty of examples elsewhere.
In a production setting, however, I recommend against reinventing the wheel along with associated mistakes. Have a look at existing solutions, such as ZeroMQ. It supports this messaging pattern of one publisher to multiple subscribers using UDP/IP multicast but with a nice interface, cross-language compatibility and much more. See Publish/Subscribe example in Python.
Supposed I have two services that need to share and / or exchange data. Both instances are separate from each other, and they shall not know anything about where the other part is located.
Now in order for them to be able to share and / or exchange data, they need to connect to each other.
How do they find each other without the need to configure the IP addresses explicitly? In other words: How could they detect each other automatically?
Basically, I have two ideas:
Pull: You need to have a central service where they register. Then you can ask that service for the address of a service, and that service then returns those data. While this works, it has the drawback that it only shifts the problem to the next level: What if I have multiple instances of that service, and I don't want them to know each other in advance?
Push: Each service broadcasts its own address, so that other services get it to know. Each service repeats this from time to time. Drawback: This does hardly work in the internet.
Any idea of how I could solve this in an intelligent way?
PS: If you want to say so, I'm looking for a way to handle dynamic IPs without the need for a central DNS server.
The usual way is to have some fault-tolerant server where services register and can then look for other services - Curator framework implements that over zookeeper.
If you want autodiscovery then you should probably implement some sort of gossip protocol so that the servers would know which other servers are out there in a reliable way. You should keep in mind that getting gossip protocols right is tricky (e.g. some of past Amazon cloud failures where due to problems in their implementation)
"broadcast packets are not forwarded everywhere on a network, but only to devices within a broadcast domain."
If your devices are on different broadcast domains then broadcasting is not going to work.
You are probably going to have to implement your own central service, unless you can use one of the free dynamic dns servers, for example: Free
We're prototyping a jgroups-based cluster node messaging system that will replace one that was JDBC-based. There are a lot of folks in my organization who are concerned about adding more multicast traffic to an already busy network, so I'm getting some pushback on a UDP/multicast solution.
I know JGroups can be configured to be TCP only, but I do not want to have to force a configuration step into the application where each node has to be identified ahead of time in a config file.
What I'd like then is to see if we can get a hybrid working here where multicast is used ONLY for group membership operations (discovery, heartbeats, failure detection), but messaging is all TCP-based.
I'm not finding examples of that in my searches, however, and am therefore questioning whether JGroups can be configured this way.
Can it, and any example configs showing how?
Thanks!
As for discovery, using MPING you can do that - it uses IP multicast to discover new nodes, although these later respond via the main transport (TCP in your case).
As for FD/FD_ALL, I don't think it's possible, the protocols are designed to use the main transport. You'd have to write your own FD protocol, shouldn't be that complicated.
However, if you can use UDP, you probably should. It's up to you whether you send a message to one node, multiple ones or all of them - for one destination there will be unicast UDP, for few destinations (if you set the anycast option) will be multiple unicasts used and only for all nodes the UDP will reduce the network load by multicasting. It's really up to the application, UDP just allows multicasts.
Just a technical question -
Can two or more SNMP agents be run on the same port (on the same machine)?
My first instinct would be no since host:port identifies an instance of an application but I'm not sure.
Thank you!
Technically, if the OS supports it, the SO_REUSEADDR SO_REUSEPORT options may be set on a socket to allow other processes to bind to the same address/port and thus allow multiple processes to receive messages on the same address/port. But both processes would have to set the option, and I doubt any agent implementations do that because it would not make sense to do so--it would just cause headaches having both agents potentially responding to a single request. Managers won't be equipped to handle it.
However, you can instead run an SNMP proxy in the primary address/port, configured to forward requests to one of multiple agents based on query, security, or (with SNMPv3) context/engine ID parameters, and forward responses back.
Also, using AgentX, you have an SNMP master agent running on the primary address/port, and one or more SNMP sub-agents connected to the master agent. The master agent dispatches requests to the sub-agents as appropriate, merging the results into a single response, so that to the outside world it appears as a single agent. Each sub-agent typically handles a different branch of OID space (one sub-agent implementing certain module(s), another sub-agent implementing other module(s)).
But taking two agents intended to own the address/port exclusively, and forcing them to share through the REUSE options, while it may be possible, would not be wise.
You can run multiple agents on the same host and with the same port if they have differents ip address (can use a netsh script for that).
Personnaly I use the nsoftware ddl : SecureSNMP V8 edition .NET to do this.
You can look at this post : Multiple SNMP Agents with nsoftware dll
No, two agents cannot both run on the same port as seperate applications for the reasons you assumed (except with a brittle packet sniffing hack, which we'll not go into).
However, 2 agents can be accessed through the same port if there is some mechanism that handles the actual port and distributes requests based on MIB. For example the Windows SNMP service does this, allowing any number of SNMP agents to be added as "extensions" through the registry (HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SNMP\Parameters\ExtensionAgents) by writing them as DLLs and using the snmp.h headers in the platform SDK.
