How to ssh from one ec2 instance to another?

Method 1 - use the same keys on the servers:

Convert the keys to openssh format and upload the private keys to the servers. When you ssh to the destination host, specify the private key file:

ssh -i mykey.pem private.ip.of.other.server

Method 2 - Create new keys

On each server run:

ssh-keygen

Hit enter enter enter. You'll have two files:

.ssh/id_rsa
.ssh/id_rsa.pub

On Server A, cat and copy to clipboard the public key:

cat ~/.ssh/id_rsa.pub
[select and copy to your clipboard]

ssh into Server B, and append the contents of that to the it's authorized_keys file:

cat >> ~/.ssh/authorized_keys
[paste your clipboard contents]
[ctrl+d to exit]

Now ssh from server A:

ssh -i ~/.ssh/id_rsa private.ip.of.other.server

There is a 3rd and IMHO the best solution so called ssh agent forwarding:

Host <ip-or-name-of-A-server>
  ForwardAgent yes

I assume on server A and B you have your local ~/.ssh/id_rsa.pub added to server's ~/.ssh/authorized_keys

While working on server A your keys can be used in further ssh communication - e.g.:

connecting to other server with ssh client - in this case to server B,
scp (secure copy),
git - you can pull/push using your local identity to your remote git repositories
etc.

To check to see if this works:

connect to server A
check if there is socket connection for key exchange by detecting SSH_AUTH_SOCK env var:

set|grep SSH_AUTH_ # output should be something like this:
SSH_AUTH_SOCK=/tmp/ssh-sEHiRF4hls/agent.12042

Notes:

you need to have ssh agent running - linux: ps -e | grep [s]sh-agent, for windows check putty's utilities pagent and plink
reference: https://help.github.com/articles/using-ssh-agent-forwarding
troubleshooting ssh:
https://confluence.atlassian.com/display/BITBUCKET/Troubleshoot+SSH+Issues

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