== Summary ==
Enable systemd-resolved by default. glibc will perform name resolution
using nss-resolve rather than nss-dns.
== Owner ==
* Name: [[User:catanzaro| Michael Catanzaro]]
* Email: <mcatanzaro(a)redhat.com>
== Detailed Description ==
We will enable systemd-resolved by default.
# We will change the
fedora-release presets] to enable systemd-resolved instead of disable
# systemd-libs currently has
a %post scriplet] to enable nss-myhostname and nss-systemd by either
(a) modifying authselect's user-nsswitch.conf template, if authselect
is in use, or (b) directly modifying /etc/nsswitch.conf otherwise. We
will work with the systemd maintainers to enable nss-resolve here as
# We will work with the authselect maintainers to update authselect's
minimal and nis profiles to enforce nss-resolve. These profiles modify
the hosts line of /etc/resolv.conf. (By default, Fedora uses
authselect's sssd profile, which does not modify the hosts line and
therefore does not have this problem.)
# We will remove our downstream patch to systemd that prevents systemd
from symlinking /etc/resolv.conf to
/run/systemd/resolve/stub-resolv.conf on new installs. For system
upgrades, a systemd-libs %post scriptlet will be used to reassign the
symlink during upgrade. Upon detecting this symlink, NetworkManager
will automatically enable its systemd-resolved support and configure
split DNS as appropriate.
systemd-resolved has been enabled by default in Ubuntu since Ubuntu
16.10, but please note we are doing this differently than Ubuntu has.
Ubuntu does not use nss-resolve. Instead, Ubuntu uses the traditional
nss-dns provided by glibc upstream, so glibc on Ubuntu continues to
read /etc/resolv.conf, as is traditional. This extra step is not
useful and not recommended by upstream. We want to follow upstream
recommendations in using nss-resolve instead.
If you do not wish to use systemd-resolved, then manual intervention
will be required to disable it:
* Modify /etc/authselect/user-nsswitch.conf and remove `resolve
[!UNAVAIL=return]` from the hosts line. Run `authselect
apply-changes`. (If you have disabled authselect, then edit
* Disable and stop systemd-resolved.service.
* Restart the NetworkManager service. NetworkManager will then create
a traditional /etc/resolv.conf. (If you are not using NetworkManager,
you may create your own /etc/resolv.conf.)
== Benefit to Fedora ==
=== Standardization ===
Fedora will continue its history of enabling new systemd-provided
services whenever it makes sense to do so. Standardizing on upstream
systemd services is beneficial to the broader Linux ecosystem in
addition to Fedora, since standardizing reduces behavior differences
between different Linux distributions. Sadly, Fedora is no longer
leading in this area. Ubuntu has enabled systemd-resolved by default
since Ubuntu 16.10, so by the time Fedora 33 is released, we will be
three years behind Ubuntu here.
=== resolvectl ===
`resolvectl` has several useful functions (e.g. `resolvectl status` or
`resolvectl query`) that will be enabled out-of-the-box.
=== Caching ===
systemd-resolved caches DNS queries for a short while. This can
dramatically] improve performance for applications that do not already
manually cache their own DNS results. (Generally, only web browsers
bother with manually caching DNS results.)
=== Split DNS ===
When systemd-resolved is enabled, users who use multiple VPNs at the
same time will notice that DNS requests are now sent to the correct
DNS server by default. Previously, this scenario would result in
embarrassing "DNS leaks" and, depending on the order that the VPN
connections were established, possible failure to resolve private
resources. These scenarios will now work properly. For example,
consider the scenario of Distrustful Denise, who (quite reasonably)
does not trust her ISP. Denise uses a public VPN service,
, to hide her internet activity from her ISP,
but she also needs to use her employer's corporate VPN,
, in order to access internal company resources
while working from home. Using the Network panel in System Settings,
Denise has configured her employer's VPN to "use this connection only
for resources on its network." Distrustful Denise expects that her
employer's VPN will receive all traffic for corporation.example.com
and no other traffic. And while this mostly works in Fedora 32, she
discovers that it does not work properly for DNS:
* If Denise connects to public-vpn.example.com
second, she is unable to access internal
company resources. All DNS requests are sent to
public-vpn.example.com's DNS server, so she is unable to resolve names
for internal company websites.
* If Denise connects to corporation.example.com
second, then she is able to access internal
company resources. However, it only works because ''all'' her DNS
requests are sent to corporation.example.com's DNS server. Sadly for
Distrustful Denise, her employer discovers that she has been making
some embarrassing DNS requests that she had expected to go through
Whichever VPN Denise connects to first receives all DNS requests
because glibc's nss-dns module is not smart enough to split the
requests. /etc/resolv.conf is just a list of DNS servers to try in
order, and NetworkManager has no plausible way to decide which to list
first, because both ways are broken, so it just prefers whichever was
connected first. And if one server fails to respond, then the next
server in the list will be tried, likely resulting in a DNS leak. In
contrast, when systemd-resolved is enabled, it will send each DNS
request only to the correct DNS server. The DNS server that will be
used for each tun interface can be inspected using the resolvectl
Migrating away from /etc/resolv.conf will also avoid an annoying
footgun with this legacy file: only the first three listed nameservers
are respected. All further nameservers are silently ignored.
NetworkManager adds a warning comment when writing more than three
nameservers to this file, but it cannot do any better than that.
=== DNS over TLS ===
systemd-resolved supports DNS over TLS (different from DNS over
HTTPS). Although this feature will not initially be enabled by
default, using systemd-resolved will enable us to turn on DNS over TLS
in a future Fedora release, providing improved security if the user's
DNS server supports DNS over TLS.
== Scope ==
* Proposal owners: We will update Fedora presets to enable
systemd-resolved by default.
