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


     ip - Linux IPv4 protocol implementation


     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <netinet/ip.h> /* superset of previous */
     tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
     udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
     raw_socket = socket(AF_INET, SOCK_RAW, protocol);


     Linux implements the Internet Protocol, version 4, described in RFC 791
     and RFC 1122.  ip contains a level 2 multicasting  implementation  con-
     forming  to RFC 1112.  It also contains an IP router including a packet
     The programming interface is BSD-sockets compatible.  For more informa-
     tion on sockets, see socket(7).
     An IP socket is created using socket(2):
         socket(AF_INET, socket_type, protocol);
     Valid  socket types are SOCK_STREAM to open a tcp(7) socket, SOCK_DGRAM
     to open a udp(7) socket, or SOCK_RAW to open a raw(7) socket to  access
     the IP protocol directly.  protocol is the IP protocol in 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
     RFC 1700 assigned numbers.
     When a process wants to receive new incoming packets or connections, it
     should bind a socket to a local interface address  using  bind(2).   In
     this case, only one IP socket may be bound to any given local (address,
     port) pair.  When INADDR_ANY is specified in the bind call, the  socket
     will  be bound to all local interfaces.  When listen(2) is called on an
     unbound socket, the socket is automatically bound to a random free port
     with the local address set to INADDR_ANY.  When connect(2) is called on
     an unbound socket, the socket is automatically bound to a  random  free
     port  or  to  a  usable  shared  port  with  the  local  address set to
     A TCP local socket address that has been bound is unavailable for  some
     time  after  closing,  unless the SO_REUSEADDR flag has been set.  Care
     should be taken when using this flag as it makes TCP less reliable.
 Address format
     An IP socket address is defined as a combination  of  an  IP  interface
     address  and a 16-bit port number.  The basic IP protocol does not sup-
     ply port numbers, they are implemented by higher level  protocols  like
     udp(7)  and tcp(7).  On raw sockets sin_port is set to the IP protocol.
         struct sockaddr_in {
             sa_family_t    sin_family; /* address family: AF_INET */
             in_port_t      sin_port;   /* port in network byte order */
             struct in_addr sin_addr;   /* internet address */ };
         /* Internet address. */ struct in_addr {
             uint32_t       s_addr;     /* address in network byte order  */
     sin_family  is  always  set to AF_INET.  This is required; in Linux 2.2
     most networking functions return EINVAL when this setting  is  missing.
     sin_port  contains  the  port  in network byte order.  The port numbers
     below 1024 are called privileged ports (or sometimes: reserved  ports).
     Only   a   privileged  process  (on  Linux:  a  process  that  has  the
     CAP_NET_BIND_SERVICE capability in the  user  namespace  governing  its
     network  namespace)  may  bind(2)  to these sockets.  Note that the raw
     IPv4 protocol as such has no concept of a port,  they  are  implemented
     only by higher protocols like tcp(7) and udp(7).
     sin_addr  is  the IP host address.  The s_addr member of struct in_addr
     contains the host interface address in  network  byte  order.   in_addr
     should  be  assigned one of the INADDR_* values (e.g., INADDR_LOOPBACK)
     using  htonl(3)  or   set   using   the   inet_aton(3),   inet_addr(3),
     inet_makeaddr(3)  library  functions or directly with the name resolver
     (see gethostbyname(3)).
     IPv4 addresses are  divided  into  unicast,  broadcast,  and  multicast
     addresses.   Unicast  addresses  specify  a single interface of a host,
     broadcast addresses specify all  hosts  on  a  network,  and  multicast
     addresses  address all hosts in a multicast group.  Datagrams to broad-
     cast addresses can be sent  or  received  only  when  the  SO_BROADCAST
     socket flag is set.  In the current implementation, connection-oriented
     sockets are allowed to use only unicast addresses.
     Note that the address and the port are always stored  in  network  byte
     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 func-
     tions in the standard library work in network byte order.
     There are several special addresses: INADDR_LOOPBACK ( always
     refers to the local host via the loopback device; INADDR_ANY  (
     means any address for binding; INADDR_BROADCAST ( means
     any host and has the same effect on bind as INADDR_ANY  for  historical
 Socket options
     IP  supports some protocol-specific socket options that can be set with
     setsockopt(2) and read with getsockopt(2).  The socket option level for
     IP  is  IPPROTO_IP.   A  boolean integer flag is zero when it is false,
     otherwise true.
