I used the broadcaster.c from Beej's Guide to Network Programing as well as the listner.c but am unable to make have a Broadcast packet send to the server. I have checked in Wireshark and it does send. But I cannot get the listener to receive the connection. It only seems to work for localhost and 192.168.1.56 and not for 255.255.255.255 or 192.168.1.255.
If you bound the socket to the broadcast address (either manually or INADDR_BROADCAST) or INADDR_ANY on the server part, then it would receive the broadcast, otherwise Linux will not receive the packets.
Btw, routers do not forward 255.255.255.255 broadcast. Therefore, don't use it on the infrastracture mode.
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I have a TCP client and server running on Linux platform. Basically using setsocketopt I am able to mark SYNC packets. But I don't know how to apply DSCP priority to SYNC-ACK packet. I can set correct DSCP once socket is accepted on server but SYNC-ACK packet goes before application receives connected socket.
FYI, I also tried to apply DSCP on server listener socket. But Linux doesn't use that value for SYNC-ACK packets.
from this page, the SYNC_ACK is the same as SYNC which from client.
I'm trying to set up an application on Linux to receive MIDI data over UDP multicast via WiFi.
Wireshark shows that the packets sent from the MIDI controller are received by my machine (paste from wireshark).
The source code from the utility I'm using to listen for network traffic and produce ALSA midi events (called multimidicast) sets up listening sockets like this. Basically it sets up 20 sockets, binds them to ports 21928-21948, calls setsockopt() with IP_ADD_MEMBERSHIP to the group for "225.0.0.37", and then starts listening. This is, as far as I can see, in line with all tutorials and advice on how to listen for UDP multicast traffic.
However, the utility does not receive data.
If, from my PC, I send packets to the relevant ports on "225.0.0.37" (s.sendto("hello", ("225.0.0.37", 21928)) in Python), the tool still doesn't receive the data. If I send to the ports on localhost (s.sendto("hello", ("", 21928))), it does receive the data.
I've been reading and experimenting quite a lot, but I can't work out what it is I'm missing. I'm not even sure whether it's an error in the code I'm using or in my box's configuration.
Could anybody shine any light on this?
I am writing a simple sender and receiver program to transit using UDP so it's connectionless, but I am having a problem figuring out whether my receiver program needs to call bind() or the server and/or both. My receiver program(client) will sit in an infinite loop waiting to receive data from the sender(server) and then it will print out the data. I'm not quite sure what bind() does exactly besides associating an address/port with a specific socket. Why is it that I need to call bind()?
You need to call bind(2) so that the OS knows which application to route network packets to. When you call bind with a specific port for a given protocol (e.g. TCP or UDP), you're asking it "whenever you see a network packet on port XXXXX, please give it to me".
Say, for example, that two copies of your program were running, and they both wanted to listen for UDP packets on the same port. If they both call bind on the same port, then one will succeed and one will fail, since the OS can arbitrate who is bound to each port. Then, any packet received on that port will be given to whichever instance of the program succeeded at binding to that port.
when you want to make a socket a fixed address or/and port, you use bind.
See you when developing a Network Application you need to specify "Address and Port" to Bind because if you want to set it for Localhost your application is not able to communicate with the all over the network its only for your system which its communicating.. If you set it with your Network address it's not able to communicate as localhost Its only communicate with the network and If you set it to 0 then It can be use as both for localhost and Network.
I'm working on a client-server application written in C. I want to broadcast a message to all the machines available on the local network.
How can I do that using the usual socket system calls in C?
Just send the message to the broadcast address of your subnet, which for 192.168.0.0/24 is 192.168.0.255, or just broadcast to 255.255.255.255.
you have to use UDP to send a broadcast message over a network. when creating your socket using the socket() function, specify AF_INET for the family parameter and SOCK_DGRAM for the type parameter. on some systems, you have to enable the sending of broadcast packet by setting the SO_BROADCAST socket option to 1, using setsockopt().
then use the sendto() function call to send a datagram, and use 255.255.255.255 as the destination address. (for datagram sockets, you don't need to call connect(), since there is no 'connection').
in standard implementations, this address broadcasts to all computer in the local network, this means that the packet will not cross gateway boundaries and will not be received by computers using a network mask diferent from the network mask of the sending computer.
Have a look at UDP sockets.
I recomend Beej's Guide to Network Programming. Have a look at 6.3 Datagram Sockets
You can use the special address of 255.255.255.255 to send a broadcast message to every computer on the local network.
For more info see section IP Network Broadcasting.
I have an application bound to eth0, sending UDP packets on port A to 255.255.255.255. At the same time, I have a UDP server bound to eth0, 0.0.0.0 and port A.
What I want to do is to make sure that the server won't receive messages generated by the application (handled purely in software by the kernel) but it will receive messages generated by other hosts in the network.
I can't change the payload of UDP packets nor add any headers to it.
I've already implemented a solution using RTNETLINK to fetch all IP addresses of the machine I'm sitting on (and filter basing on address from recvfrom()), but I'm wondering if there might be a simpler and cleaner solution.
EDIT: I thought about something like tagging the skb - the tag would disappear after leaving a physical interface, but wouldn't if it's just routed in the software.
Any ideas?
If you can patch your Linux kernel, you could use a setsockopt() option for choosing if you want to loopback the broadcast packets you're sending or not.
This patch reuse the IP_MULTICAST_LOOP option exactly for this purpose.
Also, instead of "messing" with the IP_MULTICAST_LOOP option, you could easily add your own setsockopt() option, maybe called IP_BROADCAST_NO_LOOP. This would guarantee that you're not changing the behavior for any other application.
You can compute a checksum or CRC (better) over the payload and filter against this.
You can do this at the firewall level by dropping packets to broadcast address port A with source address of the eth0.