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:
Specifies an address to listen on for a stream
), datagram (SOCK_DGRAM
), or sequential packet
) socket, respectively. The address can be written in
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
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
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.
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.
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
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
Specifies a USB FunctionFS endpoints location
to listen on, for implementation of USB gadget functions. This expects an
absolute file system path of 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.
Takes one of udplite or sctp. Specifies a
socket protocol (IPPROTO_UDPLITE) UDP-Lite (IPPROTO_SCTP) SCTP
Takes one of default
. 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
(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
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)
details. Defaults to SOMAXCONN (128).
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
for details). If this option is used, an implicit dependency
from this socket unit on the network interface device unit
is created. Note that setting this parameter might
result in additional dependencies to be added to the unit (see above).
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.
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.
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.
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
. 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
socket may, but does not need to, call close(2)
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
, 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.
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.
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.
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.
The maximum number of connections for a service per
source IP address. This is very similar to the MaxConnections=
directive above. Disabled by default.
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)
the TCP Keepalive HOWTO
 for details.) Defaults to
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
 for details.) Defaults value is 7200 seconds (2 hours).
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
 for details.) Defaults value is 75
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
and the TCP Keepalive HOWTO
 for details.)
Defaults value is 9.
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)
Takes an integer argument controlling the priority for
all traffic sent from this socket. This controls the SO_PRIORITY socket option
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
socket option will be used (see tcp(7)
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.
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.
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
may be specified.
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)
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
Takes a boolean value. If true, allows multiple
s to this TCP or UDP port. This controls the SO_REUSEPORT socket
option. See socket(7)
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 for details.
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
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
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)
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.
Takes a boolean value. Controls the IP_TRANSPARENT socket
option. Defaults to false.
Takes a boolean value. This controls the SO_BROADCAST
socket option, which allows broadcast datagrams to be sent from this socket.
Defaults to false.
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
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
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
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.
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.
Configures the time to wait for the commands specified in
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
after another delay of this time with SIGKILL
. (See KillMode=
.) 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)
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).
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
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
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
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.
Configures a limit on how often this socket unit my 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)
on the various time units understood). The TriggerLimitBurst=
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.exec(5) and systemd.kill(5) for more