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SYSTEMD.SOCKET(5)		systemd.socket		     SYSTEMD.SOCKET(5)

NAME
       systemd.socket - Socket unit configuration

SYNOPSIS
       socket.socket

DESCRIPTION
       A unit configuration file whose name ends in ".socket" encodes
       information about an IPC or network socket or a file system FIFO
       controlled and supervised by systemd, for socket-based activation.

       This man page lists the configuration options specific to this unit
       type. See systemd.unit(5) for the common options of all unit
       configuration files. The common configuration items are configured in
       the generic [Unit] and [Install] sections. The socket specific
       configuration options are configured in the [Socket] section.

       Additional options are listed in systemd.exec(5), which define the
       execution environment the ExecStartPre=, ExecStartPost=, ExecStopPre=
       and ExecStopPost= commands are executed in, and in systemd.kill(5),
       which define the way the processes are terminated, and in
       systemd.resource-control(5), which configure resource control settings
       for the processes of the socket.

       For each socket file, a matching service file must exist, describing
       the service to start on incoming traffic on the socket (see
       systemd.service(5) for more information about .service files). The name
       of the .service unit is by default the same as the name of the .socket
       unit, but can be altered with the Service= option described below.
       Depending on the setting of the Accept= option described below, this
       .service unit must either be named like the .socket unit, but with the
       suffix replaced, unless overridden with Service=; or it must be a
       template unit named the same way. Example: a socket file foo.socket
       needs a matching service foo.service if Accept=false is set. If
       Accept=true is set, a service template file foo@.service must exist
       from which services are instantiated for each incoming connection.

       Unless DefaultDependencies= is set to false, socket units will
       implicitly have dependencies of type Requires= and After= on
       sysinit.target as well as dependencies of type Conflicts= and Before=
       on shutdown.target. These ensure that socket units pull in basic system
       initialization, and are terminated cleanly prior to system shutdown.
       Only sockets involved with early boot or late system shutdown should
       disable this option.

       Socket units will have a Before= dependency on the service which they
       trigger added implicitly. No implicit WantedBy= or RequiredBy=
       dependency from the socket to the service is added. This means that the
       service may be started without the socket, in which case it must be
       able to open sockets by itself. To prevent this, an explicit Requires=
       dependency may be added.

       Socket units may be used to implement on-demand starting of services,
       as well as parallelized starting of services. See the blog stories
       linked at the end for an introduction.

       Note that the daemon software configured for socket activation with
       socket units needs to be able to accept sockets from systemd, either
       via systemd's native socket passing interface (see sd_listen_fds(3) for
       details) or via the traditional inetd(8)-style socket passing (i.e.
       sockets passed in via standard input and output, using
       StandardInput=socket in the service file).

OPTIONS
       Socket files must include a [Socket] section, which carries information
       about the socket or FIFO it supervises. A number of options that may be
       used in this section are shared with other unit types. These options
       are documented in systemd.exec(5) and systemd.kill(5). The options
       specific to the [Socket] section of socket units are the following:

       ListenStream=, ListenDatagram=, ListenSequentialPacket=
	   Specifies an address to listen on for a stream (SOCK_STREAM),
	   datagram (SOCK_DGRAM), or sequential packet (SOCK_SEQPACKET)
	   socket, respectively. The address can be written in various
	   formats:

	   If the address starts with a slash ("/"), it is read as file system
	   socket in the AF_UNIX socket family.

	   If the address starts with an at symbol ("@"), it is read as
	   abstract namespace socket in the AF_UNIX family. The "@" is
	   replaced with a NUL character before binding. For details, see
	   unix(7).

	   If the address string is a single number, it is read as port number
	   to listen on via IPv6. Depending on the value of BindIPv6Only= (see
	   below) this might result in the service being available via both
	   IPv6 and IPv4 (default) or just via IPv6.

	   If the address string is a string in the format v.w.x.y:z, it is
	   read as IPv4 specifier for listening on an address v.w.x.y on a
	   port z.

