{ "content": [ { "type": "text", "text": "# systemd-resolved (man)\n\n## NAME\n\nsystemd-resolved.service, systemd-resolved - Network Name Resolution manager\n\n## SYNOPSIS\n\nsystemd-resolved.service\n/usr/lib/systemd/systemd-resolved\n\n## DESCRIPTION\n\nsystemd-resolved is a system service that provides network name resolution to local\napplications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an\nLLMNR and MulticastDNS resolver and responder. Local applications may submit network name\nresolution requests via three interfaces:\n\n## Sections\n\n- **NAME**\n- **SYNOPSIS**\n- **DESCRIPTION**\n- **SYNTHETIC RECORDS**\n- **PROTOCOLS AND ROUTING**\n- **SIGNALS** (1 subsections)\n- **CREDENTIALS**\n- **KERNEL COMMAND LINE**\n- **IP PORTS**\n- **SEE ALSO**\n- **NOTES**\n\nUse structuredContent.sections for detailed options, examples, and full documentation.\n" } ], "structuredContent": { "command": "systemd-resolved", "section": "", "mode": "man", "summary": "systemd-resolved.service, systemd-resolved - Network Name Resolution manager", "synopsis": "systemd-resolved.service\n/usr/lib/systemd/systemd-resolved", "tldr_summary": null, "tldr_examples": [], "tldr_source": null, "flags": [], "examples": [], "see_also": [ { "name": "systemd", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/systemd/1/json" }, { "name": "resolved.conf", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/resolved.conf/5/json" }, { "name": "dnssec-trust-anchors.d", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/dnssec-trust-anchors.d/5/json" }, { "name": "nss-resolve", "section": "8", "url": "https://www.chedong.com/phpMan.php/man/nss-resolve/8/json" }, { "name": "resolvectl", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/resolvectl/1/json" }, { "name": "resolv.conf", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/resolv.conf/5/json" }, { "name": "hosts", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/hosts/5/json" }, { "name": "systemd.network", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/systemd.network/5/json" }, { "name": "systemd-networkd.service", "section": "8", "url": "https://www.chedong.com/phpMan.php/man/systemd-networkd.service/8/json" } ], "section_outline": [ { "name": "NAME", "lines": 2, "subsections": [] }, { "name": "SYNOPSIS", "lines": 4, "subsections": [] }, { "name": "DESCRIPTION", "lines": 44, "subsections": [] }, { "name": "SYNTHETIC RECORDS", "lines": 32, "subsections": [] }, { "name": "PROTOCOLS AND ROUTING", "lines": 186, "subsections": [] }, { "name": "SIGNALS", "lines": 17, "subsections": [ { "name": "SIGRTMIN+1", "lines": 12 } ] }, { "name": "CREDENTIALS", "lines": 12, "subsections": [] }, { "name": "KERNEL COMMAND LINE", "lines": 11, "subsections": [] }, { "name": "IP PORTS", "lines": 16, "subsections": [] }, { "name": "SEE ALSO", "lines": 3, "subsections": [] }, { "name": "NOTES", "lines": 31, "subsections": [] } ], "sections": { "NAME": { "content": "systemd-resolved.service, systemd-resolved - Network Name Resolution manager\n", "subsections": [] }, "SYNOPSIS": { "content": "systemd-resolved.service\n\n/usr/lib/systemd/systemd-resolved\n", "subsections": [] }, "DESCRIPTION": { "content": "systemd-resolved is a system service that provides network name resolution to local\napplications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an\nLLMNR and MulticastDNS resolver and responder. Local applications may submit network name\nresolution requests via three interfaces:\n\n• The native, fully-featured API systemd-resolved exposes on the bus, see\norg.freedesktop.resolve1(5) and org.freedesktop.LogControl1(5) for details. Usage of this\nAPI is generally recommended to clients as it is asynchronous and fully featured (for\nexample, properly returns DNSSEC validation status and interface scope for addresses as\nnecessary for supporting link-local networking).\n\n• The glibc getaddrinfo(3) API as defined by \u001b[34mRFC3493[1] and its related resolver functions,\nincluding gethostbyname(3). This API is widely supported, including beyond the Linux\nplatform. In its current form it does not expose DNSSEC validation status information\nhowever, and is synchronous only. This API is backed by the glibc Name Service Switch\n(nss(5)). Usage of the glibc NSS module nss-resolve(8) is required in order to allow\nglibc's NSS resolver functions to resolve hostnames via systemd-resolved.