how to bind raw socket to specific interface - c

Question

My application is running on CentOS 5.5.
I'm using raw socket to send data:
sd = socket(AF_INET, SOCK_RAW, IPPROTO_RAW);
if (sd < 0) {
// Error
}
const int opt_on = 1;
rc = setsockopt(m_SocketDescriptor, IPPROTO_IP, IP_HDRINCL, &opt_on, sizeof(opt_on));
if (rc < 0) {
close(sd);
// Error
}
struct sockaddr_in sin;
memset(&sin, 0, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = my_ip_address;
if (sendto(m_SocketDescriptor, DataBuffer, (size_t)TotalSize, 0, (struct sockaddr *)&sin, sizeof(struct sockaddr)) < 0) {
close(sd);
// Error
}
How can I bind this socket to specific network interface (say eth1)?

Answer 1

const char *opt;
opt = "eth0";
const len = strnlen(opt, IFNAMSIZ);
if (len == IFNAMSIZ) {
fprintf(stderr, "Too long iface name");
return 1;
}
setsockopt(sd, SOL_SOCKET, SO_BINDTODEVICE, opt, len);
First line: set up your variable
Second line: tell the program which interface to bind to
Lines 3-5: get length of interface name and check if it's size not too big.
Six line: set the socket options for socket sd, binding to the device opt.
setsockopt prototype:
int setsockopt(int s, int level, int optname, const void *optval, socklen_t optlen);
Also, make sure you include the if.h, socket.h and string.h header files

Answer 2

As mentioned earlier, the correct thing to do is use the struct ifreq to specify the interface name. Here is my code sample.
#define SERVERPORT 5555
...
struct ifreq ifr;
/* Create the socket */
sd = socket(AF_INET, SOCK_STREAM, 0);
if (sd < 0)
{
printf("Error in socket() creation - %s", strerror(errno));
}
/* Bind to eth1 interface only - this is a private VLAN */
memset(&ifr, 0, sizeof(ifr));
snprintf(ifr.ifr_name, sizeof(ifr.ifr_name), "eth1");
if ((rc = setsockopt(sd, SOL_SOCKET, SO_BINDTODEVICE, (void *)&ifr, sizeof(ifr))) < 0)
{
perror("Server-setsockopt() error for SO_BINDTODEVICE");
printf("%s\n", strerror(errno));
close(sd);
exit(-1);
}
/* bind to an address */
memset(&serveraddr, 0x00, sizeof(struct sockaddr_in));
serveraddr.sin_family = AF_INET;
serveraddr.sin_port = htons(SERVERPORT);
serveraddr.sin_addr.s_addr = inet_addr("9.1.2.3");
int rc = bind(sd, (struct sockaddr *)&serveraddr, sizeof(serveraddr));
I would also like to add that from a security perspective, while it is good to bind the socket to an interface, it does not make sense to use INADDR_ANY as the listening IP address. Doing so would make the port appear open in netstat on all network interfaces.
Proto Recv-Q Send-Q Local Address Foreign Address State User Inode PID/Program name
tcp 0 0 0.0.0.0:5555 0.0.0.0:* LISTEN 0 210898 26996/myserver
Instead, I specified an IP address specific to the interface being used (a private VLAN). This fixed the netstat output too:
Proto Recv-Q Send-Q Local Address Foreign Address State User Inode PID/Program name
tcp 0 0 9.1.2.3:5555 0.0.0.0:* LISTEN 0 210898 26996/myserver

Answer 3

Bind socket to specific interface IP address
int bind_using_iface_ip(int fd, char *ipaddr, uint16_t port)
{
struct sockaddr_in localaddr = {0};
localaddr.sin_family = AF_INET;
localaddr.sin_port = htons(port);
localaddr.sin_addr.s_addr = inet_addr(ipaddr);
return bind(fd, (struct sockaddr*) &localaddr, sizeof(struct sockaddr_in));
}
Bind socket to specific interface name
int bind_using_iface_name(int fd, char *iface_name)
{
return setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, iface_name, strlen(iface_name))
}
In bind_using_iface_ip, to bind to any port 0 should be passed. And also if the fd is raw socket then need to pass port as 0. This bind mechanism is common for all kind of sockets like raw, dgram and stream.