troubleshooting common connection issues

This is a stub for a troubleshooting guide for SSH/SCP/SFTP, FTP, RSH, TELNET, etc related issues:

Problem 1: Unable to SSH or SCP

Common troubleshooting steps:

1) check the permissions of the authorized_keys on the target machine. Should be restrictive,644 is good. check the permissions on the directory path
2) Check the authorized_keys file on target box
3) check the permissions on the source box
4) check the private keys files on the source box
5) check the command
6) check if version 1 or 2 is restricted on any of the box
vi /etc/ssh/sshd_config on the source and target box
7) check the command

Problem 2: SSH or SCP is prompting for passphrase when it didn't use to before
or how can I make it such that passphrase is not prompted

Common troubleshooting tips:
1) restart the ssh-agent and run ssh-add
2) recreate your keys without a passphrase

Problem 3: SSH needs to be configured only to allow running of certain commands

Problem 4: I use putty to SSH, how do I convert the keys

Problem 5: SFTP fails