SYSTEMD.SOCKET(5)



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 unit, a matching service unit must exist, describing
       the service to start on incoming traffic on the socket (see
       systemd.service(5) for more information about .service units). 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=no is set. If Accept=yes
       is set, a service template 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).

       All network sockets allocated through .socket units are allocated in
       the host's network namespace (see network_namespaces(7)). This does not
       mean however that the service activated by a configured socket unit has
       to be part of the host's network namespace as well. It is supported and
       even good practice to run services in their own network namespace (for
       example through PrivateNetwork=, see systemd.exec(5)), receiving only
       the sockets configured through socket-activation from the host's
       namespace. In such a set-up communication within the host's network
       namespace is only permitted through the activation sockets passed in
       while all sockets allocated from the service code itself will be
       associated with the service's own namespace, and thus possibly subject
       to a a much more restrictive configuration.

AUTOMATIC DEPENDENCIES
   Implicit Dependencies
       The following dependencies are implicitly added:

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

       o   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.

       o   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:

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

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

       BindIPv6Only=
           Takes 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=no. 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=no, but it may do so for sockets it got
           with Accept=yes set. Setting Accept=yes 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=yes 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=no 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-system.conf(5), 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

systemd 245                                                  SYSTEMD.SOCKET(5)

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