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 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.
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 32bit 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. Note that this value is silently capped by
the "net.core.somaxconn" sysctl, which typically defaults to 4096.
By default this is set to 4294967295, so that the sysctl takes full
effect.
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 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.
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.
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[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 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".
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.
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.
TCPCongestion=
Takes a string value. Controls the TCP congestion algorithm used by
this socket. Should be one of "westwood", "reno", "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 may 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.unit(5), 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://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
systemd 252 SYSTEMD.SOCKET(5)
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