S 5.75 Protecting against overloading the fax server

Initiation responsibility: IT Security Officer, Head of IT

Implementation responsibility: Administrator, Fax Mail Centre

A fax server can become overloaded with incoming as well as outgoing fax transmissions. If the fax server is overloaded, this could mean a temporary suspension in the receipt or dispatch of fax transmissions. It is also possible that such an overload could cause the operating system or the fax server application to crash, thus rendering the fax server completely unavailable for a time.

The fax server can be said to be overloaded if all the channels provided by the communications cards are blocked with incoming and outgoing fax transmissions. The result is that no more faxes can be received or sent until a channel becomes free again. The same situation will occur if all the lines provided by the telecommunications company are busy with incoming and outgoing fax transmissions.

Before purchasing one or more fax servers, it is necessary to estimate the likely volume of faxes. Components which are capable of handling this volume should then be purchased. Furthermore, measures should be taken to ensure that sufficient telecommunications lines are available.

The fax server log files should be regularly examined in order to identify whether there are certain times when the server is being overloaded or working at the limits of its capacity.

The fax server can be overloaded if an attempt is made to send out a large number of faxes within an organisation. This could, in adverse circumstances, cause the fax server application or the operating system to crash. This could be triggered, for example, by an attempt to send out a large number of serial faxes. Trials should therefore be run during the test or pilot phase in order to determine the maximum load. A maximum size should be imposed on users for serial fax transmissions, e.g. by adopting an appropriate standard operating procedure, in order to avoid exceeding this limit. Very large serial fax transmissions should then be split into several smaller transmissions. Measures should be taken to ensure that only urgent faxes are sent at times when the load on the fax server is high, either by way of an appropriate standard operating procedure or by appropriate permission settings on the fax server. One possible solution is to stipulate that, wherever possible, faxes are timed to be transmitted at night which also has the advantage of lower telephone charges.

If the fax server is found to be blocked at certain times by the same call numbers sending large numbers of faxes, the first step will be to identify the senders and the nature of the fax transmissions concerned. If the organisation really needs these faxes, one possibility would be to arrange with the senders when they should send their faxes so that they can be received without any problems. If the faxes are not actually needed (e.g. unsolicited advertisements), attempts can be made to block the call numbers via the fax server application or via the private branch exchange. This will only be possible, however, if the caller sender identification (CSID) is not disguised or, where ISDN is used, if the call number is not withheld by the sender. If it is not possible to determine the sender's fax number, the only remaining option will be to expand the existing capacity - as described above.

Another potential problem with a fax server is hard disk capacity. However, it is unlikely that the hard disk space will be entirely exhausted by a strategic attack from outside. A single faxed DIN A4 page occupies around 70 kB. Given that most hard disks today hold several gigabytes and given the telephone charges which would be incurred, an attack of this type is improbable. In principle, all incoming and outgoing fax transmissions are stored either permanently or temporarily on the hard disk of the fax server. The further course of action depends on the fax server application and possibly on the configuration as well. One possibility, for example, is that all fax transmissions are permanently stored or archived on the hard disk of the fax server. When this mode of operation is employed, it is possible - depending on the volume of faxes - for the hard disk capacity to become exhausted very quickly. In this case steps should be taken to ensure that outgoing fax transmissions and incoming faxes which have already been read are archived as soon as possible on external data media and deleted from the fax server. The amount of memory placed at the disposal of users on the fax server should therefore be limited. A standard operating procedure should also be issued, for example, dictating that fax transmissions which are no longer required should be deleted. This applies especially to any unsolicited advertisements received. Regular checks should be made by the fax mail centre on the amount of memory available on the fax server's hard disk.

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