mirror of https://github.com/mkerrisk/man-pages
266 lines
7.5 KiB
Groff
266 lines
7.5 KiB
Groff
.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
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.\"
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.\" %%%LICENSE_START(VERBATIM_ONE_PARA)
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.\" Permission is granted to distribute possibly modified copies
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.\" of this page provided the header is included verbatim,
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.\" and in case of nontrivial modification author and date
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.\" of the modification is added to the header.
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.\" %%%LICENSE_END
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.\"
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.\" $Id: udp.7,v 1.7 2000/01/22 01:55:05 freitag Exp $
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.\"
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.TH UDP 7 2013-07-31 "Linux" "Linux Programmer's Manual"
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.SH NAME
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udp \- User Datagram Protocol for IPv4
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.SH SYNOPSIS
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.B #include <sys/socket.h>
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.br
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.B #include <netinet/in.h>
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.br
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.B #include <netinet/udp.h>
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.sp
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.B udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
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.SH DESCRIPTION
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This is an implementation of the User Datagram Protocol
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described in RFC\ 768.
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It implements a connectionless, unreliable datagram packet service.
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Packets may be reordered or duplicated before they arrive.
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UDP generates and checks checksums to catch transmission errors.
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When a UDP socket is created,
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its local and remote addresses are unspecified.
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Datagrams can be sent immediately using
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.BR sendto (2)
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or
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.BR sendmsg (2)
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with a valid destination address as an argument.
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When
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.BR connect (2)
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is called on the socket, the default destination address is set and
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datagrams can now be sent using
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.BR send (2)
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or
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.BR write (2)
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without specifying a destination address.
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It is still possible to send to other destinations by passing an
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address to
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.BR sendto (2)
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or
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.BR sendmsg (2).
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In order to receive packets, the socket can be bound to a local
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address first by using
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.BR bind (2).
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Otherwise, the socket layer will automatically assign
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a free local port out of the range defined by
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.I /proc/sys/net/ipv4/ip_local_port_range
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and bind the socket to
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.BR INADDR_ANY .
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All receive operations return only one packet.
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When the packet is smaller than the passed buffer, only that much
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data is returned; when it is bigger, the packet is truncated and the
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.B MSG_TRUNC
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flag is set.
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.B MSG_WAITALL
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is not supported.
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IP options may be sent or received using the socket options described in
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.BR ip (7).
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They are processed by the kernel only when the appropriate
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.I /proc
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parameter
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is enabled (but still passed to the user even when it is turned off).
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See
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.BR ip (7).
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When the
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.B MSG_DONTROUTE
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flag is set on sending, the destination address must refer to a local
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interface address and the packet is sent only to that interface.
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By default, Linux UDP does path MTU (Maximum Transmission Unit) discovery.
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This means the kernel
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will keep track of the MTU to a specific target IP address and return
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.B EMSGSIZE
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when a UDP packet write exceeds it.
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When this happens, the application should decrease the packet size.
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Path MTU discovery can be also turned off using the
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.B IP_MTU_DISCOVER
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socket option or the
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.I /proc/sys/net/ipv4/ip_no_pmtu_disc
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file; see
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.BR ip (7)
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for details.
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When turned off, UDP will fragment outgoing UDP packets
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that exceed the interface MTU.
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However, disabling it is not recommended
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for performance and reliability reasons.
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.SS Address format
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UDP uses the IPv4
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.I sockaddr_in
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address format described in
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.BR ip (7).
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.SS Error handling
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All fatal errors will be passed to the user as an error return even
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when the socket is not connected.
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This includes asynchronous errors
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received from the network.
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You may get an error for an earlier packet
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that was sent on the same socket.
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This behavior differs from many other BSD socket implementations
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which don't pass any errors unless the socket is connected.
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Linux's behavior is mandated by
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.BR RFC\ 1122 .
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For compatibility with legacy code, in Linux 2.0 and 2.2
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it was possible to set the
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.B SO_BSDCOMPAT
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.B SOL_SOCKET
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option to receive remote errors only when the socket has been
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connected (except for
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.B EPROTO
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and
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.BR EMSGSIZE ).
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Locally generated errors are always passed.
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Support for this socket option was removed in later kernels; see
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.BR socket (7)
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for further information.
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When the
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.B IP_RECVERR
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option is enabled, all errors are stored in the socket error queue,
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and can be received by
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.BR recvmsg (2)
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with the
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.B MSG_ERRQUEUE
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flag set.
