diff --git a/man/systemd-resolved.service.xml b/man/systemd-resolved.service.xml index 3fb0846157..a30c5f72b8 100644 --- a/man/systemd-resolved.service.xml +++ b/man/systemd-resolved.service.xml @@ -29,9 +29,10 @@ Description - systemd-resolved is a system service that provides network name resolution to local - applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR and MulticastDNS - resolver and responder. Local applications may submit network name resolution requests via three interfaces: + systemd-resolved is a system service that provides network name resolution to + local applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR + and MulticastDNS resolver and responder. Local applications may submit network name resolution requests + via three interfaces: The native, fully-featured API systemd-resolved exposes on the bus, @@ -42,144 +43,149 @@ necessary for supporting link-local networking). The glibc - getaddrinfo3 API as defined - by RFC3493 and its related resolver functions, - including gethostbyname3. This - API is widely supported, including beyond the Linux platform. In its current form it does not expose DNSSEC - validation status information however, and is synchronous only. This API is backed by the glibc Name Service - Switch (nss5). Usage of the - glibc NSS module nss-resolve8 - is required in order to allow glibc's NSS resolver functions to resolve host names via + getaddrinfo3 + API as defined by RFC3493 and its related + resolver functions, including + gethostbyname3. + This API is widely supported, including beyond the Linux platform. In its current form it does not + expose DNSSEC validation status information however, and is synchronous only. This API is backed by the + glibc Name Service Switch + (nss5). + Usage of the glibc NSS module + nss-resolve8 is + required in order to allow glibc's NSS resolver functions to resolve host names via systemd-resolved. - Additionally, systemd-resolved provides a local DNS stub listener on IP - address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly, bypassing any local - API may be directed to this stub, in order to connect them to systemd-resolved. Note however - that it is strongly recommended that local programs use the glibc NSS or bus APIs instead (as described above), - as various network resolution concepts (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped - to the unicast DNS protocol. + Additionally, systemd-resolved provides a local DNS stub listener on + IP address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly, + bypassing any local API may be directed to this stub, in order to connect them to + systemd-resolved. Note however that it is strongly recommended that local programs + use the glibc NSS or bus APIs instead (as described above), as various network resolution concepts + (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped to the unicast DNS + protocol. The DNS servers contacted are determined from the global settings in /etc/systemd/resolved.conf, the per-link static settings in /etc/systemd/network/*.network files (in case - systemd-networkd.service8 is - used), the per-link dynamic settings received over DHCP, user request made via - resolvectl1, and any DNS server - information made available by other system services. See + systemd-networkd.service8 + is used), the per-link dynamic settings received over DHCP, user request made via + resolvectl1, and any + DNS server information made available by other system services. See resolved.conf5 and - systemd.network5 for details - about systemd's own configuration files for DNS servers. To improve compatibility, - /etc/resolv.conf is read in order to discover configured system DNS servers, but only if it is - not a symlink to /run/systemd/resolve/stub-resolv.conf, - /usr/lib/systemd/resolv.conf or /run/systemd/resolve/resolv.conf (see - below). + systemd.network5 for + details about systemd's own configuration files for DNS servers. To improve compatibility, + /etc/resolv.conf is read in order to discover configured system DNS servers, but + only if it is not a symlink to /run/systemd/resolve/stub-resolv.conf, + /usr/lib/systemd/resolv.conf or + /run/systemd/resolve/resolv.conf (see below). Synthetic Records - systemd-resolved synthesizes DNS resource records (RRs) for the following cases: + systemd-resolved synthesizes DNS resource records (RRs) for the following + cases: - The local, configured hostname is resolved to - all locally configured IP addresses ordered by their scope, or - — if none are configured — the IPv4 address 127.0.0.2 (which - is on the local loopback) and the IPv6 address ::1 (which is the - local host). + The local, configured hostname is resolved to all locally configured IP addresses + ordered by their scope, or — if none are configured — the IPv4 address 127.0.0.2 (which is on the local + loopback) and the IPv6 address ::1 (which is the local host). - The hostnames localhost and - localhost.localdomain (as well as any hostname - ending in .localhost or .localhost.localdomain) - are resolved to the IP addresses 127.0.0.1 and ::1. + The hostnames localhost and localhost.localdomain + (as well as any hostname ending in .localhost or + .localhost.localdomain) are resolved to the IP addresses 127.0.0.1 and ::1. + - The hostname _gateway is - resolved to all current default routing gateway