1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
|
<?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="systemd-resolved.service" conditional='ENABLE_RESOLVED'>
<refentryinfo>
<title>systemd-resolved.service</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>systemd-resolved.service</refentrytitle>
<manvolnum>8</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd-resolved.service</refname>
<refname>systemd-resolved</refname>
<refpurpose>Network Name Resolution manager</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>systemd-resolved.service</filename></para>
<para><filename>/usr/lib/systemd/systemd-resolved</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><command>systemd-resolved</command> is a system service that provides network name resolution to local
applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR resolver and
responder. Local applications may submit network name resolution requests via three interfaces:</para>
<itemizedlist>
<listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus. See the
<ulink url="http://www.freedesktop.org/wiki/Software/systemd/resolved">API Documentation</ulink> for
details. Usage of this API is generally recommended to clients as it is asynchronous and fully featured (for
example, properly returns DNSSEC validation status and interface scope for addresses as necessary for supporting
link-local networking).</para></listitem>
<listitem><para>The glibc
<citerefentry project='man-pages'><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry> API as defined
by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related resolver functions,
including <citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This
API is widely supported, including beyond the Linux platform. In its current form it does not expose DNSSEC
validation status information however, and is synchronous only. This API is backed by the glibc Name Service
Switch (<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>). Usage of the
glibc NSS module <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>
is required in order to allow glibc's NSS resolver functions to resolve host names via
<command>systemd-resolved</command>.</para></listitem>
<listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on IP
address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly, bypassing any local
API may be directed to this stub, in order to connect them to <command>systemd-resolved</command>. Note however
that it is strongly recommended that local programs use the glibc NSS or bus APIs instead (as described above),
as various network resolution concepts (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped
to the unicast DNS protocol.</para></listitem>
</itemizedlist>
<para>The DNS servers contacted are determined from the global settings in
<filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in
<filename>/etc/systemd/network/*.network</filename> files, the per-link dynamic settings received over DHCP and any
DNS server information made available by other system services. See
<citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details
about systemd's own configuration files for DNS servers. To improve compatibility,
<filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but only if it is
not a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> (see below).</para>
<para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following cases:</para>
<itemizedlist>
<listitem><para>The local, configured hostname is resolved to
all locally configured IP addresses ordered by their scope, or
— if none are configured — the IPv4 address 127.0.0.2 (which
is on the local loopback) and the IPv6 address ::1 (which is the
local host).</para></listitem>
<listitem><para>The hostnames <literal>localhost</literal> and
<literal>localhost.localdomain</literal> (as well as any hostname
ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
<listitem><para>The hostname <literal>gateway</literal> is
resolved to all current default routing gateway addresses,
ordered by their metric. This assigns a stable hostname to the
current gateway, useful for referencing it independently of the
current network configuration state.</para></listitem>
<listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their configured
addresses and back.</para></listitem>
</itemizedlist>
<para>Lookup requests are routed to the available DNS servers
and LLMNR interfaces according to the following rules:</para>
<itemizedlist>
<listitem><para>Lookups for the special hostname
<literal>localhost</literal> are never routed to the
network. (A few other, special domains are handled the same way.)</para></listitem>
<listitem><para>Single-label names are routed to all local
interfaces capable of IP multicasting, using the LLMNR
protocol. Lookups for IPv4 addresses are only sent via LLMNR on
IPv4, and lookups for IPv6 addresses are only sent via LLMNR on
IPv6. Lookups for the locally configured host name and the
<literal>gateway</literal> host name are never routed to
LLMNR.</para></listitem>
<listitem><para>Multi-label names are routed to all local
interfaces that have a DNS sever configured, plus the globally
configured DNS server if there is one. Address lookups from the
link-local address range are never routed to
DNS.</para></listitem>
</itemizedlist>
<para>If lookups are routed to multiple interfaces, the first
successful response is returned (thus effectively merging the
lookup zones on all matching interfaces). If the lookup failed on
all interfaces, the last failing response is returned.</para>
<para>Routing of lookups may be influenced by configuring
per-interface domain names. See
<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details. Lookups for a hostname ending in one of the
per-interface domains are exclusively routed to the matching
interfaces.</para>
<para>See the <ulink url="http://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para>
</refsect1>
<refsect1>
<title><filename>/etc/resolv.conf</filename></title>
<para>Three modes of handling <filename>/etc/resolv.conf</filename> (see
<citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
supported:</para>
<itemizedlist>
<listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists
the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
<filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to
<command>systemd-resolved</command>. This mode of operation is recommended.</para></listitem>
<listitem><para><command>systemd-resolved</command> maintains the
<filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux
programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept up-to-date,
containing information about all known DNS servers. Note the file format's limitations: it does not know a
concept of per-interface DNS servers and hence only contains system-wide DNS server definitions. Note that
<filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications, but only
through a symlink from <filename>/etc/resolv.conf</filename>. If this mode of operation is used local clients
that bypass any local DNS API will also bypass <command>systemd-resolved</command> and will talk directly to the
known DNS servers.</para> </listitem>
<listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, in which
case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode of operation
<command>systemd-resolved</command> is consumer rather than provider of this configuration
file. </para></listitem>
</itemizedlist>
<para>Note that the selected mode of operation for this file is detected fully automatically, depending on whether
<filename>/etc/resolv.conf</filename> is a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> or
lists 127.0.0.53 as DNS server.</para>
</refsect1>
<refsect1>
<title>Signals</title>
<variablelist>
<varlistentry>
<term><constant>SIGUSR1</constant></term>
<listitem><para>Upon reception of the SIGUSR1 process signal <command>systemd-resolved</command> will dump the
contents of all DNS resource record caches it maintains into the system logs.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>SIGUSR2</constant></term>
<listitem><para>Upon reception of the SIGUSR2 process signal <command>systemd-resolved</command> will flush all
caches it maintains. Note that it should normally not be necessary to request this explicitly – except for
debugging purposes – as <command>systemd-resolved</command> flushes the caches automatically anyway any time
the host's network configuration changes.</para></listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-resolve</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
|