\n\n• Additionally, systemd-resolved provides a local DNS stub listener on the IP addresses\n127.0.0.53 and 127.0.0.54 on the local loopback interface. Programs issuing DNS requests\ndirectly, bypassing any local API may be directed to this stub, in order to connect them\nto systemd-resolved. Note however that it is strongly recommended that local programs use\nthe glibc NSS or bus APIs instead (as described above), as various network resolution\nconcepts (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped to\nthe unicast DNS protocol.\n\nThe DNS stub resolver on 127.0.0.53 provides the full feature set of the local resolver,\nwhich includes offering LLMNR/MulticastDNS resolution. The DNS stub resolver on\n127.0.0.54 provides a more limited resolver, that operates in \"proxy\" mode only, i.e. it\nwill pass most DNS messages relatively unmodified to the current upstream DNS servers and\nback, but not try to process the messages locally, and hence does not validate DNSSEC, or\noffer up LLMNR/MulticastDNS. (It will translate to DNS-over-TLS communication if needed\nhowever.)\n\nThe DNS servers contacted are determined from the global settings in\n/etc/systemd/resolved.conf, the per-link static settings in /etc/systemd/network/*.network\nfiles (in case systemd-networkd.service(8) is used), the per-link dynamic settings received\nover DHCP, information provided via resolvectl(1), and any DNS server information made\navailable by other system services. See resolved.conf(5) and systemd.network(5) for details\nabout systemd's own configuration files for DNS servers. To improve compatibility,\n/etc/resolv.conf is read in order to discover configured system DNS servers, but only if it\nis not a symlink to /run/systemd/resolve/stub-resolv.conf, /usr/lib/systemd/resolv.conf or\n/run/systemd/resolve/resolv.conf (see below).\n", "subsections": [] }, "SYNTHETIC RECORDS": { "content": "systemd-resolved synthesizes DNS resource records (RRs) for the following cases:\n\n• The local, configured hostname is resolved to all locally configured IP addresses ordered\nby their scope, or — if none are configured — the IPv4 address 127.0.0.2 (which is on the\nlocal loopback interface) and the IPv6 address ::1 (which is the local host).\n\n• The hostnames \"localhost\" and \"localhost.localdomain\" as well as any hostname ending in\n\".localhost\" or \".localhost.localdomain\" are resolved to the IP addresses 127.0.0.1 and\n::1.\n\n• The hostname \"gateway\" is resolved to all current default routing gateway addresses,\nordered by their metric. This assigns a stable hostname to the current gateway, useful\nfor referencing it independently of the current network configuration state.\n\n• The hostname \"outbound\" is resolved to the local IPv4 and IPv6 addresses that are most\nlikely used for communication with other hosts. This is determined by requesting a\nrouting decision to the configured default gateways from the kernel and then using the\nlocal IP addresses selected by this decision. This hostname is only available if there is\nat least one local default gateway configured. This assigns a stable hostname to the\nlocal outbound IP addresses, useful for referencing them independently of the current\nnetwork configuration state.\n\n• The hostname \"localdnsstub\" is resolved to the IP address 127.0.0.53, i.e. the address\nthe local DNS stub (see above) is listening on.\n\n• The hostname \"localdnsproxy\" is resolved to the IP address 127.0.0.54, i.e. the address\nthe local DNS proxy (see above) is listening on.\n\n• The mappings defined in /etc/hosts are resolved to their configured addresses and back,\nbut they will not affect lookups for non-address types (like MX). Support for /etc/hosts\nmay be disabled with ReadEtcHosts=no, see resolved.conf(5).\n", "subsections": [] }, "PROTOCOLS AND ROUTING": { "content": "The lookup requests that systemd-resolved.service receives are routed to the available DNS\nservers, LLMNR, and MulticastDNS interfaces according to the following rules:\n\n• Names for which synthetic records are generated (the local hostname, \"localhost\" and\n\"localdomain\", local gateway, as listed in the previous section) and addresses configured\nin /etc/hosts are never routed to the network and a reply is sent immediately.