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IP(7)                      Linux Programmer's Manual                     IP(7)



NAME
       ip(7,8) - Linux IPv4 protocol implementation

SYNOPSIS
       #include <sys/socket.h>
       #include <netinet/in.h>
       #include <netinet/ip.h> /* superset of previous */

       tcp_socket = socket(2,7,n)(PF_INET, SOCK_STREAM, 0);
       raw_socket = socket(2,7,n)(PF_INET, SOCK_RAW, protocol);
       udp_socket = socket(2,7,n)(PF_INET, SOCK_DGRAM, protocol);

DESCRIPTION
       Linux  implements the Internet Protocol, version(1,3,5) 4, described in(1,8) RFC791
       and RFC1122.  ip(7,8) contains a level 2  multicasting  implementation  con-
       forming  to  RFC1112.  It also contains an IP router including a packet
       filter.

       The programmer's interface is BSD sockets compatible.  For more  infor-
       mation on sockets, see socket(2,7,n)(7).

       An   IP  socket(2,7,n)  is  created  by  calling  the  socket(2,7,n)(2)  function  as
       socket(2,7,n)(PF_INET,  socket_type,  protocol).   Valid  socket(2,7,n)   types   are
       SOCK_STREAM  to  open(2,3,n)  a  tcp(7)  socket(2,7,n),  SOCK_DGRAM  to open(2,3,n) a udp(7)
       socket(2,7,n), or SOCK_RAW to open(2,3,n) a raw(3x,7,8,3x cbreak)(7) socket(2,7,n) to access(2,5) the  IP  protocol
       directly.   protocol is the IP protocol in(1,8) the IP header to be received
       or sent.  The only valid values for protocol are 0 and IPPROTO_TCP  for
       TCP  sockets  and  0 and IPPROTO_UDP for UDP sockets.  For SOCK_RAW you
       may specify a valid IANA IP protocol defined in(1,8) RFC1700  assigned  num-
       bers.

       When a process wants to receive new incoming packets or connections, it
       should bind(2,n,1 builtins) a socket(2,7,n) to a local interface address using bind(2,n,1 builtins)(2).   Only
       one  IP  socket(2,7,n)  may  be bound to any given local (address, port) pair.
       When INADDR_ANY is specified in(1,8) the bind(2,n,1 builtins) call the socket(2,7,n) will be  bound
       to  all  local interfaces. When listen(1,2,7)(2) or connect(2) are called on a
       unbound socket(2,7,n) the socket(2,7,n) is automatically bound to a random(3,4,6) free  port
       with the local address set(7,n,1 builtins) to INADDR_ANY.

       A  TCP local socket(2,7,n) address that has been bound is unavailable for some
       time(1,2,n) after closing, unless the SO_REUSEADDR flag has  been  set.   Care
       should be taken when using this flag as it makes TCP less(1,3) reliable.


ADDRESS FORMAT
       An  IP  socket(2,7,n)  address  is defined as a combination of an IP interface
       address and a port number. The basic IP protocol does not  supply  port
       numbers, they are implemented by higher level protocols like udp(7) and
       tcp(7).  On raw(3x,7,8,3x cbreak) sockets sin_port is set(7,n,1 builtins) to the IP protocol.


              struct sockaddr_in {
                  sa_family_t    sin_family; /* address family: AF_INET */
                  u_int16_t      sin_port;   /* port in(1,8) network byte order */
                  struct in_addr  sin_addr;  /* internet address */
              };

              /* Internet address. */
              struct in_addr {
                  u_int32_t      s_addr;     /* address in(1,8) network byte order */
              };

       sin_family is always set(7,n,1 builtins) to AF_INET.  This is required;  in(1,8)  Linux  2.2
       most  networking  functions return EINVAL when this setting is missing.
       sin_port contains the port in(1,8) network  byte  order.  The  port  numbers
       below  1024  are  called reserved ports.  Only processes with effective
       user id 0 or the CAP_NET_BIND_SERVICE capability may bind(2,n,1 builtins)(2)  to  these
       sockets.  Note  that  the raw(3x,7,8,3x cbreak) IPv4 protocol as such has no concept of a
       port, they are only implemented by higher  protocols  like  tcp(7)  and
       udp(7).

