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514094f9 | 1 | <?xml version='1.0'?> |
3a54a157 | 2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
12b42c76 | 3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> |
db9ecf05 | 4 | <!-- SPDX-License-Identifier: LGPL-2.1-or-later --> |
091a364c | 5 | |
1ec57f33 | 6 | <refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'> |
091a364c | 7 | |
798d3a52 ZJS |
8 | <refentryinfo> |
9 | <title>systemd-resolved.service</title> | |
10 | <productname>systemd</productname> | |
798d3a52 ZJS |
11 | </refentryinfo> |
12 | ||
13 | <refmeta> | |
14 | <refentrytitle>systemd-resolved.service</refentrytitle> | |
15 | <manvolnum>8</manvolnum> | |
16 | </refmeta> | |
17 | ||
18 | <refnamediv> | |
19 | <refname>systemd-resolved.service</refname> | |
20 | <refname>systemd-resolved</refname> | |
21 | <refpurpose>Network Name Resolution manager</refpurpose> | |
22 | </refnamediv> | |
23 | ||
24 | <refsynopsisdiv> | |
25 | <para><filename>systemd-resolved.service</filename></para> | |
12b42c76 | 26 | <para><filename>/usr/lib/systemd/systemd-resolved</filename></para> |
798d3a52 ZJS |
27 | </refsynopsisdiv> |
28 | ||
29 | <refsect1> | |
30 | <title>Description</title> | |
31 | ||
b0fb800c ZJS |
32 | <para><command>systemd-resolved</command> is a system service that provides network name resolution to |
33 | local applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR | |
34 | and MulticastDNS resolver and responder. Local applications may submit network name resolution requests | |
35 | via three interfaces:</para> | |
b541146b LP |
36 | |
37 | <itemizedlist> | |
ffd10e5a ZJS |
38 | <listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus, |
39 | see | |
40 | <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
1a0e562a ZJS |
41 | and |
42 | <citerefentry><refentrytitle>org.freedesktop.LogControl1</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
ffd10e5a ZJS |
43 | for details. Usage of this API is generally recommended to clients as it is asynchronous and fully |
44 | featured (for example, properly returns DNSSEC validation status and interface scope for addresses as | |
45 | necessary for supporting link-local networking).</para></listitem> | |
b541146b LP |
46 | |
47 | <listitem><para>The glibc | |
b0fb800c ZJS |
48 | <citerefentry project='man-pages'><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry> |
49 | API as defined by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related | |
50 | resolver functions, including | |
51 | <citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>. | |
52 | This API is widely supported, including beyond the Linux platform. In its current form it does not | |
53 | expose DNSSEC validation status information however, and is synchronous only. This API is backed by the | |
54 | glibc Name Service Switch | |
55 | (<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>). | |
56 | Usage of the glibc NSS module | |
57 | <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> is | |
38b38500 | 58 | required in order to allow glibc's NSS resolver functions to resolve hostnames via |
b541146b LP |
59 | <command>systemd-resolved</command>.</para></listitem> |
60 | ||
b0fb800c ZJS |
61 | <listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on |
62 | IP address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly, | |
63 | bypassing any local API may be directed to this stub, in order to connect them to | |
64 | <command>systemd-resolved</command>. Note however that it is strongly recommended that local programs | |
65 | use the glibc NSS or bus APIs instead (as described above), as various network resolution concepts | |
66 | (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped to the unicast DNS | |
67 | protocol.</para></listitem> | |
b541146b | 68 | </itemizedlist> |
798d3a52 | 69 | |
b541146b LP |
70 | <para>The DNS servers contacted are determined from the global settings in |
71 | <filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in | |
6cdf635d | 72 | <filename>/etc/systemd/network/*.network</filename> files (in case |
b0fb800c | 73 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
9d569d5f | 74 | is used), the per-link dynamic settings received over DHCP, information provided via |
b0fb800c ZJS |
75 | <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, and any |
76 | DNS server information made available by other system services. See | |
b541146b | 77 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and |
b0fb800c ZJS |
78 | <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for |
79 | details about systemd's own configuration files for DNS servers. To improve compatibility, | |
80 | <filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but | |
81 | only if it is not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename>, | |