\n\n• Single-label names are resolved using LLMNR on all local interfaces where LLMNR is\nenabled. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6\naddresses are only sent via LLMNR on IPv6. Note that lookups for single-label synthesized\nnames are not routed to LLMNR, MulticastDNS or unicast DNS.\n\n• Queries for the address records (A and AAAA) of single-label non-synthesized names are\nresolved via unicast DNS using search domains. For any interface which defines search\ndomains, such look-ups are routed to the servers defined for that interface, suffixed\nwith each of those search domains. When global search domains are defined, such look-ups\nare routed to the global servers. For each search domain, queries are performed by\nsuffixing the name with each of the search domains in turn. Additionally, lookup of\nsingle-label names via unicast DNS may be enabled with the ResolveUnicastSingleLabel=yes\nsetting. The details of which servers are queried and how the final reply is chosen are\ndescribed below. Note that this means that address queries for single-label names are\nnever sent out to remote DNS servers by default, and resolution is only possible if\nsearch domains are defined.\n\n• Multi-label names with the domain suffix \".local\" are resolved using MulticastDNS on all\nlocal interfaces where MulticastDNS is enabled. As with LLMNR, IPv4 address lookups are\nsent via IPv4 and IPv6 address lookups are sent via IPv6.\n\n• Queries for multi-label names are routed via unicast DNS on local interfaces that have a\nDNS server configured, plus the globally configured DNS servers if there are any. Which\ninterfaces are used is determined by the routing logic based on search and route-only\ndomains, described below. Note that by default, lookups for domains with the \".local\"\nsuffix are not routed to DNS servers, unless the domain is specified explicitly as\nrouting or search domain for the DNS server and interface. This means that on networks\nwhere the \".local\" domain is defined in a site-specific DNS server, explicit search or\nrouting domains need to be configured to make lookups work within this DNS domain. Note\nthat these days, it's generally recommended to avoid defining \".local\" in a DNS server,\nas \u001b[34mRFC6762[2] reserves this domain for exclusive MulticastDNS use.\n\n• Address lookups (reverse lookups) are routed similarly to multi-label names, with the\nexception that addresses from the link-local address range are never routed to unicast\nDNS and are only resolved using LLMNR and MulticastDNS (when enabled).\n\nIf lookups are routed to multiple interfaces, the first successful response is returned (thus\neffectively merging the lookup zones on all matching interfaces). If the lookup failed on all\ninterfaces, the last failing response is returned.\n\nRouting of lookups is determined by the per-interface routing domains (search and route-only)\nand global search domains. See systemd.network(5) and resolvectl(1) for a description how\nthose settings are set dynamically and the discussion of Domains= in resolved.conf(5) for a\ndescription of globally configured DNS settings.\n\nThe following query routing logic applies for unicast DNS lookups initiated by\nsystemd-resolved.service:\n\n• If a name to look up matches (that is: is equal to or has as suffix) any of the\nconfigured routing domains (search or route-only) of any link, or the globally configured\nDNS settings, \"best matching\" routing domain is determined: the matching one with the\nmost labels. The query is then sent to all DNS servers of any links or the globally\nconfigured DNS servers associated with this \"best matching\" routing domain. (Note that\nmore than one link might have this same \"best matching\" routing domain configured, in\nwhich case the query is sent to all of them in parallel).\n\nIn case of single-label names, when search domains are defined, the same logic applies,\nexcept that the name is first suffixed by each of the search domains in turn. Note that\nthis search logic doesn't apply to any names with at least one dot. Also see the\ndiscussion about compatibility with the traditional glibc resolver below.