When attempting SSH authentication from a non-interactive environment (like an Apache user session), you might encounter this stubborn error:

read_passphrase: can't open /dev/tty: No such device or address
Permission denied (publickey)

This occurs even when using passwordless SSH keys with the -t flag. The root cause typically involves multiple factors including terminal allocation, key format, and host verification.

The debug output reveals critical authentication flow:

debug1: Offering RSA public key: nonpublic/id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 279
debug1: key_parse_private_pem: PEM_read_PrivateKey failed

This indicates the server accepts your public key but fails to process the private key. Common triggers include:

• PuTTY-generated keys in PPK format
• Incorrect file permissions (key must be 600)
• Missing host verification in known_hosts

If your key was generated with PuTTYgen, convert it to OpenSSH format:

# Install putty-tools if needed
sudo apt-get install putty-tools

# Convert PPK to OpenSSH format
puttygen id_rsa.ppk -O private-openssh -o id_rsa

Set proper permissions:

chmod 600 id_rsa
chown apache_user:apache_group id_rsa

For Apache or cron environments, use this optimized command structure:

ssh -o BatchMode=yes \
    -o StrictHostKeyChecking=no \
    -o UserKnownHostsFile=/path/to/custom_known_hosts \
    -i /path/to/converted_key \
    user@host command_to_run

Pre-populate known_hosts for non-interactive use:

ssh-keyscan -p 54367 localhost >> ~apache_user/.ssh/known_hosts

Alternatively, use SSH config:

Host localhost
    HostName localhost
    Port 54367
    User username
    IdentityFile ~/nonpublic/id_rsa
    StrictHostKeyChecking no

1. Verify key format with file id_rsa
2. Check SELinux context if applicable
3. Test with ssh -v for detailed logs
4. Ensure home directory permissions (711 for ~/.ssh)

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When attempting SSH authentication from a non-interactive environment (like Apache running as www-data), you'll encounter:

read_passphrase: can't open /dev/tty: No such device or address
Permission denied (publickey)

Let's examine the complete error pattern from the debug output (-v flag):

debug1: Offering RSA public key: nonpublic/id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 279
debug1: key_parse_private_pem: PEM_read_PrivateKey failed
debug1: read_passphrase: can't open /dev/tty

The fundamental issue stems from using a Putty-generated private key with OpenSSH. Putty uses its proprietary .ppk format, while OpenSSH expects PEM format.

Step 1: Convert Putty Key to OpenSSH Format

Install putty-tools if needed:

sudo apt-get install putty-tools  # Debian/Ubuntu
sudo yum install putty            # RHEL/CentOS

Conversion command:

puttygen nonpublic/id_rsa -O private-openssh -o openssh_key

Step 2: Verify Key Permissions

chmod 600 openssh_key
chown www-data:www-data openssh_key

Step 3: Update SSH Command

ssh -i /path/to/openssh_key \
    -o StrictHostKeyChecking=no \
    -o UserKnownHostsFile=/srv/http/.ssh/known_hosts \
    username@localhost command_to_run

Step 4: Alternative Solution Using sshpass

If you must keep the Putty format temporarily:

sshpass -p "" ssh -i nonpublic/id_rsa \
    -o BatchMode=yes \
    username@localhost command_to_run

Format	Header	Putty Compatible	OpenSSH Compatible
Putty (.ppk)	PuTTY-User-Key-File-2	Yes	No
OpenSSH (PEM)	-----BEGIN RSA PRIVATE KEY-----	No	Yes

• Always generate keys using ssh-keygen for OpenSSH compatibility
• Set proper permissions: chmod 600 for private keys
• Use -o BatchMode=yes to prevent interactive prompts
• Configure ~/.ssh/config with host-specific settings

Host myserver
    HostName localhost
    User username
    Port 54367
    IdentityFile ~/.ssh/openssh_key
    StrictHostKeyChecking no