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Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)ip.4 8.2 (Berkeley) 11/30/93 .\" $FreeBSD$ .\" .Dd September 26, 2005 .Dt IP 4 .Os .Sh NAME .Nm ip .Nd Internet Protocol .Sh SYNOPSIS .In sys/types.h .In sys/socket.h .In netinet/in.h .Ft int .Fn socket AF_INET SOCK_RAW proto .Sh DESCRIPTION .Tn IP is the transport layer protocol used by the Internet protocol family. Options may be set at the .Tn IP level when using higher-level protocols that are based on .Tn IP (such as .Tn TCP and .Tn UDP ) . It may also be accessed through a .Dq raw socket when developing new protocols, or special-purpose applications. .Pp There are several .Tn IP-level .Xr setsockopt 2 and .Xr getsockopt 2 options. .Dv IP_OPTIONS may be used to provide .Tn IP options to be transmitted in the .Tn IP header of each outgoing packet or to examine the header options on incoming packets. .Tn IP options may be used with any socket type in the Internet family. The format of .Tn IP options to be sent is that specified by the .Tn 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: .Bd -literal setsockopt(s, IPPROTO_IP, IP_OPTIONS, NULL, 0); .Ed .Pp .Dv IP_TOS and .Dv IP_TTL may be used to set the type-of-service and time-to-live fields in the .Tn IP header for .Dv SOCK_STREAM , SOCK_DGRAM , and certain types of .Dv SOCK_RAW sockets. For example, .Bd -literal 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)); .Ed .Pp .Dv 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. .Pp .Dv IP_DONTFRAG may be used to set the Don't Fragment flag on IP packets. Currently this option is respected only on .Xr udp 4 and raw .Xr ip 4 sockets, unless the .Dv IP_HDRINCL option has been set. On .Xr 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 .Er EMSGSIZE error. .Pp If the .Dv IP_RECVDSTADDR option is enabled on a .Dv SOCK_DGRAM socket, the .Xr recvmsg 2 call will return the destination .Tn IP address for a .Tn 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 .Tn IP address. The .Vt cmsghdr fields have the following values: .Bd -literal cmsg_len = sizeof(struct in_addr) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVDSTADDR .Ed .Pp The source address to be used for outgoing .Tn UDP datagrams on a socket that is not bound to a specific .Tn IP address can be specified as ancillary data with a type code of .Dv IP_SENDSRCADDR . The msg_control field in the msghdr structure should point to a buffer that contains a .Vt cmsghdr structure followed by the .Tn IP address. The cmsghdr fields should have the following values: .Bd -literal cmsg_len = sizeof(struct in_addr) cmsg_level = IPPROTO_IP cmsg_type = IP_SENDSRCADDR .Ed .Pp For convenience, .Dv IP_SENDSRCADDR is defined to have the same value as .Dv IP_RECVDSTADDR , so the .Dv IP_RECVDSTADDR control message from .Xr recvmsg 2 can be used directly as a control message for .Xr sendmsg 2 . .Pp If the .Dv IP_ONESBCAST option is enabled on a .Dv SOCK_DGRAM or a .Dv SOCK_RAW socket, the destination address of outgoing broadcast datagrams on that socket will be forced to the undirected broadcast address, .Dv 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 .Dv IFF_BROADCAST flag set. .Pp 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: .Bd -literal 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)); .Ed .Pp It is the application's responsibility to set the .Dv IP_TTL option to an appropriate value in order to prevent broadcast storms. The application must have sufficient credentials to set the .Dv SO_BROADCAST socket level option, otherwise the .Dv IP_ONESBCAST option has no effect. .Pp If the .Dv IP_RECVTTL option is enabled on a .Dv SOCK_DGRAM socket, the .Xr recvmsg 2 call will return the .Tn IP .Tn TTL (time to live) field for a .Tn UDP datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the .Tn TTL . The cmsghdr fields have the following values: .Bd -literal cmsg_len = sizeof(u_char) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVTTL .Ed .Pp If the .Dv IP_RECVIF option is enabled on a .Dv SOCK_DGRAM socket, the .Xr recvmsg 2 call returns a .Vt "struct sockaddr_dl" corresponding to the interface on which the packet was received. The .Va 