\n\n• If a query does not match any configured routing domain (either per-link or global), it\nis sent to all DNS servers that are configured on links with the DefaultRoute= option\nset, as well as the globally configured DNS server.\n\n• If there is no link configured as DefaultRoute= and no global DNS server configured, one\nof the compiled-in fallback DNS servers is used.\n\n• Otherwise the unicast DNS query fails, as no suitable DNS servers can be determined.\n\nThe DefaultRoute= option is a boolean setting configurable with resolvectl or in .network\nfiles. If not set, it is implicitly determined based on the configured DNS domains for a\nlink: if there's a route-only domain other than \"~.\", it defaults to false, otherwise to\ntrue.\n\nEffectively this means: in order to support single-label non-synthesized names, define\nappropriate search domains. In order to preferably route all DNS queries not explicitly\nmatched by routing domain configuration to a specific link, configure a \"~.\" route-only\ndomain on it. This will ensure that other links will not be considered for these queries\n(unless they too carry such a routing domain). In order to route all such DNS queries to a\nspecific link only if no other link is preferred, set the DefaultRoute= option for the link\nto true and do not configure a \"~.\" route-only domain on it. Finally, in order to ensure\nthat a specific link never receives any DNS traffic not matching any of its configured\nrouting domains, set the DefaultRoute= option for it to false.\n\nSee org.freedesktop.resolve1(5) for information about the D-Bus APIs systemd-resolved\nprovides.\n\nCOMPATIBILITY WITH THE TRADITIONAL GLIBC STUB RESOLVER\nThis section provides a short summary of differences in the resolver implemented by nss-\nresolve(8) together with systemd-resolved and the traditional stub resolver implemented in\nnss-dns.\n\n• Some names are always resolved internally (see Synthetic Records above). Traditionally\nthey would be resolved by nss-files if provided in /etc/hosts. But note that the details\nof how a query is constructed are under the control of the client library. nss-dns will\nfirst try to resolve names using search domains and even if those queries are routed to\nsystemd-resolved, it will send them out over the network using the usual rules for\nmulti-label name routing [3].\n\n• Single-label names are not resolved for A and AAAA records using unicast DNS (unless\noverridden with ResolveUnicastSingleLabel=, see resolved.conf(5)). This is similar to the\nno-tld-query option being set in resolv.conf(5).\n\n• Search domains are not used for suffixing of multi-label names. (Search domains are\nnevertheless used for lookup routing, for names that were originally specified as\nsingle-label or multi-label.) Any name with at least one dot is always interpreted as a\nFQDN. nss-dns would resolve names both as relative (using search domains) and absolute\nFQDN names. Some names would be resolved as relative first, and after that query has\nfailed, as absolute, while other names would be resolved in opposite order. The ndots\noption in /etc/resolv.conf was used to control how many dots the name needs to have to be\nresolved as relative first. This stub resolver does not implement this at all:\nmulti-label names are only resolved as FQDNs.[4]\n\n• This resolver has a notion of the special \".local\" domain used for MulticastDNS, and will\nnot route queries with that suffix to unicast DNS servers unless explicitly configured,\nsee above. Also, reverse lookups for link-local addresses are not sent to unicast DNS\nservers.\n\n• This resolver reads and caches /etc/hosts internally. (In other words, nss-resolve\nreplaces nss-files in addition to nss-dns). Entries in /etc/hosts have highest priority.\n\n• This resolver also implements LLMNR and MulticastDNS in addition to the classic unicast\nDNS protocol, and will resolve single-label names using LLMNR (when enabled) and names\nending in \".local\" using MulticastDNS (when enabled).\n\n• Environment variables $LOCALDOMAIN and $RESOPTIONS described in resolv.conf(5) are not\nsupported currently.