HylaFAX - introduction to HylaFAX server operation and file formats
HylaFAX is a system for sending and receiving facsimile.
It supports queued transmission and asynchronous
reception of facsimile.
Ancillary programs are invoked by
the system for flexibility
HylaFAX includes client and server programs to
support remote submission of jobs for transmission,
remote removal of queued jobs, and to remotely
query the status of jobs queued for transmission.
This document describes the organization of the
filesystem spooling area in which HylaFAX
server and server-related processes operate, and
introduces the various files that reside in the spooling area.
The spooling area is typically located under the
Ancillary command scripts used by the server programs
are located in the
Configuration, access control, and accounting
information are maintained in the
Outgoing jobs are described by files in the
subdirectory, while received facsimile are deposited in the
subdirectories are also used in the preparation of outbound jobs;
the latter holds files that may be freely purged while the former
holds client files that may reside on the server independent of
an associated job.
subdirectory holds jobs that have completed but have not yet been
purged or archived.
On systems with job archival support, completed jobs that have
been archived are placed in the
subdirectory holds documents that are available for polled
retrieval from the server.
subdirectory contains files that describe the capabilities
of facsimile machines that HylaFAX has called-this information is used in preparing documents for transmission.
subdirectory contains files that server processes write their
current status to.
subdirectory contains logging information about send and
special files used for communication with
HylaFAX supports multiple modems on a host.
A single process acts as central queueing agent for all outbound jobs.
Typically each modem also has a server process
that monitors the modem status and handles inbound phone calls.
Per-modem server processes communicate with the central queueing
special files; see
Any other synchronization between server processes
is done using file-level locking.
process listens for commands written to the file named
process listens for commands written to a per-device file named
is an identifier derived from the name of the device
special file to which the modem is connected; e.g.
To send a command to the queueing agent, one writes to
This is useful, for example, for submitting a job for
To send a command to a specific
file is used.
Client applications interact with a HylaFAX server machine using
a communications protocol implemented by the
program is typically started at system startup; it listens for
client requests for service and creates a process for each client.
supports the submission of outbound jobs, querying the status
of the send and receive queues, and altering parameters of
previously submitted jobs.
processes communicate with the
Commands sent to
are sent to
and responses are received on
files that each
creates in the
Each HylaFAX server machine must run the
command prior to starting up HylaFAX server processes.
verifies that the HylaFAX software has been installed correctly
and that any parameters that were specified at the time the software
was built are appropriate for the system.
Each outgoing facsimile job has a job description file
that is located in the
This file contains all the information necessary to
manage the transmission; c.f.
The actual documents that are to be sent are usually located
subdirectory (though it is also possible to reference documents
HylaFAX accepts POSTSCRIPT, PDF, PCL, and
documents for transmission (support for PCL documents
requires an external application).
Documents are automatically converted to
documents prior to transmission according to the capabilities
of the remote facsimile machine: maximum page width
and length, ability to handle 2D-encoded data, and ability
to handle high resolution (7 line/mm) data.
This remote machine capability information is stored
in files in the
If a machine has not been called before,
HylaFAX assumes the remote machine has the requested capabilities.
If a capabilities mismatch is detected while sending a facsimile
HylaFAX will disconnect and re-convert the submitted documents according
to the newly discovered capabilities.
Users may also restrict the session parameters used to format
documents on a per-job basis.
The actual transmission is handled by a
process that is initiated by the scheduler.
This program may be substituted for by specifying the
configuration parameter in the
While a job is being processed by a server process,
its job description file is locked for exclusive
program uses this information to tell if a job is actively being processed.
parameter in a server's configuration file is non-zero,
then tracing information for an outgoing job will be logged
in a file in the
Each destination machine has a separate log file named
by its canonical phone number.
The remote job submission facility includes host and user
must be present and list those hosts and users that are
permitted to queue jobs for transmission or do other operations
that alter the status of a job.
Note that it is necessary to include the ``local host''
definition (usually 127.0.0.1) if local submission
is to be permitted.
For more information consult
If an error is encountered during transmission and a subsequent
retransmission would not include the original cover page, then
HylaFAX can be configured to generate a
continuation cover page
that is prepended to the retransmitted pages.
Such cover pages are usually generated by the
command; though the exact command to use can be specified in the
configuration file read by
HylaFAX can be configured to generate a line of status information
across the top of each page of an outbound facsimile.
