CODEWITHSUNDEEP
linuxsocketsudpposixbuffer
Ahnihmuhs
Ahnihmuhs

Reputation: 107

How to put back udp packet in the receivebuffer

I'm writing a small application which includes UDP Hole-Punching for P2P-Communication implemented in Linux (Posix Sockets). It works well so far but I do have a problem with the incoming packets.

After the two clients received the peerinformation of the respective client they start sending UDP-Packets to make holes in their firewall.

And here my problem pops in: After sending the "HolePunching-Packet" the client listens (rcv) for incoming packets. If the client receives a "HolePunching-Packet", the Punching process is beeing stopped successfully and the user can start receiving/sending userdata.

But if the HolePunching-Packet of the other client is beeing held by the clients firewall, this packet will never reach the rcv-call and the current thread will be blocked until the next userdata "wakes up" the client, but this perticular packet is being consumed by the Punching-process and can't be relayed to the user.

So I'm searching for any method to put back the first received udp-packet back into the receive-buffer of the system. I can't use my own (char) buffers as a project contraint and have to find a workaround if there is no way to achieve this using posix-methods.

Do you guys know any method to do this in Posix systems?

Here is a rough schema of how my application works:

SocketClass
{
    Punching
    {
        send(Give me the IP:PORT of Client2)
        recv(Peerinformation)

        send(HolePunchingPacket to client2)
        recv(HolePunchingPacket from client2 or userdata)

        if (recvedPacket != HolePunching-Packet)
           put_back_in_recvbuffer(recvedPacket)

        done
    }

    UserCode
    {
       [...]
    }
}

Upvotes: 0

Views: 268

Answers (1)

Nikolai Fetissov
Nikolai Fetissov

Reputation: 84169

You can use MSG_PEEK flag with recv(2) in you "punching" process, and then only de-queue data when needed, but honestly, this looks like a kludge. Why not implement a clean state machine for your process and forward packets to appropriate code paths according to the state?

Upvotes: 2

Related Questions