\n\n• The nss-dns resolver maintains little state between subsequent DNS queries, and for each\nquery always talks to the first listed DNS server from /etc/resolv.conf first, and on\nfailure continues with the next until reaching the end of the list which is when the\nquery fails. The resolver in systemd-resolved.service however maintains state, and will\ncontinuously talk to the same server for all queries on a particular lookup scope until\nsome form of error is seen at which point it switches to the next, and then continuously\nstays with it for all queries on the scope until the next failure, and so on, eventually\nreturning to the first configured server. This is done to optimize lookup times, in\nparticular given that the resolver typically must first probe server feature sets when\ntalking to a server, which is time consuming. This different behaviour implies that\nlisted DNS servers per lookup scope must be equivalent in the zones they serve, so that\nsending a query to one of them will yield the same results as sending it to another\nconfigured DNS server.\n\n/ETC/RESOLV.CONF\nFour modes of handling /etc/resolv.conf (see resolv.conf(5)) are supported:\n\n• systemd-resolved maintains the /run/systemd/resolve/stub-resolv.conf file for\ncompatibility with traditional Linux programs. This file lists the 127.0.0.53 DNS stub\n(see above) as the only DNS server. It also contains a list of search domains that are in\nuse by systemd-resolved. The list of search domains is always kept up-to-date. Note that\n/run/systemd/resolve/stub-resolv.conf should not be used directly by applications, but\nonly through a symlink from /etc/resolv.conf. This file may be symlinked from\n/etc/resolv.conf in order to connect all local clients that bypass local DNS APIs to\nsystemd-resolved with correct search domains settings. This mode of operation is\nrecommended.\n\n• A static file /usr/lib/systemd/resolv.conf is provided that lists the 127.0.0.53 DNS stub\n(see above) as only DNS server. This file may be symlinked from /etc/resolv.conf in order\nto connect all local clients that bypass local DNS APIs to systemd-resolved. This file\ndoes not contain any search domains.\n\n• systemd-resolved maintains the /run/systemd/resolve/resolv.conf file for compatibility\nwith traditional Linux programs. This file may be symlinked from /etc/resolv.conf and is\nalways kept up-to-date, containing information about all known DNS servers. Note the file\nformat's limitations: it does not know a concept of per-interface DNS servers and hence\nonly contains system-wide DNS server definitions. Note that\n/run/systemd/resolve/resolv.conf should not be used directly by applications, but only\nthrough a symlink from /etc/resolv.conf. If this mode of operation is used local clients\nthat bypass any local DNS API will also bypass systemd-resolved and will talk directly to\nthe known DNS servers.\n\n• Alternatively, /etc/resolv.conf may be managed by other packages, in which case\nsystemd-resolved will read it for DNS configuration data. In this mode of operation\nsystemd-resolved is consumer rather than provider of this configuration file.\n\nNote that the selected mode of operation for this file is detected fully automatically,\ndepending on whether /etc/resolv.conf is a symlink to /run/systemd/resolve/resolv.conf or\nlists 127.0.0.53 as DNS server.\n", "subsections": [] }, "SIGNALS": { "content": "SIGUSR1\nUpon reception of the SIGUSR1 process signal systemd-resolved will dump the contents of\nall DNS resource record caches it maintains, as well as all feature level information it\nlearnt about configured DNS servers into the system logs.\n\nAdded in version 231.\n\nSIGUSR2\nUpon reception of the SIGUSR2 process signal systemd-resolved will flush all caches it\nmaintains. Note that it should normally not be necessary to request this explicitly –\nexcept for debugging purposes – as systemd-resolved flushes the caches automatically\nanyway any time the host's network configuration changes. Sending this signal to\nsystemd-resolved is equivalent to the resolvectl flush-caches command, however the latter\nis recommended since it operates in a synchronous way.\n\nAdded in version 231.