I cant seem to figure out how to do the next step for my UDP server. So far one client connects and then it sends data back and forth but when another client connects the server will take data from the new client and send it to the other.
I was going to check if the message that has been received is from a new client or not, if it is then give that client an identifier that the server uses to do some processing. Is that the correct way to do it? If so how do you do it? I know that recvfrom has "sockaddr *from" field which I assume fills that field with the data of the client but how do I use that data?
Thanks
sockaddr_in saddr;
int length = sizeof(saddr);
int read = recvfrom(your_socket, buff, 4096, 0, (sockaddr*)&saddr, &length);
if(read != -1) {
// now saddr contains the address of the client
// the g_addr is a previously captured client address
if(saddr.sin_addr.S_un.S_addr = g_addr.sin_addr.S_un.S_addr) {
// returning client?
} else {
// not yet seen client, so store address
}
}
UDP is connection-less. Try using TCP. Why did you decide to use UDP? What are your constraints?
The address in the recvfrom function will be filled with the source information from the client. In the case if udp it will give you address and port, which for IPv4 can easily be converted to a long long and stored as an identifier. IPv4 is 4 bytes + port 2 bytes. You will need to cast the pointer to sockaddr_in and then get the values of sin_port and sin_addr.
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I'm in the process of writing a project for college involving writing a chat client and server using POSIX sockets and C++.
The clients are supposed to converse with each other using P2P, such as each client has his own open UDP socket through which he sends and recieves messages from/to other clients.
My problem is 2-fold:
My UDPSocket class constructor seems to be ignoring the port number completely, binding to port 65535 regardless of the parameter.
The port is binding to IP 255.255.255.255 rather than my own IP (10.0.0.3), or at least that's what i get when I call getpeername.
To the best of my knowledge passing INADDR_ANY should bind to my local address, and passing port number 0 should make the OS choose a free port, what am I doing wrong?
This is the constructor of my UDPSocket class:
UDPSocket::UDPSocket(int port){
socket_fd = socket (AF_INET, SOCK_DGRAM, 0);
// clear the s_in struct
bzero((char *) &in, sizeof(in)); /* They say you must do this */
//sets the sin address
in.sin_family = (short)AF_INET;
in.sin_addr.s_addr = htonl(INADDR_ANY); /* WILDCARD */
in.sin_port = htons((u_short)port);
fsize = sizeof(from);
//bind the socket on the specified address
if(bind(socket_fd, (struct sockaddr *)&in, sizeof(in))<0){
perror ("Error naming channel");
}
}
This is the initialization:
m_Socket = new UDPSocket(0);
And this is the method I use to retrieve the binded address: (UDPSocket inherits Socket)
std::string Socket::GetSocketAddress()
{
struct sockaddr_in addr;
int len = sizeof(addr);
getpeername(socket_fd, (struct sockaddr*)&addr, (socklen_t*)&len);
char ipAddressBuffer[50];
memset(ipAddressBuffer, 0, sizeof(ipAddressBuffer));
sprintf(ipAddressBuffer, "%s:%d", inet_ntoa(addr.sin_addr), ntohs(addr.sin_port));
return ipAddressBuffer;
}
Any help would be greatly appreciated,
Avi.
You're using getpeername , which gives you the remote address of a connected socket. If you check the return value of getpeername(), it should indicate failure.
You need to use getsockname() instead of getpeername() to get the address of your local socket
You need to check that getsockname() succeeds.
Note that your socket is bound to the special 0.0.0.0 address, which means "all local interfaces", so that's what getsockname() will also return.
Answering the more general question "How to set up peer-to-peer communications with UDP":
With UDP sockets, while you can use connect, you generally don't want to, as that restricts you to a single peer per socket. Instead, you want to use a single unconnected UDP socket in each peer with the sendto and recvfrom system calls to send and receive packets with a different address for each packet.
The sendto function takes a packet and a peer address to send it to, while the recvfrom function returns a packet and the peer address it came from. With a single socket, there's no need to multiplexing with select or poll -- you just call recvfrom to get the next packet from any source. When you get a packet, you also get the peer address to send packets (back) to.
On startup, your peer will create a single socket and bind it to INADDR_ANY (allowing it to receive packets on any interface or broadcast address on the machine) and either the specific port assigned to you program or port 0 (allowing the OS to pick any unused port). In the latter case, you'll need to use getsockname to get the port and report it to the user. Once the socket is set up, the peer program can sendto any peer it knows about, or recvfrom any peer at all (including those it does not yet know about).
So the only tricky part is bootstrapping -- getting the first packet(s) flowing so that peers can recieve them and figure out their peer addresses to talk to. One method is specifying peer addresses on the command line when you start each peer. You'll start the first one with no arguments (as it has no peers -- yet). It will just recvfrom (after socket setup) to get packets from peers. Start the second with the address of the first as an argument. It sends a packet (or several) to the first peer, which will then know about the new peer as soon as it gets the first packet. Now start a third client with the addresses of the first two on the command line...
