mirror of
https://github.com/systemd/systemd.git
synced 2024-11-01 09:21:26 +03:00
ceeddf79b8
In some cases, caching DNS results locally is not desirable, a it makes DNS cache poisoning attacks a tad easier and also allows users on the system to determine whether or not a particular domain got visited by another user. Thus provide a new "Cache" resolved.conf option to disable it.
237 lines
12 KiB
XML
237 lines
12 KiB
XML
<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
|
|
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
|
|
|
|
<!--
|
|
This file is part of systemd.
|
|
|
|
Copyright 2014 Tom Gundersen
|
|
|
|
systemd is free software; you can redistribute it and/or modify it
|
|
under the terms of the GNU Lesser General Public License as published by
|
|
the Free Software Foundation; either version 2.1 of the License, or
|
|
(at your option) any later version.
|
|
|
|
systemd is distributed in the hope that it will be useful, but
|
|
WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
|
|
Lesser General Public License for more details.
|
|
|
|
You should have received a copy of the GNU Lesser General Public License
|
|
along with systemd; If not, see <http://www.gnu.org/licenses/>.
|
|
-->
|
|
|
|
<refentry id="resolved.conf" conditional='ENABLE_RESOLVED'
|
|
xmlns:xi="http://www.w3.org/2001/XInclude">
|
|
<refentryinfo>
|
|
<title>resolved.conf</title>
|
|
<productname>systemd</productname>
|
|
|
|
<authorgroup>
|
|
<author>
|
|
<contrib>Developer</contrib>
|
|
<firstname>Tom</firstname>
|
|
<surname>Gundersen</surname>
|
|
<email>teg@jklm.no</email>
|
|
</author>
|
|
</authorgroup>
|
|
</refentryinfo>
|
|
|
|
<refmeta>
|
|
<refentrytitle>resolved.conf</refentrytitle>
|
|
<manvolnum>5</manvolnum>
|
|
</refmeta>
|
|
|
|
<refnamediv>
|
|
<refname>resolved.conf</refname>
|
|
<refname>resolved.conf.d</refname>
|
|
<refpurpose>Network Name Resolution configuration files</refpurpose>
|
|
</refnamediv>
|
|
|
|
<refsynopsisdiv>
|
|
<para><filename>/etc/systemd/resolved.conf</filename></para>
|
|
<para><filename>/etc/systemd/resolved.conf.d/*.conf</filename></para>
|
|
<para><filename>/run/systemd/resolved.conf.d/*.conf</filename></para>
|
|
<para><filename>/usr/lib/systemd/resolved.conf.d/*.conf</filename></para>
|
|
</refsynopsisdiv>
|
|
|
|
<refsect1>
|
|
<title>Description</title>
|
|
|
|
<para>These configuration files control local DNS and LLMNR
|
|
name resolution.</para>
|
|
|
|
</refsect1>
|
|
|
|
<xi:include href="standard-conf.xml" xpointer="main-conf" />
|
|
|
|
<refsect1>
|
|
<title>Options</title>
|
|
|
|
<para>The following options are available in the <literal>[Resolve]</literal> section:</para>
|
|
|
|
<variablelist class='network-directives'>
|
|
|
|
<varlistentry>
|
|
<term><varname>DNS=</varname></term>
|
|
<listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as system DNS servers. DNS requests
|
|
are sent to one of the listed DNS servers in parallel to suitable per-link DNS servers acquired from
|
|
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
|
|
set at runtime by external applications. For compatibility reasons, if this setting is not specified, the DNS
|
|
servers listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any servers
|
|
are configured in it. This setting defaults to the empty list.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>FallbackDNS=</varname></term>
|
|
<listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as the fallback DNS servers. Any
|
|
per-link DNS servers obtained from
|
|
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
|
|
take precedence over this setting, as do any servers set via <varname>DNS=</varname> above or
|
|
<filename>/etc/resolv.conf</filename>. This setting is hence only used if no other DNS server information is
|
|
known. If this option is not given, a compiled-in list of DNS servers is used instead.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>Domains=</varname></term>
|
|
<listitem><para>A space-separated list of domains. These domains are used as search suffixes when resolving
|
|
single-label host names (domain names which contain no dot), in order to qualify them into fully-qualified
|
|
domain names (FQDNs). Search domains are strictly processed in the order they are specified, until the name
|
|
with the suffix appended is found. For compatibility reasons, if this setting is not specified, the search
|
|
domains listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any domains
|
|
are configured in it. This setting defaults to the empty list.</para>