\n", "subsections": [ { "name": "SIGRTMIN+1", "content": "Upon reception of the SIGRTMIN+1 process signal systemd-resolved will forget everything\nit learnt about the configured DNS servers. Specifically any information about server\nfeature support is flushed out, and the server feature probing logic is restarted on the\nnext request, starting with the most fully featured level. Note that it should normally\nnot be necessary to request this explicitly – except for debugging purposes – as\nsystemd-resolved automatically forgets learnt information any time the DNS server\nconfiguration changes. Sending this signal to systemd-resolved is equivalent to the\nresolvectl reset-server-features command, however the latter is recommended since it\noperates in a synchronous way.\n\nAdded in version 235.\n" } ] }, "CREDENTIALS": { "content": "systemd-resolved supports the service credentials logic as implemented by\nImportCredential=/LoadCredential=/SetCredential= (see systemd.exec(5) for details). The\nfollowing credentials are used when passed in:\n\nnetwork.dns, network.searchdomains\nMay contain a space separated list of DNS server IP addresses and DNS search domains.\nThis information is only used when no explicit configuration via\n/etc/systemd/resolved.conf, /etc/resolv.conf or the kernel command line has been\nprovided.\n\nAdded in version 253.\n", "subsections": [] }, "KERNEL COMMAND LINE": { "content": "systemd-resolved also honours two kernel command line options:\n\nnameserver=, domain=\nTakes the IP address of a DNS server (in case of nameserver=), and a DNS search domain\n(in case of domain=). May be used multiple times, to define multiple DNS servers/search\ndomains. If either of these options are specified /etc/resolv.conf will not be read and\nthe DNS= and Domains= settings of resolved.conf(5) will be ignored. These two kernel\ncommand line options hence override system configuration.\n\nAdded in version 253.\n", "subsections": [] }, "IP PORTS": { "content": "The systemd-resolved service listens on the following IP ports:\n\n• Port 53 on IPv4 addresses 127.0.0.53 and 127.0.0.54 (both are on the local loopback\ninterface \"lo\"). This is the local DNS stub, as discussed above. Both UDP and TCP are\ncovered.\n\n• Port 5353 on all local addresses, both IPv4 and IPv6 (0.0.0.0 and ::0), for MulticastDNS\non UDP. Note that even though the socket is bound to all local interfaces via the\nselected \"wildcard\" IP addresses, the incoming datagrams are filtered by the network\ninterface they are coming in on, and separate MulticastDNS link-local scopes are\nmaintained for each, taking into consideration whether MulticastDNS is enabled for the\ninterface or not.\n\n• Port 5355 on all local addresses, both IPv4 and IP6 (0.0.0.0 and ::0), for LLMNR, on both\nTCP and UDP. As with MulticastDNS filtering by incoming network interface is applied.\n", "subsections": [] }, "SEE ALSO": { "content": "systemd(1), resolved.conf(5), dnssec-trust-anchors.d(5), nss-resolve(8), resolvectl(1),\nresolv.conf(5), hosts(5), systemd.network(5), systemd-networkd.service(8)\n", "subsections": [] }, "NOTES": { "content": "1. RFC3493\nhttps://tools.ietf.org/html/rfc3493\n\n2. RFC6762\nhttps://tools.ietf.org/html/rfc6762\n\n3. For example, if /etc/resolv.conf has\n\nnameserver 127.0.0.53\nsearch foobar.com barbar.com\n\n\nand we look up \"localhost\", nss-dns will send the following queries to systemd-resolved\nlistening on 127.0.0.53:53: first \"localhost.foobar.com\", then \"localhost.barbar.com\",\nand finally \"localhost\". If (hopefully) the first two queries fail, systemd-resolved will\nsynthesize an answer for the third query.\n\nWhen using nss-dns with any search domains, it is thus crucial to always configure nss-\nfiles with higher priority and provide mappings for names that should not be resolved\nusing search domains.\n\n4. There are currently more than 1500 top-level domain names defined, and new ones are added\nregularly, often using \"attractive\" names that are also likely to be used locally. Not\nlooking up multi-label names in this fashion avoids fragility in both directions: a valid\nglobal name could be obscured by a local name, and resolution of a relative local name\ncould suddenly break when a new top-level domain is created, or when a new subdomain of a\ntop-level domain in registered. Resolving any given name as either relative or absolute\navoids this ambiguity.\n\n\nsystemd 255 SYSTEMD-RESOLVED.SERVICE(8)", "subsections": [] } } } }