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: .Bd -literal cmsg_len = sizeof(struct sockaddr_dl) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVIF .Ed .Pp .Dv 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: .Bl -tag -width IP_PORTRANGE_DEFAULT .It Dv IP_PORTRANGE_DEFAULT use the default range of values, normally .Dv IPPORT_HIFIRSTAUTO through .Dv IPPORT_HILASTAUTO . This is adjustable through the sysctl setting: .Va net.inet.ip.portrange.first and .Va net.inet.ip.portrange.last . .It Dv IP_PORTRANGE_HIGH use a high range of values, normally .Dv IPPORT_HIFIRSTAUTO and .Dv IPPORT_HILASTAUTO . This is adjustable through the sysctl setting: .Va net.inet.ip.portrange.hifirst and .Va net.inet.ip.portrange.hilast . .It Dv IP_PORTRANGE_LOW use a low range of ports, which are normally restricted to privileged processes on .Ux systems. The range is normally from .Dv IPPORT_RESERVED \- 1 down to .Li IPPORT_RESERVEDSTART in descending order. This is adjustable through the sysctl setting: .Va net.inet.ip.portrange.lowfirst and .Va net.inet.ip.portrange.lowlast . .El .Pp The range of privileged ports which only may be opened by root-owned processes may be modified by the .Va net.inet.ip.portrange.reservedlow and .Va net.inet.ip.portrange.reservedhigh sysctl settings. The values default to the traditional range, 0 through .Dv 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 .Va net.inet.ip.portrange values above. Changing these values departs from .Ux tradition and has security consequences that the administrator should carefully evaluate before modifying these settings. .Pp Ports are allocated at random within the specified port range in order to increase the difficulty of random spoofing attacks. In scenarios such as benchmarking, this behavior may be undesirable. In these cases, .Va net.inet.ip.portrange.randomized can be used to toggle randomization off. If more than .Va net.inet.ip.portrange.randomcps ports have been allocated in the last second, then return to sequential port allocation. Return to random allocation only once the current port allocation rate drops below .Va net.inet.ip.portrange.randomcps for at least .Va net.inet.ip.portrange.randomtime seconds. The default values for .Va net.inet.ip.portrange.randomcps and .Va net.inet.ip.portrange.randomtime are 10 port allocations per second and 45 seconds correspondingly. .Ss "Multicast Options" .Pp .Tn IP multicasting is supported only on .Dv AF_INET sockets of type .Dv SOCK_DGRAM and .Dv SOCK_RAW , and only on networks where the interface driver supports multicasting. .Pp The .Dv IP_MULTICAST_TTL option changes the time-to-live (TTL) for outgoing multicast datagrams in order to control the scope of the multicasts: .Bd -literal u_char ttl; /* range: 0 to 255, default = 1 */ setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl)); .Ed .Pp 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. .Pp For hosts with multiple interfaces, each multicast transmission is sent from the primary network interface. The .Dv IP_MULTICAST_IF option overrides the default for subsequent transmissions from a given socket: .Bd -literal struct in_addr addr; setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr)); .Ed .Pp where "addr" is the local .Tn IP address of the desired interface or .Dv INADDR_ANY to specify the default interface. An interface's local IP address and multicast capability can be obtained via the .Dv SIOCGIFCONF and .Dv SIOCGIFFLAGS ioctls. Normal applications should not need to use this option. .Pp 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 .Dv IP_MULTICAST_LOOP option gives the sender explicit control over whether or not subsequent datagrams are looped back: .Bd -literal u_char loop; /* 0 = disable, 1 = enable (default) */ setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop)); .Ed .Pp 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). .Pp 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. .Pp 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 .Dv IP_ADD_MEMBERSHIP option: .Bd -literal struct ip_mreq mreq; setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)); .Ed .Pp where .Fa mreq is the following structure: .Bd -literal struct