|
|
Modifying IPv4 headers
|
|
ip(7) Miscellaneous Information Manual ip(7) NAME
ip - Internet Protocol (IPv4 and IPv6) SYNOPSIS
#include <sys/socket.h> #include <netinet/in.h> The following is the socket call for AF_INET sockets: s = socket(AF_INET, SOCK_RAW, proto); The following is the socket call for AF_INET6 sockets: s = socket(AF_INET6, SOCK_RAW, proto); DESCRIPTION
The Internet Protocol (IP) is the transport layer protocol used by the Internet Protocol family. Options may be set at the IP level when using higher-level protocols that are based on IP (such as the Transmission Control Protocol (TCP) and the User Datagram Package (UDP)). You can also access the protocol through a raw socket when developing new protocols, or special purpose applications. Both IP version 4 and IP version 6 are supported. IP-level options are set with the setsockopt() function and examined with the getsockopt() function. Other options supported by the get- sockopt() and setsockopt() functions can be found in the <netinet/in.h> header file for IPv4 and in the <netinet/in6.h> header file for IPv6. Provides IPv4 options to be transmitted in the IPv4 header of each outgoing packet or examines IPv4 heaader options on incoming packets. The format of IPv4 options to be sent is that specified by the IPv4 specification, 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. IPv4 options may be used with any socket type in the Internet family. Enables a SOCK_DGRAM socket to receive the destination IPv4 address for a UDP datagram. Enables a SOCK_DGRAM socket to receive the IPv4 options for a UDP datagram. Sets the type-of-service (TOS) field in the IPv4 header for a TCP or UDP socket. For example: int tos; setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos)); You can set the type-of-service to any of the defined constants in <netinet/ip.h>. Typically used constants are: IPTOS_LOWDELAY, IPTOS_THROUGHPUT, and IPTOS_RELIABILITY. To determine the current value for this option, use the getsockopt call. For example: int tos; getsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos)); Sets the time-to-live (TTL) field in the IPv4 header for a TCP or UDP socket, by default the values are 60 and 30, respectively, for example: int ttl; setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl)); To determine the current value for this option, use the getsockopt call, for example: int ttl; getsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl)); Sets the hop limit for all subsequent IPv6 unicast packets sent on a specified socket. For example: int hoplimit = 10; if (setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS, (char *) &hoplimit, sizeof(hoplimit)) == -1) perror("setsockopt IPV6_UNICAST_HOPS"); To determine the current value for this option, use the getsockopt call. For example: int hoplimit; size_t len = sizeof(hoplimit); if (getsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS, (char *) &hoplimit, &len) == -1); perror("getsockopt IPV6_UNICAST_HOPS"); else printf("Using %d for hop limit. ", hoplimit); MULTICASTING IP multicasting is supported on AF_INET and AF_INET6 sockets of type SOCK_DGRAM and SOCK_RAW only, and only on networks where the interface driver supports multicasting. By default, if a multicast datagram is sent to a group to which the sending host itself belongs (on the out- going interface), a copy of the datagram is looped back by the IP layer for local delivery. A host must become a member of a multicast group before it can receive datagrams sent to the group. Memberships in multicast groups are dropped when the socket is closed or the process exits. IPv4 Multicasting The following IPv4 multicasting options are supported: Joins an IPv4 multicast group, for example: struct ip_mreq mreq; setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)); The mreq parameter 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 */ } The imr_interface should be INADDR_ANY to choose the default multicast interface, or the IPv4 address of a particular multicast-capable interface if the host is multihomed. Membership is associated with a sin- gle 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. Drops a membership in an IPv4 multicast group, for example: struct ip_mreq mreq; setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq)); The mreq parameter contains the same values as used to add the membership. Overrides the default for subsequent IPv4 transmissions from a given socket. By default, for hosts with multiple interfaces, each multicast transmission is sent from the primary network interface. struct in_addr addr; setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr)); The addr parameter specifies the local IPv4 address of the desired interface or INADDR_ANY to specify the default interface. An interface's local IPv4 address and multicast capability can be obtained through the SIOCGIFCONF and SIOCGIFLAGS