Ubuntu Manpage: systemd-resolved.service, systemd-resolved

Provided by: systemd_237-3ubuntu10.57_amd64

NAME

       systemd-resolved.service, systemd-resolved - Network Name Resolution manager

SYNOPSIS

       systemd-resolved.service

       /lib/systemd/systemd-resolved

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 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. See the API
Documentation[1] for details. Usage of this API is generally recommended to clients as
it is asynchronous and fully featured (for example, properly returns DNSSEC validation
status and interface scope for addresses as necessary for supporting link-local
networking).

• The glibc getaddrinfo(3) API as defined by RFC3493[2] and its related resolver
functions, including gethostbyname(3). 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 (nss(5)). Usage of the glibc NSS module nss-resolve(8) 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.

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, the per-link dynamic settings received over DHCP and any DNS server information
made available by other system services. See resolved.conf(5) and systemd.network(5) 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 or
/run/systemd/resolve/resolv.conf (see below).

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 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 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).

Lookup requests are routed to the available DNS servers and LLMNR 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.)

• 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 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.

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. See
systemd.network(5) for details. Lookups for a hostname ending in one of the per-interface
domains are exclusively routed to the matching interfaces.

See the resolved D-Bus API Documentation[1] for information about the APIs
systemd-resolved provides.

/ETC/RESOLV.CONF

Four modes of handling /etc/resolv.conf (see resolv.conf(5)) are supported:

• 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
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.

• 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.

• 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.

SIGNALS

       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.

       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 systemd-resolve --flush-caches command, however
           the latter is recommended since it operates in a synchronous way.

       SIGRTMIN+1
           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 systemd-resolve --reset-server-features command, however the latter
           is recommended since it operates in a synchronous way.

NOTES