Motivation:
Recently DnsServerAddressStreamProvider was introduced to allow control for each query as to which DNS server should be used for resolution to respect the local host's default DNS server configuration. However resolver-dns also accepts a stream of DNS servers to use by default, but this stream is not host name aware. This creates an ambiguity as to which method is used to determine the DNS server to user during resolution, and in which order. We can remove this ambiguity and provide a more general API by just supporting DnsServerAddressStreamProvider.
Modifications:
- Remove the fixed DnsServerAddresses and instead only accept a DnsServerAddressStreamProvider.
- Add utility methods to help use DnsServerAddressStreamProvider for a single entry, a list of entries, and get the default for the current machine.
Result:
Fixes https://github.com/netty/netty/issues/6573.
Motivation:
We have our own ThreadLocalRandom implementation to support older JDKs . That said we should prefer the JDK provided when running on JDK >= 7
Modification:
Using ThreadLocalRandom implementation of the JDK when possible.
Result:
Make use of JDK implementations when possible.
Motivation:
54c9ecf682 introduced a unit tests which attempted to exclude addresses which resolved to loop back addresses from an assert statement. This was done with a static check for localhost but depending on machine configuration it is possible for other interfaces to be resolved.
Modifications:
- Use InetAddress#isLoopbackAddress() instead of string match on localhost
Result:
DnsNameResolverTest#testNameServerCache is more reliable.
Motivation:
The JDK uses gethostbyname for blocking hostname resoltuion. gethostbyname can be configured on Unix systems according to [1][2]. This may impact the name server that is used to resolve particular domains or just override the default fall-back resolver. DnsNameResolver currently ignores these configuration files which means the default resolution behavior is different than the JDK. This may lead to unexpected resolution failures which succeed when using the JDK's resolver.
Modifications:
- Add an interface which can override what DnsServerAddressStream to use for a given hostname
- Provide a Unix specific implementation of this interface and implement [1][2]. Some elements may be ignored sortlist, timeout, etc...
Result:
DnsNameResolver behaves more like the JDK resolver by default.
[1] https://linux.die.net/man/5/resolver
[2] https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man5/resolver.5.html
Motivation:
HostsFileParser only retains the first address for each given hostname.
This is wrong, and it’s allowed to have both an IPv4 and an IPv6.
Modifications:
* Have `HostsFileParser` now return a `HostsFileEntries` that contains IPv4 entries and IPv6 entries
* Introduce `ResolvedAddressTypes` to describe resolved address types preferences
* Add a new `ResolvedAddressTypes` parameter to `HostsFileEntriesResolver::address` to account for address types preferences
* Change `DnsNameResolver` constructor to take a `ResolvedAddressTypes`, allowing for a null value that would use default
* Change `DnsNameResolverBuilder::resolvedAddressTypes` to take a `ResolvedAddressTypes`
* Make `DnsNameResolver::resolvedAddressTypes` return a `ResolvedAddressTypes`
* Add a static `DnsNameResolverBuilder::computeResolvedAddressTypes` to ease converting from `InternetProtocolFamily`
Result:
We now support hosts files that contains IPv4 and IPv6 pairs for a same
hostname.
Motivation:
a416b79 introduced a check for null or empty host name to be compatible with the JDK resolution. However the doResolve(String, Promise) method, and if the doResolve(String, DnsRecord[], Promise, DnsCache) method was overridden the empty/null hostname would not be correctly resolved.
Modifications:
- Move the empty/null host name check into the lowest level doResolve method in DnsNameResolver
- Remove the duplicate logic in InetNameResolver.java which can be bypassed anyways
Result:
By default (unless behavior is overridden) DnsNameResolver resolves null/empty host names to local host just like the JDK.
Motivation:
We need to ensure we release the AddressedEnvelope if we fail to notify the future (as it may be notified before because of an timeout). Otherwise we may leak.
Modifications:
Call release() if we fail to notify the future.
Result:
No more memory leak on notify failure.
Motivation:
DnsNameResolver does not handle recursive DNS and so fails if you query a DNS server (for example a ROOT dns server) which provides the correct redirect for a domain.
Modification:
Add support for redirects (a.k.a. handling of AUTHORITY section').
Result:
Its now possible to use a DNS server that redirects.