addresses, - ordered by their metric. This assigns a stable hostname to the - current gateway, useful for referencing it independently of the - current network configuration state. + The hostname _gateway is resolved to all current default routing + gateway addresses, ordered by their metric. This assigns a stable hostname to the current gateway, + useful for referencing it independently of the current network configuration state. - The mappings defined in /etc/hosts are resolved - to their configured addresses and back, but they will not affect lookups for - non-address types (like MX). + The mappings defined in /etc/hosts are resolved to their + configured addresses and back, but they will not affect lookups for non-address types (like MX). + Protocols and Routing - Lookup requests are routed to the available DNS servers, LLMNR and MulticastDNS interfaces according to the - following rules: + Lookup requests are routed to the available DNS servers, LLMNR and MulticastDNS interfaces + according to the following rules: - Lookups for the special hostname localhost are never routed to the network. (A - few other, special domains are handled the same way.) + Lookups for the special hostname localhost are never routed to the + network. (A few other, special domains are handled the same way.) - Single-label names are routed to all local interfaces capable of IP multicasting, using the LLMNR - protocol. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are only - sent via LLMNR on IPv6. Lookups for the locally configured host name and the _gateway host - name are never routed to LLMNR. + Single-label names are routed to all local interfaces capable of IP multicasting, using + the LLMNR protocol. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 + addresses are only sent via LLMNR on IPv6. Lookups for the locally configured host name and the + _gateway host name are never routed to LLMNR. - Multi-label names with the domain suffix .local are routed to all local - interfaces capable of IP multicasting, using the MulticastDNS protocol. As with LLMNR IPv4 address lookups are - sent via IPv4 and IPv6 address lookups are sent via IPv6. + Multi-label names with the domain suffix .local are routed to all + local interfaces capable of IP multicasting, using the MulticastDNS protocol. As with LLMNR IPv4 + address lookups are sent via IPv4 and IPv6 address lookups are sent via IPv6. - Other multi-label names are routed to all local interfaces that have a DNS server configured, - plus the globally configured DNS server if there is one. Address lookups from the link-local address range are - never routed to DNS. Note that by default lookups for domains with the .local suffix are not - routed to DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server - and interface. This means that on networks where the .local domain is defined in a - site-specific DNS server, explicit search or routing domains need to be configured to make lookups within this - DNS domain work. Note that today it's generally recommended to avoid defining .local in a DNS - server, as RFC6762 reserves this domain for exclusive + Other multi-label names are routed to all local interfaces that have a DNS server + configured, plus the globally configured DNS server if there is one. Address lookups from the + link-local address range are never routed to DNS. Note that by default lookups for domains with the + .local suffix are not routed to DNS servers, unless the domain is specified + explicitly as routing or search domain for the DNS server and interface. This means that on networks + where the .local domain is defined in a site-specific DNS server, explicit search or + routing domains need to be configured to make lookups within this DNS domain work. Note that today it's + generally recommended to avoid defining .local in a DNS server, as RFC6762 reserves this domain for exclusive MulticastDNS use. - If lookups are routed to multiple interfaces, the first - successful response is returned (thus effectively merging the - lookup zones on all matching interfaces). If the lookup failed on - all interfaces, the last failing response is returned. + If lookups are routed to multiple interfaces, the first successful response is returned (thus + effectively merging the lookup zones on all matching interfaces). If the lookup failed on all interfaces, + the last failing response is returned. - Routing of lookups may be influenced by configuring per-interface domain names and other settings. See + Routing of lookups may be influenced by configuring per-interface domain names and other + settings. See systemd.network5 and - resolvectl1 for details. The - following query routing logic applies for unicast DNS traffic: + resolvectl1 for + details. The following query routing logic applies for unicast DNS traffic: - If a name to look up matches (that is: is equal to or has as suffix) any of the configured search - or route-only domains of any link (or the globally configured DNS settings), the "best matching" - search/route-only domain is determined: the matching one with the most labels. The query is then sent to all DNS - servers of any links or the globally configured DNS servers associated with this "best matching" - search/route-only domain. (Note that more than one link might have this same "best matching" search/route-only - domain