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1 | <?xml version='1.0'?> | |
2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" | |
3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> | |
4 | <!-- SPDX-License-Identifier: LGPL-2.1-or-later --> | |
5 | ||
6 | <refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE' | |
7 | xmlns:xi="http://www.w3.org/2001/XInclude"> | |
8 | ||
9 | <refentryinfo> | |
10 | <title>systemd-resolved.service</title> | |
11 | <productname>systemd</productname> | |
12 | </refentryinfo> | |
13 | ||
14 | <refmeta> | |
15 | <refentrytitle>systemd-resolved.service</refentrytitle> | |
16 | <manvolnum>8</manvolnum> | |
17 | </refmeta> | |
18 | ||
19 | <refnamediv> | |
20 | <refname>systemd-resolved.service</refname> | |
21 | <refname>systemd-resolved</refname> | |
22 | <refpurpose>Network Name Resolution manager</refpurpose> | |
23 | </refnamediv> | |
24 | ||
25 | <refsynopsisdiv> | |
26 | <para><filename>systemd-resolved.service</filename></para> | |
27 | <para><filename>/usr/lib/systemd/systemd-resolved</filename></para> | |
28 | </refsynopsisdiv> | |
29 | ||
30 | <refsect1> | |
31 | <title>Description</title> | |
32 | ||
33 | <para><command>systemd-resolved</command> is a system service that provides network name resolution to | |
34 | local applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR | |
35 | and MulticastDNS resolver and responder. Local applications may submit network name resolution requests | |
36 | via three interfaces:</para> | |
37 | ||
38 | <itemizedlist> | |
39 | <listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus, | |
40 | see | |
41 | <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
42 | and | |
43 | <citerefentry><refentrytitle>org.freedesktop.LogControl1</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
44 | for details. Usage of this API is generally recommended to clients as it is asynchronous and fully | |
45 | featured (for example, properly returns DNSSEC validation status and interface scope for addresses as | |
46 | necessary for supporting link-local networking).</para></listitem> | |
47 | ||
48 | <listitem><para>The glibc | |
49 | <citerefentry project='man-pages'><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
50 | API as defined by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related | |
51 | resolver functions, including | |
52 | <citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>. | |
53 | This API is widely supported, including beyond the Linux platform. In its current form it does not | |
54 | expose DNSSEC validation status information however, and is synchronous only. This API is backed by the | |
55 | glibc Name Service Switch | |
56 | (<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>). | |
57 | Usage of the glibc NSS module | |
58 | <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> is | |
59 | required in order to allow glibc's NSS resolver functions to resolve hostnames via | |
60 | <command>systemd-resolved</command>.</para></listitem> | |
61 | ||
62 | <listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on | |
63 | the IP addresses 127.0.0.53 and 127.0.0.54 on the local loopback interface. Programs issuing DNS | |
64 | requests directly, bypassing any local API may be directed to this stub, in order to connect them to | |
65 | <command>systemd-resolved</command>. Note however that it is strongly recommended that local programs | |
66 | use the glibc NSS or bus APIs instead (as described above), as various network resolution concepts | |
67 | (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped to the unicast DNS | |
68 | protocol.</para> | |
69 | ||
70 | <para id="proxy-stub">The DNS stub resolver on 127.0.0.53 provides the full feature set of the local | |
71 | resolver, which includes offering LLMNR/MulticastDNS resolution. The DNS stub resolver on 127.0.0.54 | |
72 | provides a more limited resolver, that operates in "proxy" mode only, i.e. it will pass most DNS | |
73 | messages relatively unmodified to the current upstream DNS servers and back, but not try to process the | |
74 | messages locally, and hence does not validate DNSSEC, or offer up LLMNR/MulticastDNS. (It will | |
75 | translate to DNS-over-TLS communication if needed however.)</para></listitem> | |
76 | </itemizedlist> | |
77 | ||
78 | <para>The DNS servers contacted are determined from the global settings in | |
79 | <filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in | |