     When an invalid socket option is specified, getsockopt(2) and  setsock-
     opt(2) fail with the error ENOPROTOOPT.
     IP_ADD_MEMBERSHIP (since Linux 1.2)
            Join a multicast group.  Argument is an 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 applica-
     tion wants to join or leave.  It must be a valid multicast address  (or
     setsockopt(2) fails with the error EINVAL).  imr_address is the address
     of the local interface with which the system should join the  multicast
     group; if 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/leave  the imr_multiaddr group, or 0 to indicate any
            The ip_mreqn structure is available only since Linux  2.2.   For
            compatibility,  the  old  ip_mreq structure (present since Linux
            1.2) is still supported; it differs from ip_mreqn  only  by  not
            including  the  imr_ifindex field.  (The kernel determines which
            structure is being passed based on the size passed in optlen.)
            IP_ADD_MEMBERSHIP is valid only for setsockopt(2).
     IP_ADD_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)
            Join a multicast group and allow  receiving  data  only  from  a
            specified source.  Argument is an ip_mreq_source structure.
         struct ip_mreq_source {
             struct in_addr imr_multiaddr;  /* IP multicast group
                                               address */
             struct in_addr imr_interface;  /* IP address of local
                                               interface */
             struct in_addr imr_sourceaddr; /* IP address of
                                               multicast source */ };
     The  ip_mreq_source  structure  is  similar to ip_mreqn described under
     IP_ADD_MEMBERSIP.  The imr_multiaddr field contains the address of  the
     multicast group the application wants to join or leave.  The imr_inter-
     face field is the address of the local interface with which the  system
     should  join  the  multicast  group.  Finally, the imr_sourceaddr field
     contains the address of the source the  application  wants  to  receive
     data from.
            This  option  can be used multiple times to allow receiving data
            from more than one source.
     IP_BIND_ADDRESS_NO_PORT (since Linux 4.2)
            Inform the kernel to not reserve an ephemeral  port  when  using
            bind(2)  with  a port number of 0.  The port will later be auto-
            matically chosen at connect(2) time, in a way that allows  shar-
            ing a source port as long as the 4-tuple is unique.
     IP_BLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
            Stop  receiving multicast data from a specific source in a given
            group.  This is valid only after the application has  subscribed
            to   the  multicast  group  using  either  IP_ADD_MEMBERSHIP  or
            Argument is  an  ip_mreq_source  structure  as  described  under
     IP_DROP_MEMBERSHIP (since Linux 1.2)
            Leave  a  multicast  group.   Argument is an ip_mreqn or ip_mreq
            structure similar to IP_ADD_MEMBERSHIP.
     IP_DROP_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)
            Leave a source-specific group--that is, stop receiving data from
            a  given  multicast group that come from a given source.  If the
            application has subscribed to multiple sources within  the  same
            group,  data from the remaining sources will still be delivered.
            To  stop  receiving  data  from  all  sources   at   once,   use
            Argument  is  an  ip_mreq_source  structure  as  described under
     IP_FREEBIND (since Linux 2.4)
            If enabled, this boolean option allows binding to an IP  address
            that  is nonlocal or does not (yet) exist.  This permits listen-
            ing on a socket, without requiring the underlying network inter-
            face  or  the  specified dynamic IP address to be up at the time
            that the application is trying to bind to it.   This  option  is
            the  per-socket  equivalent of the ip_nonlocal_bind /proc inter-
            face described below.
     IP_HDRINCL (since Linux 2.0)
            If enabled, the user supplies an IP header in front of the  user
            data.   Valid  only  for  SOCK_RAW  sockets; see raw(7) for more
            information.  When this flag  is  enabled,  the  values  set  by
            IP_OPTIONS, IP_TTL, and IP_TOS are ignored.
     IP_MSFILTER (since Linux 2.4.22 / 2.5.68)
            This option provides access to the advanced full-state filtering
            API.  Argument is an ip_msfilter structure.
         struct ip_msfilter {
             struct in_addr imsf_multiaddr; /* IP multicast group
                                               address */
             struct in_addr imsf_interface; /* IP address of local
                                               interface */
             uint32_t       imsf_fmode;     /* Filter-mode */
