HylaFAX − introduction to Hyla FAX server operation and file formats


Hyla FAX 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 and configurability. Hyla FAX 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 Hyla FAX 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 directory ${SPOOL}. Ancillary command scripts used by the server programs faxq(1m), faxsend(1m), pagesend(1m), and faxgetty(1m) are located in the bin subdirectory. Configuration, access control, and accounting information are maintained in the etc and config subdirectories. Outgoing jobs are described by files in the sendq subdirectory, while received facsimile are deposited in the recvq subdirectory. The docq and temp 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. The doneq 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 On systems with job archival support, completed jobs that have been archived are placed in the archive subdirectory. The pollq subdirectory holds documents that are available for polled retrieval from the server. The info subdirectory contains files that describe the capabilities of facsimile machines that Hyla FAX has called−this information is used in preparing documents for transmission. The status subdirectory contains files that server processes write their current status to. The log subdirectory contains logging information about send and receive sessions. The client subdirectory contains FIFO special files used for communication with faxq.

Hyla FAX 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 agent using FIFO special files; see mknod(2) or mkfifo(2). Any other synchronization between server processes is done using file-level locking. The faxq process listens for commands written to the file named FIFO, while each faxgetty process listens for commands written to a per-device file named FIFO.devid (where devid is an identifier derived from the name of the device special file to which the modem is connected; e.g. ttym2 for /dev/ttym2, term_10 for /dev/term/10.) To send a command to the queueing agent, one writes to FIFO. This is useful, for example, for submitting a job for transmission. To send a command to a specific faxgetty process, the FIFO.devid file is used.

Client applications interact with a Hyla FAX server machine using a communications protocol implemented by the hfaxd(1m) program. The hfaxd program is typically started at system startup; it listens for client requests for service and creates a process for each client. hfaxd supports the submission of outbound jobs, querying the status of the send and receive queues, and altering parameters of previously submitted jobs. The hfaxd processes communicate with the faxq process through FIFO special files. Commands sent to faxq are sent to FIFO and responses are received on FIFO files that each hfaxd creates in the client subdirectory.


Each Hyla FAX server machine must run the faxsetup(1m) command prior to starting up Hyla FAX server processes. faxsetup verifys that the Hyla FAX 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 sendq subdirectory. This file contains all the information necessary to manage the transmission; c.f. sendq(4f). The actual documents that are to be sent are usually located in the docq subdirectory (though it is also possible to reference documents from the recvq directory). Hyla FAX accepts P OST S CRIPT , PDF, PCL , and TIFF documents for transmission (though support for PCL documents is not currently implemented). Documents are automatically converted to TIFF/F 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 info subdirectory. If a machine has not been called before, Hyla FAX assumes the remote machine has the requested capabilities. If a capabilities mismatch is detected while sending a facsimile Hyla FAX will disconnect and reconvert 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 faxsend(1m) process that is initiated by the scheduler. This program may be substituted for by specifying the FaxSendCmd configuration parameter in the faxq configuration file.

While a job is being processed by a server process, its job description file is locked for exclusive use with flock(2). The hfaxd(1m) program uses this information to tell if a job is actively being processed.

If the SessionTracing 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 log subdirectory. Each destination machine has a separate log file named by its canonical phone number.

The remote job submission facility includes host and user access control. The file etc/hosts.hfaxd 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 if local submission is to be permitted. For more information consult hosts.hfaxd(4f).

There are a number of controls on outbound calls that can be specified using the etc/destcontrols file (or whatever file is specified in a DestControls configuration parameter in the faxq configuration file). This file, described in destctrls(4f), allows an administrator to restrict calls by phone number and to control the time of day that calls may be placed. In addition, operational parameters such as the maximum number of pages in a facsimile transmission can be constrained on a per-destination basis.

If an error is encountered during transmission and a subsequent retransmission would not include the original cover page, then Hyla FAX can be configured to generate a continuation cover page that is prepended to the retransmitted pages. Such cover pages are usually generated by the bin/mkcover command; though the exact command to use can be specified in the configuration file read by faxq.

Hyla FAX 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. faxq 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.


faxgetty server processes can be configured to answer incoming phone calls and automatically receive facsimile. Received documents are placed in the recvq subdirectory as TIFF Class F files. The faxgetty 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 faxgetty process usually invokes the bin/faxrcvd command; though the exact command to invoke can be specified in the per-modem configuration file. The default notify command is a shell script that sends a mail message to a well known user, the FaxMaster, but one might also, for example, automatically spool the document for printing.

Hyla FAX supports a simple form of access control for receiving facsimile. Each faxgetty process may be configured to check the Transmission Subscriber Identifiers ( TSI ) of the remote fax machine against an access control list, typically etc/tsi. Only if the TSI 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 junk fax.

Hyla FAX can be configured to do copy quality checking on received facsimile data. When this feature is enabled faxgetty decodes and analyzes the received facsimile data as it is received. If data is received with too many errors, according to the setting of the MaxConsecutiveBadLines and PercentGoodLines configuration parameters, then the sender will be told to retransmit the page. 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 transcoding and can significantly reduce the space needed to store received facsimile.