80 | <filename>/etc/systemd/network/*.network</filename> files (in case | |
81 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
82 | is used), the per-link dynamic settings received over DHCP, information provided via | |
83 | <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, and any | |
84 | DNS server information made available by other system services. See | |
85 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and | |
86 | <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for | |
87 | details about systemd's own configuration files for DNS servers. To improve compatibility, | |
88 | <filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but | |
89 | only if it is not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename>, | |
90 | <filename>/usr/lib/systemd/resolv.conf</filename> or | |
91 | <filename>/run/systemd/resolve/resolv.conf</filename> (see below).</para> | |
92 | ||
93 | </refsect1> | |
94 | ||
95 | <refsect1> | |
96 | <title>Synthetic Records</title> | |
97 | ||
98 | <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following | |
99 | cases:</para> | |
100 | ||
101 | <itemizedlist> | |
102 | <listitem><para>The local, configured hostname is resolved to all locally configured IP addresses | |
103 | ordered by their scope, or — if none are configured — the IPv4 address 127.0.0.2 (which is on the local | |
104 | loopback interface) and the IPv6 address ::1 (which is the local host).</para></listitem> | |
105 | ||
106 | <listitem><para>The hostnames <literal>localhost</literal> and <literal>localhost.localdomain</literal> | |
107 | as well as any hostname ending in <literal>.localhost</literal> or | |
108 | <literal>.localhost.localdomain</literal> are resolved to the IP addresses 127.0.0.1 and ::1. | |
109 | </para></listitem> | |
110 | ||
111 | <listitem><para>The hostname <literal>_gateway</literal> is resolved to all current default routing | |
112 | gateway addresses, ordered by their metric. This assigns a stable hostname to the current gateway, | |
113 | useful for referencing it independently of the current network configuration state.</para></listitem> | |
114 | ||
115 | <listitem><para>The hostname <literal>_outbound</literal> is resolved to the local IPv4 and IPv6 | |
116 | addresses that are most likely used for communication with other hosts. This is determined by | |
117 | requesting a routing decision to the configured default gateways from the kernel and then using the | |
118 | local IP addresses selected by this decision. This hostname is only available if there is at least one | |
119 | local default gateway configured. This assigns a stable hostname to the local outbound IP addresses, | |
120 | useful for referencing them independently of the current network configuration state.</para></listitem> | |
121 | ||
122 | <listitem><para>The hostname <literal>_localdnsstub</literal> is resolved to the IP address 127.0.0.53, | |
123 | i.e. the address the local DNS stub (see above) is listening on.</para></listitem> | |
124 | ||
125 | <listitem><para>The hostname <literal>_localdnsproxy</literal> is resolved to the IP address 127.0.0.54, | |
126 | i.e. the address the local DNS proxy (see above) is listening on.</para></listitem> | |
127 | ||
128 | <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their | |
129 | configured addresses and back, but they will not affect lookups for non-address types (like MX). | |
130 | Support for <filename>/etc/hosts</filename> may be disabled with <varname>ReadEtcHosts=no</varname>, | |
131 | see <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>. | |
132 | </para></listitem> | |
133 | </itemizedlist> | |
134 | </refsect1> | |
135 | ||
136 | <refsect1> | |
137 | <title>Protocols and Routing</title> | |
138 | ||
139 | <para>The lookup requests that <filename>systemd-resolved.service</filename> receives are routed to the | |
140 | available DNS servers, LLMNR, and MulticastDNS interfaces according to the following rules:</para> | |
141 | ||
142 | <itemizedlist> | |
143 | <listitem><para>Names for which synthetic records are generated (the local hostname, | |
144 | <literal>localhost</literal> and <literal>localdomain</literal>, local gateway, as listed in the | |
145 | previous section) and addresses configured in <filename>/etc/hosts</filename> are never routed to the | |
146 | network and a reply is sent immediately.</para></listitem> | |
147 | ||
148 | <listitem><para>Single-label names are resolved using LLMNR on all local interfaces where LLMNR is | |