	   If the address string is a string in the format [x]:y, it is read
	   as IPv6 address x on a port y. Note that this might make the
	   service available via IPv4, too, depending on the BindIPv6Only=
	   setting (see below).

	   Note that SOCK_SEQPACKET (i.e.  ListenSequentialPacket=) is only
	   available for AF_UNIX sockets.  SOCK_STREAM (i.e.  ListenStream=)
	   when used for IP sockets refers to TCP sockets, SOCK_DGRAM (i.e.
	   ListenDatagram=) to UDP.

	   These options may be specified more than once in which case
	   incoming traffic on any of the sockets will trigger service
	   activation, and all listed sockets will be passed to the service,
	   regardless of whether there is incoming traffic on them or not. If
	   the empty string is assigned to any of these options, the list of
	   addresses to listen on is reset, all prior uses of any of these
	   options will have no effect.

	   It is also possible to have more than one socket unit for the same
	   service when using Service=, and the service will receive all the
	   sockets configured in all the socket units. Sockets configured in
	   one unit are passed in the order of configuration, but no ordering
	   between socket units is specified.

	   If an IP address is used here, it is often desirable to listen on
	   it before the interface it is configured on is up and running, and
	   even regardless of whether it will be up and running at any point.
	   To deal with this, it is recommended to set the FreeBind= option
	   described below.

       ListenFIFO=
	   Specifies a file system FIFO to listen on. This expects an absolute
	   file system path as argument. Behavior otherwise is very similar to
	   the ListenDatagram= directive above.

       ListenSpecial=
	   Specifies a special file in the file system to listen on. This
	   expects an absolute file system path as argument. Behavior
	   otherwise is very similar to the ListenFIFO= directive above. Use
	   this to open character device nodes as well as special files in
	   /proc and /sys.

       ListenNetlink=
	   Specifies a Netlink family to create a socket for to listen on.
	   This expects a short string referring to the AF_NETLINK family name
	   (such as audit or kobject-uevent) as argument, optionally suffixed
	   by a whitespace followed by a multicast group integer. Behavior
	   otherwise is very similar to the ListenDatagram= directive above.

       ListenMessageQueue=
	   Specifies a POSIX message queue name to listen on. This expects a
	   valid message queue name (i.e. beginning with /). Behavior
	   otherwise is very similar to the ListenFIFO= directive above. On
	   Linux message queue descriptors are actually file descriptors and
	   can be inherited between processes.

       BindIPv6Only=
	   Takes a one of default, both or ipv6-only. Controls the IPV6_V6ONLY
	   socket option (see ipv6(7) for details). If both, IPv6 sockets
	   bound will be accessible via both IPv4 and IPv6. If ipv6-only, they
	   will be accessible via IPv6 only. If default (which is the default,
	   surprise!), the system wide default setting is used, as controlled
	   by /proc/sys/net/ipv6/bindv6only, which in turn defaults to the
	   equivalent of both.

       Backlog=
	   Takes an unsigned integer argument. Specifies the number of
	   connections to queue that have not been accepted yet. This setting
	   matters only for stream and sequential packet sockets. See
	   listen(2) for details. Defaults to SOMAXCONN (128).

       BindToDevice=
	   Specifies a network interface name to bind this socket to. If set,
	   traffic will only be accepted from the specified network
	   interfaces. This controls the SO_BINDTODEVICE socket option (see
	   socket(7) for details). If this option is used, an automatic
	   dependency from this socket unit on the network interface device
	   unit (systemd.device(5) is created.

       DirectoryMode=
	   If listening on a file system socket or FIFO, the parent
	   directories are automatically created if needed. This option
	   specifies the file system access mode used when creating these
	   directories. Takes an access mode in octal notation. Defaults to
	   0755.

       SocketMode=
	   If listening on a file system socket or FIFO, this option specifies
	   the file system access mode used when creating the file node. Takes
	   an access mode in octal notation. Defaults to 0666.