You are correct: ports can't be shared.
If both the agents were designed by you, then the answer can be different.
Consider the HTTP and FTP cases, we can use host names to distinguise multiple sites on the same port, then why can't we do it for SNMP?
We can create a dispatcher who monitors port 161 for incoming traffic. Then use multiple real agents to handle those traffic behind. We can feel free to design how to distinguise them. Personally I prefer the FTP virtual host name manner and use | to distinguise agents.
Maybe I can create a demo for #SNMP Suite in the future.
But if you need to work with existing agents on the same server, then such flexibility is lost.
I'm making a network game (1v1) where in-game its p2p - no need for a game server.
However, for players to be able to "find each other", without the need to coordinate in another medium and enter IP addresses (similar to the modem days of network games), I need to have a coordination/matching server.
I can't use regular web hosting because:
The clients will communicate in UDP.
Therefore I'll need to do UDP Hole Punching to be able to go through the NAT
That would require the server to talk in UDP and know the client's IP and port
afaik with regular web hosting (php/etc) I can only get the client's IP address and can only communicate in TCP (HTTP).
Options I am currently considering:
Use a hosting solution where my program can accept UDP connection. (any recommendations?)
UDPonNAT seems to do this but uses GTalk and requires each client to have a GTalk account for this (which probably makes it an unsuitable solution)
Any ideas? Thanks :)
First, let me say that this is well out of my realm of expertise, but I found myself very interested, so I've been doing some searching and reading.
It seems that the most commonly prescribed solution for UDP NAT traversal is to use a STUN server. I did some quick searches to see if there are any companies that will just straight-up provide you with a STUN hosting solution, but if there even were any, they were buried in piles of ads for simple web hosting.
Fortunately, it seems there are several STUN servers that are already up and running and free for public use. There is a list of public STUN servers at voip-info.org.
In addition, there is plenty more information to be had if you explore SO questions tagged "nat".
I don't see any other choice than to have a dedicated server running your code. The other solutions you propose are, shall we say, less than optimal.
If you start small, virtual hosting will be fine. Costs are pretty minimal.
Rather than a full-blown dedicated server, you could just get a cheap shared hosting service and have the application interface with a PHP page, which in turn interfaces with a MySQL database backend.
For example, Lunarpages has a $3/month starter package that includes 5gb of space and 50gb of bandwidth. For something this simple, that's all you should need.
Then you just have your application poll the web page for the list of games, and submit a POST request in order to add their own game to the list.
Of course, this method requires learning PHP and MySQL if you don't already know them. And if you do it right, you can have the PHP page enter a sort of infinite loop to keep the connection open and just feed updates to the client, rather than polling the page every few seconds and wasting a lot of bandwidth. That's way outside the scope of this answer though.
Oh, and if you're looking for something absolutely free, search for a free PHP host. Those exist too! Even with an ad-supported host, your app could just grab the page and ignore the ads when you parse the list of games. I know that T35 used to be one of my favorites because their free plan doesn't track space or bandwidth (it limits the per-file size, to eliminate their service being used as a media share, but it shouldn't be a problem for PHP files). But of course, I think in the long run you'll be better off going with a paid host.
Edit: T35 also says "Free hosting allows 1 domain to be hosted, while paid offers unlimited domain hosting." So you can even just pay for a domain name and link it to them! I think in the short term, that's your best (cheapest) bet. Of course, this is all assuming you either know or are willing to learn PHP in order to make this happen. :)
There's nothing that every net connection will support. STUN is probably good, UPnP can work for this.
However, it's rumored that most firewalls can be enticed to pass almost anything through UDP port 53 (DNS). You might have to argue with the OS about your access to that port though.
Also, check out SIP, it's another protocol designed for this sort of thing. With the popularity of VOIP, there may be decent built-in support for this in more firewalls.
If you're really committed to UDP, you might also consider tunneling it over HTTP.
how about you break the problem into two parts - make a game matcher client (that is distinct from the game), which can communicate via http to your cheap/shared webhost. All gamers who wants to use the game matching function use this. THe game matcher client then launches the actual game with the correct parameters (IP, etc etc) after obtaining the info from your server.
The game will then use the standard way to UDP punch thru NAT, etc etc, as per your network code. The game dont actually need to know anything about the matcher client or matcher server - in the true sense of p2p (like torrents, once you can obtain your peer's IPs, you can even disconnect from the tracker).
That way, your problems become smaller.
An intermediate solution between hosting your own dedicated server and a strictly P2P networking environment is the gnutella model. In that model, there are superpeers that act like local servers, having known IP addresses and being connected to (and thus having knowledge of) more clients than a typical peer. This still requires you to run at least one superpeer yourself, but it gives you the option to let other people run their own superpeers.