       sin_addr  is  the  IP  host(1,5) address.  The addr member of struct in_addr
       contains the host(1,5) interface address in(1,8) network order.   in_addr  should
       be only accessed using the inet_aton(3), inet_addr(3), inet_makeaddr(3)
       library functions or directly with the name  resolver(3,5)  (see  gethostby-
       name(3)).   IPv4 addresses are divided into unicast, broadcast and mul-
       ticast addresses. Unicast addresses specify a  single  interface  of  a
       host(1,5),  broadcast addresses specify all hosts on a network and multicast
       addresses address all hosts in(1,8) a multicast group. Datagrams  to  broad-
       cast  addresses  can  be  only  sent  or received when the SO_BROADCAST
       socket(2,7,n) flag is set.  In the current implementation connection  oriented
       sockets are only allowed to use unicast addresses.

       Note  that the address and the port are always stored in(1,8) network order.
       In particular, this means that you need to call htons(3) on the  number
       that  is assigned to a port. All address/port manipulation functions in(1,8)
       the standard library work in(1,8) network order.

       There are several special addresses: INADDR_LOOPBACK (127.0.0.1) always
       refers  to the local host(1,5) via the loopback device; INADDR_ANY (0.0.0.0)
       means any address for binding; INADDR_BROADCAST (255.255.255.255) means
       any  host(1,5)  and has the same effect on bind(2,n,1 builtins) as INADDR_ANY for historical
       reasons.


SOCKET OPTIONS
       IP supports some protocol specific socket(2,7,n) options that can be set(7,n,1 builtins)  with
       setsockopt(2) and read(2,n,1 builtins) with getsockopt(2).  The socket(2,7,n) option level for
       IP is SOL_IP.  A boolean integer flag is zero when it is false,  other-
       wise true.


       IP_OPTIONS
              Sets  or  get  the  IP options to be sent with every packet from
              this socket.  The arguments are a pointer  to  a  memory  buffer
              containing the options and the option length.  The setsockopt(2)
              call sets the IP options associated with a socket.  The  maximum
              option  size  for  IPv4  is 40 bytes. See RFC791 for the allowed
              options. When  the  initial  connection  request  packet  for  a
              SOCK_STREAM  socket(2,7,n)  contains IP options, the IP options will be
              set(7,n,1 builtins) automatically to the options from the  initial  packet  with
              routing  headers  reversed.  Incoming packets are not allowed to
              change options after the connection is  established.   The  pro-
              cessing  of  all  incoming source routing options is disabled by
              default and can be  enabled  by  using  the  accept_source_route
              sysctl.   Other  options like timestamps are still handled.  For
              datagram sockets, IP options can be only set(7,n,1 builtins) by the local  user.
              Calling  getsockopt(2)  with  IP_OPTIONS  puts(3,n)  the  current  IP
              options used for sending into the supplied buffer.


       IP_PKTINFO
              Pass an IP_PKTINFO ancillary message  that  contains  a  pktinfo
              structure  that  supplies  some  information  about the incoming
              packet.  This only works for  datagram  oriented  sockets.   The
              argument  is a flag that tells the socket(2,7,n) whether the IP_PKTINFO
              message should be passed or not. The message itself can only  be
              sent/retrieved as control message with a packet using recvmsg(2)
              or sendmsg(2).


              struct in_pktinfo {
                  unsigned int   ipi_ifindex;  /* Interface index */
                  struct in_addr ipi_spec_dst; /* Local address */
                  struct in_addr ipi_addr;     /* Header Destination address */
              };

              ipi_ifindex is the unique index of the interface the packet  was
              received  on.   ipi_spec_dst  is the local address of the packet
              and ipi_addr is the destination address in(1,8)  the  packet  header.
              If  IP_PKTINFO  is  passed to sendmsg(2) and ipi_spec_dst is not
              zero, then it is used as the local source address for the  rout-
              ing  table  lookup  and  for setting up IP source route options.
              When ipi_ifindex is not zero the primary local  address  of  the
              interface specified by the index overwrites ipi_spec_dst for the
              routing table lookup.

