HADOOP-18087. fix bugs when looking up record from upstream DNS servers.

[eubnara]

Description of PR

When query A record which is chained by CNAME, YARN Registry DNS Server does not properly respond. Some CNAME records are missing.

For example, "repo.maven.apache.org" is chaned as follows:

repo.maven.apache.org. 21317 IN CNAME repo.apache.maven.org. repo.apache.maven.org. 20114 IN CNAME maven.map.fastly.net. maven.map.fastly.net. 7 IN A 199.232.192.215 maven.map.fastly.net. 7 IN A 199.232.196.215

If ask A record for "repo.maven.apache.org" using "dig" or "nslookup", YARN Registry DNS Server will give answers similar to this: (10.1.2.3, 10.8.8.8 IP is virtual)

$ nslookup repo.maven.apache.org 10.1.2.3
Server:		10.1.2.3
Address:	10.1.2.3#53

Non-authoritative answer:
repo.maven.apache.org	canonical name = repo.apache.maven.org.
Name:	maven.map.fastly.net
Address: 151.101.196.215
** server can't find repo.apache.maven.org: NXDOMAIN

The reason why you can see "NXDOMAIN", "nslookup" will query "A" & "AAAA" records. If there is no answer from other dns server, "answers == null" but YARN Registry DNS Server has a bug. There is no null handling.

    // Forward lookup to primary DNS servers
    Record[] answers = getRecords(name, type);
    try {
      for (Record r : answers) {
        if (!response.findRecord(r)) {
          if (r.getType() == Type.SOA) {
            response.addRecord(r, Section.AUTHORITY);
          } else {
            response.addRecord(r, Section.ANSWER);
          }
        }
        if (r.getType() == Type.CNAME) {
          Name cname = r.getName();
          if (iterations < 6) {
            remoteLookup(response, cname, type, iterations + 1);
          }
        }
      }
    } catch (NullPointerException e) {
      return Rcode.NXDOMAIN;
    } catch (Throwable e) {
      return Rcode.SERVFAIL;
    }
    return Rcode.NOERROR;

It should be like this:

nslookup repo.maven.apache.org 10.8.8.8
Server:		10.8.8.8
Address:	10.8.8.8#53

Non-authoritative answer:
repo.maven.apache.org	canonical name = repo.apache.maven.org.
repo.apache.maven.org	canonical name = maven.map.fastly.net.
Name:	maven.map.fastly.net
Address: 151.101.196.215

How was this patch tested?

• apply this patch to YARN Registry DNS Server in our cluster & test with dig and nslookup

For code changes:

• [x] Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• [ ] Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• [ ] If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• [ ] If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?

[eubnara]

https://github.com/apache/hadoop/pull/3935

[hadoop-yetus]

:broken_heart: -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 :ok:	reexec	15m 4s		Docker mode activated.
			_ Prechecks _
+1 :green_heart:	dupname	0m 0s		No case conflicting files found.
+0 :ok:	codespell	0m 1s		codespell was not available.
+0 :ok:	detsecrets	0m 1s		detect-secrets was not available.
+1 :green_heart:	@author	0m 0s		The patch does not contain any @author tags.
+1 :green_heart:	test4tests	0m 0s		The patch appears to include 1 new or modified test files.
			_ trunk Compile Tests _
+1 :green_heart:	mvninstall	33m 37s		trunk passed
+1 :green_heart:	compile	16m 15s		trunk passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 :green_heart:	compile	16m 15s		trunk passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
+1 :green_heart:	checkstyle	0m 42s		trunk passed
+1 :green_heart:	mvnsite	0m 50s		trunk passed
+1 :green_heart:	javadoc	0m 51s		trunk passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 :green_heart:	javadoc	0m 47s		trunk passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
-1 :x:	spotbugs	1m 10s	/branch-spotbugs-hadoop-common-project_hadoop-registry-warnings.html	hadoop-common-project/hadoop-registry in trunk has 19 extant spotbugs warnings.
+1 :green_heart:	shadedclient	26m 40s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 :green_heart:	mvninstall	0m 22s		the patch passed
+1 :green_heart:	compile	15m 10s		the patch passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 :green_heart:	javac	15m 10s		the patch passed
+1 :green_heart:	compile	16m 16s		the patch passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
+1 :green_heart:	javac	16m 16s		the patch passed
+1 :green_heart:	blanks	0m 0s		The patch has no blanks issues.
-0 :warning:	checkstyle	0m 40s	/results-checkstyle-hadoop-common-project_hadoop-registry.txt	hadoop-common-project/hadoop-registry: The patch generated 15 new + 1 unchanged - 0 fixed = 16 total (was 1)
+1 :green_heart:	mvnsite	0m 47s		the patch passed
-1 :x:	javadoc	0m 46s	/results-javadoc-javadoc-hadoop-common-project_hadoop-registry-jdkUbuntu-21.0.7+6-Ubuntu-0ubuntu120.04.txt	hadoop-common-project_hadoop-registry-jdkUbuntu-21.0.7+6-Ubuntu-0ubuntu120.04 with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04 generated 5 new + 136 unchanged - 0 fixed = 141 total (was 136)
-1 :x:	javadoc	0m 46s	/results-javadoc-javadoc-hadoop-common-project_hadoop-registry-jdkUbuntu-17.0.15+6-Ubuntu-0ubuntu120.04.txt	hadoop-common-project_hadoop-registry-jdkUbuntu-17.0.15+6-Ubuntu-0ubuntu120.04 with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04 generated 5 new + 129 unchanged - 0 fixed = 134 total (was 129)
-1 :x:	spotbugs	1m 19s	/new-spotbugs-hadoop-common-project_hadoop-registry.html	hadoop-common-project/hadoop-registry generated 1 new + 17 unchanged - 2 fixed = 18 total (was 19)
+1 :green_heart:	shadedclient	26m 57s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 :green_heart:	unit	1m 30s		hadoop-registry in the patch passed.
+1 :green_heart:	asflicense	1m 1s		The patch does not generate ASF License warnings.
		182m 39s

Reason	Tests
SpotBugs	module:hadoop-common-project/hadoop-registry
	Unknown bug pattern DCN_NULLPOINTER_EXCEPTION in org.apache.hadoop.registry.server.dns.RegistryDNS.getRecords(Name, int) At RegistryDNS.java:org.apache.hadoop.registry.server.dns.RegistryDNS.getRecords(Name, int) At RegistryDNS.java:[line 1190]

Subsystem	Report/Notes
Docker	ClientAPI=1.52 ServerAPI=1.52 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8091/1/artifact/out/Dockerfile
GITHUB PR	https://github.com/apache/hadoop/pull/8091
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux 3dd397b0fd91 5.15.0-156-generic #166-Ubuntu SMP Sat Aug 9 00:02:46 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 683b944943662ca2af3a5953ecd7428768ec4173
Default Java	Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
Multi-JDK versions	/usr/lib/jvm/java-21-openjdk-amd64:Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04 /usr/lib/jvm/java-17-openjdk-amd64:Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8091/1/testReport/
Max. process+thread count	640 (vs. ulimit of 5500)
modules	C: hadoop-common-project/hadoop-registry U: hadoop-common-project/hadoop-registry
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8091/1/console
versions	git=2.25.1 maven=3.9.11 spotbugs=4.9.7
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