* Other developers: This change requires coordination with the systemd
and authselect maintainers.
* Release engineering: [https://pagure.io/releng/issue/9367
* Policies and guidelines: none required
* Trademark approval: N/A (not needed for this Change)
== Upgrade/compatibility impact ==
systemd-resolved will be enabled automatically when upgrading to
Fedora 33. After upgrade, /etc/resolv.conf will be managed by systemd
and symlinked to /run/systemd/resolve/stub-resolv.conf. '''glibc will
no longer look at /etc/resolv.conf when performing name resolution.'''
Instead, glibc will communicate directly with systemd-resolved via
nss-resolve. systemd adds a large warning comment to the top of
/etc/resolv.conf to warn system administrators that changes to this
file will be ignored; however, scripts that edit this file manually
will break. Because this file is usually managed by NetworkManager,
impact to Fedora users will be limited to users who have manually
disabled NetworkManager; such users are expected to be experienced
system administrators who should be comfortable adapting to the change
(or disabling systemd-resolved).
Any applications that bypass glibc and read /etc/resolv.conf directly
will still work because /etc/resolv.conf will point to
systemd-resolved's stub resolver running on 127.0.0.53. Nevertheless,
/etc/resolv.conf is provided only for compatibility purposes, and
applications should prefer to use either glibc or the systemd-resolved
D-Bus API instead; see systemd-resolved(8) for details.
In short, '''applications that read /etc/resolv.conf will continue to
work as before.''' Applications that write to it will no longer work
as expected, but this only previously worked if NetworkManager is
disabled, a non-default configuration. It remains possible to disable
systemd-resolved if desired. Otherwise, any custom system
administration scripts that manage /etc/resolv.conf will need to be
=== DNSSEC ===
systemd-resolved's DNSSEC support is known to cause compatibility
problems with certain network access points. Per recommendation from
the systemd developers, we will change the default value of this
setting in Fedora from the upstream default `DNSSEC=allow-downgrade`
to `DNSSEC=no` by building systemd with the build option
`-Ddefault-dnssec=no`. The upstream default attempts to use DNSSEC if
it is working, but automatically disable it otherwise, allowing
man-in-the-middle attackers to disable DNSSEC. Sadly, even the
allow-downgrade setting suffers known compatibility problems. Because
Fedora is not prepared to handle an influx of DNSSEC-related bug
reports, we will disable this feature altogether. We anticipate that
enabling DNSSEC by default will not be possible in the foreseeable
future, or perhaps ever. Instead, enabling DNS over TLS (when
supported by the DNS server) seems likely in the near future.
=== Multicast DNS ===
systemd-resolved's multicast DNS support conflicts with Avahi. Per
recommendation from the systemd developers, we will change the default
value of this setting in Fedora from the upstream default
`MulticastDNS=yes` to `MulticastDNS=resolve`. Multicast DNS resolving
will be enabled, but responding will be disabled. This will require
adding a new systemd build option to control the default value of the
MulticastDNS setting, similar to the existing `default-dnssec` and
`default-dns-over-tls` build options.
== How To Test ==
Load any website in a web browser. If you succeed, then name
resolution probably works.
Try using `resolvectl status` and, for example, `resolvectl query
fedoraproject.org`, to see how they work and sanity-check their
Users who use multiple VPNs at the same time are encouraged to test
DNS in a multiple VPN scenario, to ensure that DNS requests are sent
to the expected DNS server.
== User Experience ==
See the Benefit to Fedora section, above, for direct benefits to users
who use multiple VPNs at the same time.
Users will be able to use the resolvectl tool and the functionality it provides.
/etc/resolv.conf will now be managed by systemd rather than by
NetworkManager. As before, this file must not be modified directly
when it is managed.
== Dependencies ==
In Fedora, /etc/nsswitch.conf is managed by authselect. By default,
authselect uses the sssd profile to generate configuration compatible
with sssd. In this mode of operation, it does not modify the hosts
line in /etc/nsswitch.conf. This is also true if using the winbind
profile instead of the sssd profile. However, authselect's minimal and
nis profiles do modify the hosts line. These authselect profiles must
be updated to enable nss-resolved. If you are using authselect in one
of these modes, it will not be possible to cleanly disable
systemd-resolved because the hosts line in /etc/nsswitch.conf will be
clobbered whenever 'authselect apply-changes' is run. If you wish to
disable systemd-resolved and you are using authselect in one of these
modes, then you should stop using authselect. This is not expected to
cause many problems because virtually all Fedora users will be using
the default sssd profile.
We do not need to directly make any changes to the /etc/nsswitch.conf
shipped by glibc. Changes will be applied in the systemd-libs %post
== Contingency Plan ==
The contingency plan, in the unlikely event of unexpected problems, is
simply to revert any changes and not enable systemd-resolved.
* Contingency deadline: beta freeze
* Blocks release? No
* Blocks product? No
== Documentation ==
* systemd-resolved is documented in several manpages: resolvectl(1),
resolved.conf(5), nss-resolve(8), systemd-resolved(8).
== Release Notes ==
systemd-resolved is now enabled by default. systemd-resolved provides
a system-level DNS cache that can substantially improve performance
for applications that do not cache their own DNS results, allows
correct handling of split DNS scenarios such as when multiple VPNs are
in use, and will allow Fedora to enable DNS over TLS in the future.
/etc/resolv.conf will now be managed by systemd-resolved rather than
by NetworkManager. /etc/resolv.conf will no longer be read when
performing name resolution using glibc; however, it is still provided
for compatibility with applications that manually read this file to
perform name resolution. Writing to /etc/resolv.conf will no longer
work as expected.
He / Him / His
Senior Program Manager, Fedora & CentOS Stream