Socket files must include a [Socket] section, which carries
information about the socket or FIFO it supervises. A number of
options that may be used in this section are shared with other
unit types. These options are documented in systemd.exec(5) and
systemd.kill(5). The options specific to the [Socket] section of
socket units are the following:
ListenStream=, ListenDatagram=, ListenSequentialPacket=
Specifies an address to listen on for a stream (SOCK_STREAM),
datagram (SOCK_DGRAM), or sequential packet (SOCK_SEQPACKET)
socket, respectively. The address can be written in various
formats:
If the address starts with a slash ("/"), it is read as file
system socket in the AF_UNIX socket family.
If the address starts with an at symbol ("@"), it is read as
abstract namespace socket in the AF_UNIX family. The "@" is
replaced with a NUL character before binding. For details,
see unix(7).
If the address string is a single number, it is read as port
number to listen on via IPv6. Depending on the value of
BindIPv6Only= (see below) this might result in the service
being available via both IPv6 and IPv4 (default) or just via
IPv6.
If the address string is a string in the format "v.w.x.y:z",
it is 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
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 integer argument. Specifies the number of
connections to queue that have not been accepted yet. This
setting matters only for stream and sequential packet
sockets. See listen(2) for details. Defaults to SOMAXCONN
(128).
BindToDevice=
Specifies a network interface name to bind this socket to. If
set, traffic will only be accepted from the specified network
interfaces. This controls the SO_BINDTODEVICE socket option
(see socket(7) for details). If this option is used, an
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.
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.
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 HOWTOfalse.
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
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
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
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.txt
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", "veno",
"cubic", "lp" or any other available algorithm supported by
the IP stack. This setting applies only to stream sockets.
ExecStartPre=, ExecStartPost=
Takes one or more command lines, which are executed before or
after the listening sockets/FIFOs are created and bound,
respectively. The first token of the command line must be an
absolute filename, then followed by arguments for the
process. Multiple command lines may be specified following
the same scheme as used for ExecStartPre= of service unit
files.
ExecStopPre=, ExecStopPost=
Additional commands that are executed before or after the
listening sockets/FIFOs are closed and removed, respectively.
Multiple command lines may be specified following the same
scheme as used for ExecStartPre= of service unit files.
TimeoutSec=
Configures the time to wait for the commands specified in
ExecStartPre=, ExecStartPost=, ExecStopPre= and ExecStopPost=
to finish. If a command does not exit within the configured
time, the socket will be considered failed and be shut down
again. All commands still running will be terminated forcibly
via SIGTERM, and after another delay of this time with
SIGKILL. (See KillMode= in systemd.kill(5).) Takes a
unit-less value in seconds, or a time span value such as
"5min 20s". Pass "0" to disable the timeout logic. Defaults
to DefaultTimeoutStartSec= from the manager configuration
file (see systemd-system.conf(5)).
Service=
Specifies the service unit name to activate on incoming
traffic. This setting is only allowed for sockets with
Accept=no. It defaults to the service that bears the same
name as the socket (with the suffix replaced). In most cases,
it should not be necessary to use this option. 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 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) 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.exec(5) and systemd.kill(5) for more settings.
