systemd.socket(5) — Linux manual page


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

NAME         top

       systemd.socket - Socket unit configuration

SYNOPSIS         top


DESCRIPTION         top

       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

       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 about the precise protocol used and the order in which
       the file descriptors are passed) or via 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.


   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

   Default Dependencies
       The following dependencies are added unless DefaultDependencies=no is

       •   Socket units automatically gain a Before= dependency on

       •   Socket units automatically gain a pair of After= and Requires=
           dependency on, and a pair of Before= and
           Conflicts= dependencies on 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         top

       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

           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

           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 interpreted as IPv4 address v.w.x.y and port z.

           If the address string is a string in the format "[x]:y", it is
           interpreted as IPv6 address x and port y. An optional interface
           scope (interface name or number) may be specified after a "%"
           symbol: "[x]:y%dev". Interface scopes are only useful with
           link-local addresses, because the kernel ignores them in other
           cases. Note that if an address is specified as IPv6, it might
           still 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.

           Specifies a file system FIFO (see fifo(7) for details) to listen
           on. This expects an absolute file system path as argument.
           Behavior otherwise is very similar to the ListenDatagram=
           directive above.

           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/.

           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.

           Specifies a POSIX message queue name to listen on (see
           mq_overview(7) for details). 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.

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

           Takes one of udplite or sctp. The socket will use the UDP-Lite
           (IPPROTO_UDPLITE) or SCTP (IPPROTO_SCTP) protocol, respectively.

           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.

           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).

           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 is created (see systemd.device(5)). 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

           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.

           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

           Takes a boolean argument. If yes, a service instance is spawned
           for each incoming connection and only the connection socket is
           passed to it. If no, 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 no. 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.

           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.

           Takes a boolean argument. May only be used when Accept=no. If
           yes, the socket's buffers are cleared after the triggered service
           exited. This causes any pending data to be flushed and any
           pending incoming connections to be rejected. If no, the socket's
           buffers won't be cleared, permitting the service to handle any
           pending connections after restart, which is the usually expected
           behaviour. Defaults to no.

           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.

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

           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.

           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).

           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.

           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.

           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.

           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.).

           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

           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.

           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.

           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.)

           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.

           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

           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 "yes". Also note that this
           option is useful only when MLS/MCS SELinux policy is deployed.
           Defaults to "false".

           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.

           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/IPV6_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.

           Takes a boolean value. Controls the
           IP_TRANSPARENT/IPV6_TRANSPARENT socket option. Defaults to false.

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

           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

           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.

           Takes a boolean value. This controls the IP_PKTINFO,
           options, which enable reception of additional per-packet metadata
           as ancillary message, on AF_INET, AF_INET6, AF_UNIX and AF_PACKET
           sockets. Defaults to false.

           Takes one of "off", "us" (alias: "usec", "µs") or "ns" (alias:
           "nsec"). This controls the SO_TIMESTAMP or SO_TIMESTAMPNS socket
           options, and enables whether ingress network traffic shall carry
           timestamping metadata. Defaults to off.

           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.

           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)).

           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).

           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

           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.

           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         top

       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),

       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         top

        1. USB FunctionFS

        2. TCP Keepalive HOWTO

        3. Smack.txt

        4. Socket Activation

        5. Socket Activation, part II

        6. Converting inetd Services

        7. Socket Activated Internet Services and OS Containers

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug
       report for this manual page, see
       ⟨⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨⟩ on 2020-11-01.  (At that
       time, the date of the most recent commit that was found in the repos‐
       itory was 2020-11-01.)  If you discover any rendering problems in
       this HTML version of the page, or you believe there is a better or
       more up-to-date source for the page, or you have corrections or im‐
       provements to the information in this COLOPHON (which is not part of
       the original manual page), send a mail to

systemd 247                                                SYSTEMD.SOCKET(5)

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