How to get the ip address of the accepted in-bound socket? - c

Question

My question is:
Server will create a socket, bind to a given port and with address = INADDR_ANY.
listen() & accept() the new connection. Then, we can get the client's ip-address
from accept().
Now, I want to know the ip-address of the Server, since the host of the server has
multiple NIC on it.
How to know the ip-address of the network interface with which the accepted in-bound socket is from?
I tried getsockname, it gave me the port number, but the ip is all-zero.
Update: Here is the code:
Server.c (header files are removed)
int main(void)
{
struct sockaddr_in stSockAddr;
int res, addr_len, SocketFD, ConnectFD;
struct sockaddr_in addr;
SocketFD = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
if(-1 == SocketFD)
{
perror("can not create socket");
//exit(EXIT_FAILURE);
return -1;
}
memset(&stSockAddr, 0, sizeof stSockAddr);
stSockAddr.sin_family = AF_INET;
stSockAddr.sin_port = htons(49335);
stSockAddr.sin_addr.s_addr = INADDR_ANY;
if(-1 == bind(SocketFD,(struct sockaddr *)&stSockAddr, sizeof stSockAddr))
{
perror("error bind failed");
close(SocketFD);
return -1;
}
printf("going to listen!\n");
if(-1 == listen(SocketFD, 10))
{
perror("error listen failed");
close(SocketFD);
//exit(EXIT_FAILURE);
return -1;
}
ConnectFD = accept(SocketFD, NULL, NULL);
if(0 > ConnectFD)
{
perror("error accept failed");
close(SocketFD);
//exit(EXIT_FAILURE);
return -1;
}
addr.sin_family = AF_INET;
res = getsockname (ConnectFD, (struct sockaddr *)&addr, &addr_len);
// if you remove the following comment, that means, if you call
// two times of getsockname, the result will be correct.
//res = getsockname (ConnectFD, (struct sockaddr *)&addr, &addr_len);
printf("addr:%x\n", addr.sin_addr.s_addr);
while(1) {
if (getchar() == 'q')
break;
}
close(ConnectFD);
close(SocketFD);
return 0;
}
Below is client.c:
int main(void)
{
struct sockaddr_in stSockAddr;
int Res;
int SocketFD = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
if (-1 == SocketFD)
{
perror("cannot create socket");
exit(EXIT_FAILURE);
}
memset(&stSockAddr, 0, sizeof stSockAddr);
stSockAddr.sin_family = AF_INET;
stSockAddr.sin_port = htons(49335);
Res = inet_pton(AF_INET, "192.168.1.102", &stSockAddr.sin_addr);
if (0 > Res)
{
perror("error: first parameter is not a valid address family");
close(SocketFD);
exit(EXIT_FAILURE);
}
else if (0 == Res)
{
perror("char string (second parameter does not contain valid ipaddress");
close(SocketFD);
exit(EXIT_FAILURE);
}
if (-1 == connect(SocketFD, (struct sockaddr *)&stSockAddr, sizeof stSockAddr))
{
perror("connect failed");
close(SocketFD);
exit(EXIT_FAILURE);
}
/* perform read write operations ... */
printf("client sockfd is successful\n");
while(1) {
if (getchar() == 'q')
break;
}
shutdown(SocketFD, SHUT_RDWR);
close(SocketFD);
return 0;
}

Answer 1

Use getsockname(2) on the socket returned from accept(2), not the socket returned from bind(2).

Answer 2

getsockname() gets name of passed socket.
in this example, you pass the socket which is created by
accept function in server.
this socket is in server side, so it's name & address is
related to server side.
if you want to know "who was connected to me"
you must use getpeername() instead of getsockname.
good luck

Answer 3

if you remove the following comment, that means, if you call
two times of getsockname, the result will be correct.
res = getsockname (ConnectFD, (struct sockaddr *)&addr, &addr_len);
you must init addr_len.
addr_len = sizeof(addr);