CONTENTS

SYNOPSIS

DESCRIPTION

There are several IP-level setsockopt(2) and getsockopt(2) options. IP_OPTIONS may be used 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 will be 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);

IP_TOS and IP_TTL may be used to set the type-of-service and time-to-live fields in the IP header for SOCK_STREAM, SOCK_DGRAM, and certain types of SOCK_RAW 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));

IP_MINTTL may be used to set the minimum acceptable TTL a packet must have when received on a socket. All packets with a lower TTL are silently dropped. This option is only really useful when set to 255 preventing packets from outside the directly connected networks reaching local listeners on sockets.

IP_DONTFRAG may be used to set the Don’t Fragment flag on IP packets. Currently this option is respected only on udp(4) and Raw ip(4) sockets, unless the IP_HDRINCL option has been set. On tcp(4) sockets the Don’t Fragment flag is controlled by the Path MTU Discovery option. Sending a packet larger than the MTU size of the egress interface, determined by the destination address, returns an EMSGSIZE error.

If the IP_RECVDSTADDR option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call will return the destination IP address for a UDP datagram. The
.Vt msg_control field in the
.Vt msghdr structure points to a buffer that contains a
.Vt cmsghdr structure followed by the IP address. The
.Vt cmsghdr fields have the following values:
cmsg_len = sizeof(struct in_addr)
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVDSTADDR

The source address to be used for outgoing UDP datagrams on a socket that is not bound to a specific IP address can be specified as ancillary data with a type code of IP_SENDSRCADDR. The msg_control field in the msghdr structure should point to a buffer that contains a
.Vt cmsghdr structure followed by the IP address. The cmsghdr fields should have the following values:
cmsg_len = sizeof(struct in_addr)
cmsg_level = IPPROTO_IP
cmsg_type = IP_SENDSRCADDR

For convenience, IP_SENDSRCADDR is defined to have the same value as IP_RECVDSTADDR, so the IP_RECVDSTADDR control message from recvmsg(2) can be used directly as a control message for sendmsg(2).

If the IP_ONESBCAST option is enabled on a SOCK_DGRAM or a SOCK_RAW socket, the destination address of outgoing broadcast datagrams on that socket will be forced to the undirected broadcast address, INADDR_BROADCAST, before transmission. This is in contrast to the default behavior of the system, which is to transmit undirected broadcasts via the first network interface with the IFF_BROADCAST flag set.

This option allows applications to choose which interface is used to transmit an undirected broadcast datagram. For example, the following code would force an undirected broadcast to be transmitted via the interface configured with the broadcast address 192.168.2.255:
char msg[512];
struct sockaddr_in sin;
u_char onesbcast = 1; /* 0 = disable (default), 1 = enable */

setsockopt(s, IPPROTO_IP, IP_ONESBCAST, &onesbcast, sizeof(onesbcast));
sin.sin_addr.s_addr = inet_addr("192.168.2.255");
sin.sin_port = htons(1234);
sendto(s, msg, sizeof(msg), 0, &sin, sizeof(sin));

It is the application’s responsibility to set the IP_TTL option to an appropriate value in order to prevent broadcast storms. The application must have sufficient credentials to set the SO_BROADCAST socket level option, otherwise the IP_ONESBCAST option has no effect.

If the IP_RECVTTL option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call will return the IP TTL (time to live) field for a UDP datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the TTL. The cmsghdr fields have the following values:
cmsg_len = sizeof(u_char)
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVTTL

If the IP_RECVIF option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call returns a
.Vt "struct sockaddr_dl" corresponding to the interface on which the packet was received. The msg_control field in the
.Vt msghdr structure points to a buffer that contains a
.Vt cmsghdr structure followed by the
.Vt "struct sockaddr_dl" . The
.Vt cmsghdr fields have the following values:
cmsg_len = sizeof(struct sockaddr_dl)
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVIF

IP_PORTRANGE may be used to set the port range used for selecting a local port number on a socket with an unspecified (zero) port number. It has the following possible values:

The range of privileged ports which only may be opened by root-owned processes may be modified by the net.inet.ip.portrange.reservedlow and net.inet.ip.portrange.reservedhigh sysctl settings. The values default to the traditional range, 0 through IPPORT_RESERVED - 1 (0 through 1023), respectively. Note that these settings do not affect and are not accounted for in the use or calculation of the other net.inet.ip.portrange values above. Changing these values departs from Unix tradition and has security consequences that the administrator should carefully evaluate before modifying these settings.

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.

The IP_MULTICAST_TTL option changes 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 are not forwarded beyond the local network. Multicast datagrams with a TTL of 0 will not be transmitted on any network, but may be delivered locally if the sending host belongs to the destination group and if multicast loopback has not 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. An interface’s local IP address and multicast capability can be obtained via the SIOCGIFCONF and SIOCGIFFLAGS ioctls. Normal applications should not 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 daemon), by eliminating the overhead of receiving their own transmissions. It should generally not 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 does not 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; /* IP multicast address of group */
struct in_addr imr_interface; /* local IP address of interface */
}

imr_interface should be set to INADDR_ANY to choose the default multicast interface, or the IP address of a particular multicast-capable interface if the host is multihomed. Since
.Fx 4.4 , if the imr_interface member is within the network range 0.0.0.0/8, it is treated as an interface index in the system interface MIB, as per the RIP Version 2 MIB Extension (RFC-1724).

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));

Raw IP sockets are connectionless, and are normally used with the sendto(2) and recvfrom(2) calls, though the connect(2) call may also be used to fix the destination for future packets (in which case the read(2) or recv(2) and write(2) or send(2) system calls may be used).

If proto 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 non-zero, 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/in_systm.h>
#include <netinet/ip.h>

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

Unlike previous BSD releases, 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;

The following errors specific to IP may occur when setting or getting IP options:

The following errors may occur when attempting to send IP datagrams via a "raw socket" with the IP_HDRINCL option set:

SEE ALSO

HISTORY