SYSTEMD.SOCKET(5)systemd.socketSYSTEMD.SOCKET(5)NAMEsystemd.socket - systemd socket configuration files
SYNOPSISsystemd.socketDESCRIPTION
A unit configuration file whose name ends in .socket encodes
information about an IPC or network socket or a file system FIFO
controlled and supervised by systemd, for socket-based activation.
This man page lists the configuration options specific to this unit
type. See systemd.unit(5) for the common options of all unit
configuration files. The common configuration items are configured in
the generic [Unit] and [Install] sections. The socket specific
configuration options are configured in the [Socket] section.
Additional options are listed in systemd.exec(5), which define the
execution environment the ExecStartPre=, ExecStartPost=, ExecStopPre=
and ExecStoptPost= commands are executed in.
For each socket file a matching service file (see systemd.service(5)
for details) must exist, describing the service to start on incoming
traffic on the socket. Depending on the setting of Accept= (see below),
this must either be named like the socket unit, but with the suffix
replaced; or it must be a template file named the same way. Example: a
socket file foo.socket needs a matching service foo.service if
Accept=false is set. If Accept=true is set a service template file
foo@.service must exist from which services are instantiated for each
incoming connection.
Unless DefaultDependencies= is set to false, socket units will
implicitly have dependencies of type Requires= and After= on
sysinit.target as well as dependencies of type Conflicts= and Before=
on shutdown.target. These ensure that socket units pull in basic system
initialization, and are terminated cleanly prior to system shutdown.
Only sockets involved with early boot or late system shutdown should
disable this option.
Socket units may be used to implement on-demand starting of services,
as well as parallelized starting of services.
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) or via the traditional inetd(8)-style socket passing (i.e.
sockets passed in via STDIN and STDOUT, using StandardInput=socket in
the service file).
OPTIONS
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). 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) resp. sequential packet (SOCK_SEQPACKET)
socket. 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 for both IPv4 and 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 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 whether there is incoming traffic on them or not.
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 whether it will be up and running ever at all. To
deal with this it is recommended to set the FreeBind= option
described below.
ListenFIFO=
Specifies a file system FIFO to listen on. This expects an absolute
file system path as argument. Behaviour 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. Behaviour
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. Behaviour
otherwise is very similar to the ListenDatagram= directive above.
ListenMessageQueue=
Specifies a POSIX message queue name to listen on. This expects a
valid message queue name (i.e. beginning with /). Behaviour
otherwise is very similar to the ListenFIFO= directive above. On
Linux message queue descriptors are actually file descriptors and
can be inherited between processes.
BindIPv6Only=
Takes a 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.
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 automatic
dependency from this socket unit on the network interface device
unit (systemd.device(5) is created.
DirectoryMode=
If listening on a file system socket of 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.
SocketMode=
If listening on a file system socket of 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.
Accept=
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 false.
For performance reasons, it is recommended to write new daemons
only in a way that is suitable for Accept=false. This option is
mostly useful to allow daemons designed for usage with inetd(8), to
work unmodified with systemd socket activation.
MaxConnections=
The maximum number of connections to simultaneously run services
instances for, when Accept=true 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 for
sockets configured with Accept=no or datagram sockets. Defaults to
64.
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[1] for details.) 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.).
ReceiveBuffer=, SendBuffer=
Takes an integer argument controlling the receive resp. send buffer
sizes of this socket. This controls the SO_RCVBUF resp. SO_SNDBUF
socket options (see socket(7) for details.).
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.
PipeSize=
Takes an integer value. Controls the pipe buffer size of FIFOs
configured in this socket unit. See fcntl(2) for details.
MessageQueueMaxMessages=, MessageQueueMessageSize=
These two settings take integer values and control the mq_maxmsg
resp. mq_msgsize field 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 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 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.
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 (resp.
after) the listening sockets/FIFOs are created and bound. The first
token of the command line must be an absolute file name, 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 (resp. after) the
listening sockets/FIFOs are closed and removed. 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=
below.) 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 90s.
KillMode=
Specifies how processes of this socket unit shall be killed. One of
control-group, process, none.
This option is mostly equivalent to the KillMode= option of service
files. See systemd.service(5) for details.
KillSignal=
Specifies which signal to use when killing a process of this
socket. Defaults to SIGTERM.
SendSIGKILL=
Specifies whether to send SIGKILL to remaining processes after a
timeout, if the normal shutdown procedure left processes of the
socket around. Takes a boolean value. Defaults to "yes".
Service=
Specifies the service unit name to activate on incoming traffic.
This defaults to the service that bears the same name as the socket
(ignoring the different suffixes). In most cases it should not be
necessary to use this option.
SEE ALSOsystemd(1), systemctl(8), systemd.unit(5), systemd.exec(5),
systemd.service(5)AUTHOR
Lennart Poettering <lennart@poettering.net>
Developer
NOTES
1. TCP Keepalive HOWTO
http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/
systemd 03/16/2012 SYSTEMD.SOCKET(5)
