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NAME    [Toc]    [Back]

       udp - User Datagram Protocol for IPv4

SYNOPSIS    [Toc]    [Back]

       #include <sys/socket.h>
       #include <netinet/in.h>
       udp_socket = socket(PF_INET, SOCK_DGRAM, 0);

DESCRIPTION    [Toc]    [Back]

       This  is  an  implemention  of  the User Datagram Protocol described in
       RFC768. It implements a connectionless, unreliable datagram packet service.
   Packets	may be reordered or duplicated before they arrive. UDP
       generates and checks checksums to catch transmission errors.

       When a UDP socket is  created,  its  local  and	remote	addresses  are
       unspecified.   Datagrams  can  be  sent	immediately using sendto(2) or
       sendmsg(2) with a valid destination address as an argument.  When  con-
       nect(2)	is called on the socket the default destination address is set
       and datagrams can now be sent using send(2) or write(2) without	specifying
  an  destination  address.  It is still possible to send to other
       destinations by passing an address  to  sendto(2)  or  sendmsg(2).   In
       order  to  receive  packets the socket can be bound to an local address
       first by using bind(2).	Otherwise the socket layer will  automatically
       assign	a   free   local   port   out	of   the   range   defined  by
       net.ipv4.ip_local_port_range and bind the socket to INADDR_ANY.

       All receive operations return only one  packet.	 When  the  packet  is
       smaller than the passed buffer only that much data is returned, when it
       is bigger the packet is	truncated  and	the  MSG_TRUNC	flag  is  set.
       MSG_WAITALL is not supported.

       IP  options  may be sent or received using the socket options described
       in ip(7).  They are only processed by the kernel when  the  appropriate
       sysctl  is enabled (but still passed to the user even when it is turned
       off). See ip(7).

       When the MSG_DONTROUTE flag is set on sending the  destination  address
       must refer to an local interface address and the packet is only sent to
       that interface.

       UDP fragments a packet when its total length exceeds the interface  MTU
       (Maximum Transmission Unit).  A more network friendly alternative is to
       use path MTU discovery as described in the IP_PMTU_DISCOVER section  of

ADDRESS FORMAT    [Toc]    [Back]

       UDP uses the IPv4 sockaddr_in address format described in ip(7).

ERROR HANDLING    [Toc]    [Back]

       All  fatal  errors  will  be passed to the user as an error return even
       when the socket is not connected.  This	includes  asynchronous	errors
       received  from  the network. You may get an error for an earlier packet
       that was sent on the same socket.  This	behaviour  differs  from  many
       other BSD socket implementations which don't pass any errors unless the
       socket is connected.  Linux's behaviour is mandated by RFC1122.

       For compatibility with legacy code it is possible to set the SO_BSDCOM-
       PAT SOL_SOCKET option to receive remote errors only when the socket has
       been connected (except for EPROTO and EMSGSIZE).  It is better  to  fix
       the code to handle errors properly than to enable this option.  Locally
       generated errors are always passed.

       When the IP_RECVERR option is enabled all  errors  are  stored  in  the
       socket  error  queue  and  can  be  received  by  recvmsg(2)  with  the
       MSG_ERRQUEUE flag set.

IOCTLS    [Toc]    [Back]

       These ioctls can be accessed using ioctl(2).  The correct syntax is:

	      int value;
	      error = ioctl(tcp_socket, ioctl_type, &value);

       SIOCINQ    [Toc]    [Back]
	      Gets a pointer to an integer as argument. Returns  the  size  of
	      the  next pending datagram in the integer in bytes, or 0 when no
	      datagram is pending.

       SIOCOUTQ    [Toc]    [Back]
	      Returns the number of data bytes in the local send  queue.  Only
	      supported with Linux 2.4 and above.

       In addition all ioctls documented in ip(7) and socket(7) are supported.

ERRORS    [Toc]    [Back]

       All errors documented for socket(7) or ip(7) may be returned by a  send
       or receive on a UDP socket.

       ECONNREFUSED  No  receiver was associated with the destination address.
       This might be caused by a previous packet sent over the socket.

VERSIONS    [Toc]    [Back]

       IP_RECVERR is a new feature in Linux 2.2

CREDITS    [Toc]    [Back]

       This man page was written by Andi Kleen.

SEE ALSO    [Toc]    [Back]

       ip(7), socket(7), raw(7)

       RFC768 for the User Datagram protocol.
       RFC1122 for the host requirements.
       RFC1191 for a description of path MTU discovery.

Linux Man Page			  1998-10-02				UDP(7)
[ Back ]
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