This information, termed a ``tagline'', typically includes the
sender's identity (i.e. phone number), the time and date of the
transmission, and the page number.
The exact format of the tagline is configurable and applications
can override the default configuration parameters on a per-job basis.
Note that in the United States the law
requires that a tagline that identifies the sender's phone number
must appear on each transmitted page of facsimile.
Facsimile transmitted to receivers that accept variable-length pages
may have short pages ``chopped''.
That is, if a page has a significant amount of trailing whitespace
and the receiver accepts variable-length pages then only the top
part of the page will be transmitted.
can be configured so that only the last page of
each document is potentially chopped, all pages are potentially
chopped, or chopping is disabled.
The minimum whitespace threshold is also configurable.
Applications can override the default configuration parameters
on a per-job basis.
server processes can be configured to answer incoming
phone calls and automatically receive facsimile.
Received documents are placed in the
Class F files.
processes can be configured to make these files publicly
accessible, or they can be made private in which case
an administrator must manage their delivery and/or the assignment
of ownership to particular users.
When a facsimile is received, the
process usually invokes the
command; though the exact command to invoke can be specified
in the per-modem configuration file.
command is a shell script that sends a mail
message to a well known user, the
but one might also, for example, automatically spool the
document for printing.
HylaFAX supports a simple form of access control for receiving facsimile.
process may be configured to check the
Transmission Subscriber Identifiers (TSI)
of the remote fax machine against an access control list, typically
Only if the
is matched by a regular expression pattern in the file,
is the remote machine permitted to transmit a document.
This mechanism can be used, for example, to guard against
HylaFAX can be configured to do
copy quality checking
on received facsimile data.
When this feature is enabled
decodes and analyzes the received facsimile data as it is received.
If data is received with too many errors, according to the setting
configuration parameters, then the sender will be told to retransmit
When copy quality checking is enabled it is also possible to force
received facsimile data to be saved with a different compression
scheme than was used for transmission.
This function is known as
and can significantly reduce the space needed to store received facsimile.
HylaFAX supports the polled retrieval of facsimile documents.
Documents that are received because of a poll request are
stored in the
subdirectory and also delivered directly to the requester using the
command; though the exact command to invoke can be specified
script typically encodes the binary facsimile data and
returns it to the user via electronic mail.
INBOUND CALL HANDLING
In environments where Caller-ID information is available,
HylaFAX also supports a call screening facility similar to the
access control facility.
can be configured to check the phone number of each caller
against an access control list, typically
Only if the number is matched by a regular expression pattern
in the file is the call answered.
All Caller ID information is logged, irregardless of whether
or not it is used to screen incoming calls.
is also capable of using distinctive ring information
to identify whether an inbound call is voice, data, or fax.
for a description of this facility.
Most fax modems also support non-facsimile communication.
HylaFAX uses the locking mechanism employed by
processes will transparently ``get out of the way''
when an application wants to use a modem for an outgoing call.
In addition, HylaFAX can be configured to deduce whether an incoming
call is for facsimile or data use.
If a call from a data modem is recognized and the
parameter is specified in the configuration file,
will invoke the
program so that caller may login to the system.
Similar functionality is also available for invoking
a ``voice getty'' process, though auto-detection of inbound
voice calls is less extensive.
HylaFAX maintains status information in several forms.
General status information for each server process is maintained
subdirectory and returned to users by the
facility is used by all server processed
for logging status and error diagnostics.
The server processes may also be configured to log various
kinds of debugging and tracing information; c.f.
parameter description in
Any problems encountered when transmitting a facsimile
are described in messages returned to the user by electronic mail.
A user may also request notification by mail when a
job is requeued; for example, because a call failed.
Notification by electronic mail is implemented by the
command script; though the name of the script may be overridden
utility provides (remote) status of jobs queued
for transmission, jobs received, and the general
status of server processes.
contains status information about all facsimile sent and
received on a machine.
This file is in a simple
format that is easy to manipulate with programs such as
to generate accounting information.
for information about the format.
for example scripts that print summarized accounting information.
process supports a event monitoring facility that can be
access via the
This facility permits clients to register interest in various
events and receive ``realtime notification'' when such events occur
on the server.
Using this facility it is/should-be simple to construct applications
that do things like monitor modem status and use.
MODEM STATE CHANGES
In normal operation each modem is managed by a HylaFAX server
process such as
These processes communicate with the central scheduler process
to notify it when a modem is ready for use, busy for outbound use,
or possibly in an unusable state (either purposely marked unavailable
or potentially found to be wedged).