       Accept=
	   Takes a boolean argument. If true, a service instance is spawned
	   for each incoming connection and only the connection socket is
	   passed to it. If false, all listening sockets themselves are passed
	   to the started service unit, and only one service unit is spawned
	   for all connections (also see above). This value is ignored for
	   datagram sockets and FIFOs where a single service unit
	   unconditionally handles all incoming traffic. Defaults to false.
	   For performance reasons, it is recommended to write new daemons
	   only in a way that is suitable for Accept=false. A daemon listening
	   on an AF_UNIX socket may, but does not need to, call close(2) on
	   the received socket before exiting. However, it must not unlink the
	   socket from a file system. It should not invoke shutdown(2) on
	   sockets it got with Accept=false, but it may do so for sockets it
	   got with Accept=true set. Setting Accept=true is mostly useful to
	   allow daemons designed for usage with inetd(8) to work unmodified
	   with systemd socket activation.

       MaxConnections=
	   The maximum number of connections to simultaneously run services
	   instances for, when Accept=true is set. If more concurrent
	   connections are coming in, they will be refused until at least one
	   existing connection is terminated. This setting has no effect on
	   sockets configured with Accept=false or datagram sockets. Defaults
	   to 64.

       KeepAlive=
	   Takes a boolean argument. If true, the TCP/IP stack will send a
	   keep alive message after 2h (depending on the configuration of
	   /proc/sys/net/ipv4/tcp_keepalive_time) for all TCP streams accepted
	   on this socket. This controls the SO_KEEPALIVE socket option (see
	   socket(7) and the TCP Keepalive HOWTO[1] for details.) Defaults to
	   false.

       Priority=
	   Takes an integer argument controlling the priority for all traffic
	   sent from this socket. This controls the SO_PRIORITY socket option
	   (see socket(7) for details.).

       ReceiveBuffer=, SendBuffer=
	   Takes an integer argument controlling the receive or send buffer
	   sizes of this socket, respectively. This controls the SO_RCVBUF and
	   SO_SNDBUF socket options (see socket(7) for details.). The usual
	   suffixes K, M, G are supported and are understood to the base of
	   1024.

       IPTOS=
	   Takes an integer argument controlling the IP Type-Of-Service field
	   for packets generated from this socket. This controls the IP_TOS
	   socket option (see ip(7) for details.). Either a numeric string or
	   one of low-delay, throughput, reliability or low-cost may be
	   specified.

       IPTTL=
	   Takes an integer argument controlling the IPv4 Time-To-Live/IPv6
	   Hop-Count field for packets generated from this socket. This sets
	   the IP_TTL/IPV6_UNICAST_HOPS socket options (see ip(7) and ipv6(7)
	   for details.)

       Mark=
	   Takes an integer value. Controls the firewall mark of packets
	   generated by this socket. This can be used in the firewall logic to
	   filter packets from this socket. This sets the SO_MARK socket
	   option. See iptables(8) for details.

       ReusePort=
	   Takes a boolean value. If true, allows multiple bind(2)s to this
	   TCP or UDP port. This controls the SO_REUSEPORT socket option. See
	   socket(7) for details.

       SmackLabel=, SmackLabelIPIn=, SmackLabelIPOut=
	   Takes a string value. Controls the extended attributes
	   "security.SMACK64", "security.SMACK64IPIN" and
	   "security.SMACK64IPOUT", respectively, i.e. the security label of
	   the FIFO, or the security label for the incoming or outgoing
	   connections of the socket, respectively. See Smack.txt[2] for
	   details.

       PipeSize=
	   Takes an size in bytes. Controls the pipe buffer size of FIFOs
	   configured in this socket unit. See fcntl(2) for details. The usual
	   suffixes K, M, G are supported and are understood to the base of
	   1024.

       MessageQueueMaxMessages=, MessageQueueMessageSize=
	   These two settings take integer values and control the mq_maxmsg
	   field or the mq_msgsize field, respectively, when creating the
	   message queue. Note that either none or both of these variables
	   need to be set. See mq_setattr(3) for details.