configured, in which case the query is sent to all of them in parallel). + If a name to look up matches (that is: is equal to or has as suffix) any of the + configured search or route-only domains of any link (or the globally configured DNS settings), the + "best matching" search/route-only domain is determined: the matching one with the most labels. The + query is then sent to all DNS servers of any links or the globally configured DNS servers associated + with this "best matching" search/route-only domain. (Note that more than one link might have this same + "best matching" search/route-only domain configured, in which case the query is sent to all of them in + parallel). - If a query does not match any configured search/route-only domain (neither per-link nor global), - it is sent to all DNS servers that are configured on links with the "DNS default route" option set, as well as - the globally configured DNS server. + If a query does not match any configured search/route-only domain (neither per-link nor + global), it is sent to all DNS servers that are configured on links with the "DNS default route" option + set, as well as the globally configured DNS server. - If there is no link configured as "DNS default route" and no global DNS server configured, the - compiled-in fallback DNS server is used. + If there is no link configured as "DNS default route" and no global DNS server + configured, the compiled-in fallback DNS server is used. - Otherwise the query is failed as no suitable DNS servers could be determined. + Otherwise the query is failed as no suitable DNS servers could be determined. + - The "DNS default route" option is a boolean setting configurable with resolvectl or in - .network files. If not set, it is implicitly determined based on the configured DNS domains - for a link: if there's any route-only domain (not matching ~.) it defaults to false, otherwise - to true. + The "DNS default route" option is a boolean setting configurable with resolvectl + or in .network files. If not set, it is implicitly determined based on the + configured DNS domains for a link: if there's any route-only domain (not matching ~.) + it defaults to false, otherwise to true. Effectively this means: in order to preferably route all DNS queries not explicitly matched by - search/route-only domain configuration to a specific link, configure a ~. route-only domain on - it. This will ensure that other links will not be considered for the queries (unless they too carry such a - route-only domain). In order to route all such DNS queries to a specific link only in case no other link is - preferable, then set the "DNS default route" option for the link to true, and do not configure a - ~. route-only domain on it. Finally, in order to ensure that a specific link never receives any - DNS traffic not matching any of its configured search/route-only domains, set the "DNS default route" option for it - to false. + search/route-only domain configuration to a specific link, configure a ~. route-only + domain on it. This will ensure that other links will not be considered for the queries (unless they too + carry such a route-only domain). In order to route all such DNS queries to a specific link only in case + no other link is preferable, then set the "DNS default route" option for the link to true, and do not + configure a ~. route-only domain on it. Finally, in order to ensure that a specific + link never receives any DNS traffic not matching any of its configured search/route-only domains, set the + "DNS default route" option for it to false. - See the resolved D-Bus API - Documentation for information about the APIs systemd-resolved provides. + See the resolved D-Bus API + Documentation for information about the APIs systemd-resolved provides. + @@ -191,40 +197,41 @@ systemd-resolved maintains the - /run/systemd/resolve/stub-resolv.conf file for compatibility with traditional Linux - programs. This file may be symlinked from /etc/resolv.conf. This file lists the 127.0.0.53 - DNS stub (see above) as the only DNS server. It also contains a list of search domains that are in use by - systemd-resolved. The list of search domains is always kept up-to-date. Note that - /run/systemd/resolve/stub-resolv.conf should not be used directly by applications, but only - through a symlink from /etc/resolv.conf. This file may be symlinked from - /etc/resolv.conf in order to connect all local clients that bypass local DNS APIs to - systemd-resolved with correct search domains settings. This mode of operation is + /run/systemd/resolve/stub-resolv.conf file for compatibility with traditional + Linux programs. This file may be symlinked from /etc/resolv.conf. This file lists + the 127.0.0.53 DNS stub (see above) as the only DNS server. It also contains a list of search domains + that are in use by systemd-resolved. The list of search domains is always kept up-to-date. Note that + /run/systemd/resolve/stub-resolv.conf should not be used directly by applications, + but only through a symlink from /etc/resolv.conf. This file may be symlinked from + /etc/resolv.conf in order to connect all local clients that bypass local DNS APIs + to systemd-resolved with correct search domains settings. This mode of operation is recommended. A static file /usr/lib/systemd/resolv.conf is provided that lists