Ubuntu Manpage: systemd.socket - Socket unit configuration

Provided by: systemd_204-5ubuntu20.31_amd64

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.

       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) 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
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.

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.
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.

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.

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.

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=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.

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 or send buffer sizes of this socket, respectively.
This controls the SO_RCVBUF and 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.

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.

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 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 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 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 90s.

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.

Check systemd.exec(5) and systemd.kill(5) for more settings.

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

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

SEE ALSO

NOTES