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?

Solution

/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.

Comments

[Amre]

There would no problem with using public dns, directly in the masters and slave files. As for using the private ip, I had just replaced the public dns with the corresponding private ip in the /etc/hosts file, before I saw your answer, but it gave me the same error.

[Amre]

but I did use ip-10.x.x.x, rather than 10.x.x.x, could that have been the issue?

[Amre]

I can ssh using the private ip if i do ssh ip-10.x.x.x., but not ssh 10.x.x.x

[SSaikia_JtheRocker]

"but I did use ip-10.x.x.x, rather than 10.x.x.x" - I didn't get you?

[Amre]

The aws console tells you the private ip but with "ip-" infront of the address

[SSaikia_JtheRocker]

Can you ping 10.x.x.x?

[SSaikia_JtheRocker]

Did you map the private ip address of both in each of the machines in /etc/hosts? i.e. your both '{IP_address_1st_instance} node1' and '{IP_address_2st_instance} node2' mapping should go in every machine's /etc/hosts

[Amre]

I can't ping "10.x.x.x", but I can ping "ip-10.x.x.x"

[Amre]

I was able to start everything when i replaced the "node1" and "node2" with the actual ips, but I'm having a hadoop problem now, it tells me the connection to localhost is refused, even though I can do a passphraseless ssh into localhost

[SSaikia_JtheRocker]

do you have localhost mapped to 127.0.0.1 and nothing else in the /etc/hosts?

[SSaikia_JtheRocker]

let us continue this discussion in chat

[Amre]

My work network let me open the chat page, but the I just rmemebered, i had forgotten to staart mapreduce when I did the example. Thanks for your help, putting the ip worked.

[SSaikia_JtheRocker]

Glad that it woreked!