82 | <filename>/usr/lib/systemd/resolv.conf</filename> or | |
83 | <filename>/run/systemd/resolve/resolv.conf</filename> (see below).</para> | |
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84 | |
85 | </refsect1> | |
86 | ||
87 | <refsect1> | |
88 | <title>Synthetic Records</title> | |
b541146b | 89 | |
78bc7025 | 90 | <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following |
b0fb800c | 91 | cases:</para> |
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92 | |
93 | <itemizedlist> | |
b0fb800c ZJS |
94 | <listitem><para>The local, configured hostname is resolved to all locally configured IP addresses |
95 | ordered by their scope, or — if none are configured — the IPv4 address 127.0.0.2 (which is on the local | |
9e1804b2 | 96 | loopback interface) and the IPv6 address ::1 (which is the local host).</para></listitem> |
b0fb800c ZJS |
97 | |
98 | <listitem><para>The hostnames <literal>localhost</literal> and <literal>localhost.localdomain</literal> | |
9e1804b2 ZJS |
99 | as well as any hostname ending in <literal>.localhost</literal> or |
100 | <literal>.localhost.localdomain</literal> are resolved to the IP addresses 127.0.0.1 and ::1. | |
b0fb800c ZJS |
101 | </para></listitem> |
102 | ||
103 | <listitem><para>The hostname <literal>_gateway</literal> is resolved to all current default routing | |
104 | gateway addresses, ordered by their metric. This assigns a stable hostname to the current gateway, | |
105 | useful for referencing it independently of the current network configuration state.</para></listitem> | |
2f166bb7 LP |
106 | |
107 | <listitem><para>The hostname <literal>_outbound</literal> is resolved to the local IPv4 and IPv6 | |
108 | addresses that are most likely used for communication with other hosts. This is determined by | |
109 | requesting a routing decision to the configured default gateways from the kernel and then using the | |
110 | local IP addresses selected by this decision. This hostname is only available if there is at least one | |
111 | local default gateway configured. This assigns a stable hostname to the local outbound IP addresses, | |
112 | useful for referencing them independently of the current network configuration state.</para></listitem> | |
b0fb800c ZJS |
113 | |
114 | <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their | |
115 | configured addresses and back, but they will not affect lookups for non-address types (like MX). | |
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116 | Support for <filename>/etc/hosts</filename> may be disabled with <varname>ReadEtcHosts=no</varname>, |
117 | see <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>. | |
b0fb800c | 118 | </para></listitem> |
2dc6b11d | 119 | </itemizedlist> |
396c716c LP |
120 | </refsect1> |
121 | ||
122 | <refsect1> | |
123 | <title>Protocols and Routing</title> | |
2dc6b11d | 124 | |
32f82ccc ZJS |
125 | <para>The lookup requests that <filename>systemd-resolved.service</filename> receives are routed to the |
126 | available DNS servers, LLMNR, and MulticastDNS interfaces according to the following rules:</para> | |
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127 | |
128 | <itemizedlist> | |
9e1804b2 ZJS |
129 | <listitem><para>Names for which synthetic records are generated (the local hostname, |
130 | <literal>localhost</literal> and <literal>localdomain</literal>, local gateway, as listed in the | |
131 | previous section) and addresses configured in <filename>/etc/hosts</filename> are never routed to the | |
132 | network and a reply is sent immediately.</para></listitem> | |
133 | ||
134 | <listitem><para>Single-label names are resolved using LLMNR on all local interfaces where LLMNR is | |
135 | enabled. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are | |
78bc7025 | 136 | only sent via LLMNR on IPv6. Note that lookups for single-label synthesized names are not routed to |
9e1804b2 ZJS |
137 | LLMNR, MulticastDNS or unicast DNS.</para></listitem> |
138 | ||
78bc7025 | 139 | <listitem><para>Queries for the address records (A and AAAA) of single-label non-synthesized names are |
9e1804b2 ZJS |
140 | resolved via unicast DNS using search domains. For any interface which defines search domains, such |
141 | look-ups are routed to that interface, suffixed with each of the search domains defined on that | |
142 | interface in turn. When global search domains are defined, such look-ups are routed to all interfaces, | |
143 | suffixed by each of the global search domains in turn. Additionally, lookup of single-label names via | |
144 | unicast DNS may be enabled with the <varname>ResolveUnicastSingleLabel=yes</varname> setting. The | |
145 | details of which servers are queried and how the final reply is chosen are described below. Note that | |
