systemd.socket — systemd socket configuration files
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 ExecStoptPost= commands are executed in.
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.
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). 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) resp. sequential packet (SOCK_SEQPACKET) socket. 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 ampersand (@) 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 for both IPv4 and 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 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 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.
Specifies a file system FIFO to listen on. This expects an absolute file system path as argument. Behaviour otherwise is very similar to the ListenDatagram= directive above.
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.
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 automatic dependency from this socket unit on the network interface device unit (systemd.device(5) is created.
If listening on a file system socket of 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.
If listening on a file system socket of 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.
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.
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=no or datagram sockets. Defaults to 64.
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 for details.) 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 an integer argument controlling the receive resp. send buffer sizes of this socket. This controls the SO_RCVBUF resp. SO_SNDBUF socket options (see socket(7) for details.).
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 an integer value. Controls the pipe buffer size of FIFOs configured in this socket unit. See fcntl(2) 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 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 one or more command lines, which are executed before (resp. after) the listening sockets/FIFOs are created and bound. 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.
Additional commands that are executed before (resp. after) the listening sockets/FIFOs are closed and removed. 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= below.) 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 60s.
Specifies how processes of this socket unit shall be killed. One of control-group, process-group, process, none.
This option is mostly equivalent to the KillMode= option of service files. See systemd.service(5) for details.
systemd(1), systemctl(8), systemd.unit(5), systemd.exec(5), systemd.service(5)