Since commit 925b484 ("No fallback nameservers for internal
resolver"), if the host's resolv.conf has no nameservers and
no servers are supplied via config, the internal resolver will
not use Google's DNS - so the container will not be able to
resolve external DNS requests.
That can happen when container's are "restart-always" and the
docker daemon starts before the host's DNS is configured.
So, to highlight the issue (which may not be an error, but
probably is), include a warning in the container's resolv.conf
file.
Also, log a warning - logs currently say "No non-localhost DNS
nameservers are left in resolv.conf. Using default external
servers". But, that's misleading because it's from an initial
resolv.conf setup, before the internal resolver configured without
those fallbacks - we'll drop the fallbacks completely once the
default bridge has an internal resolver).
Signed-off-by: Rob Murray <rob.murray@docker.com>
The internal resolver now uses any namesever found in the host's
/etc/resolv.conf as an external nameserver, and it's accessed
from the host's network namespace.
Before this change, when no external nameservers were found (so
the host had no entries in /etc/resolv.conf) Google's DNS servers
were used as fallbacks, always accessed from the container's
network namespace. If a container's initial set of endpoints had
IPv6 enabled, the IPv6 nameservers were included.
Now we have IPv6-only networks, a similar exception would be
needed for Google's IPv4 nameservers... don't include them if
there are no IPv4 endpoints.
However, only the initial set of endpoints was considered. As
networks are connected/disconnected, IPv4 or IPv6 connectivity
may be lost.
Unlike nameservers read from the host's /etc/resolv.conf, there
is no way to tell which fallback nameservers (v4/v6) might work
from the host's namespace. So, using the host's namespace isn't
a good solution.
Since we want to get away from using fallback nameservers anyway,
this change removes them.
If a host has no /etc/resolv.conf entries, but a container does
need to use DNS, it'll need to be configured with servers via
'--dns'.
Signed-off-by: Rob Murray <rob.murray@docker.com>
The internal resolver needs to know whether to make requets
to external DNS servers from the container's network namespace
or the host's.
The original rule was that requests were always made from the
container's namespace, unless the nameserver was on a localhost
address on the host. IPv6 nameservers were left in the container's
/etc/resolv.conf.
Commit 4e8d9a4 modified that so that IPv6 nameservers were also
used as external nameservers. The internal resolver accessed
them from the host namespace if the container's initial set of
endpoints were IPv4-only, or the nameserver address contained
a zone-id, (or the nameserver was on the IPv6 loopback address).
That would break if initial IPv6 endpoints were disconnected from
the container, leaving it with no IPv6 address.
Once IPv6-only networks are allowed, another exception would need
to be made for IPv4 nameservers (they'd need to be accessed from
the host's namespace).
Instead of doing that ... this change simplifies things, if a
nameserver address is read from the host's /etc/resolv.conf, it'll
work in the host's namespace. So, the rule is now simply that
nameservers read from the host's resolv.conf are accessed from the
host's namespace. DNS servers added as overrides ('--dns') are
accessed from the container's namespace (as before).
Signed-off-by: Rob Murray <rob.murray@docker.com>
This reverts commit d365702dbd.
Because buildkit doesn't run an internal resolver, and it bases its
/etc/resolv.conf on the host's ... when buildkit is run in a container
that has 'nameserver 127.0.0.11', its build containers will use Google's
DNS servers as a fallback (unless the build container uses host
networking).
Before, when the 127.0.0.11 resolver was not used for the default network,
the buildkit container would have inherited a site-local nameserver. So,
the build containers it created would also have inherited that DNS
server - and they'd be able to resolve site-local hostnames.
By replacing the site-local nameserver with Google's, we broke access
to local DNS and its hostnames.
Signed-off-by: Rob Murray <rob.murray@docker.com>
Don't fall-back to Google's DNS servers in a network that has an
internal resolver.
Now the default bridge uses the internal resolver, the only reason a
network started by the daemon should end up without any upstream
servers is if the host's resolv.conf doesn't list any. In this case,
the '--dns' option can be used to explicitly configure nameservers
for a container if necessary.
(Note that buildkit's containers do not have an internal resolver, so
they will still set up Google's nameservers if the host has no
resolvers that can be used in the container's namespace.)
Signed-off-by: Rob Murray <rob.murray@docker.com>
When configuring the internal DNS resolver - rather than keep IPv6
nameservers read from the host's resolv.conf in the container's
resolv.conf, treat them like IPv4 addresses and use them as upstream
resolvers.
For IPv6 nameservers, if there's a zone identifier in the address or
the container itself doesn't have IPv6 support, mark the upstream
addresses for use in the host's network namespace.
Signed-off-by: Rob Murray <rob.murray@docker.com>
Replace regex matching/replacement and re-reading of generated files
with a simple parser, and struct to remember and manipulate the file
content.
Annotate the generated file with a header comment saying the file is
generated, but can be modified, and a trailing comment describing how
the file was generated and listing external nameservers.
Always start with the host's resolv.conf file, whether generating config
for host networking, or with/without an internal resolver - rather than
editing a file previously generated for a different use-case.
Resolves an issue where rewrites of the generated file resulted in
default IPv6 nameservers being unnecessarily added to the config.
Signed-off-by: Rob Murray <rob.murray@docker.com>