146 | this means that address queries for single-label names are never sent out to remote DNS servers by | |
147 | default, and resoulution is only possible if search domains are defined.</para></listitem> | |
148 | ||
149 | <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are resolved using | |
150 | MulticastDNS on all local interfaces where MulticastDNS is enabled. As with LLMNR, IPv4 address lookups | |
151 | are sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem> | |
152 | ||
153 | <listitem><para>Queries for multi-label names are routed via unicast DNS on local interfaces that have | |
154 | a DNS server configured, plus the globally configured DNS servers if there are any. Which interfaces | |
155 | are used is determined by the routing logic based on search and route-only domains, described below. | |
156 | Note that by default, lookups for domains with the <literal>.local</literal> suffix are not routed to | |
157 | DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server | |
158 | and interface. This means that on networks where the <literal>.local</literal> domain is defined in a | |
159 | site-specific DNS server, explicit search or routing domains need to be configured to make lookups work | |
160 | within this DNS domain. Note that these days, it's generally recommended to avoid defining | |
161 | <literal>.local</literal> in a DNS server, as <ulink | |
162 | url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive | |
6cdf635d | 163 | MulticastDNS use.</para></listitem> |
9d569d5f | 164 | |
9e1804b2 ZJS |
165 | <listitem><para>Address lookups (reverse lookups) are routed similarly to multi-label names, with the |
166 | exception that addresses from the link-local address range are never routed to unicast DNS and are only | |
167 | resolved using LLMNR and MulticastDNS (when enabled).</para></listitem> | |
2dc6b11d LP |
168 | </itemizedlist> |
169 | ||
b0fb800c ZJS |
170 | <para>If lookups are routed to multiple interfaces, the first successful response is returned (thus |
171 | effectively merging the lookup zones on all matching interfaces). If the lookup failed on all interfaces, | |
172 | the last failing response is returned.</para> | |
2dc6b11d | 173 | |
9e1804b2 ZJS |
174 | <para>Routing of lookups is determined by the per-interface routing domains (search and route-only) and |
175 | global search domains. See | |
2e88625f | 176 | <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> and |
9e1804b2 ZJS |
177 | <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for a |
178 | description how those settings are set dynamically and the discussion of <varname>Domains=</varname> in | |
179 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> for a | |
180 | description of globally configured DNS settings.</para> | |
181 | ||
32f82ccc ZJS |
182 | <para>The following query routing logic applies for unicast DNS lookups initiated by |
183 | <filename>systemd-resolved.service</filename>:</para> | |
2e88625f LP |
184 | |
185 | <itemizedlist> | |
b0fb800c | 186 | <listitem><para>If a name to look up matches (that is: is equal to or has as suffix) any of the |
9e1804b2 ZJS |
187 | configured routing domains (search or route-only) of any link, or the globally configured DNS settings, |
188 | "best matching" routing domain is determined: the matching one with the most labels. The query is then | |
189 | sent to all DNS servers of any links or the globally configured DNS servers associated with this "best | |
190 | matching" routing domain. (Note that more than one link might have this same "best matching" routing | |
191 | domain configured, in which case the query is sent to all of them in parallel).</para> | |
9d569d5f ZJS |
192 | |
193 | <para>In case of single-label names, when search domains are defined, the same logic applies, except | |
bace6883 | 194 | that the name is first suffixed by each of the search domains in turn. Note that this search logic |
377a9545 | 195 | doesn't apply to any names with at least one dot. Also see the discussion about compatibility with |
bace6883 | 196 | the traditional glibc resolver below.</para></listitem> |
b0fb800c | 197 | |
9e1804b2 ZJS |
198 | <listitem><para>If a query does not match any configured routing domain (either per-link or global), it |
199 | is sent to all DNS servers that are configured on links with the <varname>DefaultRoute=</varname> | |
200 | option set, as well as the globally configured DNS server.</para></listitem> | |
b0fb800c | 201 | |
9e1804b2 ZJS |
202 | <listitem><para>If there is no link configured as <varname>DefaultRoute=</varname> and no global DNS |
203 | server configured, one of the compiled-in fallback DNS servers is used.</para></listitem> | |
b0fb800c | 204 | |
9e1804b2 | 205 | <listitem><para>Otherwise the unicast DNS query fails, as no suitable DNS servers can be determined. |
