1SYSTEMD-RESOLVED.SERVICE(8)systemd-resolved.serviceSYSTEMD-RESOLVED.SERVICE(8)
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NAME

6       systemd-resolved.service, systemd-resolved - Network Name Resolution
7       manager
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SYNOPSIS

10       systemd-resolved.service
11
12       /usr/lib/systemd/systemd-resolved
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DESCRIPTION

15       systemd-resolved is a system service that provides network name
16       resolution to local applications. It implements a caching and
17       validating DNS/DNSSEC stub resolver, as well as an LLMNR and
18       MulticastDNS resolver and responder. Local applications may submit
19       network name resolution requests via three interfaces:
20
21       ·   The native, fully-featured API systemd-resolved exposes on the bus.
22           See the API Documentation[1] for details. Usage of this API is
23           generally recommended to clients as it is asynchronous and fully
24           featured (for example, properly returns DNSSEC validation status
25           and interface scope for addresses as necessary for supporting
26           link-local networking).
27
28       ·   The glibc getaddrinfo(3) API as defined by RFC3493[2] and its
29           related resolver functions, including gethostbyname(3). This API is
30           widely supported, including beyond the Linux platform. In its
31           current form it does not expose DNSSEC validation status
32           information however, and is synchronous only. This API is backed by
33           the glibc Name Service Switch (nss(5)). Usage of the glibc NSS
34           module nss-resolve(8) is required in order to allow glibc's NSS
35           resolver functions to resolve host names via systemd-resolved.
36
37       ·   Additionally, systemd-resolved provides a local DNS stub listener
38           on IP address 127.0.0.53 on the local loopback interface. Programs
39           issuing DNS requests directly, bypassing any local API may be
40           directed to this stub, in order to connect them to
41           systemd-resolved. Note however that it is strongly recommended that
42           local programs use the glibc NSS or bus APIs instead (as described
43           above), as various network resolution concepts (such as link-local
44           addressing, or LLMNR Unicode domains) cannot be mapped to the
45           unicast DNS protocol.
46
47       The DNS servers contacted are determined from the global settings in
48       /etc/systemd/resolved.conf, the per-link static settings in
49       /etc/systemd/network/*.network files (in case systemd-
50       networkd.service(8) is used), the per-link dynamic settings received
51       over DHCP, user request made via resolvectl(1), and any DNS server
52       information made available by other system services. See
53       resolved.conf(5) and systemd.network(5) for details about systemd's own
54       configuration files for DNS servers. To improve compatibility,
55       /etc/resolv.conf is read in order to discover configured system DNS
56       servers, but only if it is not a symlink to
57       /run/systemd/resolve/stub-resolv.conf, /usr/lib/systemd/resolv.conf or
58       /run/systemd/resolve/resolv.conf (see below).
59

SYNTHETIC RECORDS

61       systemd-resolved synthesizes DNS resource records (RRs) for the
62       following cases:
63
64       ·   The local, configured hostname is resolved to all locally
65           configured IP addresses ordered by their scope, or — if none are
66           configured — the IPv4 address 127.0.0.2 (which is on the local
67           loopback) and the IPv6 address ::1 (which is the local host).
68
69       ·   The hostnames "localhost" and "localhost.localdomain" (as well as
70           any hostname ending in ".localhost" or ".localhost.localdomain")
71           are resolved to the IP addresses 127.0.0.1 and ::1.
72
73       ·   The hostname "_gateway" is resolved to all current default routing
74           gateway addresses, ordered by their metric. This assigns a stable
75           hostname to the current gateway, useful for referencing it
76           independently of the current network configuration state.
77
78       ·   The mappings defined in /etc/hosts are resolved to their configured
79           addresses and back, but they will not affect lookups for
80           non-address types (like MX).
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PROTOCOLS AND ROUTING