       IP_RECVTOS
              If enabled the IP_TOS ancillary message is passed with  incoming
              packets.  It  contains  a  byte which specifies the Type of Ser-
              vice/Precedence field of the packet header.  Expects  a  boolean
              integer flag.


       IP_RECVTTL
              When this flag is set(7,n,1 builtins) pass a IP_RECVTTL control message with the
              time(1,2,n) to live field of the received packet as a  byte.  Not  sup-
              ported for SOCK_STREAM sockets.


       IP_RECVOPTS
              Pass all incoming IP options to the user in(1,8) a IP_OPTIONS control
              message. The routing header and other options are already filled
              in(1,8) for the local host. Not supported for SOCK_STREAM sockets.


       IP_RETOPTS
              Identical  to  IP_RECVOPTS  but  returns raw(3x,7,8,3x cbreak) unprocessed options
              with timestamp and route record options not filled in(1,8)  for  this
              hop.


       IP_TOS Set or receive the Type-Of-Service (TOS) field that is sent with
              every IP packet originating from this socket. It is used to pri-
              oritize  packets  on the network.  TOS is a byte. There are some
              standard TOS flags defined: IPTOS_LOWDELAY  to  minimize  delays
              for  interactive  traffic, IPTOS_THROUGHPUT to optimize through-
              put, IPTOS_RELIABILITY to optimize for  reliability,  IPTOS_MIN-
              COST  should  be  used for "filler data" where slow transmission
              doesn't matter.  At most one of these TOS values can  be  speci-
              fied.  Other bits are invalid and shall be cleared.  Linux sends
              IPTOS_LOWDELAY datagrams first by default, but the exact  behav-
              iour  depends  on the configured queueing discipline.  Some high
              priority levels may require an effective user id  of  0  or  the
              CAP_NET_ADMIN  capability.   The  priority  can also be set(7,n,1 builtins) in(1,8) a
              protocol independent way by the (SOL_SOCKET, SO_PRIORITY) socket(2,7,n)
              option (see socket(2,7,n)(7)).


       IP_TTL Set  or  retrieve the current time(1,2,n) to live field that is send(2,n) in(1,8)
              every packet send(2,n) from this socket.


       IP_HDRINCL
              If enabled the user supplies an ip(7,8) header in(1,8) front of  the  user
              data.  Only  valid  for  SOCK_RAW  sockets.  See raw(3x,7,8,3x cbreak)(7) for more
              information. When  this  flag  is  enabled  the  values  set(7,n,1 builtins)  by
              IP_OPTIONS, IP_TTL and IP_TOS are ignored.


       IP_RECVERR (defined in(1,8) <linux/errqueue.h>)
              Enable extended reliable error(8,n) message passing.  When enabled on
              a datagram socket(2,7,n) all generated errors will be queued in(1,8) a  per-
              socket(2,7,n)  error(8,n)  queue.  When  the  user  receives an error(8,n) from a
              socket(2,7,n)  operation  the  errors  can  be  received   by   calling
              recvmsg(2) with the MSG_ERRQUEUE flag set. The sock_extended_err
              structure describing the error(8,n) will be  passed  in(1,8)  a  ancillary
              message  with the type IP_RECVERR and the level SOL_IP.  This is
              useful for reliable error(8,n) handling on unconnected sockets.   The
              received  data  portion  of  the  error(8,n) queue(1,3) contains the error(8,n)
              packet.