Motivation:
DnsNameResolver will return the domain / host name as ascii code using punycode (https://tools.ietf.org/html/rfc3492). This is different to what the JDK does which always convert it to unicode. We should do the same by default but allow to also not do it.
Modifications:
- Add new builder method on DnsNameResolverBuilder which allow to disable / enable converting. Default is to convert just like the JDK does.
- Add unit tests for it.
Result:
DnsNameResolver and JDK impl behave the same way.
Motivation:
When an empty hostname is used in DnsNameResolver.resolve*(...) it will never notify the future / promise. The root cause is that we not correctly guard against errors of IDN.toASCII(...) which will throw an IllegalArgumentException when it can not parse its input. That said we should also handle an empty hostname the same way as the JDK does and just use "localhost" when this happens.
Modifications:
- If the try to resolve an empty hostname we use localhost
- Correctly guard against errors raised by IDN.toASCII(...) so we will always noify the future / promise
- Add unit test.
Result:
DnsNameResolver.resolve*(...) will always notify the future.
Motivation:
Currently Netty does not wrap socket connect, bind, or accept
operations in doPrivileged blocks. Nor does it wrap cases where a dns
lookup might happen.
This prevents an application utilizing the SecurityManager from
isolating SocketPermissions to Netty.
Modifications:
I have introduced a class (SocketUtils) that wraps operations
requiring SocketPermissions in doPrivileged blocks.
Result:
A user of Netty can grant SocketPermissions explicitly to the Netty
jar, without granting it to the rest of their application.
Motivation:
When we follow CNAME records we should respect resolvedAddressTypes and only query A / AAAA depending on which address types are expected.
Modifications:
Check if we should query A / AAAA when follow CNAMEs depending on resolvedAddressTypes.
Result:
Correct behaviour when follow CNAMEs.
Motivation:
We miss checking if DnsCache is null in DnsNameResolver constructor which will later then lead to a NPE. Better fail fast here.
Modifications:
Check for null and if so throw a NPE.
Result:
Fail fast.
Motivation:
In later Java8 versions our Atomic*FieldUpdater are slower then the JDK implementations so we should not use ours anymore. Even worse the JDK implementations provide for example an optimized version of addAndGet(...) using intrinsics which makes it a lot faster for this use-case.
Modifications:
- Remove methods that return our own Atomic*FieldUpdaters.
- Use the JDK implementations everywhere.
Result:
Faster code.
Motivation:
We should not use the InternalThreadLocalMap where access may be done from outside the EventLoop as this may create a lot of memory usage while not be reused anyway.
Modifications:
Not use InternalThreadLocalMap in places where the code-path will likely be executed from outside the EventLoop.
Result:
Less memory bloat.
Motivation:
Make small refactoring for recently merged PR #5867 to make the code more flexible and expose aggressive round robin as a NameResolver too with proper code reuse.
Modifications:
Round robin is a method of hostname resolving - so Round robin related code fully moved to RoundRobinInetAddressResolver implements NameResolver<InetAddress>, RoundRobinInetSocketAddressResolver is deleted as a separate class, instance with the same functionality could be created by calling #asAddressResolver.
Result:
New forced Round Robin code exposed not only as an AddressResolver but as a NameResolver too, more proper code and semantic reusing of InetNameResolver and InetSocketAddressResolver classes.
Motivation:
Suppose the domain `foo.example.com` resolves to the following ip
addresses `10.0.0.1`, `10.0.0.2`, `10.0.0.3`. Round robin DNS works by
having each client probabilistically getting a different ordering of
the set of target IP’s, so connections from different clients (across
the world) would be split up across each of the addresses. Example: In
a `ChannelPool` to manage connections to `foo.example.com`, it may be
desirable for high QPS applications to spread the requests across all
available network addresses. Currently, Netty’s resolver would return
only the first address (`10.0.0.1`) to use. Let say we are making
dozens of connections. The name would be resolved to a single IP and
all of the connections would be made to `10.0.0.1`. The other two
addresses would not see any connections. (they may see it later if new
connections are made and `10.0.0.2` is the first in the list at that
time of a subsequent resolution). In these changes, I add support to
select a random one of the resolved addresses to use on each resolve
call, all while leveraging the existing caching and inflight request
detection. This way in my example, the connections would be make to
random selections of the resolved IP addresses.