b0fb800c | 206 | </para></listitem> |
2e88625f LP |
207 | </itemizedlist> |
208 | ||
9e1804b2 ZJS |
209 | <para>The <varname>DefaultRoute=</varname> option is a boolean setting configurable with |
210 | <command>resolvectl</command> or in <filename>.network</filename> files. If not set, it is implicitly | |
211 | determined based on the configured DNS domains for a link: if there's a route-only domain other than | |
212 | <literal>~.</literal>, it defaults to false, otherwise to true.</para> | |
2e88625f | 213 | |
78bc7025 | 214 | <para>Effectively this means: in order to support single-label non-synthesized names, define appropriate |
9e1804b2 ZJS |
215 | search domains. In order to preferably route all DNS queries not explicitly matched by routing domain |
216 | configuration to a specific link, configure a <literal>~.</literal> route-only domain on it. This will | |
217 | ensure that other links will not be considered for these queries (unless they too carry such a routing | |
218 | domain). In order to route all such DNS queries to a specific link only if no other link is preferred, | |
219 | set the <varname>DefaultRoute=</varname> option for the link to true and do not configure a | |
9d569d5f | 220 | <literal>~.</literal> route-only domain on it. Finally, in order to ensure that a specific link never |
9e1804b2 ZJS |
221 | receives any DNS traffic not matching any of its configured routing domains, set the |
222 | <varname>DefaultRoute=</varname> option for it to false.</para> | |
b0fb800c | 223 | |
21006e0e ZJS |
224 | <para>See |
225 | <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
226 | for information about the D-Bus APIs <filename>systemd-resolved</filename> provides.</para> | |
798d3a52 ZJS |
227 | </refsect1> |
228 | ||
bace6883 ZJS |
229 | <refsect1> |
230 | <title>Compatibility with the traditional glibc stub resolver</title> | |
231 | ||
232 | <para>This section provides a short summary of differences in the stub resolver implemented by | |
233 | <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> together | |
75909cc7 | 234 | with <command>systemd-resolved</command> and the traditional stub resolver implemented in |
d8096172 | 235 | <filename>nss-dns</filename>.</para> |
bace6883 ZJS |
236 | |
237 | <itemizedlist> | |
238 | <listitem><para>Some names are always resolved internally (see Synthetic Records above). Traditionally | |
32f82ccc ZJS |
239 | they would be resolved by <filename>nss-files</filename> if provided in |
240 | <filename>/etc/hosts</filename>. But note that the details of how a query is constructed are under the | |
241 | control of the client library. <filename>nss-dns</filename> will first try to resolve names using | |
242 | search domains and even if those queries are routed to <filename>systemd-resolved</filename>, it will | |
243 | send them out over the network using the usual rules for multi-label name routing <footnote><para>For | |
244 | example, if <filename>/etc/nsswitch.conf</filename> has <programlisting>nameserver 127.0.0.53 | |
245 | search foobar.com barbar.com | |
246 | </programlisting>and we look up <literal>localhost</literal>, <filename>nss-dns</filename> will send | |
247 | the following queries to <filename>systemd-resolved</filename> listening on 127.0.0.53:53: first | |
248 | <literal>localhost.foobar.com</literal>, then <literal>localhost.barbar.com</literal>, and finally | |
249 | <literal>localhost</literal>. If (hopefully) the first two queries fail, | |
250 | <filename>systemd-resolved</filename> will synthesize an answer for the third query.</para> | |
251 | ||
252 | <para>When using <filename>nss-dns</filename> with any search domains, it is thus crucial to always | |
253 | configure <filename>nss-files</filename> with higher priority and provide mappings for names that | |
254 | should not be resolved using search domains.</para></footnote>.</para></listitem> | |
bace6883 ZJS |
255 | |
256 | <listitem><para>Single-label names are not resolved for A and AAAA records using unicast DNS (unless | |
377a9545 | 257 | overridden with <varname>ResolveUnicastSingleLabel=</varname>, see |
bace6883 ZJS |
258 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>). |
259 | This is similar to the <option>no-tld-query</option> option being set in | |
d8096172 | 260 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>. |
bace6883 ZJS |
261 | </para></listitem> |
262 | ||
263 | <listitem><para>Search domains are not used for <emphasis>suffixing</emphasis> of multi-label names. | |
264 | (Search domains are nevertheless used for lookup <emphasis>routing</emphasis>, for names that were | |
265 | originally specified as single-label or multi-label.) Any name with at least one dot is always | |
