Not able to telnet from one AWS instance to another (to resolve master_not_discovered_exception)

Question

I have 2 AWS EC2 instances which have the same security group:

When I'm running curl -XGET 'http://localhost:9200/_cluster/state/master_node?pretty' from the nodes (works only on master), I get: master_not_discovered_exception

I tried to telnet with port 9200 from one machine to another, and it doesn't work, so I think this the root cause (I'm able to telnet from my local machine.

I followed this guideline

What else I should check?

Update:

telnet works tnx to suggested solution, however issue was still persist. I was able to fix it after following this answer

Answer 1

It would be a good idea to check the following: -Are both aws machines in the same VPC/Subnet? -If they are not, are there any ACLs configured in any of the VPCs that would prevent communication? Check VPCs and subnets. -If they are not, is routing configured between the machines? -Does the security group allow connections to TCP 9200 from any machine? or from a specific ip/range/subnet? if this is the case, make sure the machines ips are included.

-Do you have any firewall running on the machines that could prevent connectivity? ( If they are linux machines, check firewalld/iptables, and disable it shortly for a test. If they are windows machines, do the same test with windows firewall. If you do this test, make sure the machines are not reachable from external sources for the test duration.