Modifications:
I added another method `newAddressResolver` to
`DnsAddressResolverGroup` which can be overriden much like
`newNameResolver`. The current functionality which creates
`InetSocketAddressResolver` is still used. I added
`RoundRobinDnsAddressResolverGroup` which extends
DnsAddressResolverGroup and overrides the `newAddressResolver` method
to return a subclass of the `InetSocketAddressResolver`. This subclass
is called `RoundRobinInetSocketAddressResolver` and it contains logic
that takes a `resolve` request, does a `resolveAll` under the hood, and
returns a single element at random from the result of the `resolveAll`.
Result:
The existing functionality of `DnsAddressResolverGroup` is left
unchanged. All new functionality is in the
`RoundRobinInetSocketAddressResolver` which users will now have the
option to use.
Motivation:
RFC7871 defines an extension which allows to request responses for a given subset.
Modifications:
- Add DnsOptPseudoRrRecord which can act as base class for extensions based on EDNS(0) as defined in RFC6891
- Add DnsOptEcsRecord to support the Client Subnet in DNS Queries extension
- Add tests
Result:
Client Subnet in DNS Queries extension is now supported.
Motivation:
According to the Oracle documentation:
> java.net.preferIPv4Stack (default: false)
>
> If IPv6 is available on the operating system, the underlying native
> socket will be an IPv6 socket. This allows Java applications to connect
> to, and accept connections from, both IPv4 and IPv6 hosts.
>
> If an application has a preference to only use IPv4 sockets, then this
> property can be set to true. The implication is that the application
> will not be able to communicate with IPv6 hosts.
which means, if DnsNameResolver returns an IPv6 address, a user (or
Netty) will not be able to connect to it.
Modifications:
- Move the code that retrieves java.net.prefer* properties from
DnsNameResolver to NetUtil
- Add NetUtil.isIpV6AddressesPreferred()
- Revise the API documentation of NetUtil.isIpV*Preferred()
- Set the default resolveAddressTypes to IPv4 only when
NetUtil.isIpv4StackPreferred() returns true
Result:
- Fixes#5657
Motivation:
AddressResolverGroup adds a listener to the termination future of an
EventExecutor when a new AddressResolver is created. The listener calls
AddressResolver.close() when the EventExecutor is terminated to give the
AddressResolver a chance to release its resources.
When using DnsAddressResolverGroup, the AddressResolver.close() will
eventually trigger DnsNameResolver.close(), which closes its underlying
DatagramChannel.
DatagramChannel.close() (or any Channel.close()) will travel through
pipeline and trigger EventExecutor.execute() because
DnsNameResolver.close() has been invoked from a non-I/O thread.
(NB: A terminationFuture is always notified from the GlobalEventExecutor
thread.)
However, because we are doing this in the listener of the termination
future of the terminated EventLoop we are trying to execute a task upon,
the attempt to close the channel fails due to RejectedExecutionException.
Modifications:
- Do not call Channel.close() in DnsNameResolver.close() if the Channel
has been closed by EventLoop already
Result:
No more RejectedExecutionException when shutting down an event loop.
Motivation:
When a hostname cannot be resolved, the message in the UnknownHostException mentions the hostname with the last attempted search domain appended, which is kind of confusing. I would prefer to see the original hostname supplied to the method in the exception.
Modifications:
Store the pristine hostname in the resolver context and use it to create the exception message instead of the hostname with search domain.
Add unit test to check that the exception does not mention the search domain.
Result:
The exception mentions the unmodified hostname in the message.
Motivation:
The ndots = 0 is a valid value for ndots, it means that when using a non dotted name, the resolution should first try using a search and if it fails then use subdomains. Currently it is not allowed. Docker compose uses this when wiring up containers as names have usually no dots inside.
Modification:
Modify DnsNameResolver to accept ndots = 0 and handle the case in the resolution procedure. In this case a direct search is done and then a fallback on the search path is performed.
Result:
The ndots = 0 case is implemented.
Motivation:
When resolving localhost on Windows where the hosts file does not contain a localhost entry by default, the resulting InetAddress object returned by the resolver does not have the hostname set so that getHostName returns the ip address 127.0.0.1. This behaviour is inconsistent with Windows where the hosts file does contain a localhost entry and with Linux in any case. It breaks at least some unit tests.
Modifications:
Create the LOCALHOST4 and LOCALHOST6 objects with hostname localhost in addition to the address.
Add unit test domain localhost to DnsNameResolverTest to check the resolution of localhost with ipv4 at least.