149 | enabled. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are | |
150 | only sent via LLMNR on IPv6. Note that lookups for single-label synthesized names are not routed to | |
151 | LLMNR, MulticastDNS or unicast DNS.</para></listitem> | |
152 | ||
153 | <listitem><para>Queries for the address records (A and AAAA) of single-label non-synthesized names are | |
154 | resolved via unicast DNS using search domains. For any interface which defines search domains, such | |
155 | look-ups are routed to the servers defined for that interface, suffixed with each of those search | |
156 | domains. When global search domains are defined, such look-ups are routed to the global servers. For | |
157 | each search domain, queries are performed by suffixing the name with each of the search domains in | |
158 | turn. Additionally, lookup of single-label names via unicast DNS may be enabled with the | |
159 | <varname>ResolveUnicastSingleLabel=yes</varname> setting. The details of which servers are queried and | |
160 | how the final reply is chosen are described below. Note that this means that address queries for | |
161 | single-label names are never sent out to remote DNS servers by default, and resolution is only | |
162 | possible if search domains are defined.</para></listitem> | |
163 | ||
164 | <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are resolved using | |
165 | MulticastDNS on all local interfaces where MulticastDNS is enabled. As with LLMNR, IPv4 address lookups | |
166 | are sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem> | |
167 | ||
168 | <listitem><para>Queries for multi-label names are routed via unicast DNS on local interfaces that have | |
169 | a DNS server configured, plus the globally configured DNS servers if there are any. Which interfaces | |
170 | are used is determined by the routing logic based on search and route-only domains, described below. | |
171 | Note that by default, lookups for domains with the <literal>.local</literal> suffix are not routed to | |
172 | DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server | |
173 | and interface. This means that on networks where the <literal>.local</literal> domain is defined in a | |
174 | site-specific DNS server, explicit search or routing domains need to be configured to make lookups work | |
175 | within this DNS domain. Note that these days, it's generally recommended to avoid defining | |
176 | <literal>.local</literal> in a DNS server, as <ulink | |
177 | url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive | |
178 | MulticastDNS use.</para></listitem> | |
179 | ||
180 | <listitem><para>Address lookups (reverse lookups) are routed similarly to multi-label names, with the | |
181 | exception that addresses from the link-local address range are never routed to unicast DNS and are only | |
182 | resolved using LLMNR and MulticastDNS (when enabled).</para></listitem> | |
183 | </itemizedlist> | |
184 | ||
185 | <para>If lookups are routed to multiple interfaces, the first successful response is returned (thus | |
186 | effectively merging the lookup zones on all matching interfaces). If the lookup failed on all interfaces, | |
187 | the last failing response is returned.</para> | |
188 | ||
189 | <para>Routing of lookups is determined by the per-interface routing domains (search and route-only) and | |
190 | global search domains. See | |
191 | <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> and | |
192 | <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for a | |
193 | description how those settings are set dynamically and the discussion of <varname>Domains=</varname> in | |
194 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> for a | |
195 | description of globally configured DNS settings.</para> | |
196 | ||
197 | <para>The following query routing logic applies for unicast DNS lookups initiated by | |
198 | <filename>systemd-resolved.service</filename>:</para> | |
199 | ||
200 | <itemizedlist> | |
201 | <listitem><para>If a name to look up matches (that is: is equal to or has as suffix) any of the | |
202 | configured routing domains (search or route-only) of any link, or the globally configured DNS settings, | |
203 | "best matching" routing domain is determined: the matching one with the most labels. The query is then | |
204 | sent to all DNS servers of any links or the globally configured DNS servers associated with this "best | |
