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https://issues.apache.org/jira/browse/HADOOP-15129?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16361506#comment-16361506
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Ajay Kumar edited comment on HADOOP-15129 at 2/12/18 9:54 PM:
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{quote} The local IP address for the connection is indeterminate until the 
target hostname is resolved e.g. multi-homed setups. So I am okay with leaving 
this as null for now.{quote}
[~arpitagarwal], I see your point. I was thinking that localhost "UNKOWN" may 
mislead someone in wrong diagnosis about the problem. Wondering if we can we 
improve the messaging. But this can be discussed separately and doesn't need to 
be part of this jira.


was (Author: ajayydv):
{quote} The local IP address for the connection is indeterminate until the 
target hostname is resolved e.g. multi-homed setups. So I am okay with leaving 
this as null for now.{quote}
I see your point. I was thinking that localhost "UNKOWN" may mislead someone in 
wrong diagnosis about the problem. Wondering if we can we improve the 
messaging. But this can be discussed separately and doesn't need to be part of 
this jira.

> Datanode caches namenode DNS lookup failure and cannot startup
> --------------------------------------------------------------
>
>                 Key: HADOOP-15129
>                 URL: https://issues.apache.org/jira/browse/HADOOP-15129
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: ipc
>    Affects Versions: 2.8.2
>         Environment: Google Compute Engine.
> I'm using Java 8, Debian 8, Hadoop 2.8.2.
>            Reporter: Karthik Palaniappan
>            Assignee: Karthik Palaniappan
>            Priority: Minor
>         Attachments: HADOOP-15129.001.patch, HADOOP-15129.002.patch
>
>
> On startup, the Datanode creates an InetSocketAddress to register with each 
> namenode. Though there are retries on connection failure throughout the 
> stack, the same InetSocketAddress is reused.
> InetSocketAddress is an interesting class, because it resolves DNS names to 
> IP addresses on construction, and it is never refreshed. Hadoop re-creates an 
> InetSocketAddress in some cases just in case the remote IP has changed for a 
> particular DNS name: https://issues.apache.org/jira/browse/HADOOP-7472.
> Anyway, on startup, you cna see the Datanode log: "Namenode...remains 
> unresolved" -- referring to the fact that DNS lookup failed.
> {code:java}
> 2017-11-02 16:01:55,115 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: 
> Refresh request received for nameservices: null
> 2017-11-02 16:01:55,153 WARN org.apache.hadoop.hdfs.DFSUtilClient: Namenode 
> for null remains unresolved for ID null. Check your hdfs-site.xml file to 
> ensure namenodes are configured properly.
> 2017-11-02 16:01:55,156 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: 
> Starting BPOfferServices for nameservices: <default>
> 2017-11-02 16:01:55,169 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: 
> Block pool <registering> (Datanode Uuid unassigned) service to 
> cluster-32f5-m:8020 starting to offer service
> {code}
> The Datanode then proceeds to use this unresolved address, as it may work if 
> the DN is configured to use a proxy. Since I'm not using a proxy, it forever 
> prints out this message:
> {code:java}
> 2017-12-15 00:13:40,712 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: 
> Problem connecting to server: cluster-32f5-m:8020
> 2017-12-15 00:13:45,712 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: 
> Problem connecting to server: cluster-32f5-m:8020
> 2017-12-15 00:13:50,712 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: 
> Problem connecting to server: cluster-32f5-m:8020
> 2017-12-15 00:13:55,713 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: 
> Problem connecting to server: cluster-32f5-m:8020
> 2017-12-15 00:14:00,713 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: 
> Problem connecting to server: cluster-32f5-m:8020
> {code}
> Unfortunately, the log doesn't contain the exception that triggered it, but 
> the culprit is actually in IPC Client: 
> https://github.com/apache/hadoop/blob/trunk/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/Client.java#L444.
> This line was introduced in https://issues.apache.org/jira/browse/HADOOP-487 
> to give a clear error message when somebody mispells an address.
> However, the fix in HADOOP-7472 doesn't apply here, because that code happens 
> in Client#getConnection after the Connection is constructed.
> My proposed fix (will attach a patch) is to move this exception out of the 
> constructor and into a place that will trigger HADOOP-7472's logic to 
> re-resolve addresses. If the DNS failure was temporary, this will allow the 
> connection to succeed. If not, the connection will fail after ipc client 
> retries (default 10 seconds worth of retries).
> I want to fix this in ipc client rather than just in Datanode startup, as 
> this fixes temporary DNS issues for all of Hadoop.



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