Result:
The resolver returns a InetAddress object for localhost with the hostname localhost in all cases.
Motivation:
The current DnsNameResolver does not support search domains resolution. Search domains resolution is supported out of the box by the java.net resolver, making the DnsNameResolver not able to be a drop in replacement for io.netty.resolver.DefaultNameResolver.
Modifications:
The DnsNameResolverContext resolution has been modified to resolve a list of search path first when it is configured so. The resolve method now uses the following algorithm:
if (hostname is absolute (start with dot) || no search domains) {
searchAsIs
} else {
if (numDots(name) >= ndots) {
searchAsIs
}
if (searchAsIs wasn't performed or failed) {
searchWithSearchDomainsSequenciallyUntilOneSucceeds
}
}
The DnsNameResolverBuilder provides configuration for the search domains and the ndots value. The default search domains value is configured with the OS search domains using the same native configuration the java.net resolver uses.
Result:
The DnsNameResolver performs search domains resolution when they are present.
Modifications:
The DnsNameResolver Bootstrap does not bind anymore, instead it registers and use the channel directly. The localAddress has also been removed from the DnsAddressResolverGroup, DnsNameResolver and DnsNameResolverBuilder as it is not necessary anymore and the API is marked as @UnstableApi.
Result:
Dns resolution does not require anymore to bind locally.
Motivation:
If DnsNameResolver works with NoopDnsCache, IndexOutOfBoundsException will
be thrown.
Modifications:
Test if the result of DnsNameResolver.get(hostname) is empty before
accessing it's elements.
Motivation:
On Windows localhost is not in hosts file and the DNS server does not resolve this address either, i.e it is handled by the Windows API. So using a Bootstrap (among others) with the resolver based on DnsNameResolver will not resolve localhost.
Modifications:
Workaround behavior of Windows
Result:
Correctly resolve localhost on Windows when using DnsNameResolver
Motivation:
The current DnsNameResolver fails to resolve an A+CNAME answer. For example:
dig moose.rmq.cloudamqp.com
...
;; ANSWER SECTION:
moose.rmq.cloudamqp.com. 1800 IN CNAME ec2-54-152-221-139.compute-1.amazonaws.com.
ec2-54-152-221-139.compute-1.amazonaws.com. 583612 IN A 54.152.221.139
...
The resolver constructs a map of cnames but forgets the trailing "." in the values which lead to not resolve the A record.
Modifications:
Reuse the code of DefaltDnsRecordDecoder which correctly handles the trailing dot.
Result:
Correctly resolve.
Motivation:
JCTools supports both non-unsafe, unsafe versions of queues and JDK6 which allows us to shade the library in netty-common allowing it to stay "zero dependency".
Modifications:
- Remove copy paste JCTools code and shade the library (dependencies that are shaded should be removed from the <dependencies> section of the generated POM).
- Remove usage of OneTimeTask and remove it all together.
Result:
Less code to maintain and easier to update JCTools and less GC pressure as the queue implementation nt creates so much garbage
Motivation:
We use a default of 3 for maxQueriesPerResolve when using the DnsNameResolverBuilder, which is too low if you want to resolve a hostname that uses a lot of CNAME records.
Modifications:
- Use higher default (16)
- Make exception message more clear why it failed.
Result:
Be able to resolve more domains by default and be able to better trouble shoot why a resolver failed.
Motivation:
DnsAddressResolverGroup allows to override the newResolver(...) method to change the settings used by the user. We should better let the user override another method and always apply the InflightNameResolver.
Modifications:
- Mark newResolver(...) method as deprecated, we will make it private soon.
- Add newNameResolver(...) method that user can override.
Result:
Easier to extend DnsAddressResolverGroup
Related issue: #5179
Motivation:
When you attempt to make a lot of connection attempts to the same target
host at the same time and our DNS resolver does not have a record for it
in the cache, the DNS resolver will send as many DNS queries as the
number of connection attempts.
As a result, DNS server will reject or drop the requests, making the
name resolution attempt fail.
Modifications:
- Add InflightNameResolver that keeps the list of name resolution
queries and subscribes to the future of the matching query instead of
sending a duplicate query.
Result:
- The AddressResolvers created by DnsAddressResolverGroup do not send
duplicate DNS queries anymore
Motivation:
Some codecs should be considered unstable as these are relative new. For this purpose we should introduce an annotation which these codecs should us to be marked as unstable in terms of API.