205 | matching" routing domain. (Note that more than one link might have this same "best matching" routing | |
206 | domain configured, in which case the query is sent to all of them in parallel).</para> | |
207 | ||
208 | <para>In case of single-label names, when search domains are defined, the same logic applies, except | |
209 | that the name is first suffixed by each of the search domains in turn. Note that this search logic | |
210 | doesn't apply to any names with at least one dot. Also see the discussion about compatibility with | |
211 | the traditional glibc resolver below.</para></listitem> | |
212 | ||
213 | <listitem><para>If a query does not match any configured routing domain (either per-link or global), it | |
214 | is sent to all DNS servers that are configured on links with the <varname>DefaultRoute=</varname> | |
215 | option set, as well as the globally configured DNS server.</para></listitem> | |
216 | ||
217 | <listitem><para>If there is no link configured as <varname>DefaultRoute=</varname> and no global DNS | |
218 | server configured, one of the compiled-in fallback DNS servers is used.</para></listitem> | |
219 | ||
220 | <listitem><para>Otherwise the unicast DNS query fails, as no suitable DNS servers can be determined. | |
221 | </para></listitem> | |
222 | </itemizedlist> | |
223 | ||
224 | <para>The <varname>DefaultRoute=</varname> option is a boolean setting configurable with | |
225 | <command>resolvectl</command> or in <filename>.network</filename> files. If not set, it is implicitly | |
226 | determined based on the configured DNS domains for a link: if there's a route-only domain other than | |
227 | <literal>~.</literal>, it defaults to false, otherwise to true.</para> | |
228 | ||
229 | <para>Effectively this means: in order to support single-label non-synthesized names, define appropriate | |
230 | search domains. In order to preferably route all DNS queries not explicitly matched by routing domain | |
231 | configuration to a specific link, configure a <literal>~.</literal> route-only domain on it. This will | |
232 | ensure that other links will not be considered for these queries (unless they too carry such a routing | |
233 | domain). In order to route all such DNS queries to a specific link only if no other link is preferred, | |
234 | set the <varname>DefaultRoute=</varname> option for the link to true and do not configure a | |
235 | <literal>~.</literal> route-only domain on it. Finally, in order to ensure that a specific link never | |
236 | receives any DNS traffic not matching any of its configured routing domains, set the | |
237 | <varname>DefaultRoute=</varname> option for it to false.</para> | |
238 | ||
239 | <para>See | |
240 | <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
241 | for information about the D-Bus APIs <filename>systemd-resolved</filename> provides.</para> | |
242 | </refsect1> | |
243 | ||
244 | <refsect1> | |
245 | <title>Compatibility with the traditional glibc stub resolver</title> | |
246 | ||
247 | <para>This section provides a short summary of differences in the stub resolver implemented by | |
248 | <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> together | |
249 | with <command>systemd-resolved</command> and the traditional stub resolver implemented in | |
250 | <filename>nss-dns</filename>.</para> | |
251 | ||
252 | <itemizedlist> | |
253 | <listitem><para>Some names are always resolved internally (see Synthetic Records above). Traditionally | |
254 | they would be resolved by <filename>nss-files</filename> if provided in | |
255 | <filename>/etc/hosts</filename>. But note that the details of how a query is constructed are under the | |
256 | control of the client library. <filename>nss-dns</filename> will first try to resolve names using | |
257 | search domains and even if those queries are routed to <filename>systemd-resolved</filename>, it will | |
258 | send them out over the network using the usual rules for multi-label name routing <footnote><para>For | |
259 | example, if <filename>/etc/resolv.conf</filename> has <programlisting>nameserver 127.0.0.53 | |
260 | search foobar.com barbar.com | |
261 | </programlisting>and we look up <literal>localhost</literal>, <filename>nss-dns</filename> will send | |
262 | the following queries to <filename>systemd-resolved</filename> listening on 127.0.0.53:53: first | |
263 | <literal>localhost.foobar.com</literal>, then <literal>localhost.barbar.com</literal>, and finally | |
