Unix Network Programming Episode 56

int main(int argc, char **argv)
{
	int fd;
	socklen_t len;
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main function to check all socket options

static char strres[128];

static char *sock_str_flag(union val *ptr, int len)
{
    if(len!=sizeof(int))
        snprintf(strres, sizeof(strres), "size (%d) not sizeof(int)", len);
    else
        snprintf(strres, sizeof(strres), "%s", (ptr->i_val==)?"off": "on");
    return strres;
}
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sock_str_flag function: convert flag option to a string

Socket States

Some socket options have timing considerations about when to set or fetch the option versus the state of the socket. We mention these with the affected options.
The following socket options are inherited by a connected TCP socket from the listening socket (pp. 462–463 of TCPv2): SO_DEBUG, SO_DONTROUTE, SO_KEEPALIVE, SO_LINGER, SO_OOBINLINE, SO_RCVBUF, SO_RCVLOWAT, SO_SNDBUF, SO_SNDLOWAT, TCP_MAXSEG, and TCP_NODELAY. This is important with TCP because the connected socket is not returned to a server by accept until the three-way handshake is completed by the TCP layer.

Generic Socket Options

We start with a discussion of the generic socket options. These options are protocol-independent (that is, they are handled by the protocol-independent code within the kernel, not by one particular protocol module such as IPv4), but some of the options apply to only certain types of sockets. For example, even though the SO_BROADCAST socket option is called “generic,” it applies only to datagram sockets.

SO_BROADCAST Socket Option

This option enables or disables the ability of the process to send broadcast messages. Broadcasting is supported for only datagram sockets and only on networks that support the concept of a broadcast message (e.g., Ethernet, token ring, etc.). You cannot broadcast on a point-to-point link or any connection-based transport protocol such as SCTP or TCP.

SO_DEBUG Socket Option

This option is supported only by TCP. When enabled for a TCP socket, the kernel keeps track of detailed information about all the packets sent or received by TCP for the socket. These are kept in a circular buffer within the kernel that can be examined with the trpt program. Pages 916–920 of TCPv2 provide additional details and an example that uses this option.

SO_DONTROUTE Socket Option

This option specifies that outgoing packets are to bypass the normal routing mechanisms of the underlying protocol. For example, with IPv4, the packet is directed to the appropriate local interface, as specified by the network and subnet portions of the destination address. If the local interface cannot be determined from the destination address (e.g., the destination is not on the other end of a point-to-point link, or is not on a shared network), ENETUNREACH is returned.