|
|
|
|
<para>Specified domain names may optionally be prefixed with <literal>~</literal>. In this case they do not
|
|
define a search path, but preferably direct DNS queries for the indicated domains to the DNS servers configured
|
|
with the system <varname>DNS=</varname> setting (see above), in case additional, suitable per-link DNS servers
|
|
are known. If no per-link DNS servers are known using the <literal>~</literal> syntax has no effect. Use the
|
|
construct <literal>~.</literal> (which is composed of <literal>~</literal> to indicate a routing domain and
|
|
<literal>.</literal> to indicate the DNS root domain that is the implied suffix of all DNS domains) to use the
|
|
system DNS server defined with <varname>DNS=</varname> preferably for all domains.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>LLMNR=</varname></term>
|
|
<listitem><para>Takes a boolean argument or
|
|
<literal>resolve</literal>. Controls Link-Local Multicast Name
|
|
Resolution support (<ulink
|
|
url="https://tools.ietf.org/html/rfc4795">RFC 4794</ulink>) on
|
|
the local host. If true, enables full LLMNR responder and
|
|
resolver support. If false, disables both. If set to
|
|
<literal>resolve</literal>, only resolution support is enabled,
|
|
but responding is disabled. Note that
|
|
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
|
|
also maintains per-link LLMNR settings. LLMNR will be
|
|
enabled on a link only if the per-link and the
|
|
global setting is on.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>DNSSEC=</varname></term>
|
|
<listitem><para>Takes a boolean argument or
|
|
<literal>allow-downgrade</literal>. If true all DNS lookups are
|
|
DNSSEC-validated locally (excluding LLMNR and Multicast
|
|
DNS). If the response to a lookup request is detected to be invalid
|
|
a lookup failure is returned 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>allow-downgrade</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, additional trust anchors may be
|
|
defined with
|
|
<citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
|
|
Trust anchors may change at 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>allow-downgrade</literal> the resolver will
|
|
automatically turn off 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 known 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>allow-downgrade</literal>.</para>
|
|
|
|
<para>In addition to this global DNSSEC setting
|
|
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
|
|
also maintains per-link DNSSEC settings. For system DNS
|
|
servers (see above), only the global DNSSEC setting is in
|
|
effect. For per-link DNS servers the per-link
|
|
setting is in effect, unless it is unset in which case the
|
|
global setting is used instead.</para>
|
|
|
|
<para>Site-private DNS zones generally conflict with DNSSEC
|
|
operation, unless a negative (if the private zone is not
|
|
signed) or positive (if the private zone is signed) trust
|
|
anchor is configured for them. If
|
|
<literal>allow-downgrade</literal> mode is selected, it is
|
|
attempted to detect site-private DNS zones using top-level
|
|
domains (TLDs) that are not known by the DNS root server. This
|
|
logic does not work in all private zone setups.</para>
|
|
|
|
<para>Defaults to off.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>Cache=</varname></term>
|
|
<listitem><para>Takes a boolean argument. If "yes" (the default),
|
|
resolving a domain name which already got queried earlier will re-use
|
|
the previous result as long as that is still valid, and thus does not
|
|
need to do an actual network request.</para>
|
|
|
|
<para>However, local caching slightly increases the chance of a
|
|
successful DNS poisoning attack, and might also be a privacy problem in
|
|
some environments: By measuring the time it takes to resolve a
|
|
particular network name, a user can determine whether any other user on
|
|
the same machine recently visited that name. If either of these is a
|
|
concern, you may disable the local caching. Be aware that this comes at
|
|
a performance cost, which is <emphasis>very</emphasis> high with DNSSEC.
|
|
</para></listitem>
|
|
</varlistentry>
|
|
|
|
</variablelist>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>See Also</title>
|
|
<para>
|
|
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
|
|
<citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>4</manvolnum></citerefentry>
|
|
</para>
|
|
</refsect1>
|
|
|
|
</refentry>
|