Greetings to the Kafka Community!

I'm a newbie in Kafka and only recently went beyond a local installation
described in the Quickstart. I have faced a weird issue that I can't
explain.

I want to deploy on two machines:
- Machine #1 runs ZooKeeper and a single Kafka broker. I use default
configuration files.
- Machine #2 runs a single instance of Kafka Connect. I use the source
connector for Ignite/GridGain (
https://www.confluent.io/hub/gridgain/ignite-connector). I doubt it
matters, though, as the issue seems to be related to connectivity between
the connector and the broker.

After starting the ZooKeeper and the broker on machine #1, I did the
following on machine #2:

1. Updated plugin.path and bootstrap.servers parameters in the
connect-standalone.properties; bootstrap.servers points to the IP of the #1.
2. Created a properties file for the GridGain connector.
3. Start the connector using this command:

bin/connect-standalone.sh config/connect-standalone.properties
config/gridgain-kafka-connect-source.properties

The issue is that it doesn't connect to the remote broker, and still tries
localhost ignoring the bootstrap.servers settings. The output looks like
this:

[2020-04-27 19:37:08,334] INFO StandaloneConfig values:
        ...
        bootstrap.servers = [98.1.96.147:9092]
        ...
[2020-04-27 19:37:08,334] INFO Creating Kafka admin client
(org.apache.kafka.connect.util.ConnectUtils:43)
[2020-04-27 19:37:08,337] INFO AdminClientConfig values:
        bootstrap.servers = [98.1.96.147:9092]
        ...
[2020-04-27 19:37:08,950] WARN [AdminClient clientId=adminclient-1]
Connection to node 0 (localhost/127.0.0.1:9092) could not be established.
Broker may not be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:09,052] WARN [AdminClient clientId=adminclient-1]
Connection to node 0 (localhost/127.0.0.1:9092) could not be established.
Broker may not be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:09,253] WARN [AdminClient clientId=adminclient-1]
Connection to node 0 (localhost/127.0.0.1:9092) could not be established.
Broker may not be available. (org.apache.kafka.clients.NetworkClient:756)

The warning goes on for a while until the process stops after a timeout.

Does anyone have any idea why Kafka Connect can try to establish a
connection with the localhost, although it has picked up the correct IP
from the configuration file?

The issue doesn't seem to be reproducible on any environment, so I
would appreciate any pointers on how to investigate this further.

Kafka's version is 2.4.1. Config files and the log are attached. If
anything else is needed, let me know.

