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SYSTEMD.SOCKET(5) systemd.socket SYSTEMD.SOCKET(5)
NAME
systemd.socket - Socket unit configuration
SYNOPSIS
socket.socket
DESCRIPTION
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 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.
For each socket file, a matching service file must exist, describing the service to start
on incoming traffic on the socket (see systemd.service(5) for more information about
.service files). 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=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 will have a Before= dependency on the service which they trigger added
implicitly. 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.
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.
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 standard input and output, 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) 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 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 below).
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 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. 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.
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, 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 automatic
dependency from this socket unit on the network interface device unit
(systemd.device(5) is created.
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 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. 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=false, but it may do so for sockets
it got with Accept=true set. Setting Accept=true 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, 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.
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 on
sockets configured with Accept=false 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.
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[1] for details.) Defaults value is 7200 seconds
(2 hours).
KeepAliveIntervalSec=
Takes time (in seconds) as argument between individual keepalive probes, if the socket
option SO_KEEPALIVE has been set on this socket seconds as argument. This controls the
TCP_KEEPINTVL socket option (see socket(7) and the TCP Keepalive HOWTO[1] for
details.) Defaults value is 75 seconds.
KeepAliveProbes=
Takes integer as argument. It's 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[1] for details.)
Defaults value is 9.
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 behaviour of honouring 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[2] for details.
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 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 "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 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 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.
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. Defaults to the empty list.
Check systemd.exec(5) and systemd.kill(5) for more settings.
SEE ALSO
systemd(1), systemctl(1), systemd.unit(5), systemd.exec(5), systemd.kill(5),
systemd.resource-control(5), systemd.service(5), systemd.directives(7)
For more extensive descriptions see the "systemd for Developers" series: Socket
Activation[3], Socket Activation, part II[4], Converting inetd Services[5], Socket
Activated Internet Services and OS Containers[6].
NOTES
1. TCP Keepalive HOWTO
http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/
2. Smack.txt
https://www.kernel.org/doc/Documentation/security/Smack.txt
3. Socket Activation
http://0pointer.de/blog/projects/socket-activation.html
4. Socket Activation, part II
http://0pointer.de/blog/projects/socket-activation2.html
5. Converting inetd Services
http://0pointer.de/blog/projects/inetd.html
6. Socket Activated Internet Services and OS Containers
http://0pointer.de/blog/projects/socket-activated-containers.html
systemd 219 SYSTEMD.SOCKET(5)
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