]>
Commit | Line | Data |
---|---|---|
6e866b33 | 1 | <?xml version='1.0'?> |
bb4f798a | 2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
e735f4d4 | 3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> |
a032b68d | 4 | <!-- SPDX-License-Identifier: LGPL-2.1-or-later --> |
60f067b4 | 5 | |
f5e65279 | 6 | <refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'> |
60f067b4 | 7 | |
e735f4d4 MP |
8 | <refentryinfo> |
9 | <title>systemd-resolved.service</title> | |
10 | <productname>systemd</productname> | |
e735f4d4 MP |
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> | |
26 | <para><filename>/usr/lib/systemd/systemd-resolved</filename></para> | |
27 | </refsynopsisdiv> | |
28 | ||
29 | <refsect1> | |
30 | <title>Description</title> | |
31 | ||
a10f5d05 MB |
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> | |
5a920b42 MP |
36 | |
37 | <itemizedlist> | |
a10f5d05 MB |
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> | |
41 | and | |
42 | <citerefentry><refentrytitle>org.freedesktop.LogControl1</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
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> | |
5a920b42 MP |
46 | |
47 | <listitem><para>The glibc | |
a10f5d05 MB |
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 | |
58 | required in order to allow glibc's NSS resolver functions to resolve hostnames via | |
5a920b42 MP |
59 | <command>systemd-resolved</command>.</para></listitem> |
60 | ||
a10f5d05 | 61 | <listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on |
ea0999c9 MB |
62 | the IP addresses 127.0.0.53 and 127.0.0.54 on the local loopback interface. Programs issuing DNS |
63 | requests directly, bypassing any local API may be directed to this stub, in order to connect them to | |
a10f5d05 MB |
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 | |
ea0999c9 MB |
67 | protocol.</para> |
68 | ||
69 | <para id="proxy-stub">The DNS stub resolver on 127.0.0.53 provides the full feature set of the local | |
70 | resolver, which includes offering LLMNR/MulticastDNS resolution. The DNS stub resolver on 127.0.0.54 | |
71 | provides a more limited resolver, that operates in "proxy" mode only, i.e. it will pass most DNS | |
72 | messages relatively unmodified to the current upstream DNS servers and back, but not try to process the | |
73 | messages locally, and hence does not validate DNSSEC, or offer up LLMNR/MulticastDNS. (It will | |
74 | translate to DNS-over-TLS communication if needed however.)</para></listitem> | |
5a920b42 MP |
75 | </itemizedlist> |
76 | ||
77 | <para>The DNS servers contacted are determined from the global settings in | |
78 | <filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in | |
b012e921 | 79 | <filename>/etc/systemd/network/*.network</filename> files (in case |
a10f5d05 MB |
80 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
81 | is used), the per-link dynamic settings received over DHCP, information provided via | |
82 | <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, and any | |
83 | DNS server information made available by other system services. See | |
5a920b42 | 84 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and |
a10f5d05 MB |
85 | <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for |
86 | details about systemd's own configuration files for DNS servers. To improve compatibility, | |
87 | <filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but | |
88 | only if it is not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename>, | |
89 | <filename>/usr/lib/systemd/resolv.conf</filename> or | |
90 | <filename>/run/systemd/resolve/resolv.conf</filename> (see below).</para> | |
6e866b33 MB |
91 | |
92 | </refsect1> | |
93 | ||
94 | <refsect1> | |
95 | <title>Synthetic Records</title> | |
e735f4d4 | 96 | |
3a6ce677 | 97 | <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following |
a10f5d05 | 98 | cases:</para> |
13d276d0 MP |
99 | |
100 | <itemizedlist> | |
a10f5d05 MB |
101 | <listitem><para>The local, configured hostname is resolved to all locally configured IP addresses |
102 | ordered by their scope, or — if none are configured — the IPv4 address 127.0.0.2 (which is on the local | |
a032b68d | 103 | loopback interface) and the IPv6 address ::1 (which is the local host).</para></listitem> |
a10f5d05 MB |
104 | |
105 | <listitem><para>The hostnames <literal>localhost</literal> and <literal>localhost.localdomain</literal> | |
a032b68d MB |
106 | as well as any hostname ending in <literal>.localhost</literal> or |