83       Lookup requests are routed to the available DNS servers, LLMNR and
84       MulticastDNS interfaces according to the following rules:
85
86       ·   Lookups for the special hostname "localhost" are never routed to
87           the network. (A few other, special domains are handled the same
88           way.)
89
90       ·   Single-label names are routed to all local interfaces capable of IP
91           multicasting, using the LLMNR protocol. Lookups for IPv4 addresses
92           are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are
93           only sent via LLMNR on IPv6. Lookups for the locally configured
94           host name and the "_gateway" host name are never routed to LLMNR.
95
96       ·   Multi-label names with the domain suffix ".local" are routed to all
97           local interfaces capable of IP multicasting, using the MulticastDNS
98           protocol. As with LLMNR IPv4 address lookups are sent via IPv4 and
99           IPv6 address lookups are sent via IPv6.
100
101       ·   Other multi-label names are routed to all local interfaces that
102           have a DNS server configured, plus the globally configured DNS
103           server if there is one. Address lookups from the link-local address
104           range are never routed to DNS. Note that by default lookups for
105           domains with the ".local" suffix are not routed to DNS servers,
106           unless the domain is specified explicitly as routing or search
107           domain for the DNS server and interface. This means that on
108           networks where the ".local" domain is defined in a site-specific
109           DNS server, explicit search or routing domains need to be
110           configured to make lookups within this DNS domain work. Note that
111           today it's generally recommended to avoid defining ".local" in a
112           DNS server, as RFC6762[3] reserves this domain for exclusive
113           MulticastDNS use.
114
115       If lookups are routed to multiple interfaces, the first successful
116       response is returned (thus effectively merging the lookup zones on all
117       matching interfaces). If the lookup failed on all interfaces, the last
118       failing response is returned.
119
120       Routing of lookups may be influenced by configuring per-interface
121       domain names and other settings. See systemd.network(5) and
122       resolvectl(1) for details. The following query routing logic applies
123       for unicast DNS traffic:
124
125       ·   If a name to look up matches (that is: is equal to or has as
126           suffix) any of the configured search or route-only domains of any
127           link (or the globally configured DNS settings), the "best matching"
128           search/route-only domain is determined: the matching one with the
129           most labels. The query is then sent to all DNS servers of any links
130           or the globally configured DNS servers associated with this "best
131           matching" search/route-only domain. (Note that more than one link
132           might have this same "best matching" search/route-only domain
133           configured, in which case the query is sent to all of them in
134           parallel).
135
136       ·   If a query does not match any configured search/route-only domain
137           (neither per-link nor global), it is sent to all DNS servers that
138           are configured on links with the "DNS default route" option set, as
139           well as the globally configured DNS server.
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141       ·   If there is no link configured as "DNS default route" and no global
142           DNS server configured, the compiled-in fallback DNS server is used.
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144       ·   Otherwise the query is failed as no suitable DNS servers could be
145           determined.
146
147       The "DNS default route" option is a boolean setting configurable with
148       resolvectl or in .network files. If not set, it is implicitly
149       determined based on the configured DNS domains for a link: if there's
150       any route-only domain (not matching "~.") it defaults to false,
151       otherwise to true.
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153       Effectively this means: in order to preferably route all DNS queries
154       not explicitly matched by search/route-only domain configuration to a
155       specific link, configure a "~."  route-only domain on it. This will
156       ensure that other links will not be considered for the queries (unless
157       they too carry such a route-only domain). In order to route all such
158       DNS queries to a specific link only in case no other link is
159       preferable, then set the "DNS default route" option for the link to
160       true, and do not configure a "~."  route-only domain on it. Finally, in
161       order to ensure that a specific link never receives any DNS traffic not
162       matching any of its configured search/route-only domains, set the "DNS
163       default route" option for it to false.
164
165       See the resolved D-Bus API Documentation[1] for information about the
166       APIs systemd-resolved provides.
167