ip_mreq { struct in_addr imr_multiaddr; /* IP multicast address of group */ struct in_addr imr_interface; /* local IP address of interface */ } .Ed .Pp .Va imr_interface should be set to .Dv INADDR_ANY to choose the default multicast interface, or the .Tn IP address of a particular multicast-capable interface if the host is multihomed. Since .Fx 4.4 , if the .Va imr_interface member is within the network range .Li 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). .Pp 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 .Dv IP_MAX_MEMBERSHIPS (currently 20) memberships may be added on a single socket. .Pp To drop a membership, use: .Bd -literal struct ip_mreq mreq; setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq)); .Ed .Pp where .Fa mreq contains the same values as used to add the membership. Memberships are dropped when the socket is closed or the process exits. .\"----------------------- .Ss "Raw IP Sockets" .Pp Raw .Tn IP sockets are connectionless, and are normally used with the .Xr sendto 2 and .Xr recvfrom 2 calls, though the .Xr connect 2 call may also be used to fix the destination for future packets (in which case the .Xr read 2 or .Xr recv 2 and .Xr write 2 or .Xr send 2 system calls may be used). .Pp If .Fa proto is 0, the default protocol .Dv IPPROTO_RAW is used for outgoing packets, and only incoming packets destined for that protocol are received. If .Fa proto is non-zero, that protocol number will be used on outgoing packets and to filter incoming packets. .Pp Outgoing packets automatically have an .Tn IP header prepended to them (based on the destination address and the protocol number the socket is created with), unless the .Dv IP_HDRINCL option has been set. Incoming packets are received with .Tn IP header and options intact. .Pp .Dv IP_HDRINCL indicates the complete IP header is included with the data and may be used only with the .Dv SOCK_RAW type. .Bd -literal #include #include int hincl = 1; /* 1 = on, 0 = off */ setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl)); .Ed .Pp Unlike previous .Bx releases, the program must set all the fields of the IP header, including the following: .Bd -literal ip->ip_v = IPVERSION; ip->ip_hl = hlen >> 2; ip->ip_id = 0; /* 0 means kernel set appropriate value */ ip->ip_off = offset; .Ed .Pp The .Va ip_len and .Va ip_off fields .Em must be provided in host byte order . All other fields must be provided in network byte order. See .Xr byteorder 3 for more information on network byte order. If the .Va ip_id field is set to 0 then the kernel will choose an appropriate value. If the header source address is set to .Dv INADDR_ANY , the kernel will choose an appropriate address. .Sh ERRORS A socket operation may fail with one of the following errors returned: .Bl -tag -width Er .It Bq Er EISCONN when trying to establish a connection on a socket which already has one, or when trying to send a datagram with the destination address specified and the socket is already connected; .It Bq Er ENOTCONN when trying to send a datagram, but no destination address is specified, and the socket has not been connected; .It Bq Er ENOBUFS when the system runs out of memory for an internal data structure; .It Bq Er EADDRNOTAVAIL when an attempt is made to create a socket with a network address for which no network interface exists. .It Bq Er EACCES when an attempt is made to create a raw IP socket by a non-privileged process. .El .Pp The following errors specific to .Tn IP may occur when setting or getting .Tn IP options: .Bl -tag -width Er .It Bq Er EINVAL An unknown socket option name was given. .It Bq Er EINVAL The IP option field was improperly formed; an option field was shorter than the minimum value or longer than the option buffer provided. .El .Pp The following errors may occur when attempting to send .Tn IP datagrams via a .Dq raw socket with the .Dv IP_HDRINCL option set: .Bl -tag -width Er .It Bq Er EINVAL The user-supplied .Va ip_len field was not equal to the length of the datagram written to the socket. .El .Sh SEE ALSO .Xr getsockopt 2 , .Xr recv 2 , .Xr send 2 , .Xr byteorder 3 , .Xr icmp 4 , .Xr inet 4 , .Xr intro 4 .Sh HISTORY The .Nm protocol appeared in .Bx 4.2 .