I'm trying to implement a peer-to-peer chat application using UDP and I was wondering about how to scale the program to multiple users.
As I understand it, UDP needs only one socket to send and receive data using the recvfrom and sendto functions. Using the data from the address fields passed to these functions, I can determine which user I'm communicating with.
I was wondering if I could create multiple UDP sockets on the same port for each peer that I'm talking to. That way, if data comes from a peer X, then the data goes to the UDP port and gets passed to the appropriate socket that is 'bound' to X's address.
Is there anyway I could do this while still using UDP?
Yes, you can specify SO_REUSEADDR (SO_REUSEPORT on Linux) before binding the UDP socket (all sockets including the first), and then connect each socket to the appropriate target, but it's really not necessary. Just dispatch each message arriving on a single socket according to its source-address.
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I'm attempting to have two daemons running on the same port and IP but one is a server and the other is a client. Is there a method using socket options that would allow each socket to have a copy of the packet and let the daemons filter out the messages based on the protocol? It looks like reuse address blocks the first configured port and reuse port might just balance the packet between the two daemons.
Otherwise, I guess create another daemons to read the socket and send the packets to the correct daemon.
Thanks
You are correct at the end, you will need to have a third part that binds to the port and forwards the packets to the correct daemon.
The other way to do it would be to use three ports and use a firewall to redirect from the front end port to the backend ports; but that is a lot more complicated and not portable. But in the end you could use QOS or something. There is a wide range of possible types of use case behind the word protocol.
If the UDP packets you are receiving are multicast or broadcast packets, then you can set SO_REUSEADDR (and, for BSD-based OS's, SO_REUSEPORT) on the socket (using setsockopt()) before bind()-ing the socket, and then both sockets will receive a copy of each incoming UDP packet. (If the UDP packets are regular old unicast packets, OTOH, then doing the above will result in each received packet being received by only one of the two UDP sockets, which isn't what you want).
Note, however: You refer to the two daemons as being a "client" and a "server" -- the implication of those terms is that the two daemons are going to communicate with each other. If that is the case, then the typical approach would be to have the server-daemon bind to a well-known port number, and the client-daemon could bind to any port number (e.g. it could pass 0 as the port-number to bind(), and let the OS choose an available port-number for it). Then the client-daemon could start the conversation by sending one or more UDP packets to the server's well-known port number, and the server can find out which port the client is sending from (and therefore what port to send reply packets to) by examining the fifth argument of its recvfrom() call. In this case there is no need for the two programs to bind to the same port, and therefore no need for packet-forwarding.
I have a linux computer with a code in C that must communicate in UDP with 4 differents equipments. The computer sends differents commands to each equipment and receives responses, sometimes in parallel ...
I am a perfect beginner, and managed to communicate with one equipment using UDP socket. But now, i'm looking for a way to communicate with all these equipments, what i would like to call "multiple socket", but i don't know where to look/ what word to search to find a way ...
My linux computer is the client and all the equipment servers. I only have one eth port on the computer and will have to use a switch to have access to all the equipment. I would like to create functions like :
sendcmd(IPnumber, PORTnumber, cmd , ...)
readbuff(IPnumber, PORTnumber, buff, ...)
so i can choose which IP will received cmd ... i don't know if it's possible or if i need to open the socket, then close and redo the operation with another IP ...
So, if I ever managed to make myself understood, where should I look for a solution to my problem?
Thank you !
You can use a single UDP socket for your scenario. You can keep the socket open for the lifetime of your application.
UDP is not connection oriented. UDP sockets are also not classified into client sockets and server sockets. UDP sockets are always bound to a local port, either implicitly (typically for pure clients) or explicitly (which is usually the case for servers). In your case you do not care about the port for your UDP client.
To send to your four UDP server you can use sendto(). This lets you specify the destination IP address and port the UDP packet gets sent to.
To receive from your four UDP servers you can use recvfrom(). This will tell the IP address and port where the UDP packet came from.
You most likely want to have a receive loop of some kind. If you want to do anything else in your application you most likely want to either make recvfrom() non-blocking or you want to have the receive loop in its own thread. But this goes beyond your question.
