systemd.socket

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 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.

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

       PassPacketInfo=
           Takes a boolean value. This controls the IP_PKTINFO, IPV6_RECVPKTINFO
           and NETLINK_PKTINFO socket options, which enable reception of
           additional per-packet metadata as ancillary message, on AF_INET,
           AF_INET6 and AF_UNIX sockets. 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 246                                                    SYSTEMD.SOCKET(5)