Hyla FAX supports the polled retrieval of facsimile documents. Documents that are received because of a poll request are stored in the recvq subdirectory and also delivered directly to the requester using the bin/pollrcvd command; though the exact command to invoke can be specified with the PollRcvdCmd configuration parameter. The pollrcvd script typically encodes the binary facsimile data and returns it to the user via electronic mail.


In environments where Caller-ID information is available, Hyla FAX also supports a call screening facility similar to the TSI access control facility. faxgetty can be configured to check the phone number of each caller against an access control list, typically etc/cid. 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.

faxgetty is also capable of using distinctive ring information to identify whether an inbound call is voice, data, or fax. Consult the RingData, RingFax, and RingVoice parameters in hylafax-config(4f) for a description of this facility.


Most fax modems also support non-facsimile communication. Hyla FAX uses the locking mechanism employed by uucp(1C), cu(1C), slip(1m), and ppp(1m). Any faxgetty processes will transparently ‘‘get out of the way’’ when an application wants to use a modem for an outgoing call. In addition, Hyla FAX 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 GettyArgs parameter is specified in the configuration file, faxgetty will invoke the getty(1m) 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.


Hyla FAX maintains status information in several forms. General status information for each server process is maintained in the status subdirectory and returned to users by the faxstat(1) program. The syslog(3) 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. the ServerTracing parameter description in hylafax-config(4f).

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 bin/notify command script; though the name of the script may be overridden with the NotifyCmd configuration parameter.

The faxstat utility provides (remote) status of jobs queued for transmission, jobs received, and the general status of server processes.

The file etc/xferfaxlog contains status information about all facsimile sent and received on a machine. This file is in a simple ASCII format that is easy to manipulate with programs such as awk(1), to generate accounting information. See xferfaxlog(4f) for information about the format. See xferfaxstats(1m) and recvstats(1m) for example scripts that print summarized accounting information.

Finally, the hfaxd process supports a event monitoring facility that can be access via the faxwatch(1m) program. 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.


In normal operation each modem is managed by a Hyla FAX server process such as faxgetty. 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 faxstate(1m) program. The faxconfig(1m) program can also be used to dynamically make changes to configuration parameters that may cause a modem to be treated differently (e.g. setting RingsBeforeAnswer to zero will cause faxgetty to not answer incoming calls).

When Hyla FAX is used in a send-only configuration there are no faxgetty processes and communication must be done directly with the faxq process. The faxstate program can still be used to manipulate modem use for outbound jobs but the faxconfig 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 faxalter(1m) program. A job’s scheduling priority may also be altered by faxq 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 for use. Modem readiness is usually communicated to faxq by per-modem faxgetty processes. In a send-only environment however this is not possible; instead modems configured for use with faxmodem are considered always ready for use unless they are presently assigned to an outbound job or their state is explicitly changed through the faxstate(1m) program (faxstate 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 jobs first. Modem priority is statically configured through configuration files, the faxmodem program, and the faxconfig program. If multiple modems share the same priority value, then faxq(1m) will allocate jobs to them in a round-robin fashon.


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 suspended state, and may be returned to this state at any time that they are not actively being processed (e.g. a faxsend 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 pending (delayed waiting for a future time to send), sleeping (delayed waiting for a scheduled timeout), blocked (delayed by concurrent activity to the same destination), or ready (ready for transmission, waiting only for available resources). When a job is actively processed by the faxsend program its state is marked active. Jobs that have completed, either successfully or unsuccessfully are placed in a done state and their job description files are moved to the doneq subdirectory. Clients may still access the state of jobs that are done; until a ‘‘cleaner process’’ either purges them from the system or archives their state. 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.


Hyla FAX server programs read configuration information from a configuration file. Multiple files are used, one for the faxq 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 faxgetty 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, Hyla FAX server programs accept commands on their FIFO special files to alter configuration parameters in the running executable (the faxconfig(1m) 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 for communicating with a faxgetty process


command that prints information about received facsimile


command to force server to quit


faxd command for handling newly received facsimile


faxd command for generating continuation cover pages


faxd command for doing user notification


faxd command for delivering facsimile received by poll


faxd command for converting P OST S CRIPT to TIFF


documents available for transmission


server setup file created by faxsetup


caller id access control list


configuration data for <devid>


hosts that may submit jobs for transmission


fax machine receive access control list


log of facsimile sent and received


data base of remote fax machine capabilities


FIFO special files created by client processes


prototype configuration files used by faxaddmodem


session logging records


received facsimile


descriptions of jobs queued for transmission


descriptions of jobs that are done


server status information


temporary files created when submitting a job


database of archived jobs


faxsetup(1m), faxq(1m), faxgetty(1m), hfaxd(1m), faxsend(1m), faxrcvd(1m), faxconfig(1m), faxmodem(1m), faxstate(1m), notify(1m), pollrcvd(1m), recvstats(1m), xferfaxstats(1m), archive(4f), hylafax-config(4f), dialrules(4f), doneq(4f), hosts.hfaxd(4f), hylafax-info(4f), hylafax-log(4f), tsi(4f), recvq(4f), sendq(4f), status(4f), xferfaxlog(4f),

Extensive documentation is available in online at http://www.hylafax.org/. Many of these materials are also included in the software distribution.