the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from - /etc/resolv.conf in order to connect all local clients that bypass local DNS APIs to - systemd-resolved. This file does not contain any search domains. + /etc/resolv.conf in order to connect all local clients that bypass local DNS APIs + to systemd-resolved. This file does not contain any search domains. + systemd-resolved maintains the /run/systemd/resolve/resolv.conf file for compatibility with traditional Linux - programs. This file may be symlinked from /etc/resolv.conf and is always kept up-to-date, - containing information about all known DNS servers. Note the file format's limitations: it does not know a - concept of per-interface DNS servers and hence only contains system-wide DNS server definitions. Note that - /run/systemd/resolve/resolv.conf should not be used directly by applications, but only - through a symlink from /etc/resolv.conf. If this mode of operation is used local clients - that bypass any local DNS API will also bypass systemd-resolved and will talk directly to the - known DNS servers. + programs. This file may be symlinked from /etc/resolv.conf and is always kept + up-to-date, containing information about all known DNS servers. Note the file format's limitations: it + does not know a concept of per-interface DNS servers and hence only contains system-wide DNS server + definitions. Note that /run/systemd/resolve/resolv.conf should not be used + directly by applications, but only through a symlink from /etc/resolv.conf. If + this mode of operation is used local clients that bypass any local DNS API will also bypass + systemd-resolved and will talk directly to the known DNS servers. - Alternatively, /etc/resolv.conf may be managed by other packages, in which - case systemd-resolved will read it for DNS configuration data. In this mode of operation - systemd-resolved is consumer rather than provider of this configuration + Alternatively, /etc/resolv.conf may be managed by other packages, + in which case systemd-resolved will read it for DNS configuration data. In this mode + of operation systemd-resolved is consumer rather than provider of this configuration file. - Note that the selected mode of operation for this file is detected fully automatically, depending on whether - /etc/resolv.conf is a symlink to /run/systemd/resolve/resolv.conf or - lists 127.0.0.53 as DNS server. + Note that the selected mode of operation for this file is detected fully automatically, depending + on whether /etc/resolv.conf is a symlink to + /run/systemd/resolve/resolv.conf or lists 127.0.0.53 as DNS server. @@ -235,20 +242,21 @@ SIGUSR1 Upon reception of the SIGUSR1 process signal - systemd-resolved will dump the contents of all DNS resource record caches it maintains, as - well as all feature level information it learnt about configured DNS servers into the system - logs. + systemd-resolved will dump the contents of all DNS resource record caches it + maintains, as well as all feature level information it learnt about configured DNS servers into the + system logs. SIGUSR2 Upon reception of the SIGUSR2 process signal - systemd-resolved will flush all caches it maintains. Note that it should normally not be - necessary to request this explicitly – except for debugging purposes – as systemd-resolved - flushes the caches automatically anyway any time the host's network configuration changes. Sending this signal - to systemd-resolved is equivalent to the resolvectl flush-caches - command, however the latter is recommended since it operates in a synchronous way. + systemd-resolved will flush all caches it maintains. Note that it should normally + not be necessary to request this explicitly – except for debugging purposes – as + systemd-resolved flushes the caches automatically anyway any time the host's + network configuration changes. Sending this signal to systemd-resolved is + equivalent to the resolvectl flush-caches command, however the latter is + recommended since it operates in a synchronous way. @@ -256,13 +264,14 @@ Upon reception of the SIGRTMIN+1 process signal systemd-resolved will forget everything it learnt about the configured DNS - servers. Specifically any information about server feature support is flushed out, and the server feature - probing logic is restarted on the next request, starting with the most fully featured level. Note that it - should normally not be necessary to request this explicitly – except for debugging purposes – as - systemd-resolved automatically forgets learnt information any time the DNS server - configuration changes. Sending this signal to systemd-resolved is equivalent to the - resolvectl reset-server-features command, however the latter is recommended since it - operates in a synchronous way. + servers. Specifically any information about server feature support is flushed out, and the server + feature probing logic is restarted on the next request, starting with the most fully featured + level. Note that it should normally not be necessary to request this explicitly – except for + debugging purposes – as systemd-resolved automatically forgets learnt information + any time the DNS server configuration changes. Sending this signal to + systemd-resolved is equivalent to the resolvectl + reset-server-features command, however the latter is recommended since it operates in a + synchronous way.