[ntp:questions] ntpd 4.2.7p230 defaults to ignoring ntpdc queries

Dave Hart hart at ntp.org
Thu Nov 3 21:14:14 UTC 2011

For a long time, ntpq and its mostly text-based mode 6 (control)
protocol have been preferred over ntpdc and its mode 7 (private
request) protocol for runtime queries and configuration.  There has
been a goal of deprecating ntpdc, previously held back by numerous
capabilities exposed by ntpdc with no ntpq equivalent.  I have been
adding commands to ntpq to cover these cases, and I believe I've
covered them all, though I've not compared command-by-command

As I've said previously, the binary mode 7 protocol involves a lot of
hand-rolled structure layout and byte-swapping code in both ntpd and
ntpdc which is hard to get right.  As ntpd grows and changes, the
changes are difficult to expose via ntpdc while maintaining forward
and backward compatibility between ntpdc and ntpd.  In contrast,
ntpq's text-based, label=value approach involves more code reuse and
allows compatible changes without extra work in most cases.

Mode 7 has always been defined as vendor/implementation-specific while
mode 6 is described in RFC 1305 and intended to be open to interop
with other implementations.  There is an early draft of an updated
mode 6 description that likely will join the other NTPv4 RFCs
eventually. [1]

For these reasons, ntpd 4.2.7p230 by default disables processing of
ntpdc queries, reducing ntpd's attack surface and functionally
deprecating ntpdc.  If you are in the habit of using ntpdc for certain
operations, please try the ntpq equivalent.  If there's no equivalent,
please open a bug report at http://bugs.ntp.org./

Dave Hart

[1] http://tools.ietf.org/html/draft-odonoghue-ntpv4-control-01

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