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), systemd.kill(5) and
systemd.resource-control(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. "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.
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 (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.
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",
"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.
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.
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.
Check systemd.unit(5), systemd.exec(5), and
systemd.kill(5) for more settings.