107 | <literal>.localhost.localdomain</literal> are resolved to the IP addresses 127.0.0.1 and ::1. | |
a10f5d05 MB |
108 | </para></listitem> |
109 | ||
110 | <listitem><para>The hostname <literal>_gateway</literal> is resolved to all current default routing | |
111 | gateway addresses, ordered by their metric. This assigns a stable hostname to the current gateway, | |
112 | useful for referencing it independently of the current network configuration state.</para></listitem> | |
113 | ||
8b3d4ff0 MB |
114 | <listitem><para>The hostname <literal>_outbound</literal> is resolved to the local IPv4 and IPv6 |
115 | addresses that are most likely used for communication with other hosts. This is determined by | |
116 | requesting a routing decision to the configured default gateways from the kernel and then using the | |
117 | local IP addresses selected by this decision. This hostname is only available if there is at least one | |
118 | local default gateway configured. This assigns a stable hostname to the local outbound IP addresses, | |
119 | useful for referencing them independently of the current network configuration state.</para></listitem> | |
120 | ||
a10f5d05 MB |
121 | <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their |
122 | configured addresses and back, but they will not affect lookups for non-address types (like MX). | |
123 | Support for <filename>/etc/hosts</filename> may be disabled with <varname>ReadEtcHosts=no</varname>, | |
124 | see <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>. | |
125 | </para></listitem> | |
13d276d0 | 126 | </itemizedlist> |
6e866b33 MB |
127 | </refsect1> |
128 | ||
129 | <refsect1> | |
130 | <title>Protocols and Routing</title> | |
13d276d0 | 131 | |
3a6ce677 BR |
132 | <para>The lookup requests that <filename>systemd-resolved.service</filename> receives are routed to the |
133 | available DNS servers, LLMNR, and MulticastDNS interfaces according to the following rules:</para> | |
13d276d0 MP |
134 | |
135 | <itemizedlist> | |
a032b68d MB |
136 | <listitem><para>Names for which synthetic records are generated (the local hostname, |
137 | <literal>localhost</literal> and <literal>localdomain</literal>, local gateway, as listed in the | |
138 | previous section) and addresses configured in <filename>/etc/hosts</filename> are never routed to the | |
139 | network and a reply is sent immediately.</para></listitem> | |
140 | ||
141 | <listitem><para>Single-label names are resolved using LLMNR on all local interfaces where LLMNR is | |
142 | enabled. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are | |
3a6ce677 | 143 | only sent via LLMNR on IPv6. Note that lookups for single-label synthesized names are not routed to |
a032b68d MB |
144 | LLMNR, MulticastDNS or unicast DNS.</para></listitem> |
145 | ||
3a6ce677 | 146 | <listitem><para>Queries for the address records (A and AAAA) of single-label non-synthesized names are |
a032b68d | 147 | resolved via unicast DNS using search domains. For any interface which defines search domains, such |
8b3d4ff0 MB |
148 | look-ups are routed to the servers defined for that interface, suffixed with each of those search |
149 | domains. When global search domains are defined, such look-ups are routed to the global servers. For | |
150 | each search domain, queries are performed by suffixing the name with each of the search domains in | |
151 | turn. Additionally, lookup of single-label names via unicast DNS may be enabled with the | |
152 | <varname>ResolveUnicastSingleLabel=yes</varname> setting. The details of which servers are queried and | |
153 | how the final reply is chosen are described below. Note that this means that address queries for | |
0e1568bb | 154 | single-label names are never sent out to remote DNS servers by default, and resolution is only |
8b3d4ff0 | 155 | possible if search domains are defined.</para></listitem> |
a032b68d MB |
156 | |
157 | <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are resolved using | |
158 | MulticastDNS on all local interfaces where MulticastDNS is enabled. As with LLMNR, IPv4 address lookups | |
159 | are sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem> | |
160 | ||
161 | <listitem><para>Queries for multi-label names are routed via unicast DNS on local interfaces that have | |
162 | a DNS server configured, plus the globally configured DNS servers if there are any. Which interfaces | |