Modifications:
- Add UnstableApi annotation and use it on codecs that are not stable
- Move http2.hpack to http2.internal.hpack as it is internal.
Result:
Better document unstable APIs.
Motivation:
There were some warning in the resolver-dns code base.
Modifications:
- Fix javadocs
- Use the base class to call static method.
Result:
Cleaner code.
Related: #4771
Motivation:
A malicious or misconfigured DNS server can send the CNAME records that
resolve into each other, causing an unexpected infinite loop in
DnsNameResolverContext.onResponseCNAME().
Modifications:
- Remove the dereferenced CNAME from the alias map so that infinite loop
is impossible.
- Fix inspection warnings and typos in DnsNameResolverTest
Result:
Fixes#4771
Motivation:
Current DnsNameResolver api don't allow to define additional records in DNS query.
It can be useful in many cases. For example when we want to query dns server with
real client address (EDNS-CLIENT-SUBNET extension:
http://tools.ietf.org/html/draft-vandergaast-edns-client-subnet-02 )
Modifications:
This change add new query methods with list of additional DnsRecord-s for query.
Result:
It is possible to create dns query with EDNS-CLIENT-SUBNET extension for example.
Motivation:
See #3411. A reusable ArrayList in InternalThreadLocalMap can avoid allocations in the following pattern:
```
List<...> list = new ArrayList<...>();
add something to list but never use InternalThreadLocalMap
return list.toArray(new ...[list.size()]);
```
Modifications:
Add a reusable ArrayList to InternalThreadLocalMap and update codes to use it.
Result:
Reuse a thread local ArrayList to avoid allocations.
Motivation:
DnsNameResolverBuilder#nameServerAddresses isn’t initialized with a
default value. In most cases, user will want
DefaultDnsServerAddresses#defaultAddresses.
Modifications:
Initialize DnsNameResolverBuilder#nameServerAddresses with
DefaultDnsServerAddresses#defaultAddresses
Result:
DnsNameResolverBuilder more convenient usage.
Motivation:
As "getHostName" may do a reverse name lookup and return a host name based on the system configured name lookup service, testResolveIp may fail in some special environment. See #4720
Modifications:
Use getHostAddress instead of getHostName
Result:
testResolveIp works in all environments
Motivation:
Caching is currently nested in DnsResolver.
It should also be possible to extend DnsResolver to ba able to pass a different cache on each resolution attemp.
Modifications:
* Introduce DnsCache, NoopDnsCache and DefaultDnsCache. The latter contains all the current caching logic that was extracted.
* Introduce protected versions of doResolve and doResolveAll that can be used as extension points to build resolvers that bypass the main cache and use a different one on each resolution.
Result:
Isolated caching logic. Better extensibility.
Motivation:
There are some wrong links and tags in javadoc.
Modifications:
Fix the wrong links and tags in javadoc.
Result:
These links will work correctly in javadoc.
Motivation:
On contrary to `DefaultNameResolver`, `DnsNameResolver` doesn't currently honor hosts file.
Modifications:
* Introduce `HostsFileParser` that parses `/etc/hosts` or `C:\Windows\system32\drivers\etc\hosts` depending on the platform
* Introduce `HostsFileEntriesResolver` that uses the former to resolve host names
* Make `DnsNameResolver` check his `HostsFileEntriesResolver` prior to trying to resolve names against the DNS server
* Introduce `DnsNameResolverBuilder` so we now have a builder for `DnsNameResolver`s
* Additionally introduce a `CompositeNameResolver` that takes several `NameResolver`s and tries to resolve names by delegating sequentially
* Change `DnsNameResolver.asAddressResolver` to return a composite and honor hosts file
Result:
Hosts file support when using `DnsNameResolver`.
Consistent behavior with JDK implementation.
Motivation:
As discussed in #4529, NameResolver design shouldn't be resolving SocketAddresses (or String name + port) and return InetSocketAddresses. It should resolve String names and return InetAddresses.
This SocketAddress to InetSocketAddresses resolution is actually a different concern, used by Bootstrap.
Modifications:
Extract SocketAddress to InetSocketAddresses resolution concern to a new class hierarchy named AddressResolver.
These AddressResolvers delegate to NameResolvers.
Result:
Better separation of concerns.