             uint32_t       imsf_numsrc;    /* Number of sources in
                                               the following array */
             struct in_addr imsf_slist[1];  /* Array of source
                                               addresses */ };
     There are two macros, MCAST_INCLUDE and  MCAST_EXCLUDE,  which  can  be
     used  to  specify  the  filtering  mode.   Additionally,  the IP_MSFIL-
     TER_SIZE(n) macro exists to determine how  much  memory  is  needed  to
     store ip_msfilter structure with n sources in the source list.
            For  the full description of multicast source filtering refer to
            RFC 3376.
     IP_MTU (since Linux 2.2)
            Retrieve the current known  path  MTU  of  the  current  socket.
            Returns an integer.
            IP_MTU  is valid only for getsockopt(2) and can be employed only
            when the socket has been connected.
     IP_MTU_DISCOVER (since Linux 2.2)
            Set or receive the Path MTU  Discovery  setting  for  a  socket.
            When  enabled,  Linux will perform Path MTU Discovery as defined
            in RFC 1191 on SOCK_STREAM sockets.  For  non-SOCK_STREAM  sock-
            ets,  IP_PMTUDISC_DO forces the don't-fragment flag to be set on
            all outgoing packets.  It is the user's responsibility to packe-
            tize  the  data in MTU-sized chunks and to do the retransmits if
            necessary.  The kernel will  reject  (with  EMSGSIZE)  datagrams
            that  are bigger than the known path MTU.  IP_PMTUDISC_WANT will
            fragment a datagram if needed according to the path MTU, or will
            set the don't-fragment flag otherwise.
            The  system-wide default can be toggled between IP_PMTUDISC_WANT
            and IP_PMTUDISC_DONT by writing (respectively, zero and  nonzero
            values) to the /proc/sys/net/ipv4/ip_no_pmtu_disc file.
            Path MTU discovery value   Meaning
            IP_PMTUDISC_WANT           Use per-route settings.
            IP_PMTUDISC_DONT           Never do Path MTU Discovery.
            IP_PMTUDISC_DO             Always do Path MTU Discovery.
            IP_PMTUDISC_PROBE          Set DF but ignore Path MTU.
            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
            option  (e.g.,  after an EMSGSIZE error occurred).  The path MTU
            may change over time.  For connectionless sockets with many des-
            tinations,  the  new  MTU  for  a  given destination can also be
            accessed using the error queue (see IP_RECVERR).   A  new  error
            will be queued for every incoming MTU update.
            While  MTU  discovery is in progress, initial packets from data-
            gram 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 (headers
            up to 64 kilobytes long) and let it shrink  by  updates  of  the
            path MTU.
            To  get  an initial estimate of the path MTU, connect a datagram
            socket to the destination address using connect(2) and  retrieve
            the MTU by calling getsockopt(2) with the IP_MTU option.
            It is possible to implement RFC 4821 MTU probing with SOCK_DGRAM
            or SOCK_RAW sockets by  setting  a  value  of  IP_PMTUDISC_PROBE
            (available  since Linux 2.6.22).  This is also particularly use-
            ful for diagnostic tools  such  as  tracepath(8)  that  wish  to
            deliberately  send  probe  packets larger than the observed Path
     IP_MULTICAST_ALL (since Linux 2.6.31)
            This option can be used to modify the delivery policy of  multi-
            cast  messages  to  sockets  bound  to  the  wildcard INADDR_ANY
            address.  The argument is a boolean integer (defaults to 1).  If
            set  to  1, the socket will receive messages from all the groups
            that have been joined globally on the whole system.   Otherwise,
            it  will  deliver  messages  only from the groups that have been
            explicitly joined (for example via the IP_ADD_MEMBERSHIP option)
            on this particular socket.
     IP_MULTICAST_IF (since Linux 1.2)
            Set  the  local device for a multicast socket.  The argument for
            setsockopt(2) is an ip_mreqn or (since Linux 3.5) ip_mreq struc-
            ture  similar  to  IP_ADD_MEMBERSHIP,  or  an in_addr structure.
            (The kernel determines which structure is being passed based  on
            the  size passed in optlen.)  For getsockopt(2), the argument is
            an in_addr structure.
     IP_MULTICAST_LOOP (since Linux 1.2)
            Set or read a boolean integer argument that  determines  whether
            sent  multicast packets should be looped back to the local sock-
     IP_MULTICAST_TTL (since Linux 1.2)
            Set or read the time-to-live value of outgoing multicast packets
            for  this socket.  It is very important for multicast packets to
            set 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_NODEFRAG (since Linux 2.6.36)