163 | are used is determined by the routing logic based on search and route-only domains, described below. | |
164 | Note that by default, lookups for domains with the <literal>.local</literal> suffix are not routed to | |
165 | DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server | |
166 | and interface. This means that on networks where the <literal>.local</literal> domain is defined in a | |
167 | site-specific DNS server, explicit search or routing domains need to be configured to make lookups work | |
168 | within this DNS domain. Note that these days, it's generally recommended to avoid defining | |
169 | <literal>.local</literal> in a DNS server, as <ulink | |
170 | url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive | |
b012e921 | 171 | MulticastDNS use.</para></listitem> |
a10f5d05 | 172 | |
a032b68d MB |
173 | <listitem><para>Address lookups (reverse lookups) are routed similarly to multi-label names, with the |
174 | exception that addresses from the link-local address range are never routed to unicast DNS and are only | |
175 | resolved using LLMNR and MulticastDNS (when enabled).</para></listitem> | |
13d276d0 MP |
176 | </itemizedlist> |
177 | ||
a10f5d05 MB |
178 | <para>If lookups are routed to multiple interfaces, the first successful response is returned (thus |
179 | effectively merging the lookup zones on all matching interfaces). If the lookup failed on all interfaces, | |
180 | the last failing response is returned.</para> | |
13d276d0 | 181 | |
a032b68d MB |
182 | <para>Routing of lookups is determined by the per-interface routing domains (search and route-only) and |
183 | global search domains. See | |
6e866b33 | 184 | <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> and |
a032b68d MB |
185 | <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for a |
186 | description how those settings are set dynamically and the discussion of <varname>Domains=</varname> in | |
187 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> for a | |
188 | description of globally configured DNS settings.</para> | |
189 | ||
3a6ce677 BR |
190 | <para>The following query routing logic applies for unicast DNS lookups initiated by |
191 | <filename>systemd-resolved.service</filename>:</para> | |
6e866b33 MB |
192 | |
193 | <itemizedlist> | |
a10f5d05 | 194 | <listitem><para>If a name to look up matches (that is: is equal to or has as suffix) any of the |
a032b68d MB |
195 | configured routing domains (search or route-only) of any link, or the globally configured DNS settings, |
196 | "best matching" routing domain is determined: the matching one with the most labels. The query is then | |
197 | sent to all DNS servers of any links or the globally configured DNS servers associated with this "best | |
198 | matching" routing domain. (Note that more than one link might have this same "best matching" routing | |
199 | domain configured, in which case the query is sent to all of them in parallel).</para> | |
a10f5d05 MB |
200 | |
201 | <para>In case of single-label names, when search domains are defined, the same logic applies, except | |
a032b68d MB |
202 | that the name is first suffixed by each of the search domains in turn. Note that this search logic |
203 | doesn't apply to any names with at least one dot. Also see the discussion about compatibility with | |
204 | the traditional glibc resolver below.</para></listitem> | |
a10f5d05 | 205 | |
a032b68d MB |
206 | <listitem><para>If a query does not match any configured routing domain (either per-link or global), it |
207 | is sent to all DNS servers that are configured on links with the <varname>DefaultRoute=</varname> | |
208 | option set, as well as the globally configured DNS server.</para></listitem> | |
a10f5d05 | 209 | |
a032b68d MB |
210 | <listitem><para>If there is no link configured as <varname>DefaultRoute=</varname> and no global DNS |
211 | server configured, one of the compiled-in fallback DNS servers is used.</para></listitem> | |
a10f5d05 | 212 | |
a032b68d | 213 | <listitem><para>Otherwise the unicast DNS query fails, as no suitable DNS servers can be determined. |
a10f5d05 | 214 | </para></listitem> |
6e866b33 MB |
215 | </itemizedlist> |
216 | ||
a032b68d MB |
217 | <para>The <varname>DefaultRoute=</varname> option is a boolean setting configurable with |
218 | <command>resolvectl</command> or in <filename>.network</filename> files. If not set, it is implicitly | |
219 | determined based on the configured DNS domains for a link: if there's a route-only domain other than | |