Note that new AddressResolvers generate a bit more allocations because of the intermediate Promise and List<InetAddress>.
Motivation:
There's no way to override the default settings of the DnsNameResolvers
created by DnsNameResolverGroup because DnsNameResolverGroup is final.
Modifications:
- Make DnsNameResolverGroup non-final
- Add a new overridable protected method 'newResolver()' so that a user
can override it to create an alternative DnsNameResolver instance or
set the non-default properties
Result:
A user can configure the DnsNameResolver.
Motivation:
Each server should be checked for every record type. Currently, if there
are only two configured servers and the first is down, it is impossible
to query for IPv4 addresses because the second server is only ever
queried for type AAAA.
Modifications:
Do not cycle DNS servers while cycling DNS record types (A and AAAA)
Result:
Name resolution is less fragile when the number of available DNS servers
is 2.
Related: #3972
Motivation:
DnsNameResolver limits the number of concurrent in-progress DNS queries
to 65536 regardless the number of DNS servers it communicates with. When
the number of available DNS servers are more than just one, we end up
using much less (65536 / numDnsServers) query IDs per DNS server, which
is non-optimal.
Modifications:
- Replace the query ID and context management with
DnsQueryContextManager
- Eash DNS server gets its own query ID space
Result:
Much bigger query ID space, and thus there's less chance of getting the
'query ID space exhaustion' error
Motivation:
When DnsNameResolverContext succeeds to get the address(es), it cancels
the promise of other queries in progress.
Unlike expectation, DnsNameResolverContext.query() attempts to retry
even when the query has failed due to cancellation.
As a result, the resolver sends unnecessary extra queries to a DNS
server and then tries to mark the promised that's been fulfilled
already, leading to unnecessarily verbose 'failed to notify success to a
promise' messages.
Modifications:
Do not perform an extra query when the previous query has failed due to
cancellation
Result:
DnsNameResolver does not send unnecessary extra queries and thus does
not log the 'failed to notify success to a promise' message.
Motivation:
As relaying on external DNS Server can result to test-failures we should better use a mock DNS Server for the dns tests.
Modifications:
- Refactor the DnsNameResolverTest to use a mock DNS Server which is using apacheds.
- Allow to disable adding an opt resources as some servers not support it.
Result:
More stable testsuite.
Motivation:
Some DNS servers in DnsNameResolverTest are outdated and some of them
returns NoError for non-existent domains.
Modifications:
- Update the DNS server list from http://meo.ws/dnsrec.php again
- Update the web-scraper script
Result:
DnsNameResolverTest.testNegativeTtl() should not fail anymore.
Motivation:
DNS lookups in DnsNameResolverTest can take longer than expected due to
retries. The hard limit of 5 seconds is being applied to
testNegativeTtl(), making the first uncached lookup cause a timeout.
Modifications:
Do not use JUnit's Timeout annotation but implement simple timeout
mechanism that apples only to cached lookups.
Result:
testNegativeTtl() should not fail when an initial negative lookup
requires a retry.
Motivation:
- DNS lookup sometimes hang because it does not call
tryToFinishResolve()
- Comodo Secure DNS handles negative lookup incorrectly.
Modifications:
- Add missing tryToFinishResolve()
- Remove Comodo Secure DNS servers from the list
Result
- DNS lookup does not hang on non-existent domain names
- More reliable DnsNameResolverTest
Related: #4065
Motivation:
DnsNameResolver was using a special Iterable/Iterator implementation
that yields an infinite stream of DNS server addresses. However, this
seems to cause confusion.
Modifications:
- Make DnsServerAddresses an abstract class with an abstract stream()
method that returns DnsServerAddressStream
- Add DnsServerAddressStream that yields DNS server address infinitely
- Remove DnsServerResolver(Group) constructors that accept only a single
server address, which wasn't very useful in practice
- Extract the DnsServerAddresses implementations to top level
- DnsServerAddresses.defaultAddresses() now returns DnsServerAddresses.
- Add DnsServerAddresses.defaultAddressList() instead
Result:
Less confusion and more explicitness
Motivation:
DNS servers seem to reply with ServFail(2) response code when it is
busy.
Modifications:
- Retry when response code is ServFail instead of failing the test
- Try all DNS servers instead of retrying twice only
Result:
testQueryMx() is less likely to fail due to public DNS server problems