            If enabled (argument is nonzero),  the  reassembly  of  outgoing
            packets  is disabled in the netfilter layer.  The argument is an
            This option is valid only for SOCK_RAW sockets.
     IP_OPTIONS (since Linux 2.0)
            Set or get the IP options to be sent with every packet from this
            socket.  The arguments are a pointer to a memory buffer contain-
            ing 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 RFC 791 for  the  allowed
            options.   When  the  initial  connection  request  packet for a
            SOCK_STREAM socket contains IP options, the IP options  will  be
            set  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
            /proc interface.  Other options like timestamps are  still  han-
            dled.   For  datagram sockets, IP options can be only set by the
            local user.  Calling getsockopt(2) with IP_OPTIONS puts the cur-
            rent IP options used for sending into the supplied buffer.
     IP_PKTINFO (since Linux 2.2)
            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 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 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_RECVERR (since Linux 2.2)
            Enable extended reliable error message passing.  When enabled on
            a datagram socket, all generated errors will be queued in a per-
            socket  error  queue.   When  the  user receives an error from a
            socket  operation,  the  errors  can  be  received  by   calling
            recvmsg(2)    with    the    MSG_ERRQUEUE    flag    set.    The
            sock_extended_err structure describing the error will be  passed
            in  an  ancillary message with the type IP_RECVERR and the level
            IPPROTO_IP.  This is  useful  for  reliable  error  handling  on
            unconnected  sockets.   The  received  data portion of the error
            queue contains the error packet.
            The IP_RECVERR  control  message  contains  a  sock_extended_err
                #define  SO_EE_ORIGIN_NONE     0  #define SO_EE_ORIGIN_LOCAL
                1 #define SO_EE_ORIGIN_ICMP    2 #define  SO_EE_ORIGIN_ICMP6
                struct sock_extended_err {
                    uint32_t ee_errno;   /* error number */
                    uint8_t  ee_origin;  /* where the error originated */
                    uint8_t  ee_type;    /* type */
                    uint8_t  ee_code;    /* code */
                    uint8_t  ee_pad;
                    uint32_t ee_info;    /* additional information */
                    uint32_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 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
            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
            IP uses the sock_extended_err structure as follows: ee_origin is
            set to SO_EE_ORIGIN_ICMP for errors received as an ICMP  packet,
            or  SO_EE_ORIGIN_LOCAL  for  locally  generated errors.  Unknown
            values should be ignored.  ee_type and ee_code are set 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, 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 originated from  the  network,  all  IP
            options  (IP_OPTIONS,  IP_TTL,  etc.)  enabled on the socket and
            contained in the error packet are passed  as  control  messages.
            The  payload of the packet causing the error is returned as nor-
            mal payload.  Note that TCP has no error queue; MSG_ERRQUEUE  is
            not  permitted  on SOCK_STREAM sockets.  IP_RECVERR is valid for
            TCP, but all errors are returned by socket  function  return  or
            SO_ERROR only.
            For raw 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_RECVOPTS (since Linux 2.2)
            Pass all incoming IP options to the user in a IP_OPTIONS control
            message.   The  routing  header  and  other  options are already
            filled in for the local host.   Not  supported  for  SOCK_STREAM
     IP_RECVORIGDSTADDR (since Linux 2.6.29)
            This boolean option enables the IP_ORIGDSTADDR ancillary message
            in recvmsg(2), in which the kernel returns the original destina-
            tion address of the datagram being received.  The ancillary mes-
            sage contains a struct sockaddr_in.
     IP_RECVTOS (since Linux 2.2)
            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 (since Linux 2.2)
            When  this  flag  is set, pass a IP_TTL control message with the
            time-to-live field of the received packet as a byte.   Not  sup-
            ported for SOCK_STREAM sockets.
     IP_RETOPTS (since Linux 2.2)
            Identical  to  IP_RECVOPTS,  but returns raw unprocessed options
            with timestamp and route record options not filled in  for  this
     IP_ROUTER_ALERT (since Linux 2.2)
            Pass all to-be forwarded packets with the IP Router Alert option
            set to this socket.  Valid only for raw sockets.  This  is  use-