220 | <literal>~.</literal>, it defaults to false, otherwise to true.</para> | |
a10f5d05 | 221 | |
3a6ce677 | 222 | <para>Effectively this means: in order to support single-label non-synthesized names, define appropriate |
a032b68d MB |
223 | search domains. In order to preferably route all DNS queries not explicitly matched by routing domain |
224 | configuration to a specific link, configure a <literal>~.</literal> route-only domain on it. This will | |
225 | ensure that other links will not be considered for these queries (unless they too carry such a routing | |
226 | domain). In order to route all such DNS queries to a specific link only if no other link is preferred, | |
227 | set the <varname>DefaultRoute=</varname> option for the link to true and do not configure a | |
a10f5d05 | 228 | <literal>~.</literal> route-only domain on it. Finally, in order to ensure that a specific link never |
a032b68d MB |
229 | receives any DNS traffic not matching any of its configured routing domains, set the |
230 | <varname>DefaultRoute=</varname> option for it to false.</para> | |
a10f5d05 | 231 | |
a032b68d MB |
232 | <para>See |
233 | <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
234 | for information about the D-Bus APIs <filename>systemd-resolved</filename> provides.</para> | |
235 | </refsect1> | |
236 | ||
237 | <refsect1> | |
238 | <title>Compatibility with the traditional glibc stub resolver</title> | |
239 | ||
240 | <para>This section provides a short summary of differences in the stub resolver implemented by | |
241 | <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> together | |
3a6ce677 BR |
242 | with <command>systemd-resolved</command> and the traditional stub resolver implemented in |
243 | <filename>nss-dns</filename>.</para> | |
a032b68d MB |
244 | |
245 | <itemizedlist> | |
246 | <listitem><para>Some names are always resolved internally (see Synthetic Records above). Traditionally | |
3a6ce677 BR |
247 | they would be resolved by <filename>nss-files</filename> if provided in |
248 | <filename>/etc/hosts</filename>. But note that the details of how a query is constructed are under the | |
249 | control of the client library. <filename>nss-dns</filename> will first try to resolve names using | |
250 | search domains and even if those queries are routed to <filename>systemd-resolved</filename>, it will | |
251 | send them out over the network using the usual rules for multi-label name routing <footnote><para>For | |
626cb2db | 252 | example, if <filename>/etc/resolv.conf</filename> has <programlisting>nameserver 127.0.0.53 |
3a6ce677 BR |
253 | search foobar.com barbar.com |
254 | </programlisting>and we look up <literal>localhost</literal>, <filename>nss-dns</filename> will send | |
255 | the following queries to <filename>systemd-resolved</filename> listening on 127.0.0.53:53: first | |
256 | <literal>localhost.foobar.com</literal>, then <literal>localhost.barbar.com</literal>, and finally | |
257 | <literal>localhost</literal>. If (hopefully) the first two queries fail, | |
258 | <filename>systemd-resolved</filename> will synthesize an answer for the third query.</para> | |
259 | ||
260 | <para>When using <filename>nss-dns</filename> with any search domains, it is thus crucial to always | |
261 | configure <filename>nss-files</filename> with higher priority and provide mappings for names that | |
262 | should not be resolved using search domains.</para></footnote>.</para></listitem> | |
a032b68d MB |
263 | |
264 | <listitem><para>Single-label names are not resolved for A and AAAA records using unicast DNS (unless | |
265 | overridden with <varname>ResolveUnicastSingleLabel=</varname>, see | |
266 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>). | |
267 | This is similar to the <option>no-tld-query</option> option being set in | |
3a6ce677 | 268 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>. |
a032b68d MB |
269 | </para></listitem> |
270 | ||
271 | <listitem><para>Search domains are not used for <emphasis>suffixing</emphasis> of multi-label names. | |
272 | (Search domains are nevertheless used for lookup <emphasis>routing</emphasis>, for names that were | |
273 | originally specified as single-label or multi-label.) Any name with at least one dot is always | |
274 | interpreted as a FQDN. <filename>nss-dns</filename> would resolve names both as relative (using search | |