              The IP_RECVERR  control  message  contains  a  sock_extended_err
              structure:

              #define SO_EE_ORIGIN_NONE       0
              #define SO_EE_ORIGIN_LOCAL      1
              #define SO_EE_ORIGIN_ICMP       2
              #define SO_EE_ORIGIN_ICMP6      3

              struct sock_extended_err {
                  u_int32_t       ee_errno;   /* error(8,n) number */
                  u_int8_t        ee_origin;  /* where the error(8,n) originated */
                  u_int8_t        ee_type;    /* type */
                  u_int8_t        ee_code;    /* code */
                  u_int8_t        ee_pad;
                  u_int32_t       ee_info;    /* additional information */
                  u_int32_t       ee_data;    /* other data */
                  /* More data may follow */
              };

              struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

              ee_errno contains the errno number of the queued error.  ee_ori-
              gin is the origin code of where the error(8,n) originated.  The other
              fields are protocol specific. The macro SO_EE_OFFENDER returns a
              pointer to the address of the network  object  where  the  error(8,n)
              originated  from  given  a pointer to the ancillary message.  If
              this address is not known, the sa_family member of the  sockaddr
              contains  AF_UNSPEC  and  the  other  fields of the sockaddr are
              undefined.

              IP uses the sock_extended_err structure as follows: ee_origin is
              set(7,n,1 builtins)  to SO_EE_ORIGIN_ICMP for errors received as an ICMP packet,
              or SO_EE_ORIGIN_LOCAL for locally generated errors. Unknown val-
              ues  should  be  ignored.   ee_type and ee_code are set(7,n,1 builtins) from the
              type and code fields of the ICMP header.  ee_info  contains  the
              discovered  MTU  for EMSGSIZE errors.  The message also contains
              the sockaddr_in of the node  caused  the  error(8,n),  which  can  be
              accessed  with the SO_EE_OFFENDER macro. The sin_family field of
              the SO_EE_OFFENDER address is  AF_UNSPEC  when  the  source  was
              unknown.   When  the  error(8,n)  originated from the network, all IP
              options (IP_OPTIONS, IP_TTL, etc.) enabled  on  the  socket(2,7,n)  and
              contained  in(1,8)  the  error(8,n) packet are passed as control messages.
              The payload of the packet causing the error(8,n) is returned as  nor-
              mal  payload.  Note that TCP has no error(8,n) queue(1,3); MSG_ERRQUEUE is
              illegal on SOCK_STREAM sockets.  Thus all errors are returned by
              socket(2,7,n) function return or SO_ERROR only.

              For raw(3x,7,8,3x cbreak) sockets, IP_RECVERR enables passing of all received ICMP
              errors to the application, otherwise errors are only reported on
              connected sockets

              It  sets  or  retrieves  an  integer  boolean  flag.  IP_RECVERR
              defaults to off.


       IP_MTU_DISCOVER
              Sets or receives the Path MTU Discovery setting  for  a  socket.
              When  enabled,  Linux will perform Path MTU Discovery as defined
              in(1,8) RFC1191 on this socket. The don't fragment flag is set(7,n,1 builtins) on all
              outgoing  datagrams.   The  system-wide default is controlled by
              the ip_no_pmtu_disc sysctl(2,5,8) for SOCK_STREAM sockets, and disabled
              on  all  others.  For  non  SOCK_STREAM sockets it is the user's
              responsibility to packetize the data in(1,8) MTU sized chunks and  to
              do the retransmits if(3,n) necessary.  The kernel will reject packets
              that are bigger than the known path MTU  if(3,n)  this  flag  is  set(7,n,1 builtins)
              (with EMSGSIZE ).


              Path MTU discovery flags   Meaning
              IP_PMTUDISC_WANT           Use per-route settings.
              IP_PMTUDISC_DONT           Never do Path MTU Discovery.
              IP_PMTUDISC_DO             Always do Path MTU Discovery.


              When  PMTU  discovery  is enabled the kernel automatically keeps
              track of the path MTU per destination host.   When  it  is  con-
              nected  to  a  specific peer with connect(2) the currently known
              path MTU can be retrieved conveniently using the  IP_MTU  socket(2,7,n)
              option  (e.g.  after  a EMSGSIZE error(8,n) occurred).  It may change
              over time.  For connectionless sockets  with  many  destinations
              the  new  also  MTU for a given destination can also be accessed
              using the error(8,n) queue(1,3) (see IP_RECVERR).  A  new  error(8,n)  will  be
              queued for every incoming MTU update.