            ful,  for  instance,  for  user-space  RSVP daemons.  The tapped
            packets are not forwarded  by  the  kernel;  it  is  the  user's
            responsibility  to  send  them  out  again.   Socket  binding is
            ignored, such packets are only filtered by protocol.  Expects an
            integer flag.
     IP_TOS (since Linux 1.0)
            Set or receive the Type-Of-Service (TOS) field that is sent with
            every IP packet originating from this socket.   It  is  used  to
            prioritize  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
            throughput,  IPTOS_RELIABILITY  to  optimize  for   reliability,
            IPTOS_MINCOST should be used for "filler data" where slow trans-
            mission doesn't matter.  At most one of these TOS values can  be
            specified.   Other bits are invalid and shall be cleared.  Linux
            sends IPTOS_LOWDELAY datagrams first by default, but  the  exact
            behavior  depends  on  the configured queueing discipline.  Some
            high-priority  levels  may  require  superuser  privileges  (the
            CAP_NET_ADMIN capability).
     IP_TRANSPARENT (since Linux 2.6.24)
            Setting this boolean option enables transparent proxying on this
            socket.  This socket option allows the  calling  application  to
            bind to a nonlocal IP address and operate both as a client and a
            server with the foreign address as the  local  endpoint.   NOTE:
            this requires that routing be set up in a way that packets going
            to the foreign address are routed through the TProxy box  (i.e.,
            the system hosting the application that employs the IP_TRANSPAR-
            ENT socket option).  Enabling this socket option requires  supe-
            ruser privileges (the CAP_NET_ADMIN capability).
            TProxy redirection with the iptables TPROXY target also requires
            that this option be set on the redirected socket.
     IP_TTL (since Linux 1.0)
            Set or retrieve the current time-to-live field that is  used  in
            every packet sent from this socket.
     IP_UNBLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
            Unblock  previously  blocked multicast source.  Returns EADDRNO-
            TAVAIL when given source is not being blocked.
            Argument is  an  ip_mreq_source  structure  as  described  under
 /proc interfaces
     The  IP  protocol  supports a set of /proc interfaces to configure some
     global parameters.  The parameters can be accessed by reading or  writ-
     ing  files  in the directory /proc/sys/net/ipv4/.  Interfaces described
     as Boolean take an integer value, with a nonzero value ("true") meaning
     that  the  corresponding  option is enabled, and a zero value ("false")
     meaning that the option is disabled.
     ip_always_defrag (Boolean; since Linux 2.2.13)
            [New with kernel 2.2.13; in earlier kernel versions this feature
            was  controlled  at  compile time by the CONFIG_IP_ALWAYS_DEFRAG
            option; this option is not present in 2.4.x and later]
            When this boolean flag is enabled (not equal 0), incoming  frag-
            ments  (parts  of  IP  packets that arose when some host 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 they are about to be  forwarded.
            Enable  only  if running either a firewall that is the sole link
            to your network or a transparent proxy; never ever use it for  a
            normal  router or host.  Otherwise, fragmented communication can
            be disturbed if  the  fragments  travel  over  different  links.
            Defragmentation also has a large memory and CPU time cost.
            This is automagically turned on when masquerading or transparent
            proxying are configured.
     ip_autoconfig (since Linux 2.2 to 2.6.17)
            Not documented.
     ip_default_ttl (integer; default: 64; since Linux 2.2)
            Set the default time-to-live value of  outgoing  packets.   This
            can be changed per socket with the IP_TTL option.
     ip_dynaddr (Boolean; default: disabled; since Linux 2.0.31)
            Enable  dynamic  socket address and masquerading entry rewriting
            on interface address change.  This is useful for  dialup  inter-
            face  with changing IP addresses.  0 means no rewriting, 1 turns
            it on and 2 enables verbose mode.
     ip_forward (Boolean; default: disabled; since Linux 1.2)
            Enable IP forwarding with a boolean flag.  IP forwarding can  be
            also set on a per-interface basis.
     ip_local_port_range (since Linux 2.2)
            This  file  contains  two integers that define the default local
            port range allocated to sockets that are not explicitly bound to
            a  port number--that is, the range used for ephemeral ports.  An
            ephemeral port is allocated to a socket in the following circum-
  • the port number in a socket address is specified as 0 when