275 | domains) and absolute FQDN names. Some names would be resolved as relative first, and after that query | |
276 | has failed, as absolute, while other names would be resolved in opposite order. The | |
277 | <varname>ndots</varname> option in <filename>/etc/resolv.conf</filename> was used to control how many | |
278 | dots the name needs to have to be resolved as relative first. This stub resolver does not implement | |
3a6ce677 BR |
279 | this at all: multi-label names are only resolved as FQDNs.<footnote><para>There are currently more than |
280 | 1500 top-level domain names defined, and new ones are added regularly, often using "attractive" names | |
281 | that are also likely to be used locally. Not looking up multi-label names in this fashion avoids | |
282 | fragility in both directions: a valid global name could be obscured by a local name, and resolution of | |
283 | a relative local name could suddenly break when a new top-level domain is created, or when a new | |
284 | subdomain of a top-level domain in registered. Resolving any given name as either relative or absolute | |
9d669329 | 285 | avoids this ambiguity.</para></footnote></para></listitem> |
a032b68d MB |
286 | |
287 | <listitem><para>This resolver has a notion of the special <literal>.local</literal> domain used for | |
288 | MulticastDNS, and will not route queries with that suffix to unicast DNS servers unless explicitly | |
289 | configured, see above. Also, reverse lookups for link-local addresses are not sent to unicast DNS | |
290 | servers.</para></listitem> | |
291 | ||
292 | <listitem><para>This resolver reads and caches <filename>/etc/hosts</filename> internally. (In other | |
293 | words, <filename>nss-resolve</filename> replaces <filename>nss-files</filename> in addition to | |
294 | <filename>nss-dns</filename>). Entries in <filename>/etc/hosts</filename> have highest priority.</para> | |
295 | </listitem> | |
296 | ||
297 | <listitem><para>This resolver also implements LLMNR and MulticastDNS in addition to the classic unicast | |
298 | DNS protocol, and will resolve single-label names using LLMNR (when enabled) and names ending in | |
299 | <literal>.local</literal> using MulticastDNS (when enabled).</para></listitem> | |
300 | ||
301 | <listitem><para>Environment variables <varname>$LOCALDOMAIN</varname> and | |
302 | <varname>$RES_OPTIONS</varname> described in | |
3a6ce677 BR |
303 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> |
304 | are not supported currently.</para></listitem> | |
a032b68d | 305 | </itemizedlist> |
e735f4d4 MP |
306 | </refsect1> |
307 | ||
5a920b42 MP |
308 | <refsect1> |
309 | <title><filename>/etc/resolv.conf</filename></title> | |
310 | ||
52ad194e | 311 | <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see |
8a584da2 | 312 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are |
5a920b42 MP |
313 | supported:</para> |
314 | ||
315 | <itemizedlist> | |
52ad194e | 316 | <listitem><para><command>systemd-resolved</command> maintains the |
a10f5d05 | 317 | <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional |
086111aa LB |
318 | Linux programs. This file lists the 127.0.0.53 DNS stub (see above) as the only DNS server. It also |
319 | contains a list of search domains that are in use by systemd-resolved. The list of search domains is | |
320 | always kept up-to-date. Note that <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not | |
321 | be used directly by applications, but only through a symlink from | |
322 | <filename>/etc/resolv.conf</filename>. This file may be symlinked from | |
a10f5d05 MB |
323 | <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs |
324 | to <command>systemd-resolved</command> with correct search domains settings. This mode of operation is | |
52ad194e MB |
325 | recommended.</para></listitem> |
326 | ||
5a920b42 MP |
327 | <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists |
328 | the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from | |
a10f5d05 MB |
329 | <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs |
330 | to <command>systemd-resolved</command>. This file does not contain any search domains. | |
331 | </para></listitem> | |
5a920b42 MP |
332 | |
333 | <listitem><para><command>systemd-resolved</command> maintains the | |
334 | <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux | |
a10f5d05 MB |
335 | programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept |
336 | up-to-date, containing information about all known DNS servers. Note the file format's limitations: it | |
