mirror of
https://github.com/systemd/systemd.git
synced 2024-11-27 12:13:33 +08:00
37d7a7d984
Change the resolved.conf Cache option to a tri-state "no, no-negative, yes" values.
If a lookup returns SERVFAIL systemd-resolved will cache the result for 30s (See 201d995),
however, there are several use cases on which this condition is not acceptable (See systemd#5552 comments)
and the only workaround would be to disable cache entirely or flush it , which isn't optimal.
This change adds the 'no-negative' option when set it avoids putting in cache
negative answers but still works the same heuristics for positive answers.
Signed-off-by: Jorge Niedbalski <jnr@metaklass.org>
274 lines
15 KiB
XML
274 lines
15 KiB
XML
<?xml version='1.0'?>
|
|
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
|
|
<!-- SPDX-License-Identifier: LGPL-2.1+ -->
|
|
|
|
<refentry id="resolved.conf" conditional='ENABLE_RESOLVE'
|
|
xmlns:xi="http://www.w3.org/2001/XInclude">
|
|
<refentryinfo>
|
|
<title>resolved.conf</title>
|
|
<productname>systemd</productname>
|
|
</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 4795</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>MulticastDNS=</varname></term>
|
|
<listitem><para>Takes a boolean argument or
|
|
<literal>resolve</literal>. Controls Multicast DNS support (<ulink
|
|
url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on
|
|
the local host. If true, enables full Multicast DNS 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 Multicast DNS settings. Multicast DNS 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 <literal>allow-downgrade</literal></para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>DNSOverTLS=</varname></term>
|
|
<listitem>
|
|
<para>Takes a boolean argument or <literal>opportunistic</literal>.
|
|
If true all connections to the server will be encrypted. Note that
|
|
this mode requires a DNS server that supports DNS-over-TLS and has
|
|
a valid certificate for it's IP. If the DNS server does not support
|
|
DNS-over-TLS all DNS requests will fail. When set to <literal>opportunistic</literal>
|
|
DNS request are attempted to send encrypted with DNS-over-TLS.
|
|
If the DNS server does not support TLS, DNS-over-TLS is disabled.
|
|
Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
|
|
attacks, where an attacker might be able to trigger a downgrade
|
|
to non-encrypted mode by synthesizing a response that suggests
|
|
DNS-over-TLS was not supported. If set to false, DNS lookups
|
|
are send over UDP.</para>
|
|
|
|
<para>Note that DNS-over-TLS requires additional data to be
|
|
send for setting up an encrypted connection, and thus results
|
|
in a small DNS look-up time penalty.</para>
|
|
|
|
<para>Note as the resolver is not capable of authenticating
|
|
the server, it is vulnerable for "man-in-the-middle" attacks.</para>
|
|
|
|
<para>In addition to this global DNSOverTLS setting
|
|
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
|
|
also maintains per-link DNSOverTLS settings. For system DNS
|
|
servers (see above), only the global DNSOverTLS 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>Defaults to off.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>Cache=</varname></term>
|
|
<listitem><para>Takes a boolean or <literal>no-negative</literal> as argument. If <literal>yes</literal> (the default), resolving a domain name
|
|
which already got queried earlier will return the previous result as long as it is still valid, and thus does
|
|
not result in a new network request. Be aware that turning off caching comes at a performance penalty, which
|
|
is particularly high when DNSSEC is used.</para>
|
|
If <literal>no-negative</literal>, only positive answers are cached.
|
|
|
|
<para>Note that caching is turned off implicitly if the configured DNS server is on a host-local IP address
|
|
(such as 127.0.0.1 or ::1), in order to avoid duplicate local caching.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>DNSStubListener=</varname></term>
|
|
<listitem><para>Takes a boolean argument or one of <literal>udp</literal> and <literal>tcp</literal>. If
|
|
<literal>udp</literal>, a DNS stub resolver will listen for UDP requests on address 127.0.0.53
|
|
port 53. If <literal>tcp</literal>, the stub will listen for TCP requests on the same address and port. If
|
|
<literal>yes</literal> (the default), the stub listens for both UDP and TCP requests. If <literal>no</literal>, the stub
|
|
listener is disabled.</para>
|
|
|
|
<para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
|
|
in use.</para></listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><varname>ReadEtcHosts=</varname></term>
|
|
<listitem><para>Takes a boolean argument. If <literal>yes</literal> (the default), the DNS stub resolver will read
|
|
<filename>/etc/hosts</filename>, and try to resolve hosts or address by using the entries in the file before
|
|
sending query to DNS servers.</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>
|