Passwordless Remote Login

Overview: The goal is to log in to a remote server (e.g., spiedie.binghamton.edu) from Mac/Windows/Linux without entering a password, and to make VS Code Remote-SSH connections more stable. This is achieved by replacing password-based authentication with public/private key authentication (公钥/私钥认证).

SSH “passwordless login” does not skip identity verification — it replaces password-based authentication with key-based authentication.

1) Two Key Files#

Private Key (私钥): stored on your local machine Example: ~/.ssh/id_ed25519 Must never be leaked or shared
Public Key (公钥): can be sent to the server Example: ~/.ssh/id_ed25519.pub Safe to share openly / copy to servers

2) How Does the Server Remember You?#

The server stores your public key in:

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~/.ssh/authorized_keys

Once added, the server will allow anyone who holds the corresponding private key to log in.

3) What Happens During Authentication (Simplified Flow)#

You initiate a connection: ssh user@host
The server looks up your public key in authorized_keys
The server sends a random challenge
Your machine signs the challenge using your private key (the private key never leaves your machine)
The server verifies the signature using your public key
Verification passes → Login succeeds (no password prompt)

2. Step-by-Step Setup#

The following examples use:

Username: xli49
Host: spiedie.binghamton.edu

1) Step 1: Generate an SSH Key on Your Local Machine#

Check whether a key already exists:

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ls ~/.ssh/id_ed25519 ~/.ssh/id_rsa 2>/dev/null

If not, generate one (ed25519 recommended):

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ssh-keygen -t ed25519 -C "xli49@spiedie.binghamton.edu"

Note: For a completely password-free experience, press Enter to leave the passphrase empty. For better security, set a passphrase and use it with Keychain / ssh-agent.

2) Step 2: Copy the Public Key to the Server#

✅ Recommended (when ssh-copy-id is available):

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ssh-copy-id xli49@spiedie.binghamton.edu

Enter your password once — that’s the last time.

Manual Method (if `ssh-copy-id` is not available)#

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cat ~/.ssh/id_ed25519.pub | ssh xli49@spiedie.binghamton.edu \
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"mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys && chmod 700 ~/.ssh && chmod 600 ~/.ssh/authorized_keys"

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ssh xli49@spiedie.binghamton.edu

✅ If no password prompt appears → setup successful.

3. Configuring `~/.ssh/config` (Recommended)#

Edit the config file:

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nano ~/.ssh/config

Recommended configuration (using the full hostname as the Host):

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Host spiedie.binghamton.edu
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    HostName spiedie.binghamton.edu
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    User xli49
4
    ServerAliveInterval 300
5
    ServerAliveCountMax 120

After saving, connect with just:

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ssh spiedie.binghamton.edu

4. When Is `IdentityFile` Needed?#

1) Usually Not Required#

SSH automatically tries the following keys in order:

~/.ssh/id_ed25519
~/.ssh/id_rsa
Any keys already loaded into ssh-agent

If your key is in one of these default locations, you typically do not need to specify IdentityFile.

2) When You Must Specify `IdentityFile`#

You have multiple keys and SSH might pick the wrong one
Your key is not in a default path
The server requires a specific key

Example:

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Host spiedie.binghamton.edu
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    HostName spiedie.binghamton.edu
3
    User xli49
4
    IdentityFile ~/.ssh/id_ed25519

5. Preventing VS Code Remote-SSH Disconnections (KeepAlive)#

Common causes of disconnection:

The school network or firewall clears long-idle connections
The remote session is considered idle when there is no output for an extended period

Solution: enable SSH heartbeat packets (KeepAlive):

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ServerAliveInterval 30
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ServerAliveCountMax 120

What this means:

Send a heartbeat packet every 30 seconds
Allow up to 120 consecutive non-responses (~1 hour) before disconnecting

6. Troubleshooting#

1) Check Which Key SSH Is Using#

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ssh -v xli49@spiedie.binghamton.edu

Look for lines like:

Offering public key: ...
Authentication succeeded

2) Check the Default Shell on the Server#

Run remotely to rule out misconfiguration:

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echo $SHELL
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getent passwd xli49 | cut -d: -f7
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which zsh

💡 One-line Takeaway
Generate a key with ssh-keygen -t ed25519, copy it to the server once with ssh-copy-id, add ServerAliveInterval 30 to ~/.ssh/config, and you'll never type a password or suffer a dropped VS Code connection again.

I. SSH Passwordless Login — Key-Based Authentication#

1. Core Concept: How SSH Passwordless Login Works#