| IP(4) | AerieBSD 1.0 Refernce Manual | IP(4) |
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 and SOCK_DGRAM sockets. For example,
int tos = IPTOS_LOWDELAY; /* see*/ 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 socket, the recvmsg(2) call will return 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 = CMSG_LEN(sizeof(struct in_addr)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVDSTADDR
If the IP_RECVDSTPORT option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call will return the destination port for a UDP datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the port in 16-bit network byte order. The cmsghdr fields have the following values:
cmsg_len = CMSG_LEN(sizeof(u_int16_t)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVDSTPORT
If the IP_RECVTTL option is enabled on a SOCK_DGRAM or SOCK_RAW socket, the recvmsg(2) call will return the TTL of the received datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the TTL value. The cmsghdr fields have the following values:
cmsg_len = CMSG_LEN(sizeof(struct in_addr)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVTTL
The IP_MINTTL option may be used on SOCK_STREAM sockets to discard packets with a TTL lower than the option value. This can be used to implement the Generalized TTL Security Mechanism (GTSM) according to RFC 3682. To discard all packets with a TTL lower than 255:
int minttl = 255; setsockopt(s, IPPROTO_IP, IP_MINTTL, &minttl, sizeof(minttl));
The IP_PORTRANGE option causes the default allocation policy for when the kernel is asked to choose a free port number. Three choices are available:
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 ioctl(2/Ns). 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; /* multicast group to join */
struct in_addr imr_interface; /* interface to join on */
}
imr_interface should be INADDR_ANY to choose the default multicast interface, or 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 4095) 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.
#includeint 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 = htons(offset); ip->ip_len = htons(len);
Additionally note that starting with OpenBSD 2.1, the ip_off and ip_len fields are in network byte order. If the header source address is set to INADDR_ANY, the kernel will choose an appropriate address.
| AerieBSD 1.0 Reference Manual | December 26 2008 | IP(4) |