xenial (5) systemd.socket.5.gz

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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).

AUTOMATIC DEPENDENCIES

       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.

       If DefaultDependencies=yes is set (the default), socket units automatically gain a Before= dependency on
       sockets.target. They also 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.

       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).

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.

       ListenUSBFunction=
           Specifies a USB FunctionFS[1] endpoint location to listen on, for implementation of USB gadget
           functions. This expects an absolute file system path 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 automatic 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.

       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 honouring 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. Defaults to the 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=, TriggerLimitIntervalBurst=
           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 5s (See systemd.time(7) for details on the
           various time units available). The TriggerLimitBurst= setting takes an integer value and specifies
           the numer of permitted activations per time interval, and defaults to 2500 (thus by default
           permitting 2500 activations per 5s). 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