              While MTU discovery is in(1,8) progress initial packets from datagram
              sockets may be dropped.  Applications using UDP should be  aware
              of this and not take it into account for their packet retransmit
              strategy.

              To bootstrap the path MTU discovery process on unconnected sock-
              ets it is possible to start with a big datagram size (up to 64K-
              headers bytes long) and let it shrink by  updates  of  the  path
              MTU.

              To  get  an  initial estimate of the path MTU connect a datagram
              socket(2,7,n) to the destination address using connect(2) and  retrieve
              the MTU by calling getsockopt(2) with the IP_MTU option.


       IP_MTU Retrieve the current known path MTU of the current socket.  Only
              valid when the socket(2,7,n) has been connected.  Returns  an  integer.
              Only valid as a getsockopt(2).

       IP_ROUTER_ALERT
              Pass all to-be forwarded packets with the IP Router Alert option
              set(7,n,1 builtins) to this socket. Only valid for raw(3x,7,8,3x cbreak) sockets. This is  useful,
              for  instance,  for  user space RSVP daemons. The tapped packets
              are not forwarded by the kernel, it is the users(1,5)  responsibility
              to  send(2,n) them out again. Socket binding is ignored, such packets
              are only filtered by protocol.  Expects an integer flag.

       IP_MULTICAST_TTL
              Set or reads the time-to-live value of outgoing multicast  pack-
              ets  for this socket. It is very important for multicast packets
              to set(7,n,1 builtins) the smallest TTL possible.  The default is 1 which  means
              that  multicast packets don't leave the local network unless the
              user program explicitly requests it. Argument is an integer.

       IP_MULTICAST_LOOP
              Sets or reads a boolean integer argument whether sent  multicast
              packets should be looped back to the local sockets.

       IP_ADD_MEMBERSHIP
              Join a multicast group. Argument is a struct ip_mreqn structure.

              struct ip_mreqn {
                  struct in_addr imr_multiaddr; /* IP multicast group address */
                  struct in_addr imr_address;   /* IP address of local interface */
                  int            imr_ifindex;   /* interface index */
              };

              imr_multiaddr contains the address of the  multicast  group  the
              application  wants  to join(1,n) or leave.  It must be a valid multi-
              cast address.  imr_address is the address of the local interface
              with  which the system should join(1,n) the multicast group; if(3,n) it is
              equal to INADDR_ANY an appropriate interface is  chosen  by  the
              system.   imr_ifindex  is  the  interface index of the interface
              that should join(1,n)/leave the imr_multiaddr group, or 0 to indicate
              any interface.

              For compatibility, the old ip_mreq structure is still supported.
              It differs from ip_mreqn only by not including  the  imr_ifindex
              field. Only valid as a setsockopt(2).

       IP_DROP_MEMBERSHIP
              Leave  a  multicast  group.  Argument  is an ip_mreqn or ip_mreq
              structure similar to IP_ADD_MEMBERSHIP.

       IP_MULTICAST_IF
              Set the local device for a  multicast  socket.  Argument  is  an
              ip_mreqn or ip_mreq structure similar to IP_ADD_MEMBERSHIP.

              When   an  invalid  socket(2,7,n)  option  is  passed,  ENOPROTOOPT  is
              returned.