266 | interpreted as a FQDN. <filename>nss-dns</filename> would resolve names both as relative (using search | |
267 | domains) and absolute FQDN names. Some names would be resolved as relative first, and after that query | |
268 | has failed, as absolute, while other names would be resolved in opposite order. The | |
269 | <varname>ndots</varname> option in <filename>/etc/resolv.conf</filename> was used to control how many | |
270 | dots the name needs to have to be resolved as relative first. This stub resolver does not implement | |
9273d8e6 ZJS |
271 | this at all: multi-label names are only resolved as FQDNs.<footnote><para>There are currently more than |
272 | 1500 top-level domain names defined, and new ones are added regularly, often using "attractive" names | |
273 | that are also likely to be used locally. Not looking up multi-label names in this fashion avoids | |
274 | fragility in both directions: a valid global name could be obscured by a local name, and resolution of | |
275 | a relative local name could suddenly break when a new top-level domain is created, or when a new | |
276 | subdomain of a top-level domain in registered. Resolving any given name as either relative or absolute | |
277 | avoids this ambiguity.)</para></footnote></para></listitem> | |
bace6883 ZJS |
278 | |
279 | <listitem><para>This resolver has a notion of the special <literal>.local</literal> domain used for | |
280 | MulticastDNS, and will not route queries with that suffix to unicast DNS servers unless explicitly | |
281 | configured, see above. Also, reverse lookups for link-local addresses are not sent to unicast DNS | |
282 | servers.</para></listitem> | |
283 | ||
284 | <listitem><para>This resolver reads and caches <filename>/etc/hosts</filename> internally. (In other | |
285 | words, <filename>nss-resolve</filename> replaces <filename>nss-files</filename> in addition to | |
286 | <filename>nss-dns</filename>). Entries in <filename>/etc/hosts</filename> have highest priority.</para> | |
287 | </listitem> | |
288 | ||
289 | <listitem><para>This resolver also implements LLMNR and MulticastDNS in addition to the classic unicast | |
290 | DNS protocol, and will resolve single-label names using LLMNR (when enabled) and names ending in | |
291 | <literal>.local</literal> using MulticastDNS (when enabled).</para></listitem> | |
292 | ||
293 | <listitem><para>Environment variables <varname>$LOCALDOMAIN</varname> and | |
294 | <varname>$RES_OPTIONS</varname> described in | |
d8096172 ZJS |
295 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> |
296 | are not supported currently.</para></listitem> | |
bace6883 ZJS |
297 | </itemizedlist> |
298 | </refsect1> | |
299 | ||
b541146b LP |
300 | <refsect1> |
301 | <title><filename>/etc/resolv.conf</filename></title> | |
302 | ||
e6b2d948 | 303 | <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see |
0a07667d | 304 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are |
b541146b LP |
305 | supported:</para> |
306 | ||
307 | <itemizedlist> | |
e6b2d948 | 308 | <listitem><para><command>systemd-resolved</command> maintains the |
b0fb800c ZJS |
309 | <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional |
310 | Linux programs. This file may be symlinked from <filename>/etc/resolv.conf</filename>. This file lists | |
311 | the 127.0.0.53 DNS stub (see above) as the only DNS server. It also contains a list of search domains | |
312 | that are in use by systemd-resolved. The list of search domains is always kept up-to-date. Note that | |
313 | <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not be used directly by applications, | |
314 | but only through a symlink from <filename>/etc/resolv.conf</filename>. This file may be symlinked from | |
315 | <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs | |
316 | to <command>systemd-resolved</command> with correct search domains settings. This mode of operation is | |
e6b2d948 DJL |
317 | recommended.</para></listitem> |
318 | ||
b541146b LP |
319 | <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists |
320 | the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from | |
b0fb800c ZJS |
321 | <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs |
322 | to <command>systemd-resolved</command>. This file does not contain any search domains. | |
323 | </para></listitem> | |
b541146b LP |
324 | |
325 | <listitem><para><command>systemd-resolved</command> maintains the | |
326 | <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux | |
b0fb800c ZJS |
327 | programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept |
328 | up-to-date, containing information about all known DNS servers. Note the file format's limitations: it | |
329 | does not know a concept of per-interface DNS servers and hence only contains system-wide DNS server | |
