Internet Protocol


#include <sys/socket.h>
#include <netinet/in.h>

int socket( AF_INET, 
            proto );


IP is the transport layer protocol used by the Internet protocol family. You may set options at the IP level when you're using higher-level protocols based on IP, such as TCP and UDP. You may also access IP through a “raw socket” (when you're developing new protocols or special-purpose applications).

There are several IP-level setsockopt() and getsockopt() options. You can use IP_OPTIONS to provide IP options to be transmitted in the IP header of each outgoing packet or to examine the header options on incoming packets. IP options may be used with any socket type in the Internet family. The format of IP options to be sent is that specified by the IP protocol specification (RFC 791), with one exception: the list of addresses for Source Route options must include the first-hop gateway at the beginning of the list of gateways. The first-hop gateway address is extracted from the option list and the size adjusted accordingly before use. To disable previously specified options, use a zero-length buffer:

setsockopt(s, IPPROTO_IP, IP_OPTIONS, NULL, 0);

You can use IP_TOS and IP_TTL to set the type-of-service and time-to-live fields in the IP header for SOCK_STREAM and SOCK_DGRAM sockets. For example:

int tos = IPTOS_LOWDELAY;       /* see <netinet/ip.h> */
setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));

int ttl = 60;                   /* max = 255 */
setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));

If the IP_RECVDSTADDR option is enabled on a SOCK_DGRAM or SOCK_RAW socket, the recvmsg() call returns the destination IP address for a UDP datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the IP address. The cmsghdr fields have the following values:

cmsg_len = sizeof(struct cmsghdr) + sizeof(struct in_addr)
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVDSTADDR

If the IP_RECVIF option is enabled on a SOCK_DGRAM or SOCK_RAW socket, the recvmsg() call returns a struct sockaddr_dl corresponding to the interface on which the packet was received. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the struct sockaddr_dl. The cmsghdr fields have the following values:

cmsg_len = sizeof(struct cmsghdr) + sizeof(struct sockaddr_dl)
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVIF

Raw IP sockets are connectionless, and are normally used with the sendto() and recvfrom() calls, although you can also use connect() to fix the destination for future packets (in which case you can use the read() or recv() and write() or send() system calls).

If the proto parameter to socket() is 0, the default protocol IPPROTO_RAW is used for outgoing packets, and only incoming packets destined for that protocol are received. If proto is nonzero, that protocol number will be used on outgoing packets and to filter incoming packets.

Outgoing packets automatically have an IP header prepended to them (based on the destination address and the protocol number the socket is created with), unless the IP_HDRINCL option has been set. Incoming packets are received with IP header and options intact.

IP_HDRINCL indicates the complete IP header is included with the data and may be used only with the SOCK_RAW type.

#include <netinet/ip.h>

int hincl = 1;                  /* 1 = on, 0 = off */
setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl));

The program must set all the fields of the IP header, including the following:

ip->ip_v = IPVERSION;
ip->ip_hl = hlen >> 2;
ip->ip_id = 0;  /* 0 means kernel set appropriate value */
ip->ip_off = offset;

If the header source address is set to INADDR_ANY, the kernel chooses an appropriate address.


IP multicasting is supported only on AF_INET sockets of type SOCK_DGRAM and SOCK_RAW, and only on networks where the interface driver supports multicasting.

Multicast Options

Change the time-to-live (TTL) for outgoing multicast datagrams in order to control the scope of the multicasts:
u_char ttl;     /* range: 0 to 255, default = 1 */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));

Datagrams with a TTL of 1 aren't forwarded beyond the local network. Multicast datagrams with a TTL of 0 aren't transmitted on any network, but may be delivered locally if the sending host belongs to the destination group and if multicast loopback hasn't been disabled on the sending socket (see below). Multicast datagrams with TTL greater than 1 may be forwarded to other networks if a multicast router is attached to the local network.

For hosts with multiple interfaces, each multicast transmission is sent from the primary network interface. The IP_MULTICAST_IF option overrides the default for subsequent transmissions from a given socket:
struct in_addr addr;
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));

where addr is the local IP address of the desired interface or INADDR_ANY to specify the default interface. You can get an interface's local IP address and multicast capability by sending the SIOCGIFCONF and SIOCGIFFLAGS requests to ioctl(). Normal applications shouldn't need to use this option.

If a multicast datagram is sent to a group to which the sending host itself belongs (on the outgoing interface), a copy of the datagram is, by default, looped back by the IP layer for local delivery. The IP_MULTICAST_LOOP option gives the sender explicit control over whether or not subsequent datagrams are looped back:
u_char loop;    /* 0 = disable, 1 = enable (default) */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));

This option improves performance for applications that may have no more than one instance on a single host (such as a router demon), by eliminating the overhead of receiving their own transmissions. It shouldn't generally be used by applications for which there may be more than one instance on a single host (such as a conferencing program) or for which the sender doesn't belong to the destination group (such as a time querying program).

A multicast datagram sent with an initial TTL greater than 1 may be delivered to the sending host on a different interface from that on which it was sent, if the host belongs to the destination group on that other interface. The loopback control option has no effect on such delivery.

A host must become a member of a multicast group before it can receive datagrams sent to the group. To join a multicast group, use the IP_ADD_MEMBERSHIP option:
struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));

where mreq is the following structure:

struct ip_mreq {
   struct in_addr imr_multiaddr; /* multicast group to join */
   struct in_addr imr_interface; /* interface to join on */

Set imr_interface to INADDR_ANY to choose the default multicast interface, or to the IP address of a particular multicast-capable interface if the host is multihomed. Membership is associated with a single interface; programs running on multihomed hosts may need to join the same group on more than one interface. Up to IP_MAX_MEMBERSHIPS (currently 20) memberships may be added on a single socket.

To drop a membership, use:
struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));

where mreq contains the same values as used to add the membership. Memberships are dropped when the socket is closed or the process exits.

Based on:

RFC 791


A descriptor referencing the socket, or -1 if an error occurs (errno is set).


You tried to create a socket with a network address for which no network interface exists.
You tried to establish a connection on a socket that already has one or to send a datagram with the destination address specified, but the socket is already connected.
The system ran out of memory for an internal data structure.
You tried to send a datagram, but no destination address was specified and the socket hasn't been connected.
Note: The following error specific to IP may occur when setting or getting IP options:
An unknown socket option name was given. The IP option field was improperly formed—an option field was shorter than the minimum value or longer than the option buffer provided.