ioctls. Normal applications should not need to use this option. Specifies whether or not subsequent IPv6 datagrams are looped back, giving the sender explicit control, for example: 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. However, 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) generally should not use this option. An IPv4 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. Changes the time-to-live (TTL) in the header for outgoing IPv4 multicast datagrams in order to con- trol the scope of the multicasts; for example: u_char ttl; /* range: 0 to 255, default = 1 */ setsockopt(s, IPPROTO_IP, IP_MUL- TICAST_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 the IP_MULTICAST_LOOP option). Multicast datagrams with TTL greater than 1 may be forwarded to other networks if a multicast router is attached to the local network. IPv6 Multicasting The following IPv6 multicasting options are supported: Joins an IPv6 multicast group on the specified interface, for example: #include <netinet/in.h> struct ipv6_mreq imr6; imr6.ipv6mr_interface = if_index; if (setsockopt(s, IPPROTO_IPV6, IPV6_JOIN_GROUP, (char *)&imr6, sizeof(imr6)) < 0) { perror("setsockopt IPV6_JOIN_GROUP"); } The ipv6_mreq parameter is the following structure: struct ipv6_mreq { struct in6_addr ipv6mr_multiaddr; /* IPv6 multicast address */ unsigned int ipv6mr_interface; /* interface index; 0 => * pick a default interface * that supports IP multicasts */ }; The ipv6mr_interface should be either zero to choose the default multicast interface or the interface index of a particular interface if the host is multihomed. Membership is asso- ciated with a single interface; programs running on multihomed hosts may need to join the same group on more than one interface. Up to IPV6_MAX_MEMBERSHIPS (currently 20) may be added to a single socket. Drops a membership in an IPv6 multicast group, for example: #include <netinet/in.h> struct ipv6_mreq imr6; if (setsockopt(s, IPPROTO_IPV6, IPV6_LEAVE_GROUP, (char *)&imr6, sizeof(imr6)) < 0) { perror("setsockopt IPV6_LEAVE_GROUP"); } The imr6 parameter contains the same values as used to add the membership. Changes the hop limit for outgoing IPv6 multicast packets; for example: int hoplimit = 255; /* range 0 to 255; hop limit = -1 sets the * hoplimit to the default value of 1 */ if (setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, &hoplimit, sizeof(hoplimit)) < 0) { perror("setsockopt IPV6_MULTICAST_HOPS"); } Sets the interface to use for outgoing IPv6 multicast packets: u_int if_index = 1; if (setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_IF, &if_index, sizeof(ifindex)) < 0) perror("setsockopt IPV6_MULTICAST_IF"); } The if_index parameter specifies the interface index of the desired interface or 0 to select a default interface. You can use the if_nametoindex() routine to find the interface index. Specifies whether or not subsequent IPv6 datagrams are looped back, for example: u_int on = 0; /* 0 = disable, 1 = enable; default = 1 */ if (setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_LOOP, &on, sizeof(on)) < 0){ perror("setsockopt IPV6_MULTICAST_LOOP "); } This option improves performance for applications that may have no more than one instance on a single host (such as a router dae- mon), by eliminating the overhead of receiving their own transmissions. However, 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) generally should not use this option. An IPv6 multicast datagram sent with an initial hop limit greater than 1 may be delivered to the sending host on a different inter- face 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. Raw IP Sockets Raw IP sockets are connectionless, and are normally used with the sendto() and recvfrom() calls, though the connect() call may also be used to fix the destination for future packets, in which case the read() or recv() and write() or send() functions may be used. If proto is 0 (zero), 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 is set. The IP_HDRINCL option specifies whether the IP header is provided by the sent packet. Incoming packets are received with IP header and options intact. ERRORS
If a socket operation fails, errno may be set to one of the following values: An attempt was made to create a socket with a network address for which no network interface exists. The socket is already connected. This error occurs when trying to establish connection on a socket or when trying to send a datagram with the destination address specified. The system ran out of memory for an internal data structure. The destination address of a datagram was not specified, and the socket has not been connected. The following errors 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 pro- vided. RELATED INFORMATION
Functions: getsockopt(2), send(2), recv(2) Network Information: netintro(7), icmp(7), inet(7), tcp(7). delim off ip(7)
Modifying IPv4 headers