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.

       For each socket file a matching service file (see systemd.service(5) for details) must
       exist, describing the service to start on incoming traffic on the socket. Depending on the
       setting of Accept= (see below), this must either be named like the socket unit, but with
       the suffix replaced; or it must be a template file 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 may be used to implement on-demand starting of services, as well as
       parallelized starting of services.

       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 STDIN and STDOUT, using StandardInput=socket in the
       service file).

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 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.

           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 whether it will
           be up and running ever at all. 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.

       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.

       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.

       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.

       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. This option is mostly useful
           to allow daemons designed for usage with inetd(8), to work unmodified with systemd
           socket activation.

       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 for
           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[1] for details.) 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.).

       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.).

       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.

       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[2] for details.

       PipeSize=
           Takes an integer value. Controls the pipe buffer size of FIFOs configured in this
           socket unit. See fcntl(2) for details.

       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 file name, 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 90s.

       Service=
           Specifies the service unit name to activate on incoming traffic. This defaults to the
           service that bears the same name as the socket (ignoring the different suffixes). In
           most cases it should not be necessary to use this option.

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

SEE ALSO

       systemd(1), systemctl(8), systemd.unit(5), systemd.exec(5), systemd.kill(5),
       systemd.service(5), systemd.directives(7)

       For more extensive descriptions see the "Systemd for Developers" series: Socket
       Activation[3], Socket Activation, part II[4], Converting inetd Services[5], Socket
       Activated Internet Services and OS Containers[6].

NOTES

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

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

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

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

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

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