ssh: Could not resolve hostname. Name or service not known

Question

I'm trying to set up hadoop on my amazon instances, on a 2 node cluster. Each instance has a public dns, which I use reference them. So in the /etc/hosts files on both machines I append lines like this:

{public dns of 1st instance} node1
{public dns of 2st instance} node2

I'm also able to ssh into each instance from the other by simply doing:

ssh {public dns of the other instance}

In the the hadoop/conf/slaves on the first instance file I have:

localhost
node2

When I start the script bin/start-dfs.sh It's able to start the namenode, datanode, and secondary namenode on the master, but it says:

node2: ssh: Could not resolve hostname node2: Name or service not known

The same it printed out if I try:

ssh node2

I guess the question is how do I tell it to associate node2 with the public dns of the second instance. Is it not enough to append the

{public dns of 2st instance} node2

line to the /etc/hosts file? Do I have to reboot the instances?

Answer 1

/etc/hosts kind of act like a local DNS, when you don't have real DNS associated with an IP address.

Do you really need a {public dns of 1st instance} node1 mapping if you can use {public dns of 1st instance} directly in the slave and master files?

Moreover, it's better to use the private IP addresses of amazon instances instead of using the public IP addresses. You can do a ifconfig in the terminal of each instances and determine their private IP addresses if any. They will probably basically will start with 10.x.x.x/172.x.x.x/192.x.x.x? You can probably then map those instead in /etc/hosts in each of the amazon instances.

So, your /etc/hosts in each machine should look something like -

Machine-1:

{IP_address_1st_instance} node1
{IP_address_2st_instance} node2

Machine-2:

{IP_address_1st_instance} node1
{IP_address_2st_instance} node2

And, this is so that the Amazon instances(machines) can resolve each other, if you are anyhow planning to map them.