Ubuntu Manpage: systemd.socket - Socket unit configuration

Provided by: systemd_255.4-1ubuntu8.5_amd64

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 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 unit files may include [Unit] and [Install] sections, which are described in systemd.unit(5).

Socket unit 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 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.

ListenFIFO=
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.

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 (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.

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 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.

Added in version 227.

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

Added in version 229.

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 32-bit 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 4294967295. Note that this value is silently capped by the
"net.core.somaxconn" sysctl, which typically defaults to 4096, so typically the sysctl is the setting
that actually matters.

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.

Added in version 214.

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 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.

Note that depending on this setting the services activated by units of this type are either regular
services (in case of Accept=no) or instances of templated services (in case of Accept=yes). See the
Description section above for a more detailed discussion of the naming rules of triggered services.

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.

It is recommended to set CollectMode=inactive-or-failed for service instances activated via
Accept=yes, to ensure that failed connection services are cleaned up and released from memory, and do
not accumulate.

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.

Added in version 227.

FlushPending=
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.

Added in version 247.

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.

Added in version 232.

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.) Default value is 7200 seconds (2 hours).

Added in version 216.

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.) Default value is 75 seconds.

Added in version 216.

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.) Default value is 9.

Added in version 216.

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.

Added in version 216.

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.

Added in version 216.

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.

Added in version 206.

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[3] for details.

Added in version 196.

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 applies only when
activated service is passed in single socket file descriptor, i.e. service instances that have
standard input connected to a socket or services triggered by exactly one socket unit. Also note that
this option is useful only when MLS/MCS SELinux policy is deployed. Defaults to "false".

Added in version 217.

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/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.

Transparent=
Takes a boolean value. Controls the IP_TRANSPARENT/IPV6_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, NETLINK_PKTINFO or
PACKET_AUXDATA socket 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.

Added in version 246.

Timestamping=
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.

Added in version 247.

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.

Added in version 214.

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.

Added in version 214.

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.

Added in version 227.

TriggerLimitIntervalSec=, TriggerLimitBurst=
Configures a limit on how often this socket unit may be activated within a specific time interval.
The TriggerLimitIntervalSec= setting 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.

Compare with PollLimitIntervalSec=/PollLimitBurst= described below, which implements a temporary
slowdown if a socket unit is flooded with incoming traffic, as opposed to the permanent failure state
TriggerLimitIntervalSec=/TriggerLimitBurst= results in.

Added in version 230.

PollLimitIntervalSec=, PollLimitBurst=
Configures a limit on how often polling events on the file descriptors backing this socket unit will
be considered. This pair of settings is similar to TriggerLimitIntervalSec=/TriggerLimitBurst= but
instead of putting a (fatal) limit on the activation frequency puts a (transient) limit on the
polling frequency. The expected parameter syntax and range are identical to that of the
aforementioned options, and can be disabled the same way.

If the polling limit is hit polling is temporarily disabled on it until the specified time window
passes. The polling limit hence slows down connection attempts if hit, but unlike the trigger limit
won't cause permanent failures. It's the recommended mechanism to deal with DoS attempts through
packet flooding.

The polling limit is enforced per file descriptor to listen on, as opposed to the trigger limit which
is enforced for the entire socket unit. This distinction matters for socket units that listen on
multiple file descriptors (i.e. have multiple ListenXYZ= stanzas).

These setting defaults to 150 (in case of Accept=yes) and 15 (otherwise) polling events per 2s. This
is considerably lower than the default values for the trigger limit (see above) and means that the
polling limit should typically ensure the trigger limit is never hit, unless one of them is
reconfigured or disabled.

Added in version 255.

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

NOTES

        1. USB FunctionFS
           https://docs.kernel.org/usb/functionfs.html

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

        3. Smack
           https://docs.kernel.org/admin-guide/LSM/Smack.html

        4. Socket Activation
           https://0pointer.de/blog/projects/socket-activation.html

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

        6. Converting inetd Services
           https://0pointer.de/blog/projects/inetd.html

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

NAME

SYNOPSIS

DESCRIPTION

AUTOMATIC DEPENDENCIES

OPTIONS

SEE ALSO

NOTES