BIND 10 #910: TSIG + TC bit support

[BIND 10 Development]

#910: TSIG + TC bit support
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                   Reporter:         |                 Owner:  jinmei
  jinmei                             |                Status:  accepted
                       Type:         |             Milestone:
  enhancement                        |  Sprint-20110802
                   Priority:  major  |            Resolution:
                  Component:         |             Sensitive:  0
  DNSPacket API                      |           Sub-Project:  DNS
                   Keywords:         |  Estimated Difficulty:  4.0
            Defect Severity:  N/A    |           Total Hours:  0
Feature Depending on Ticket:         |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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Comment (by jinmei):

 trac910 is ready for review.

 This branch consists of several not-directly-related sets of changes,
 which unfortunately makes the total diff a bit big.

 - the first two commits extend TSIGContext class so that the Message
   class can detect the expected length of TSIG RR.
 - most of the remaining changes are for the Message::toWire() so that
   the truncation case will be handled correctly.  Python test cases
   are straightforward mapping from the corresponding C++ tests, so
   hopefully this fact reduces review load.
 - commit 146c48 is an exception.  It's not directly related to
   TSIG+TC, but is necessary to handle a corner case scenario.  I
   considered an option of differing this change and the corner case
   handling, but that would open up a possibility of making the
   toWire() logic cause an unexpected exception, which would be a
   remotely exploitable DoS to the auth server.  So I decided it to
   include the entire set.

 The proposed change entry is as follows:
 {{{
 269.?   [func]          jinmei
         libdns++/pydnspp: TSIG signing now handles truncated DNS messages
         (i.e. with TC bit on) with TSIG correctly.
         (Trac #910, git TBD)
 }}}

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Ticket URL: <http://bind10.isc.org/ticket/910#comment:7>
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