SYSCTLS
       The IP protocol supports the sysctl(2,5,8) interface to configure some  global
       options.  The  sysctls  can  be  accessed  by  reading  or  writing the
       /proc(5,n)/sys/net/ipv4/* files or using the sysctl(2,5,8)(2) interface.

       ip_default_ttl
              Set the default time-to-live value of outgoing packets. This can
              be changed per socket(2,7,n) with the IP_TTL option.

       ip_forward
              Enable  IP  forwarding with a boolean flag. IP forwarding can be
              also set(7,n,1 builtins) on a per interface basis.

       ip_dynaddr
              Enable dynamic socket(2,7,n) address and masquerading  entry  rewriting
              on interface address change. This is useful for dialup interface
              with changing IP addresses.  0 means no rewriting, 1 turns it on
              and 2 enables verbose mode.

       ip_autoconfig
              Not documented.

       ip_local_port_range
              Contains  two  integers that define the default local port range
              allocated to sockets. Allocation starts with  the  first  number
              and  ends  with  the  second number.  Note that these should not
              conflict with the ports used by masquerading (although the  case
              is  handled). Also arbitary choices may cause problems with some
              firewall packet filters that make assumptions  about  the  local
              ports  in(1,8)  use.   First  number should be at least >1024, better
              >4096 to avoid clashes with well known  ports  and  to  minimize
              firewall problems.

       ip_no_pmtu_disc
              If  enabled,  don't  do  Path  MTU  Discovery for TCP sockets by
              default. Path MTU discovery may fail if(3,n) misconfigured  firewalls
              (that  drop all ICMP packets) or misconfigured interfaces (e.g.,
              a point-to-point link(1,2) where the both ends  don't  agree  on  the
              MTU)  are on the path. It is better to fix the broken routers on
              the path than to turn off Path MTU Discovery  globally,  because
              not doing it incurs a high cost to the network.

       ipfrag_high_thresh, ipfrag_low_thresh
              If the amount of queued IP fragments reaches ipfrag_high_thresh,
              the queue(1,3) is pruned  down  to  ipfrag_low_thresh.   Contains  an
              integer with the number of bytes.

       ip_always_defrag
              [New  with  Kernel 2.2.13; in(1,8) earlier kernel version(1,3,5) the feature
              was controlled at compile time(1,2,n)  by  the  CONFIG_IP_ALWAYS_DEFRAG
              option]

              When  this  boolean frag is enabled (not equal 0) incoming frag-
              ments (parts of IP packets that arose  when  some  host(1,5)  between
              origin  and  destination decided that the packets were too large
              and cut them into pieces)  will  be  reassembled  (defragmented)
              before  being processed, even if(3,n) they are about to be forwarded.

              Only enable if(3,n) running either a firewall that is the  sole  link(1,2)
              to  your network or a transparent proxy; never ever turn on here
              for a normal router or host. Otherwise fragmented  communication
              may  me disturbed when the fragments would travel over different
              links. Defragmentation also has a  large  memory  and  CPU  time(1,2,n)
              cost.

              This is automagically turned on when masquerading or transparent
              proxying are configured.

       neigh/*
              See arp(7,8)(7).

IOCTLS
       All ioctls described in(1,8) socket(2,7,n)(7) apply to ip.

       The ioctls to configure firewalling are documented in(1,8) ipfw(4) from  the
       ipchains package.

       Ioctls  to  configure generic device parameters are described in(1,8) netde-
       vice(7).

NOTES
       Be very careful with the SO_BROADCAST option - it is not privileged  in(1,8)
       Linux. It is easy to overload the network with careless broadcasts. For
       new application protocols it is better to use a multicast group instead
       of broadcasting. Broadcasting is discouraged.

       Some  other  BSD  sockets  implementations  provide  IP_RCVDSTADDR  and
       IP_RECVIF socket(2,7,n) options to get the destination address and the  inter-
       face  of  received datagrams. Linux has the more general IP_PKTINFO for
       the same task.


ERRORS
       ENOTCONN
              The operation is only defined on a  connected  socket(2,7,n),  but  the
              socket(2,7,n) wasn't connected.

       EINVAL Invalid argument passed.  For send(2,n) operations this can be caused
              by sending to a blackhole route.

       EMSGSIZE
              Datagram is bigger than an MTU on the  path  and  it  cannot  be
              fragmented.