330 | definitions. Note that <filename>/run/systemd/resolve/resolv.conf</filename> should not be used | |
331 | directly by applications, but only through a symlink from <filename>/etc/resolv.conf</filename>. If | |
332 | this mode of operation is used local clients that bypass any local DNS API will also bypass | |
333 | <command>systemd-resolved</command> and will talk directly to the known DNS servers.</para></listitem> | |
334 | ||
335 | <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, | |
336 | in which case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode | |
337 | of operation <command>systemd-resolved</command> is consumer rather than provider of this configuration | |
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338 | file. </para></listitem> |
339 | </itemizedlist> | |
340 | ||
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341 | <para>Note that the selected mode of operation for this file is detected fully automatically, depending |
342 | on whether <filename>/etc/resolv.conf</filename> is a symlink to | |
343 | <filename>/run/systemd/resolve/resolv.conf</filename> or lists 127.0.0.53 as DNS server.</para> | |
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344 | </refsect1> |
345 | ||
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346 | <refsect1> |
347 | <title>Signals</title> | |
348 | ||
349 | <variablelist> | |
350 | <varlistentry> | |
351 | <term><constant>SIGUSR1</constant></term> | |
352 | ||
d55b0463 | 353 | <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal |
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354 | <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it |
355 | maintains, as well as all feature level information it learnt about configured DNS servers into the | |
356 | system logs.</para></listitem> | |
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357 | </varlistentry> |
358 | ||
359 | <varlistentry> | |
360 | <term><constant>SIGUSR2</constant></term> | |
361 | ||
d55b0463 | 362 | <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal |
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363 | <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally |
364 | not be necessary to request this explicitly – except for debugging purposes – as | |
365 | <command>systemd-resolved</command> flushes the caches automatically anyway any time the host's | |
366 | network configuration changes. Sending this signal to <command>systemd-resolved</command> is | |
367 | equivalent to the <command>resolvectl flush-caches</command> command, however the latter is | |
368 | recommended since it operates in a synchronous way.</para></listitem> | |
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369 | </varlistentry> |
370 | ||
371 | <varlistentry> | |
372 | <term><constant>SIGRTMIN+1</constant></term> | |
373 | ||
374 | <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal | |
375 | <command>systemd-resolved</command> will forget everything it learnt about the configured DNS | |
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376 | servers. Specifically any information about server feature support is flushed out, and the server |
377 | feature probing logic is restarted on the next request, starting with the most fully featured | |
378 | level. Note that it should normally not be necessary to request this explicitly – except for | |
379 | debugging purposes – as <command>systemd-resolved</command> automatically forgets learnt information | |
380 | any time the DNS server configuration changes. Sending this signal to | |
381 | <command>systemd-resolved</command> is equivalent to the <command>resolvectl | |
382 | reset-server-features</command> command, however the latter is recommended since it operates in a | |
383 | synchronous way.</para></listitem> | |
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384 | </varlistentry> |
385 | </variablelist> | |
386 | </refsect1> | |
387 | ||
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388 | <refsect1> |
389 | <title>See Also</title> | |
390 | <para> | |
391 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
392 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
b5a8703f | 393 | <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
437293cf | 394 | <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
b69f810c | 395 | <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, |
1c18f60a | 396 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
394bac4f | 397 | <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
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398 | <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
399 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
400 | </para> | |
401 | </refsect1> | |
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402 | |
403 | </refentry> |