Modem usage can be explicitly controlled with the
program can also be used to dynamically make changes to configuration
parameters that may cause a modem to be treated differently (e.g.
to zero will cause
to not answer incoming calls).
When HylaFAX is used in a send-only configuration there are no
processes and communication must be done directly with the
program can still be used to manipulate modem use for outbound
jobs but the
program is not frequently needed.
Outbound jobs are scheduled by a single process.
Jobs have a ``scheduling priority'' that is assigned at the
time the job is submitted.
This priority can be changed at any time the job is not actively
being processed using the
A job's scheduling priority may also be altered by
in response to certain scheduling events (e.g. after a failed
attempt to send).
Modems are assigned to outbound jobs if they are deemed ready
Modem readiness is usually communicated to
In a send-only environment however this is not possible; instead
modems configured for use with
are considered always ready for use unless they are presently
assigned to an outbound job or their state is explicitly
changed through the
can also be used in a send-recv environment).
Each modem has a ``modem priority'' in the range [0..255].
Modems with a lower priority number are assigned to outbound
Modem priority is statically configured through configuration
program, and the
program. If multiple modems share the same priority value,
will allocate jobs to them in a round-robin
Outbound jobs are considered to be in one of several states
that reflect their treatment by the central scheduling process.
Jobs are initially created in a
state, and may be returned to this state at any time that they
are not actively being processed (e.g. a
program is running to process the job).
Jobs that are suspended are not processed by the scheduler; and their
internal state may be safely altered by the owner or a system
administrator. Suspending and then releasing a job has the effect
of requeueing the job, meaning that it will end up at the bottom of
queue for that job's priority.
Jobs that are ready for processing by the scheduler are ``submitted''
and their state is changed to be either
(delayed waiting for a future time to send),
(delayed waiting for a scheduled timeout),
(delayed by concurrent activity to the same destination),
(ready for transmission, waiting only for available resources).
When a job is actively processed by the
program its state is marked
Jobs that have completed, either successfully or unsuccessfully are
placed in a
state and their job description files are moved to the
Clients may still access the state of jobs that are done; until a
``cleaner process'' either purges them from the system or archives
This delayed removal of a completed job's state permits clients to
resubmit failed jobs using previously transmitted documents and other
job state information.
The exact mechanics of how and when done jobs are processed is
system-dependent; for example, how long a job is left in the done
queue before being purged, and whether job archival support is present.
HylaFAX server programs read configuration information from a
Multiple files are used, one for the
program and one for each modem.
Long-running server programs all automatically re-read their
configuration file if it is modified.
Typically this re-reading is done frequently enough that
administrators do not need to be aware of exactly when it takes place.
However in some esoteric cases the file may not be read when
expected (the one important case is that the
process reads its configuration file only when answering a
call or when resetting a modem; this means that
it will not recognize changes when the modem is idle).
In addition to the static configuration files, HylaFAX server
programs accept commands on their
special files to alter configuration parameters in the running
program can be used to dynamically change configuration parameters).
Values set in this way however are lost when the process exits
or if the configuration file is re-read.
Automatic routing of incoming facsimile is desirable.
FIFO fifo for job submission
FIFO.<devid> fifo for communicating with a faxgetty process
/usr/sbin/faxinfo command that prints information about received facsimile
/usr/sbin/faxquit command to force server to quit
bin/faxrcvd faxd command for handling newly received facsimile
bin/mkcover faxd command for generating continuation cover pages
bin/notify faxd command for doing user notification
bin/pollrcvd faxd command for delivering facsimile received by poll
bin/ps2fax faxd command for converting POSTSCRIPT to TIFF
docq/doc* documents available for transmission
etc/setup.cache server setup file created by faxsetup
etc/cid caller id access control list
etc/config.<devid> configuration data for <devid>
etc/hosts.hfaxd hosts that may submit jobs for transmission
etc/tsi fax machine receive access control list
etc/xferfaxlog log of facsimile sent and received
info/* data base of remote fax machine capabilities
client/* FIFO special files created by client processes
config/* prototype configuration files used by faxaddmodem
log/* session logging records
recvq/fax* received facsimile
sendq/q* descriptions of jobs queued for transmission
doneq/q* descriptions of jobs that are done
status/* server status information
tmp/* temporary files created when submitting a job
archive/* database of archived jobs