The most important aspect of UDP is that it is not a protocol (despite its name which is misleading). It is one puzzle piece for a protocol. It is a tool to develop your own protocol. But I assume you already have a specific protocol at hand defined by your peripherals.
(I'm new to network programming, and I am working on C in Linux)
I followed Beej's guide for a simple UDP listener-talker and I know how to create a socket and send that to a destination (with calls to getaddrinfo() and socket() using SOCK_DGRAM)
See http://beej.us/guide/bgnet/output/html/multipage/clientserver.html#datagram.
In my distributed app, I will need to send a message to multiple peers (reliable multicast).
My question is: do I need to create a socket for each of the peers? I'm worried about scalability. Or should I create the socket, use it and destroy it (close it) after each message?
In summary, is there a good way to send a UDP packet to multiple destinations, periodically?
Thanks for the help!
For UDP, you need but one local socket. You can send a packet to any destination you like from that one, single socket.
Also, you don't need to destroy and recreate the socket after each message. Just keep the socket open, and keep sending messages.
I am writing on UDP server/client application.
I want my single server to handle 40 clients at a time. For this, I want to create 40 dedicated threads, each dedicated for one single client. Since there are 40 threads one for each client, I want to create 40 dedicated sockets as well.
But the problem that:
I don't know what will be the 40 IP addresses to which I shall bind() my sockets. (since as far as I now, I have to bind() to my Server\s IP address.) Normally I bind() to "INADDR_ANY" when there is only single socket.
But what should be the IP addresses at which I should bind() each of my 40 sockets?
Please help me. Any comment/ help is appreciated.
One common way to do this with UDP is:
Server bind() to a well known port.
Client sends the initial packet to that well known port
Server receive the first packet from a client on the well known port.
Server creates a new socket with a random port
Server replies to the client from this new socket.
Client receives the reply, notices it comes from another port than the well known
server port, and uses that port as the destination for further communication.
You'll use the getpeername() call to learn the remote address.
Keep in mind that UDP is connection-less, you'll need some way to signal the end or time out you sockets.
bind only needs the local address, not the remote address.
If you want one socket for each client, then you'll need to use different ports for each (using bind). That way, each client can send its traffic to a dedicated port, and you can have a thread for each socket/port.
It's probably a better idea to only have one socket (and one port) though, and have logic in your code to assign traffic to a thread based on the remote address (retrieved using recvfrom eg.).
The usual way is to bind a single socket and accept incoming connections. Each connection will be assigned a unique socket by accept.
As you are using UDP, I would simply use TCP as described above to let the clients know of their respective server UDP addresses.
Create a single listening socket in a dedicated listening thread.
When it receives a new packet, use the packet's remote addr/port, or put a unique clientID in the packet payload, to uniquely identify the client.
Create a new thread for that client if one does not already exist, pass the packet to that thread for further processing, and go back to listening.
If a given client thread does not receive any packets for awhile, it can terminate itself.
Well, my question may look like a basic stuff, but i am new to network programming side.
I wish to know:
1) Is it always required to bind a socket in order to receive message from that? I saw a sniffer code (raw socket) one in which directly it is invoking recvfrom and another piece of code in which it is invoking bind and then a receive.
2) What is the difference between the AF_* and PF_* family? Is the later related to POSIX?
Which is the one recommended ?
No, you don't need to bind().
If you're using a TCP or UDP socket where you are planning to either connect() or send a packet to a destination with sendto(), the kernel will automatically bind the socket to a suitable port number when you try to connect or send. This is generally the preferred way. bind()ing client sockets is considered harmful.
The same is also true of AF_UNIX sockets - the client side does not need to bind, and should not do so normally.
For a client that only sends data, a bind is not necessary.
For a client that is sending a request to a server, and then receiving a response, a bind is not necessary. The server can use the IP address and port number that the incoming data came from.
For a client that is only receiving data, or is receiving data before it sends out data back to the server, a bind is necessary. How will the server know where to send data to? In this sense the "client" is acting like a "server" that needs a place (bound port) for the data to come to.
I don't know about Linux, but on Windows, if recvfrom() is called on an unbound socket, it will fail with an WSAEINVAL error.