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man: add a description of handling of single-label names 

It turns out that our man page didn't describe the handling of single-label
names almost at all. This probably adds to the confusion regarding the subject.
So let's first describe what our current implementation is doing.

Quoting https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/:
> Applications and platforms that apply a suffix search list to a single-label
> name are in conformance with IETF standards track RFCs. Furthermore,
> applications and platforms that do not query DNS for a TLD are in conformance
> with IETF standards track recommendations

Current behaviour is in line with that recommendation.

For #13763.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2020-04-15 20:10:39 +02:00
parent 035e3cb9f9
commit 9d569d5fcb
1 changed files with 53 additions and 28 deletions
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81
man/systemd-resolved.service.xml
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@ -69,7 +69,7 @@
<filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in
<filename>/etc/systemd/network/*.network</filename> files (in case
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
is used), the per-link dynamic settings received over DHCP, user request made via
is used), the per-link dynamic settings received over DHCP, information provided via
<citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, and any
DNS server information made available by other system services. See
<citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
@ -104,6 +104,8 @@
<listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their
configured addresses and back, but they will not affect lookups for non-address types (like MX).
Support for <filename>/etc/hosts</filename> may be disabled with <varname>ReadEtcHosts=no</varname>,
see <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
</itemizedlist>
</refsect1>
@ -111,32 +113,48 @@
<refsect1>
<title>Protocols and Routing</title>
<para>Lookup requests are routed to the available DNS servers, LLMNR and MulticastDNS interfaces
<para>Lookup requests are routed to the available DNS servers, LLMNR, and MulticastDNS interfaces
according to the following rules:</para>
<itemizedlist>
<listitem><para>Lookups for the special hostname <literal>localhost</literal> are never routed to the
network. (A few other, special domains are handled the same way.)</para></listitem>
<listitem><para>Names for which synthetic records are generated (as listed in the previous section) are
never routed to the network and a reply is sent immediately. In particular this means that lookups for
<literal>localhost</literal> are never routed to the network.</para></listitem>
<listitem><para>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 hostname and the
<literal>_gateway</literal> hostname are never routed to LLMNR.</para></listitem>
<listitem><para>Single-label names are routed to all local interfaces capable of IP multicasting, where
LLMNR is not disabled, 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 hostname and the <literal>_gateway</literal> hostname are never routed to LLMNR.
</para></listitem>
<listitem><para>Multi-label names with the domain suffix <literal>.local</literal> 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.</para></listitem>
local interfaces capable of IP multicasting, where MulticastDNS is not disabled, using the MulticastDNS
protocol. As with LLMNR, IPv4 address lookups are sent via IPv4 and IPv6 address lookups are sent via
IPv6.</para></listitem>
<listitem><para>Resolution of address records (A and AAAA) via unicast DNS (i.e. not LLMNR or
MulticastDNS) for non-synthesized single-label names is only allowed for non-top-level domains. This
means that such records can only be resolved when search domains are defined. For any interface which
defines search domains, such look-ups are routed to that interface, suffixed with each of the search
domains defined on that interface in turn. When global search domains are defined, such look-ups are
routed to all interfaces, suffixed by each of the global search domains in turn. The details of which
servers are queried and how the final reply is chosen are described below. Note that this means that
address queries for single-label names are never sent out to remote DNS servers, and if no search
domains are defined, resolution will fail.</para></listitem>
<listitem><para>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
<literal>.local</literal> 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 <literal>.local</literal> 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 <literal>.local</literal> in a DNS server, as <ulink
url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive
configured, plus the globally configured DNS servers if there are any. Note that by default, lookups for
domains with the <literal>.local</literal> 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 <literal>.local</literal> 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
these days, it's generally recommended to avoid defining <literal>.local</literal> in a DNS server, as
<ulink url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive
MulticastDNS use.</para></listitem>
<listitem><para>Address lookups are routed similarly to multi-label names, with the exception that
addresses from the link-local address range are never routed to unicast DNS and are only resolved using
LLMNR and MulticastDNS (when enabled).</para></listitem>
</itemizedlist>
<para>If lookups are routed to multiple interfaces, the first successful response is returned (thus
@ -151,12 +169,18 @@
<itemizedlist>
<listitem><para>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
configured search or route-only domains of any link (see
<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>),
or the globally configured DNS settings (see the discussion of <varname>Domains=</varname> in
<citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>),
"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).</para></listitem>
parallel).</para>
<para>In case of single-label names, when search domains are defined, the same logic applies, except
that the name is first suffixed by the search domain.</para></listitem>
<listitem><para>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
@ -174,14 +198,15 @@
configured DNS domains for a link: if there's any route-only domain (not matching <literal>~.</literal>)
it defaults to false, otherwise to true.</para>
<para>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 <literal>~.</literal> 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 <literal>~.</literal> 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.</para>
<para>Effectively this means: in order to support single-label non-synthetized names, define appropriate
search domains. In order to preferably route all DNS queries not explicitly matched by search/route-only
domain configuration to a specific link, configure a <literal>~.</literal> route-only domain on it. This
will ensure that other links will not be considered for these queries (unless they too carry such a
route-only domain). In order to route all such DNS queries to a specific link only if no other link
is preferable, set the "DNS default route" option for the link to true and do not configure a
<literal>~.</literal> 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.</para>
<para>See the <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved">resolved D-Bus API
Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.