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man: add basic documentation for resolved.conf's DNSSEC= switch

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Lennart Poettering 2016-01-05 00:31:32 +01:00
parent 6f8a2c6817
commit 519d39deee
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man/resolved.conf.xml
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@ -124,6 +124,61 @@
global setting is on.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>DNSSEC=</varname></term>
<listitem><para>Takes a boolean argument or
<literal>downgrade-ok</literal>. If true all DNS lookups are
DNSSEC-validated locally. If a response for a lookup request
is detected invalid this is returned as lookup failure to
applications. Note that this mode requires a DNS server that
supports DNSSEC. If the DNS server does not properly support
DNSSEC all validations will fail. If set to
<literal>downgrade-ok</literal> DNSSEC validation is
attempted, but if the server does not support DNSSEC properly,
DNSSEC mode is automatically disabled. Note that this mode
makes DNSSEC validation vulnerable to "downgrade" attacks,
where an attacker might be able to trigger a downgrade to
non-DNSSEC mode by synthesizing a DNS response that suggests
DNSSEC was not supported. If set to false, DNS lookups are not
DNSSEC validated.</para>
<para>Note that DNSSEC validation requires retrieval of
additional DNS data, and thus results in a small DNS look-up
time penalty.</para>
<para>DNSSEC requires knowledge of "trust anchors" to prove
data integrity. The trust anchor for the Internet root domain
is built into the resolver. However, trust anchors may change
in regular intervals, and old trust anchors may be revoked. In
such a case DNSSEC validation is not possible until new trust
anchors are configured locally or the resolver software
package is updated with the new root trust anchor. In effect,
when the built-in trust anchor is revoked and
<varname>DNSSEC=</varname> is true, all further lookups will
fail, as it cannot be proved anymore whether lookups are
correctly signed, or validly unsigned. If
<varname>DNSSEC=</varname> is set to
<literal>downgrade-ok</literal> the resolver will
automatically turn of DNSSEC validation in such a case.</para>
<para>Client programs looking up DNS data will be informed
whether lookups could be verified using DNSSEC, or whether the
returned data could not be verified (either because the data
was found unsigned in the DNS, or the DNS server did not
support DNSSEC or no appropriate trust anchors were known). In
the latter case it is assumed that client programs employ a
secondary scheme to validate the returned DNS data, should
this be required.</para>
<para>It is recommended to set <varname>DNSSEC=</varname> to
true on systems where it is kown that the DNS server supports
DNSSEC correctly, and where software or trust anchor updates
happen regularly. On other systems it is recommended to set
<varname>DNSSEC=</varname> to
<literal>missing-ok</literal>.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>