Summary

  • RFC 1696 defined SNMP-managed objects for dial-up modems. Its mdmCCRingsBeforeAnswer value was writable: setting it to zero meant the modem would not answer an incoming ring.
  • The same MIB let a manager write onHook, busiedOut or reset to the line-state object. The 1994 memo left security issues undiscussed; RFC 4450 later included it among old Proposed Standards moved to Historic.

The telephone rings at a modem bank. Whether a particular box answers can be decided by a number in a management system. The four authors of the August 1994 Modem Management Information Base called that object mdmCCRingsBeforeAnswer. Its default was one ring. Set it to zero, and the modem would not go off-hook to take the call. The command did not require a hand on the cable.

The specification was meant to give network managers a minimum set of objects to monitor and control dial-up devices. Each modem had a managed row and a stable index; the manager could inspect manufacturer details, line state, protocol capabilities, call outcomes and counters. The same vocabulary also contained writes. mdmLineState allowed a management command to put a modem on-hook, mark it busied-out or reset it from any state. A reset returned it to a new, implementation-dependent state. Those were operating actions, not just labels in an inventory.

Other fields could make an access line behave differently without changing the wiring. mdmLineCarrierLossTime set how long the modem waited after carrier loss before hanging up, in tenths of a second; the value 255 meant it would not hang up on that event. An inactivity timeout, measured in minutes, could disconnect an idle asynchronous call; zero disabled that timer. A requested protocol capability could be marked disabled, optional or preferred. The MIB reported a separate granted value, so a manager’s request and the modem’s accepted configuration were not the same fact.

The counters made the control surface tempting. A manager could read unanswered rings, connection failures, completed calls and retrains, then change a writable setting. That is a useful repair loop if the person making the change owns the line and knows the caller’s cost. It is also the place where an innocent-looking management object becomes a customer-facing switch. Setting rings-before-answer to zero can stop new callers reaching that modem. Setting a line busied-out takes it out of ordinary service.

The original memo’s Security Considerations section says only that security issues are not discussed. That is a limit of the document, not a report of a breach. The RFC Editor now lists RFC 1696 as Historic. In 2006, RFC 4450’s review of older Proposed Standards placed it on the reclassification list after asking whether implementers would build or maintain them. Neither record describes a named dial-up failure caused by these writes.

The story is the assignment of authority. In the dial-up era, a phone line looked local and tangible. Its answer behavior could be expressed as a remote write to a managed object. The inventory screen and the off switch shared a data model; only the permissions and operating record distinguished observation from intervention.

Sources