264 | <literal>localhost</literal>. If (hopefully) the first two queries fail, | |
265 | <filename>systemd-resolved</filename> will synthesize an answer for the third query.</para> | |
266 | ||
267 | <para>When using <filename>nss-dns</filename> with any search domains, it is thus crucial to always | |
268 | configure <filename>nss-files</filename> with higher priority and provide mappings for names that | |
269 | should not be resolved using search domains.</para></footnote>.</para></listitem> | |
270 | ||
271 | <listitem><para>Single-label names are not resolved for A and AAAA records using unicast DNS (unless | |
272 | overridden with <varname>ResolveUnicastSingleLabel=</varname>, see | |
273 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>). | |
274 | This is similar to the <option>no-tld-query</option> option being set in | |
275 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>. | |
276 | </para></listitem> | |
277 | ||
278 | <listitem><para>Search domains are not used for <emphasis>suffixing</emphasis> of multi-label names. | |
279 | (Search domains are nevertheless used for lookup <emphasis>routing</emphasis>, for names that were | |
280 | originally specified as single-label or multi-label.) Any name with at least one dot is always | |
281 | interpreted as a FQDN. <filename>nss-dns</filename> would resolve names both as relative (using search | |
282 | domains) and absolute FQDN names. Some names would be resolved as relative first, and after that query | |
283 | has failed, as absolute, while other names would be resolved in opposite order. The | |
284 | <varname>ndots</varname> option in <filename>/etc/resolv.conf</filename> was used to control how many | |
285 | dots the name needs to have to be resolved as relative first. This stub resolver does not implement | |
286 | this at all: multi-label names are only resolved as FQDNs.<footnote><para>There are currently more than | |
287 | 1500 top-level domain names defined, and new ones are added regularly, often using "attractive" names | |
288 | that are also likely to be used locally. Not looking up multi-label names in this fashion avoids | |
289 | fragility in both directions: a valid global name could be obscured by a local name, and resolution of | |
290 | a relative local name could suddenly break when a new top-level domain is created, or when a new | |
291 | subdomain of a top-level domain in registered. Resolving any given name as either relative or absolute | |
292 | avoids this ambiguity.</para></footnote></para></listitem> | |
293 | ||
294 | <listitem><para>This resolver has a notion of the special <literal>.local</literal> domain used for | |
295 | MulticastDNS, and will not route queries with that suffix to unicast DNS servers unless explicitly | |
296 | configured, see above. Also, reverse lookups for link-local addresses are not sent to unicast DNS | |
297 | servers.</para></listitem> | |
298 | ||
299 | <listitem><para>This resolver reads and caches <filename>/etc/hosts</filename> internally. (In other | |
300 | words, <filename>nss-resolve</filename> replaces <filename>nss-files</filename> in addition to | |
301 | <filename>nss-dns</filename>). Entries in <filename>/etc/hosts</filename> have highest priority.</para> | |
302 | </listitem> | |
303 | ||
304 | <listitem><para>This resolver also implements LLMNR and MulticastDNS in addition to the classic unicast | |
305 | DNS protocol, and will resolve single-label names using LLMNR (when enabled) and names ending in | |
306 | <literal>.local</literal> using MulticastDNS (when enabled).</para></listitem> | |
307 | ||
308 | <listitem><para>Environment variables <varname>$LOCALDOMAIN</varname> and | |
309 | <varname>$RES_OPTIONS</varname> described in | |
310 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
311 | are not supported currently.</para></listitem> | |
312 | </itemizedlist> | |
313 | </refsect1> | |
314 | ||
315 | <refsect1> | |
316 | <title><filename>/etc/resolv.conf</filename></title> | |
317 | ||
318 | <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see | |
319 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are | |
320 | supported:</para> | |
321 | ||
322 | <itemizedlist> | |
323 | <listitem><para><command>systemd-resolved</command> maintains the | |
324 | <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional | |
325 | Linux programs. This file lists the 127.0.0.53 DNS stub (see above) as the only DNS server. It also | |
