1. Socket options: a summary
Tcp options, ip options and general socket options can be examined and sometimes modified.
getsockopt, setsockopt
fcntl
Ioctl
#include <sys/socket.h>
int getsockopt (int sockfd, int level, int optname, void *optval, socklen_t *optlen);
Int setsockopt(int sockfd, int level, int optname, const void *optval, socklen_t optlen);

2. #include <sys/socket.h>
int getsockopt (int sockfd, int level, int optname, void *optval, socklen_t *optlen);
Int setsockopt(int sockfd, int level, int optname, const void *optval, socklen_t optlen);
Level includes general socket option (SOL_SOCKET) and protocol-specific option (IP, TCP, etc).
Option value can be of different types : int, in_addr, timeval, … ---- that is why we use the void pointer.

3. General socket options (level = SOL_SOCKET)
Optnames:
SO_BROADCAST(int): permit sending broadcast datagram
SO_DEBUG: for debugging purpose
SO_DONROUTE: used by routing daemon to make sure a packet goes through a certain interface.
SO_ERROR: can only be got (not set), reset the error
SO_KEEPALIVE: for TCP only, automatic send keepalive message when inactive for 2 hours (can be modified).
SO_LINGER: for TCP only, determines the behavior when a close is called.
SO_OOBINLINE: put out of band data in the normal input queue.
SO_RCVBUF, SO_SNDBUF: send and receive buffer size.

4. General socket options (level = SOL_SOCKET)
Optnames:
SO_RCVLOWAT, SO_SENDLOWAT: low watermark – affect the select system call
SO_RCVTIMEO, SO_SNDTIMEO: inherent timeout time (disable by default)
SO_REUSEADDR: Allows the port to be reused – all server should do this to be restart immediately
SO_TYPE: socket type? SOCK_STREAM and SOCK_DGRAM.
SO_USELOOPBACK: sending socket gets a copy?

5. IP options:
Allows packets sent through a socket to have certain behavior, e.g source routing
Level = IPPROTO_IP
Optnames: (manipulating IP header fields)
IP_HDRINCL
IP_OPTIONS
IP_RECVDSTADDR
IP_RECVIF
IP_TOS
IP_TTL

6. IP options: Level = IPPROTO_IP IP_MULTICAST_IF IP_MULTICAST_TTL
Optnames: (IP MULTICAST related)
IP_MULTICAST_IF
IP_MULTICAST_TTL
IP_MULTICAST_LOOP
IP_ADD_MEMBERSHIP
IP_DROP_MEMBERSHIP

7. See example1.c TCP options: Level = IPPROTO_TCP
Optnames:
TCP_KEEPALIVE: set the time
TCP_MAXRT: maximum retransmission time
TCP_MAXSEG: maximum segment size
TCP_NODELAY: wait to ack or not (enable)
TCP_STDURG: where is the urgent pointer.
See example1.c