       FreeBind=
	   Takes a boolean value. Controls whether the socket can be bound to
	   non-local IP addresses. This is useful to configure sockets
	   listening on specific IP addresses before those IP addresses are
	   successfully configured on a network interface. This sets the
	   IP_FREEBIND socket option. For robustness reasons it is recommended
	   to use this option whenever you bind a socket to a specific IP
	   address. Defaults to false.

       Transparent=
	   Takes a boolean value. Controls the IP_TRANSPARENT socket option.
	   Defaults to false.

       Broadcast=
	   Takes a boolean value. This controls the SO_BROADCAST socket
	   option, which allows broadcast datagrams to be sent from this
	   socket. Defaults to false.

       PassCredentials=
	   Takes a boolean value. This controls the SO_PASSCRED socket option,
	   which allows AF_UNIX sockets to receive the credentials of the
	   sending process in an ancillary message. Defaults to false.

       PassSecurity=
	   Takes a boolean value. This controls the SO_PASSSEC socket option,
	   which allows AF_UNIX sockets to receive the security context of the
	   sending process in an ancillary message. Defaults to false.

       TCPCongestion=
	   Takes a string value. Controls the TCP congestion algorithm used by
	   this socket. Should be one of "westwood", "veno", "cubic", "lp" or
	   any other available algorithm supported by the IP stack. This
	   setting applies only to stream sockets.

       ExecStartPre=, ExecStartPost=
	   Takes one or more command lines, which are executed before or after
	   the listening sockets/FIFOs are created and bound, respectively.
	   The first token of the command line must be an absolute filename,
	   then followed by arguments for the process. Multiple command lines
	   may be specified following the same scheme as used for
	   ExecStartPre= of service unit files.

       ExecStopPre=, ExecStopPost=
	   Additional commands that are executed before or after the listening
	   sockets/FIFOs are closed and removed, respectively. Multiple
	   command lines may be specified following the same scheme as used
	   for ExecStartPre= of service unit files.

       TimeoutSec=
	   Configures the time to wait for the commands specified in
	   ExecStartPre=, ExecStartPost=, ExecStopPre= and ExecStopPost= to
	   finish. If a command does not exit within the configured time, the
	   socket will be considered failed and be shut down again. All
	   commands still running, will be terminated forcibly via SIGTERM,
	   and after another delay of this time with SIGKILL. (See KillMode=
	   in systemd.kill(5).) Takes a unit-less value in seconds, or a time
	   span value such as "5min 20s". Pass 0 to disable the timeout logic.
	   Defaults to TimeoutStartSec= from the manager configuration file.

       Service=
	   Specifies the service unit name to activate on incoming traffic.
	   This setting is only allowed for sockets with Accept=no. It
	   defaults to the service that bears the same name as the socket
	   (with the suffix replaced). In most cases, it should not be
	   necessary to use this option.

       Check systemd.exec(5) and systemd.kill(5) for more settings.

SEE ALSO
       systemd(1), systemctl(8), systemd.unit(5), systemd.exec(5),
       systemd.kill(5), systemd.resource-control(5), systemd.service(5),
       systemd.directives(7)

       For more extensive descriptions see the "systemd for Developers"
       series: Socket Activation[3], Socket Activation, part II[4], Converting
       inetd Services[5], Socket Activated Internet Services and OS
       Containers[6].

NOTES
	1. TCP Keepalive HOWTO
	   http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/

	2. Smack.txt
	   https://www.kernel.org/doc/Documentation/security/Smack.txt

	3. Socket Activation
	   http://0pointer.de/blog/projects/socket-activation.html

	4. Socket Activation, part II
	   http://0pointer.de/blog/projects/socket-activation2.html

	5. Converting inetd Services
	   http://0pointer.de/blog/projects/inetd.html

	6. Socket Activated Internet Services and OS Containers
	   http://0pointer.de/blog/projects/socket-activated-containers.html

systemd 212						     SYSTEMD.SOCKET(5)
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