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.SS /proc interfaces
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System-wide UDP parameter settings can be accessed by files in the directory
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.IR /proc/sys/net/ipv4/ .
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.TP
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.IR udp_mem " (since Linux 2.6.25)"
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This is a vector of three integers governing the number
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of pages allowed for queueing by all UDP sockets.
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.RS
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.TP 10
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.I min
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Below this number of pages, UDP is not bothered about its
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memory appetite.
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When the amount of memory allocated by UDP exceeds
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this number, UDP starts to moderate memory usage.
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.TP
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.I pressure
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This value was introduced to follow the format of
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.IR tcp_mem
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(see
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.BR tcp (7)).
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.TP
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.I max
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Number of pages allowed for queueing by all UDP sockets.
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.RE
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.IP
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Defaults values for these three items are
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calculated at boot time from the amount of available memory.
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.TP
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.IR udp_rmem_min " (integer; default value: PAGE_SIZE; since Linux 2.6.25)"
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Minimal size, in bytes, of receive buffers used by UDP sockets in moderation.
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Each UDP socket is able to use the size for receiving data,
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even if total pages of UDP sockets exceed
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.I udp_mem
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pressure.
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.TP
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.IR udp_wmem_min " (integer; default value: PAGE_SIZE; since Linux 2.6.25)"
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Minimal size, in bytes, of send buffer used by UDP sockets in moderation.
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Each UDP socket is able to use the size for sending data,
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even if total pages of UDP sockets exceed
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.I udp_mem
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pressure.
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.SS Socket options
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To set or get a UDP socket option, call
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.BR getsockopt (2)
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to read or
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.BR setsockopt (2)
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to write the option with the option level argument set to
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.BR IPPROTO_UDP .
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Unless otherwise noted,
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.I optval
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is a pointer to an
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.IR int .
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.TP
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.BR UDP_CORK " (since Linux 2.5.44)"
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If this option is enabled, then all data output on this socket
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is accumulated into a single datagram that is transmitted when
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the option is disabled.
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This option should not be used in code intended to be
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portable.
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.\" FIXME document UDP_ENCAP (new in kernel 2.5.67)
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.\" From include/linux/udp.h:
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.\" UDP_ENCAP_ESPINUDP_NON_IKE draft-ietf-ipsec-nat-t-ike-00/01
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.\" UDP_ENCAP_ESPINUDP draft-ietf-ipsec-udp-encaps-06
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.\" UDP_ENCAP_L2TPINUDP rfc2661
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.SS Ioctls
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These ioctls can be accessed using
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.BR ioctl (2).
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The correct syntax is:
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.PP
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.RS
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.nf
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.BI int " value";
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.IB error " = ioctl(" udp_socket ", " ioctl_type ", &" value ");"
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.fi
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.RE
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.TP
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.BR FIONREAD " (" SIOCINQ )
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Gets a pointer to an integer as argument.
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Returns the size of the next pending datagram in the integer in bytes,
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or 0 when no datagram is pending.
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.B Warning:
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Using
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.BR FIONREAD ,
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it is impossible to distinguish the case where no datagram is pending
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from the case where the next pending datagram contains zero bytes of data.
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It is safer to use
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.BR select (2),
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.BR poll (2),
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or
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.BR epoll (7)
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to distinguish these cases.
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.\" See http://www.securiteam.com/unixfocus/5KP0I15IKO.html
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.\" "GNUnet DoS (UDP Socket Unreachable)", 14 May 2006
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.TP
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.BR TIOCOUTQ " (" SIOCOUTQ )
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Returns the number of data bytes in the local send queue.
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Supported only with Linux 2.4 and above.
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.PP
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In addition, all ioctls documented in
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.BR ip (7)
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and
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.BR socket (7)
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are supported.
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.SH ERRORS
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All errors documented for
|
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.BR socket (7)
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or
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.BR ip (7)
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may be returned by a send or receive on a UDP socket.
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.TP
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.B ECONNREFUSED
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No receiver was associated with the destination address.
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This might be caused by a previous packet sent over the socket.
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.SH VERSIONS
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.B IP_RECVERR
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is a new feature in Linux 2.2.
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.\" .SH CREDITS
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.\" This man page was written by Andi Kleen.
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.SH SEE ALSO
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.BR ip (7),
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.BR raw (7),
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.BR socket (7),
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.BR udplite (7)
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RFC\ 768 for the User Datagram Protocol.
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.br
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RFC\ 1122 for the host requirements.
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.br
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RFC\ 1191 for a description of path MTU discovery.
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