Bug Report from www.isc.org:

Name: Greg Rabil
Email: greg.rabil@bt.com
Software Version: DHCP 4.4.1
OS: Linux
Subject:Dynamic DNS updates are done via TCP instead of UDP


Bug Detail
===========
In DHCP 4.3, all dynamic DNS updates are performed using UDP.  In DHCP 4.4, that behavior has changed to use TCP.  As far as I can tell, this change was not intentional, and instead is a bug caused by changes in the BIND library.

In both DHCP 4.3 and 4.4, the common/dns.c source file contains a macro that maps ddns_update to dns_client_startupdate.  When the ddns_modify_fwd function calls ddns_update (dns_client_startupdate), it passes DNS_CLIENTRESOPT_ALLOWRUN as the eight argument.  The bind/include/dns/client.h file defines DNS_CLIENTRESOPT_ALLOWRUN as 0x02.

The problem stems from the change in the implementation of the dns_client_startupdate function between the bind-9.9.11 package in DHCP 4.3 and the bind-9.11.2-p1 package in DHCP 4.4.  Specifically, in bind-9.9.11, the implementation of dns_client_startupdate in lib/dns/client.c references the eight parameter (options) only in this line:

        UNUSED(options);

However, in bind-9.11-2-p1, the implementation includes the line above, as well as this line:

        want_tcp = ISC_TF((options & DNS_CLIENTUPDOPT_TCP) != 0);

Note that lib/dns/include/dns/client.h defines DNS_CLIENTUPDOPT_TCP as 0x02, which then sets the 'want_tcp' boolean to true.

Since the caller (client.c in DHCP source code) is intending to set DNS_CLIENTRESOPT_ALLOWRUN flag via the options parameter, but not the DNS_CLIENTUPDOPT_TCP flag, this is the source of the problem, and a change in behavior with respect to the DDNS updates using UDP in DHCP 4.3 but TCP in DHCP 4.4.

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