| SYSTEMD.SOCKET(5) | systemd.socket | SYSTEMD.SOCKET(5) |
.SH "NAME" systemd.socket - Socket unit configuration
.SH "SYNOPSIS"
.PP socket.socket
.SH "DESCRIPTION"
.PP 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.
.PP 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.
.PP 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.
.PP 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.
.PP 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.
.PP 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.
.PP 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).
.PP By default, 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.
.PP Alternatively, it is possible to run a .socket unit in another network
namespace by setting PrivateNetwork=yes in combination with
JoinsNamespaceOf=, see systemd.exec(5) and
systemd.unit(5) for details.
.SH "AUTOMATIC DEPENDENCIES"
.SS "Implicit Dependencies"
.PP The following dependencies are implicitly added:
.PP 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).
.SS "Default Dependencies"
.PP The following dependencies are added unless DefaultDependencies=no
is set:
.SH "OPTIONS"
.PP Socket unit files may include [Unit] and [Install] sections, which are
described in systemd.unit(5).
.PP 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), systemd.kill(5) and
systemd.resource-control(5). The options specific to the [Socket]
section of socket units are the following:
.PP 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. vsock may be replaced with vsock-stream, vsock-dgram or
vsock-seqpacket to force usage of the corresponding socket type.
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.
.RE
.PP 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.
.RE
.PP 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/.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP SocketProtocol=
Takes one of udplite, sctp or mptcp. The socket will use
the UDP-Lite (IPPROTO_UDPLITE), SCTP (IPPROTO_SCTP) or MPTCP
(IPPROTO_MPTCP) protocol, respectively.
Added in version 229.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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).
.RE
.PP SocketUser=, SocketGroup=
Takes a UNIX user/group name. When specified, all AF_UNIX sockets,
FIFO nodes, and message queues 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.
.RE
.PP SocketMode=
If listening on a file system socket, FIFO, or message queue, this option
specifies the file system access mode used when creating the file node.
Takes an access mode in octal notation. Defaults to 0666.
.RE
.PP 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.
.RE
.PP 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.
Typically, for performance sensitive services, a choice of Accept=no
is preferable, since that way only the first connection will have to pay the
activation resource cost. On the other hand, for sporadically used services
Accept=yes can be preferable as it simplifies the implementation (as
the service program code only has to process a single connection instead of
handling multiple) and enables stronger security (since the various
sandboxing options can be used to isolate parallel connections from each
other, as each is serviced by a separate service instance and process).
A service 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 in particular useful for allowing 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 number. These two variables correspond to those defined by the
CGI interface for web services (see RFC 3875[2]).
For AF_UNIX socket connections, the $REMOTE_ADDR environment
variable will contain either the remote socket's file system path starting
with a slash (/) or its address in the abstract namespace starting with an
at symbol (@). If the socket is unnamed, $REMOTE_ADDR will not be
set.
If Accept=yes is used, the activated service process will have set the
$SO_COOKIE environment variable to the Linux socket cookie, formatted
as decimal integer. The socket cookie can otherwise be acquired via
getsockopt(7).
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.
.RE
.PP 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.
.RE
.PP 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 will not 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.
.RE
.PP 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.
.RE
.PP MaxConnectionsPerSource=
The maximum number of connections for a service per source IP address (in
case of IPv4/IPv6), per source CID (in case of AF_VSOCK), or source
UID (in case of AF_UNIX). This is very similar to the
MaxConnections= directive above. Defaults to 0, i.e. disabled.
Added in version 232.
.RE
.PP 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[3] for details.)
Defaults to false.
.RE
.PP 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[3]
for details.) Default value is 7200 seconds (2 hours).
Added in version 216.
.RE
.PP 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[3] for details.) Default value is 75
seconds.
Added in version 216.
.RE
.PP 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[3] for details.) Default value is 9.
Added in version 216.
.RE
.PP 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.
.RE
.PP 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.).
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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.)
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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[4] for
details.
Added in version 196.
.RE
.PP XAttrEntryPoint=, XAttrListen=, XAttrAccept=
Set an extended attribute on the socket, in the form
NAME=VALUE. The extended attribute's name must be in the user.
namespace, i.e. begin with the four characters user.. Specifiers are
expanded in both the name and the value, see systemd.unit(5) for
details on the available specifiers. These settings may be used more than
once to set multiple extended attributes; assigning the empty string resets
the list.