326 | contains a list of search domains that are in use by systemd-resolved. The list of search domains is | |
327 | always kept up-to-date. Note that <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not | |
328 | be used directly by applications, but only through a symlink from | |
329 | <filename>/etc/resolv.conf</filename>. This file may be symlinked from | |
330 | <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs | |
331 | to <command>systemd-resolved</command> with correct search domains settings. This mode of operation is | |
332 | recommended.</para></listitem> | |
333 | ||
334 | <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists | |
335 | the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from | |
336 | <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs | |
337 | to <command>systemd-resolved</command>. This file does not contain any search domains. | |
338 | </para></listitem> | |
339 | ||
340 | <listitem><para><command>systemd-resolved</command> maintains the | |
341 | <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux | |
342 | programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept | |
343 | up-to-date, containing information about all known DNS servers. Note the file format's limitations: it | |
344 | does not know a concept of per-interface DNS servers and hence only contains system-wide DNS server | |
345 | definitions. Note that <filename>/run/systemd/resolve/resolv.conf</filename> should not be used | |
346 | directly by applications, but only through a symlink from <filename>/etc/resolv.conf</filename>. If | |
347 | this mode of operation is used local clients that bypass any local DNS API will also bypass | |
348 | <command>systemd-resolved</command> and will talk directly to the known DNS servers.</para></listitem> | |
349 | ||
350 | <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, | |
351 | in which case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode | |
352 | of operation <command>systemd-resolved</command> is consumer rather than provider of this configuration | |
353 | file. </para></listitem> | |
354 | </itemizedlist> | |
355 | ||
356 | <para>Note that the selected mode of operation for this file is detected fully automatically, depending | |
357 | on whether <filename>/etc/resolv.conf</filename> is a symlink to | |
358 | <filename>/run/systemd/resolve/resolv.conf</filename> or lists 127.0.0.53 as DNS server.</para> | |
359 | </refsect1> | |
360 | ||
361 | <refsect1> | |
362 | <title>Signals</title> | |
363 | ||
364 | <variablelist> | |
365 | <varlistentry> | |
366 | <term><constant>SIGUSR1</constant></term> | |
367 | ||
368 | <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal | |
369 | <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it | |
370 | maintains, as well as all feature level information it learnt about configured DNS servers into the | |
371 | system logs.</para></listitem> | |
372 | </varlistentry> | |
373 | ||
374 | <varlistentry> | |
375 | <term><constant>SIGUSR2</constant></term> | |
376 | ||
377 | <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal | |
378 | <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally | |
379 | not be necessary to request this explicitly – except for debugging purposes – as | |
380 | <command>systemd-resolved</command> flushes the caches automatically anyway any time the host's | |
381 | network configuration changes. Sending this signal to <command>systemd-resolved</command> is | |
382 | equivalent to the <command>resolvectl flush-caches</command> command, however the latter is | |
383 | recommended since it operates in a synchronous way.</para></listitem> | |
384 | </varlistentry> | |
385 | ||
386 | <varlistentry> | |
387 | <term><constant>SIGRTMIN+1</constant></term> | |
388 | ||
389 | <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal | |
390 | <command>systemd-resolved</command> will forget everything it learnt about the configured DNS | |
391 | servers. Specifically any information about server feature support is flushed out, and the server | |
392 | feature probing logic is restarted on the next request, starting with the most fully featured | |
393 | level. Note that it should normally not be necessary to request this explicitly – except for | |
394 | debugging purposes – as <command>systemd-resolved</command> automatically forgets learnt information | |
395 | any time the DNS server configuration changes. Sending this signal to | |
396 | <command>systemd-resolved</command> is equivalent to the <command>resolvectl | |