Regards,
Val
[2020-04-27 19:37:08,334] INFO StandaloneConfig values:
        access.control.allow.methods =
        access.control.allow.origin =
        admin.listeners = null
        bootstrap.servers = [98.1.96.147:9092]
        client.dns.lookup = default
        config.providers = []
        connector.client.config.override.policy = None
        header.converter = class 
org.apache.kafka.connect.storage.SimpleHeaderConverter
        internal.key.converter = class 
org.apache.kafka.connect.json.JsonConverter
        internal.value.converter = class 
org.apache.kafka.connect.json.JsonConverter
        key.converter = class org.apache.kafka.connect.json.JsonConverter
        listeners = null
        metric.reporters = []
        metrics.num.samples = 2
        metrics.recording.level = INFO
        metrics.sample.window.ms = 30000
        offset.flush.interval.ms = 10000
        offset.flush.timeout.ms = 5000
        offset.storage.file.filename = standalone.offsets
        plugin.path = [/xxx/kafka_2.11-2.4.1/connect/gridgain-kafka-connect]
        rest.advertised.host.name = null
        rest.advertised.listener = null
        rest.advertised.port = null
        rest.extension.classes = []
        rest.host.name = null
        rest.port = 8083
        ssl.client.auth = none
        task.shutdown.graceful.timeout.ms = 5000
        value.converter = class org.apache.kafka.connect.json.JsonConverter
 (org.apache.kafka.connect.runtime.standalone.StandaloneConfig:347)
[2020-04-27 19:37:08,334] INFO Creating Kafka admin client 
(org.apache.kafka.connect.util.ConnectUtils:43)
[2020-04-27 19:37:08,337] INFO AdminClientConfig values:
        bootstrap.servers = [98.1.96.147:9092]
        client.dns.lookup = default
        client.id =
        connections.max.idle.ms = 300000
        metadata.max.age.ms = 300000
        metric.reporters = []
        metrics.num.samples = 2
        metrics.recording.level = INFO
        metrics.sample.window.ms = 30000
        receive.buffer.bytes = 65536
        reconnect.backoff.max.ms = 1000
        reconnect.backoff.ms = 50
        request.timeout.ms = 120000
        retries = 5
        retry.backoff.ms = 100
        sasl.client.callback.handler.class = null
        sasl.jaas.config = null
        sasl.kerberos.kinit.cmd = /usr/bin/kinit
        sasl.kerberos.min.time.before.relogin = 60000
        sasl.kerberos.service.name = null
        sasl.kerberos.ticket.renew.jitter = 0.05
        sasl.kerberos.ticket.renew.window.factor = 0.8
        sasl.login.callback.handler.class = null
        sasl.login.class = null
        sasl.login.refresh.buffer.seconds = 300
        sasl.login.refresh.min.period.seconds = 60
        sasl.login.refresh.window.factor = 0.8
        sasl.login.refresh.window.jitter = 0.05
        sasl.mechanism = GSSAPI
        security.protocol = PLAINTEXT
        security.providers = null
        send.buffer.bytes = 131072
        ssl.cipher.suites = null
        ssl.enabled.protocols = [TLSv1.2, TLSv1.1, TLSv1]
        ssl.endpoint.identification.algorithm = https
        ssl.key.password = null
        ssl.keystore.location = null
        ssl.keystore.password = null
        ssl.keystore.type = JKS
        ssl.protocol = TLS
        ssl.provider = null
        ssl.secure.random.implementation = null
        ssl.trustmanager.algorithm = PKIX
        ssl.truststore.location = null
        ssl.truststore.password = null
        ssl.truststore.type = JKS
 (org.apache.kafka.clients.admin.AdminClientConfig:347)
[2020-04-27 19:37:08,382] WARN The configuration 'offset.flush.interval.ms' was 
supplied but isn't a known config. 
(org.apache.kafka.clients.admin.AdminClientConfig:355)
[2020-04-27 19:37:08,382] WARN The configuration 'key.converter.schemas.enable' 
was supplied but isn't a known config. 
(org.apache.kafka.clients.admin.AdminClientConfig:355)
[2020-04-27 19:37:08,383] WARN The configuration 'offset.storage.file.filename' 
was supplied but isn't a known config. 
(org.apache.kafka.clients.admin.AdminClientConfig:355)
[2020-04-27 19:37:08,383] WARN The configuration 
'value.converter.schemas.enable' was supplied but isn't a known config. 
(org.apache.kafka.clients.admin.AdminClientConfig:355)
[2020-04-27 19:37:08,383] WARN The configuration 'plugin.path' was supplied but 
isn't a known config. (org.apache.kafka.clients.admin.AdminClientConfig:355)
[2020-04-27 19:37:08,383] WARN The configuration 'value.converter' was supplied 
but isn't a known config. (org.apache.kafka.clients.admin.AdminClientConfig:355)
[2020-04-27 19:37:08,383] WARN The configuration 'key.converter' was supplied 
but isn't a known config. (org.apache.kafka.clients.admin.AdminClientConfig:355)
[2020-04-27 19:37:08,383] INFO Kafka version: 2.4.1 
(org.apache.kafka.common.utils.AppInfoParser:117)
[2020-04-27 19:37:08,383] INFO Kafka commitId: c57222ae8cd7866b 
(org.apache.kafka.common.utils.AppInfoParser:118)
[2020-04-27 19:37:08,383] INFO Kafka startTimeMs: 1588030628383 
(org.apache.kafka.common.utils.AppInfoParser:119)
[2020-04-27 19:37:08,950] WARN [AdminClient clientId=adminclient-1] Connection 
to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not 
be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:09,052] WARN [AdminClient clientId=adminclient-1] Connection 
to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not 
be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:09,253] WARN [AdminClient clientId=adminclient-1] Connection 
to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not 
be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:09,555] WARN [AdminClient clientId=adminclient-1] Connection 
to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not 
be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:09,957] WARN [AdminClient clientId=adminclient-1] Connection 
to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not 
be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:10,860] WARN [AdminClient clientId=adminclient-1] Connection 
to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not 
be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:11,964] WARN [AdminClient clientId=adminclient-1] Connection 
to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not 
be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:12,867] WARN [AdminClient clientId=adminclient-1] Connection 
to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not 
be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:13,870] WARN [AdminClient clientId=adminclient-1] Connection 
to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not 
be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:15,073] WARN [AdminClient clientId=adminclient-1] Connection 
to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not 
be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:16,177] WARN [AdminClient clientId=adminclient-1] Connection 
to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not 
be available. (org.apache.kafka.clients.NetworkClient:756)
[2020-04-27 19:37:17,079] WARN [AdminClient clientId=adminclient-1] Connection 
to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not 
be available. (org.apache.kafka.clients.NetworkClient:756)

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