calling bind(2);

  • listen(2) is called on a stream socket that was not previ-

ously bound;

  • connect(2) was called on a socket that was not previously


  • sendto(2) is called on a datagram socket that was not previ-

ously bound.

            Allocation  of  ephemeral  ports starts with the first number in
            ip_local_port_range and ends with the  second  number.   If  the
            range  of ephemeral ports is exhausted, then the relevant system
            call returns an error (but see BUGS).
            Note that the port range in ip_local_port_range should not  con-
            flict  with the ports used by masquerading (although the case is
            handled).  Also, arbitrary choices may cause problems with  some
            firewall  packet  filters  that make assumptions about the local
            ports in use.  The first number should be at least greater  than
            1024,  or  better, greater than 4096, to avoid clashes with well
            known ports and to minimize firewall problems.
     ip_no_pmtu_disc (Boolean; default: disabled; since Linux 2.2)
            If enabled, don't do Path  MTU  Discovery  for  TCP  sockets  by
            default.  Path MTU discovery may fail if misconfigured firewalls
            (that drop all ICMP packets) or misconfigured interfaces  (e.g.,
            a  point-to-point  link  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.
     ip_nonlocal_bind (Boolean; default: disabled; since Linux 2.4)
            If set, allows processes to bind(2) to  nonlocal  IP  addresses,
            which can be quite useful, but may break some applications.
     ip6frag_time (integer; default: 30)
            Time in seconds to keep an IPv6 fragment in memory.
     ip6frag_secret_interval (integer; default: 600)
            Regeneration  interval (in seconds) of the hash secret (or life-
            time for the hash secret) for IPv6 fragments.
     ipfrag_high_thresh (integer), ipfrag_low_thresh (integer)
            If the amount of queued IP fragments reaches ipfrag_high_thresh,
            the  queue  is  pruned  down  to ipfrag_low_thresh.  Contains an
            integer with the number of bytes.
            See arp(7).
     All ioctls described in socket(7) apply to ip.
     Ioctls to configure generic device parameters are described  in  netde-