397 | reset-server-features</command> command, however the latter is recommended since it operates in a | |
398 | synchronous way.</para></listitem> | |
399 | </varlistentry> | |
400 | </variablelist> | |
401 | </refsect1> | |
402 | ||
403 | <refsect1> | |
404 | <title>Credentials</title> | |
405 | ||
406 | <para><command>systemd-resolved</command> supports the service credentials logic as implemented by | |
407 | <varname>ImportCredential=</varname>/<varname>LoadCredential=</varname>/<varname>SetCredential=</varname> | |
408 | (see <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>1</manvolnum></citerefentry> for | |
409 | details). The following credentials are used when passed in:</para> | |
410 | ||
411 | <variablelist class='system-credentials'> | |
412 | <varlistentry> | |
413 | <term><varname>network.dns</varname></term> | |
414 | <term><varname>network.search_domains</varname></term> | |
415 | ||
416 | <listitem><para>May contain a space separated list of DNS server IP addresses and DNS search | |
417 | domains. This information is only used when no explicit configuration via | |
418 | <filename>/etc/systemd/resolved.conf</filename>, <filename>/etc/resolv.conf</filename> or the kernel | |
419 | command line has been provided.</para></listitem> | |
420 | </varlistentry> | |
421 | </variablelist> | |
422 | </refsect1> | |
423 | ||
424 | <refsect1> | |
425 | <title>Kernel Command Line</title> | |
426 | ||
427 | <para><command>systemd-resolved</command> also honours two kernel command line options:</para> | |
428 | ||
429 | <variablelist class='kernel-commandline-options'> | |
430 | <varlistentry> | |
431 | <term><varname>nameserver=</varname></term> | |
432 | <term><varname>domain=</varname></term> | |
433 | ||
434 | <listitem><para>Takes the IP address of a DNS server (in case of <varname>nameserver=</varname>), and | |
435 | a DNS search domain (in case of <varname>domain=</varname>). May be used multiple times, to define | |
436 | multiple DNS servers/search domains. If either of these options are specified | |
437 | <filename>/etc/resolv.conf</filename> will not be read and the <varname>DNS=</varname> and | |
438 | <varname>Domains=</varname> settings of | |
439 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
440 | will be ignored. These two kernel command line options hence override system | |
441 | configuration.</para></listitem> | |
442 | </varlistentry> | |
443 | </variablelist> | |
444 | </refsect1> | |
445 | ||
446 | <refsect1> | |
447 | <title>IP Ports</title> | |
448 | ||
449 | <para>The <command>systemd-resolved</command> service listens on the following IP ports:</para> | |
450 | ||
451 | <itemizedlist> | |
452 | <listitem><para>Port 53 on IPv4 addresses 127.0.0.53 and 127.0.0.54 (both are on the local loopback | |
453 | interface <literal>lo</literal>). This is the local DNS stub, as discussed above. Both UDP and TCP are | |
454 | covered.</para></listitem> | |
455 | ||
456 | <listitem><para>Port 5353 on all local addresses, both IPv4 and IPv6 (0.0.0.0 and ::0), for | |
457 | MulticastDNS on UDP. Note that even though the socket is bound to all local interfaces via the selected | |
458 | "wildcard" IP addresses, the incoming datagrams are filtered by the network interface they are coming | |
459 | in on, and separate MulticastDNS link-local scopes are maintained for each, taking into consideration | |
460 | whether MulticastDNS is enabled for the interface or not.</para></listitem> | |
461 | ||
462 | <listitem><para>Port 5355 on all local addresses, both IPv4 and IP6 (0.0.0.0 and ::0), for LLMNR, on | |
463 | both TCP and UDP. As with MulticastDNS filtering by incoming network interface is applied.</para></listitem> | |
464 | </itemizedlist> | |
465 | </refsect1> | |
466 | ||
467 | <refsect1> | |
468 | <title>See Also</title> | |
469 | <para> | |
470 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
471 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
472 | <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
473 | <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>, | |
474 | <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
475 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
476 | <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
477 | <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
478 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
479 | </para> | |
480 | </refsect1> | |
481 | ||
482 | </refentry> |