The s6-fdholder-daemon program
s6-fdholder-daemon is a fd-holding daemon, i.e. a
It listens on a Unix domain socket, then
accepts client connections;
it stores file descriptors on behalf of clients, along with an identifier
for every file descriptor stored, and possibly an expiration date (after
which the file descriptor will be forgotten). It also allows clients to
retrieve a file descriptor by its identifier.
s6-fdholder-daemon [ -1 ] [ -v verbosity ] [ -d | -D ] [ -c maxconn ] [ -n maxfds ] [ -b backlog ] [ -G gidlist ] [ -g gid ] [ -u uid ] [ -U ] [ -t clienttimeout ] [ -T lameducktimeout ] [ -i rulesdir | -x rulesfile ] path
- s6-fdholder-daemon parses the options and arguments it is given, and
builds a new command line with them. It then executes into that new
- The first program s6-fdholder-daemon executes into is
It will create and bind a Unix domain socket to path, then
execute into the rest of the command line.
- If a privilege-dropping operation has been requested, the
program that s6-ipcserver-socketbinder executes into is
It will drop the root privileges, then execute into the rest of the
- The last program in the chain is
s6-fdholderd. It is executed into
by s6-applyuidgid, or directly by s6-ipcserver-socketbinder if no
privilege-dropping operation has been requested. s6-fdholderd is
the long-lived process, the daemon itself, performing fd holding and
accepting connections from clients.
- -1 : write a newline to stdout, before
closing it, right after binding and listening to the Unix socket.
If stdout is suitably redirected, this can be used by monitoring
programs to check when the server is ready to accept connections.
- -v verbosity : be quiet, normally
verbose, or more verbose, depending on if verbosity is 0,
1, or more. The default is 1.
- -d : allow instant rebinding to the same path
even if it has been used not long ago - this is the SO_REUSEADDR flag to
and is generally used with server programs. This is the default. Note that
path will be deleted if it already exists at program start time.
- -D : disallow instant rebinding to the same path.
- -c maxconn : accept at most
maxconn concurrent client connections. Default is 16. It is
impossible to set it higher than the value of the S6_FDHOLDER_MAX macro,
which is 256. Client connections to this server are short-lived, so this
number needs not be too high. Every client connection eats up
one available file descriptor, so it is best for maxconn to be
as small as possible.
- -n maxfds : store at most
maxfds file descriptors. Default is 1000.
It is impossible to set it higher than the number of files that can
be opened by the s6-fdholder-daemon process minus a few descriptors
needed for correct s6-fdholderd
operation. Before running s6-fdholder-daemon, make sure to
properly adjust the
of openable files of the current process.
- -b backlog : set a maximum of
backlog backlog connections on the socket. Extra
connection attempts will rejected by the kernel.
- -G gidlist : change s6-fdholder-daemon's
supplementary group list to gidlist after binding the socket.
This is only valid when run as root. gidlist must be a
comma-separated list of numerical group IDs.
- -g gid : change s6-fdholder-daemon's groupid
to gid after binding the socket. This is only valid when run
- -u uid : change s6-fdholder-daemon's userid
to uid after binding the socket. This is only valid when run
- -U : change s6-fdholder-daemon's user id, group id and
supplementary group list
according to the values of the UID, GID and GIDLIST environment variables
after binding the socket. This is only valid when run as root.
This can be used with the
program to easily script a service that binds to a privileged socket
then drops its privileges to those of a named non-root account.
- -t clienttimeout : disconnect a client
if it's in the middle of an operation and it has not written or read any
data in clienttimeout milliseconds. By default, clienttimeout
is 0, which means infinite.
- -T lameducktimeout : give clients
lameducktimeout milliseconds to finish their current operation
before exiting after s6-fdholderd has received a SIGTERM. By default,
lameducktimeout is 0, which means infinite.
- -x rulesfile : read access rights
configuration from CDB file rulesfile.
- -i rulesdir : read access rights
configuration from the filesystem in directory rulesdir.
- s6-fdholder-daemon does not interpret its options itself. It just
dispatches them to the appropriate program on the command line that
- From the user's point of view, s6-fdholder-daemon behaves like a
long-lived process, even if the long-lived process itself is called
s6-fdholderd. Every operational detail
of s6-fdholderd applies to s6-fdholder-daemon as well; in particular,
make sure to properly
configure the clients'
- s6-fdholder-daemon is meant to be used in an s6 run script, as
a supervised local service. It does not fork itself or write to syslog.
However, it can be run under any infrastructure, including other
supervision infrastructures, OpenRC, systemd, or SysV scripts.