Provided by: systemd_237-3ubuntu10_amd64 bug

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.

       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).

IMPLICIT DEPENDENCIES

       The following dependencies are implicitly added:

       ·   Socket units automatically gain a Before= dependency on the service units they
           activate.

       ·   Socket units referring to file system paths (such as AF_UNIX sockets or FIFOs)
           implicitly gain Requires= and After= dependencies on all mount units necessary to
           access those paths.

       ·   Socket units using the BindToDevice= setting automatically gain a BindsTo= and After=
           dependency on the device unit encapsulating the specified network interface.

       Additional implicit dependencies may be added as result of execution and resource control
       parameters as documented in systemd.exec(5) and systemd.resource-control(5).

DEFAULT DEPENDENCIES

       The following dependencies are added unless DefaultDependencies=no is set:

       ·   Socket units automatically gain a Before= dependency on sockets.target.

       ·   Socket units automatically gain a pair of After= and Requires= dependency on
           sysinit.target, and a pair of Before= and Conflicts= dependencies on shutdown.target.
           These dependencies ensure that the socket unit is started before normal services at
           boot, and is stopped on shutdown. Only sockets involved with early boot or late system
           shutdown should disable DefaultDependencies= option.

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).

           If the address string is a string in the format "vsock:x:y", it is read as CID "x" on
           a port "y" address in the AF_VSOCK family. The CID is a unique 32-bit integer
           identifier in AF_VSOCK analogous to an IP address. Specifying the CID is optional, and
           may be set to the empty string.

           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.

       ListenUSBFunction=
           Specifies a USB FunctionFS[1] endpoints location to listen on, for implementation of
           USB gadget functions. This expects an absolute file system path of functionfs mount
           point as the argument. Behavior otherwise is very similar to the ListenFIFO= directive
           above. Use this to open the FunctionFS endpoint ep0. When using this option, the
           activated service has to have the USBFunctionDescriptors= and USBFunctionStrings=
           options set.

       SocketProtocol=
           Takes a one of udplite or sctp. Specifies a socket protocol (IPPROTO_UDPLITE) UDP-Lite
           (IPPROTO_SCTP) SCTP socket respectively.

       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 implicit
           dependency from this socket unit on the network interface device unit
           (systemd.device(5) is created. Note that setting this parameter might result in
           additional dependencies to be added to the unit (see above).

       SocketUser=, SocketGroup=
           Takes a UNIX user/group name. When specified, all AF_UNIX sockets and FIFO nodes in
           the file system are owned by the specified user and group. If unset (the default), the
           nodes are owned by the root user/group (if run in system context) or the invoking
           user/group (if run in user context). If only a user is specified but no group, then
           the group is derived from the user's default group.

       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.

       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.

       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.

           For IPv4 and IPv6 connections, the REMOTE_ADDR environment variable will contain the
           remote IP address, and REMOTE_PORT will contain the remote port. This is the same as
           the format used by CGI. For SOCK_RAW, the port is the IP protocol.

       Writable=
           Takes a boolean argument. May only be used in conjunction with ListenSpecial=. If
           true, the specified special file is opened in read-write mode, if false, in read-only
           mode. Defaults to false.

       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.

       MaxConnectionsPerSource=
           The maximum number of connections for a service per source IP address. This is very
           similar to the MaxConnections= directive above. Disabled by default.

       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[2] for details.) Defaults to false.

       KeepAliveTimeSec=
           Takes time (in seconds) as argument. The connection needs to remain idle before TCP
           starts sending keepalive probes. This controls the TCP_KEEPIDLE socket option (see
           socket(7) and the TCP Keepalive HOWTO[2] for details.) Defaults value is 7200 seconds
           (2 hours).

       KeepAliveIntervalSec=
           Takes time (in seconds) as argument between individual keepalive probes, if the socket
           option SO_KEEPALIVE has been set on this socket. This controls the TCP_KEEPINTVL
           socket option (see socket(7) and the TCP Keepalive HOWTO[2] for details.) Defaults
           value is 75 seconds.

       KeepAliveProbes=
           Takes an integer as argument. It is the number of unacknowledged probes to send before
           considering the connection dead and notifying the application layer. This controls the
           TCP_KEEPCNT socket option (see socket(7) and the TCP Keepalive HOWTO[2] for details.)
           Defaults value is 9.

       NoDelay=
           Takes a boolean argument. TCP Nagle's algorithm works by combining a number of small
           outgoing messages, and sending them all at once. This controls the TCP_NODELAY socket
           option (see tcp(7) 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.).