/ETC/RESOLV.CONF

169       Four modes of handling /etc/resolv.conf (see resolv.conf(5)) are
170       supported:
171
172       ·   systemd-resolved maintains the
173           /run/systemd/resolve/stub-resolv.conf file for compatibility with
174           traditional Linux programs. This file may be symlinked from
175           /etc/resolv.conf. This file lists the 127.0.0.53 DNS stub (see
176           above) as the only DNS server. It also contains a list of search
177           domains that are in use by systemd-resolved. The list of search
178           domains is always kept up-to-date. Note that
179           /run/systemd/resolve/stub-resolv.conf should not be used directly
180           by applications, but only through a symlink from /etc/resolv.conf.
181           This file may be symlinked from /etc/resolv.conf in order to
182           connect all local clients that bypass local DNS APIs to
183           systemd-resolved with correct search domains settings. This mode of
184           operation is recommended.
185
186       ·   A static file /usr/lib/systemd/resolv.conf is provided that lists
187           the 127.0.0.53 DNS stub (see above) as only DNS server. This file
188           may be symlinked from /etc/resolv.conf in order to connect all
189           local clients that bypass local DNS APIs to systemd-resolved. This
190           file does not contain any search domains.
191
192       ·   systemd-resolved maintains the /run/systemd/resolve/resolv.conf
193           file for compatibility with traditional Linux programs. This file
194           may be symlinked from /etc/resolv.conf and is always kept
195           up-to-date, containing information about all known DNS servers.
196           Note the file format's limitations: it does not know a concept of
197           per-interface DNS servers and hence only contains system-wide DNS
198           server definitions. Note that /run/systemd/resolve/resolv.conf
199           should not be used directly by applications, but only through a
200           symlink from /etc/resolv.conf. If this mode of operation is used
201           local clients that bypass any local DNS API will also bypass
202           systemd-resolved and will talk directly to the known DNS servers.
203
204       ·   Alternatively, /etc/resolv.conf may be managed by other packages,
205           in which case systemd-resolved will read it for DNS configuration
206           data. In this mode of operation systemd-resolved is consumer rather
207           than provider of this configuration file.
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209       Note that the selected mode of operation for this file is detected
210       fully automatically, depending on whether /etc/resolv.conf is a symlink
211       to /run/systemd/resolve/resolv.conf or lists 127.0.0.53 as DNS server.
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SIGNALS

214       SIGUSR1
215           Upon reception of the SIGUSR1 process signal systemd-resolved will
216           dump the contents of all DNS resource record caches it maintains,
217           as well as all feature level information it learnt about configured
218           DNS servers into the system logs.
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220       SIGUSR2
221           Upon reception of the SIGUSR2 process signal systemd-resolved will
222           flush all caches it maintains. Note that it should normally not be
223           necessary to request this explicitly – except for debugging
224           purposes – as systemd-resolved flushes the caches automatically
225           anyway any time the host's network configuration changes. Sending
226           this signal to systemd-resolved is equivalent to the resolvectl
227           flush-caches command, however the latter is recommended since it
228           operates in a synchronous way.
229
230       SIGRTMIN+1
231           Upon reception of the SIGRTMIN+1 process signal systemd-resolved
232           will forget everything it learnt about the configured DNS servers.
233           Specifically any information about server feature support is
234           flushed out, and the server feature probing logic is restarted on
235           the next request, starting with the most fully featured level. Note
236           that it should normally not be necessary to request this explicitly
237           – except for debugging purposes – as systemd-resolved automatically
238           forgets learnt information any time the DNS server configuration
239           changes. Sending this signal to systemd-resolved is equivalent to
240           the resolvectl reset-server-features command, however the latter is
241           recommended since it operates in a synchronous way.
242

SEE ALSO

244       systemd(1), resolved.conf(5), dnssec-trust-anchors.d(5), nss-
245       resolve(8), resolvectl(1), resolv.conf(5), hosts(5),
246       systemd.network(5), systemd-networkd.service(8)
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NOTES

249        1. API Documentation
250           https://www.freedesktop.org/wiki/Software/systemd/resolved
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252        2. RFC3493
253           https://tools.ietf.org/html/rfc3493
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255        3. RFC6762
256           https://tools.ietf.org/html/rfc6762
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260systemd 245                                        SYSTEMD-RESOLVED.SERVICE(8)
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