     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 without
            superuser  privileges (the CAP_NET_ADMIN capability); binding to
            a   privileged   port   without   superuser   privileges    (the
            CAP_NET_BIND_SERVICE capability).
            Tried to bind to an address already in use.
            A  nonexistent  interface  was requested or the requested source
            address was not local.
     EAGAIN Operation on a nonblocking socket would block.
            A connection operation on a nonblocking  socket  is  already  in
            A connection was closed during an accept(2).
            No  valid  routing  table entry matches the destination address.
            This error can be caused by an ICMP message from a remote router
            or for the local routing table.
     EINVAL Invalid argument passed.  For send operations this can be caused
            by sending to a blackhole route.
            connect(2) was called on an already connected socket.
            Datagram is bigger than an MTU on the  path  and  it  cannot  be
            Not  enough free memory.  This often means that the memory allo-
            cation is limited by the socket buffer limits, not by the system
            memory, but this is not 100% consistent.
     ENOENT SIOCGSTAMP was called on a socket where no packet arrived.
     ENOPKG A kernel subsystem was not configured.
            Invalid socket option passed.
            The  operation  is  defined  only on a connected socket, but the
            socket wasn't connected.
     EPERM  User doesn't have permission to set high priority,  change  con-
            figuration, or send signals to the requested process or group.
     EPIPE  The connection was unexpectedly closed or shut down by the other
            The socket is not configured  or  an  unknown  socket  type  was
     Other  errors may be generated by the overlaying protocols; see tcp(7),
     raw(7), udp(7), and socket(7).


     Be very careful with the SO_BROADCAST option - it is not privileged  in
     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 options to get the destination address and the  inter-
     face  of received datagrams.  Linux has the more general IP_PKTINFO for
     the same task.
     Some BSD sockets implementations also provide an IP_RECVTTL option, but
     an  ancillary  message with type IP_RECVTTL is passed with the incoming
     packet.  This is different from the IP_TTL option used in Linux.
     Using the SOL_IP socket options level isn't portable; BSD-based  stacks
     use the IPPROTO_IP level.
     INADDR_ANY  (  and INADDR_BROADCAST ( are byte-
      This means htonl(3) has no effect on them.
     For  compatibility  with  Linux  2.0,  the   obsolete   socket(AF_INET,
     SOCK_PACKET,  protocol)  syntax  is still supported to open a packet(7)
     socket.  This is deprecated and should be replaced by socket(AF_PACKET,
     SOCK_RAW,  protocol)  instead.   The  main  difference is the new sock-
     addr_ll address structure for generic link layer information instead of
     the old sockaddr_pkt.


     There are too many inconsistent error values.
     The  error used to diagnose exhaustion of the ephemeral port range dif-
     fers across the various system calls (connect(2),  bind(2),  listen(2),
     sendto(2)) that can assign ephemeral ports.
     The  ioctls  to  configure IP-specific interface options and ARP tables
     are not described.
     Receiving  the  original  destination  address  with  MSG_ERRQUEUE   in
     msg_name by recvmsg(2) does not work in some 2.2 kernels.


     recvmsg(2),   sendmsg(2),   byteorder(3),   ipfw(4),   capabilities(7),
     icmp(7), ipv6(7), netlink(7), raw(7), socket(7), tcp(7), udp(7), ip(8)
     RFC 791 for the original IP specification.  RFC 1122 for the IPv4  host
     requirements.  RFC 1812 for the IPv4 router requirements.


     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at

Linux 2018-02-02 IP(7)

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