       EACCES The  user  tried  to  execute an operation without the necessary
              permissions.  These include: Sending a  packet  to  a  broadcast
              address  without  having  the  SO_BROADCAST flag set.  Sending a
              packet via a prohibit route.  Modifying firewall settings  with-
              out CAP_NET_ADMIN or effective user id 0.  Binding to a reserved
              port without the CAP_NET_BIND_SERVICE capacibility or  effective
              user id 0.


       EADDRINUSE
              Tried to bind(2,n,1 builtins) to an address already in(1,8) use.

       ENOPROTOOPT and EOPNOTSUPP
              Invalid socket(2,7,n) option passed.

       EPERM  User  doesn't  have permission to set(7,n,1 builtins) high priority, change con-
              figuration, or send(2,n) signals to the requested process or group.

       EADDRNOTAVAIL
              A non-existent interface was requested or the  requested  source
              address was not local.

       EAGAIN Operation on a non-blocking socket(2,7,n) would block.

       ESOCKTNOSUPPORT
              The  socket(2,7,n)  is  not  configured  or  an unknown socket(2,7,n) type was
              requested.

       EISCONN
              connect(2) was called on an already connected socket.

       EALREADY
              An connection operation on a non-blocking socket(2,7,n) is  already  in(1,8)
              progress.

       ECONNABORTED
              A connection was closed during an accept(2,8)(2).

       EPIPE  The connection was unexpectedly closed or shut down by the other
              end.

       ENOENT SIOCGSTAMP was called on a socket(2,7,n) where no packet arrived.

       EHOSTUNREACH
              No valid routing table entry matches  the  destination  address.
              This  error(8,n) can be caused by a ICMP message from a remote router
              or for the local routing table.

       ENODEV Network device not available or not capable of sending IP.

       ENOPKG A kernel subsystem was not configured.

       ENOBUFS, ENOMEM
              Not enough free memory.  This often means that the memory  allo-
              cation is limited by the socket(2,7,n) buffer limits, not by the system
              memory, but this is not 100% consistent.

       Other errors may be generated by the overlaying protocols; see  tcp(7),
       raw(3x,7,8,3x cbreak)(7), udp(7) and socket(2,7,n)(7).

VERSIONS
       IP_PKTINFO,   IP_MTU,   IP_MTU_DISCOVER,   IP_PKTINFO,  IP_RECVERR  and
       IP_ROUTER_ALERT are new options in(1,8) Linux 2.2.  They are also all  Linux
       specific and should not be used in(1,8) programs intended to be portable.

       struct ip_mreqn is new in(1,8) Linux 2.2.  Linux 2.0 only supported ip_mreq.

       The sysctls were introduced with Linux 2.2.

COMPATIBILITY
       For  compatibility  with  Linux  2.0,  the   obsolete   socket(2,7,n)(PF_INET,
       SOCK_RAW,  protocol)  syntax  is  still  supported  to open(2,3,n) a packet(7)
       socket. This is deprecated and should be replaced by  socket(2,7,n)(PF_PACKET,
       SOCK_RAW, protocol) instead. The main difference is the new sockaddr_ll
       address structure for generic link(1,2) layer information instead of the old
       sockaddr_pkt.

BUGS
       There are too many inconsistent error(8,n) values.

       The  ioctls  to  configure IP-specific interface options and ARP tables
       are not described.

       Some versions of glibc forget to declare in_pktinfo.   Workaround  cur-
       rently is to copy it into your program from this man(1,5,7) page.

       Receiving   the  original  destination  address  with  MSG_ERRQUEUE  in(1,8)
       msg_name by recvmsg(2) does not work in(1,8) some 2.2 kernels.

SEE ALSO
       recvmsg(2), sendmsg(2), ipfw(4), capabilities(7),  netlink(3,7)(7),  raw(3x,7,8,3x cbreak)(7),
       socket(2,7,n)(7), tcp(7), udp(7)

       RFC791 for the original IP specification.
       RFC1122 for the IPv4 host(1,5) requirements.
       RFC1812 for the IPv4 router requirements.




Linux Man Page                    2001-06-19                             IP(7)

References for this manual (incoming links)