The three variables control where the extended attributes are applied:
XAttrEntryPoint= sets them on the file-system inode an AF_UNIX
socket is bound to (i.e. the socket node visible in the file system at the
specified listening path; this only applies to AF_UNIX sockets),
XAttrListen= sets them on the listening socket, and
XAttrAccept= sets them on the connection sockets accepted off the
listening socket (and is thus only relevant in combination with
Accept=yes).
This is primarily useful to tag sockets so that they can be discovered and
classified by other tools. For example, Varlink entrypoint sockets are
supposed to be tagged with user.varlink=entrypoint via
XAttrEntryPoint=, which makes them discoverable via varlinkctl
list-sockets, see varlinkctl(1).
These settings require kernel support for extended attributes on socket
inodes (available since Linux 7.0). Extended attributes that cannot be
applied are logged at debug level and otherwise ignored.
Added in version 262.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP MessageQueueMaxMessages=, MessageQueueMessageSize=
These two settings control the mq_maxmsg field or the mq_msgsize field,
respectively, when creating the message queue.
MessageQueueMaxMessages= takes an integer value.
MessageQueueMessageSize= takes a size in bytes, and the usual
suffixes K, M, G are supported and understood to the base of 1024. Note that
either none or both of these variables need to be set. See
mq_setattr(3) for details.
.RE
.PP 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.
.RE
.PP Transparent=
Takes a boolean value. Controls the
IP_TRANSPARENT/IPV6_TRANSPARENT socket option. Defaults to
false.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP PassPIDFD=
Takes a boolean value. This controls the SO_PASSPIDFD socket option,
which allows AF_UNIX sockets to receive the pidfd of the sending
process in an ancillary message. Defaults to false.
Added in version 258.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP AcceptFileDescriptors=
Takes a boolean value. This controls the SO_PASSRIGHTS socket option,
which when disabled prohibits the peer from sending SCM_RIGHTS
ancillary messages (aka file descriptors) via AF_UNIX sockets.
Defaults to true.
Added in version 258.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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)).
.RE
.PP 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).
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP 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) when
Accept=no, connection otherwise.
Added in version 227.
.RE
.PP 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.
.RE
.PP 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 will not 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.
.RE
.PP DeferTrigger=
Takes a boolean argument, or patient. May only be used when Accept=no.
If enabled, job mode lenient instead of replace is used when triggering the
service, which means currently activating/running units that conflict with
the service won't be disturbed/brought down. Furthermore, if a conflict
exists, the socket unit will wait for current job queue to complete and
potentially defer the activation by then. An upper limit of total time to
wait can be configured via DeferTriggerMaxSec=. If set to yes,
the socket unit will fail if all jobs have finished or the timeout has been
reached but the conflict remains. If patient, always wait until
DeferTriggerMaxSec= elapses. Defaults to no.
This setting is particularly useful if the socket unit should stay active
across switch-root/soft-reboot operations while the triggered service is
stopped.
Added in version 258.
.RE
.PP DeferTriggerMaxSec=
Configures the maximum time to defer the triggering when DeferTrigger=
is enabled. If the service cannot be activated within the specified time,
the socket will be considered failed and get terminated. Takes a unit-less
value in seconds, or a time span value such as "5min 20s". Pass 0
or infinity to disable the timeout logic (the default).
Added in version 258.
.RE
.PP PassFileDescriptorsToExec=
Takes a boolean argument. Defaults to off. If enabled, file descriptors
created by the socket unit are passed to ExecStartPost=,
ExecStopPre=, and ExecStopPost= commands from the socket unit.
The passed file descriptors can be accessed with sd_listen_fds(3) as
if the commands were invoked from the associated service units. Note that
ExecStartPre= command cannot access socket file descriptors.
Added in version 256.
.RE
.PP Check systemd.unit(5), systemd.exec(5), and
systemd.kill(5) for more settings.
.SH "SEE ALSO"
.PP 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)
.PP For more extensive descriptions see the "systemd for
Developers" series: Socket Activation[5], Socket Activation,
part II[6], Converting inetd Services[7], Socket Activated
Internet Services and OS Containers[8].
NOTES
- 1.
- USB FunctionFS
- 2.
- RFC 3875
- 3.
- TCP Keepalive HOWTO
- 4.
- Smack
- 5.
- Socket Activation
- 6.
- Socket Activation, part II
- 7.
- Converting inetd Services
- 8.
- Socket Activated Internet Services and OS Containers
| systemd 262.2 |