I want to limit access to the device for more than 4 clients on IP address.
struct sockaddr_in peerAddr;
SOCK_LEN_TYPE peerAddrLen = sizeof(peerAddr);
// Yes, socket is free, try to accept a connection on it
connectionSocketArr[sockIdx] = accept(listenSocket, (struct sockaddr *) &peerAddr,
&peerAddrLen);
You can use the sockIdx variable to see how many clients are currently connected.
Instead of storing the socket returned by accept directly in the array, store it in a temporary variable. If sockIdx is larger than 3 then the new client is not allowed to connect, so send a message to the client stating that and close the socket. Otherwise store the socket in the array and increase sockIdx.
Suppose I have a simple winsock server that has a listening socket, and then when a connection is accepted, it stores the socket in an array of sockets (to allow multiple connections).
How can I get the IP address of a specific connection? Is it stored in the socket handle?
As long, as the socket stays connected, you can get both own socket address and peer one.
getsockname will give you local name (i.e. from your side of a pipe)
getpeername will give you peer name (i.e. distant side of a pipe)
This information is available only when the socket is opened/connected, so it is good to to store it somewhere if it can be used after peer disconnects.
Yes it is stored in the socketaddr_in struct, you can extract it using:
SOCKADDR_IN client_info = {0};
int addrsize = sizeof(client_info);
// get it during the accept call
SOCKET client_sock = accept(serv, (struct sockaddr*)&client_info, &addrsize);
// or get it from the socket itself at any time
getpeername(client_sock, &client_info, sizeof(client_info));
char *ip = inet_ntoa(client_info.sin_addr);
printf("%s", ip);
I have a socket server that listens and accepts connections from client, which works as follow:
... do some pre-processing (socket, binds, etc)
//listen to client
if (listen(sockfd, BACKLOG) == -1) {
perror("listen");
exit(1);
}
printf("server: waiting for connections...\n");
while(1) { // main accept() loop
sin_size = sizeof client_addr;
new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
if (new_fd == -1) {
perror("accept");
continue;
}
//do something .....
.....
}
How can I restrict the server so it only accepts connection from specific IP addresses? For instance, I can create a text file containing a white list of IP addresses to accept, in the following format:
202.168.2.5 - 202.168.2.127
92.104.3.1 - 92.104.4.254
//and so on
So basically I want to reject connection from all the IP addresses not included in the whitelist. If the socket library API does not support this, I am okay with the idea of accepting the connections first, then just immediately close the socketfd if the peeraddress is not in the whitelist. But how to perform this, how can I check that a specific IP address is within the range specified in my whitelist? Any examples would be appreciated.
You want to call getpeername to get the address information from the client. Then check if their IP address is found in the whitelist. If not, disconnect them.
In order to check that their ip address lies within a given range, you want to convert the address bytes into one number. You can do that with the following:
unsigned int n = bytes[0] << 24 | bytes[1] << 16 | bytes[2] << 8 | bytes[3];
If the lower bound of the address range is A, and the upper bound is B, and the client's ip address is X, then they are white listed if (A <= X && X <= B).
If each range of ip addresses tests false, then they aren't on the white list and you should disconnect them.
Not sure what the question is here, or rather what the problem is. The client's address will be in their_addr, so just search your whitelist for that. If not found, close. You will probably want to either convert their_addr into the same format as your whitelist entries, or possibly vice versa.
On Windows only, you can use WSAAccept() instead of accept(). WSAAccept() has a parameter that you can pass a callback function to. Before a new connection is accepted, the callback is invoked with the addresses and QOS values for that connection. The callback can then return CF_ACCEPT, CF_DEFER, or CF_REJECT as needed.
I am hooking WSASend, and WSARecv in C++ using the same method I've used to hook the client's WSASend and WSARecv functions. In the client I am able to get the IP, Port, and Socket from the SOCKET structure passed by WSASend/WSARecv; however, for the server when I try to use getpeername or getsockname() they both return the error 10057 (Socket not connected)...
I'm fairly sure that the hook is correct on the server, since it prints the bytes successfully, and I'm also sure the socket SHOULD be valid seeing how client and server establish a successful connection.
Is there a way to resolve this problem by any other alternative methods? I've been looking around the internet to find a solution, but I haven't seen anyone with the same problem.
I've tried this:
sockaddr *address = new sockaddr;
int peer_len;
getpeername(s, address, &peer_len);
int err = WSAGetLastError();
if(err==0)
{
char *Str = inet_ntoa(((sockaddr_in*)address)->sin_addr);
printf("[%s", Str);
printf(":%d]",ntohs(((sockaddr_in*)address)->sin_port));
}
else
{
printf("Error %i\n",err);
}
(Using both getpeername and getsockname)Both result in the same socket not connected error.
I'm planning on using the packets the C++ dll gets and forward the information to the C# dll since it'll be easier to manage on that (for me anyways), but I'd need to distinguish each packet with it's socket id.
You can only do that on the connected socket, i.e. the one returned from the accept() call, not on the listening "server" socket.