337 | does not know a concept of per-interface DNS servers and hence only contains system-wide DNS server | |
338 | definitions. Note that <filename>/run/systemd/resolve/resolv.conf</filename> should not be used | |
339 | directly by applications, but only through a symlink from <filename>/etc/resolv.conf</filename>. If | |
340 | this mode of operation is used local clients that bypass any local DNS API will also bypass | |
341 | <command>systemd-resolved</command> and will talk directly to the known DNS servers.</para></listitem> | |
342 | ||
343 | <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, | |
344 | in which case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode | |
345 | of operation <command>systemd-resolved</command> is consumer rather than provider of this configuration | |
5a920b42 MP |
346 | file. </para></listitem> |
347 | </itemizedlist> | |
348 | ||
a10f5d05 MB |
349 | <para>Note that the selected mode of operation for this file is detected fully automatically, depending |
350 | on whether <filename>/etc/resolv.conf</filename> is a symlink to | |
351 | <filename>/run/systemd/resolve/resolv.conf</filename> or lists 127.0.0.53 as DNS server.</para> | |
5a920b42 MP |
352 | </refsect1> |
353 | ||
354 | <refsect1> | |
355 | <title>Signals</title> | |
356 | ||
357 | <variablelist> | |
358 | <varlistentry> | |
359 | <term><constant>SIGUSR1</constant></term> | |
360 | ||
f5e65279 | 361 | <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal |
a10f5d05 MB |
362 | <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it |
363 | maintains, as well as all feature level information it learnt about configured DNS servers into the | |
364 | system logs.</para></listitem> | |
5a920b42 MP |
365 | </varlistentry> |
366 | ||
367 | <varlistentry> | |
368 | <term><constant>SIGUSR2</constant></term> | |
369 | ||
f5e65279 | 370 | <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal |
a10f5d05 MB |
371 | <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally |
372 | not be necessary to request this explicitly – except for debugging purposes – as | |
373 | <command>systemd-resolved</command> flushes the caches automatically anyway any time the host's | |
374 | network configuration changes. Sending this signal to <command>systemd-resolved</command> is | |
375 | equivalent to the <command>resolvectl flush-caches</command> command, however the latter is | |
376 | recommended since it operates in a synchronous way.</para></listitem> | |
f5e65279 MB |
377 | </varlistentry> |
378 | ||
379 | <varlistentry> | |
380 | <term><constant>SIGRTMIN+1</constant></term> | |
381 | ||
382 | <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal | |
383 | <command>systemd-resolved</command> will forget everything it learnt about the configured DNS | |
a10f5d05 MB |
384 | servers. Specifically any information about server feature support is flushed out, and the server |
385 | feature probing logic is restarted on the next request, starting with the most fully featured | |
386 | level. Note that it should normally not be necessary to request this explicitly – except for | |
387 | debugging purposes – as <command>systemd-resolved</command> automatically forgets learnt information | |
388 | any time the DNS server configuration changes. Sending this signal to | |
389 | <command>systemd-resolved</command> is equivalent to the <command>resolvectl | |
390 | reset-server-features</command> command, however the latter is recommended since it operates in a | |
391 | synchronous way.</para></listitem> | |
5a920b42 MP |
392 | </varlistentry> |
393 | </variablelist> | |
394 | </refsect1> | |
395 | ||
e735f4d4 MP |
396 | <refsect1> |
397 | <title>See Also</title> | |
398 | <para> | |
399 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
400 | <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
4c89c718 | 401 | <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
d9dfd233 | 402 | <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>, |
b012e921 | 403 | <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, |
4c89c718 MP |
404 | <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
405 | <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
e735f4d4 MP |
406 | <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
407 | <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
408 | </para> | |
409 | </refsect1> | |
60f067b4 JS |
410 | |
411 | </refentry> |