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 file, a matching service file must exist, describing
the service to start on incoming traffic on the socket (see
systemd.service(5) for more information about .service files). 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=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 will have a Before= dependency on the service which they
trigger added implicitly. 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) or via the traditional inetd(8)-style socket passing (i.e.
sockets passed in via standard input and output, 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 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.
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.
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=false. 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=false, but it may do so for sockets it
got with Accept=true set. Setting Accept=true 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, 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.
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 on
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.
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[1] 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 seconds as argument. This controls the TCP_KEEPINTVL socket
option (see socket(7) and the TCP Keepalive HOWTO[1] for details.)
Defaults value is 75 seconds.
KeepAliveProbes=
Takes integer as argument. It's 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[1] 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
behaviour of honouring 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[2] 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.
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.
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. Defaults to the empty
list.
Check systemd.exec(5) and systemd.kill(5) for more settings.
SEE ALSO
systemd(1), systemctl(1), systemd.unit(5), systemd.exec(5),
systemd.kill(5), systemd.resource-control(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
systemd 219 SYSTEMD.SOCKET(5)
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