       DeferAcceptSec=
           Takes time (in seconds) as argument. If set, the listening process will be awakened
           only when data arrives on the socket, and not immediately when connection is
           established. When this option is set, the TCP_DEFER_ACCEPT socket option will be used
           (see tcp(7)), and the kernel will ignore initial ACK packets without any data. The
           argument specifies the approximate amount of time the kernel should wait for incoming
           data before falling back to the normal behavior of honoring empty ACK packets. This
           option is beneficial for protocols where the client sends the data first (e.g. HTTP,
           in contrast to SMTP), because the server process will not be woken up unnecessarily
           before it can take any action.

           If the client also uses the TCP_DEFER_ACCEPT option, the latency of the initial
           connection may be reduced, because the kernel will send data in the final packet
           establishing the connection (the third packet in the "three-way handshake").

           Disabled by default.

       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[3] for details.

       SELinuxContextFromNet=
           Takes a boolean argument. When true, systemd will attempt to figure out the SELinux
           label used for the instantiated service from the information handed by the peer over
           the network. Note that only the security level is used from the information provided
           by the peer. Other parts of the resulting SELinux context originate from either the
           target binary that is effectively triggered by socket unit or from the value of the
           SELinuxContext= option. This configuration option only affects sockets with Accept=
           mode set to "true". Also note that this option is useful only when MLS/MCS SELinux
           policy is deployed. Defaults to "false".

       PipeSize=
           Takes a 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 DefaultTimeoutStartSec= from the manager
           configuration file (see systemd-system.conf(5)).

       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. Note that setting this parameter might result in
           additional dependencies to be added to the unit (see above).

       RemoveOnStop=
           Takes a boolean argument. If enabled, any file nodes created by this socket unit are
           removed when it is stopped. This applies to AF_UNIX sockets in the file system, POSIX
           message queues, FIFOs, as well as any symlinks to them configured with Symlinks=.
           Normally, it should not be necessary to use this option, and is not recommended as
           services might continue to run after the socket unit has been terminated and it should
           still be possible to communicate with them via their file system node. Defaults to
           off.

       Symlinks=
           Takes a list of file system paths. The specified paths will be created as symlinks to
           the AF_UNIX socket path or FIFO path of this socket unit. If this setting is used,
           only one AF_UNIX socket in the file system or one FIFO may be configured for the
           socket unit. Use this option to manage one or more symlinked alias names for a socket,
           binding their lifecycle together. Note that if creation of a symlink fails this is not
           considered fatal for the socket unit, and the socket unit may still start. If an empty
           string is assigned, the list of paths is reset. Defaults to an empty list.

       FileDescriptorName=
           Assigns a name to all file descriptors this socket unit encapsulates. This is useful
           to help activated services identify specific file descriptors, if multiple fds are
           passed. Services may use the sd_listen_fds_with_names(3) call to acquire the names
           configured for the received file descriptors. Names may contain any ASCII character,
           but must exclude control characters and ":", and must be at most 255 characters in
           length. If this setting is not used, the file descriptor name defaults to the name of
           the socket unit, including its .socket suffix.

       TriggerLimitIntervalSec=, TriggerLimitBurst=
           Configures a limit on how often this socket unit my be activated within a specific
           time interval. The TriggerLimitIntervalSec= may be used to configure the length of the
           time interval in the usual time units "us", "ms", "s", "min", "h", ... and defaults to
           2s (See systemd.time(7) for details on the various time units understood). The
           TriggerLimitBurst= setting takes a positive integer value and specifies the number of
           permitted activations per time interval, and defaults to 200 for Accept=yes sockets
           (thus by default permitting 200 activations per 2s), and 20 otherwise (20 activations
           per 2s). Set either to 0 to disable any form of trigger rate limiting. If the limit is
           hit, the socket unit is placed into a failure mode, and will not be connectible
           anymore until restarted. Note that this limit is enforced before the service
           activation is enqueued.

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

SEE ALSO

       systemd(1), systemctl(1), systemd.unit(5), systemd.exec(5), systemd.kill(5),
       systemd.resource-control(5), systemd.service(5), systemd.directives(7), sd_listen_fds(3),
       sd_listen_fds_with_names(3)

       For more extensive descriptions see the "systemd for Developers" series: Socket
       Activation[4], Socket Activation, part II[5], Converting inetd Services[6], Socket
       Activated Internet Services and OS Containers[7].

NOTES

        1. USB FunctionFS
           https://www.kernel.org/doc/Documentation/usb/functionfs.txt

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

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

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

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

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

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