Mastering SSH Public‑Key Login for Efficient Multi‑Server Automation

Requirement

In daily work we often need to run the same command on many servers, such as comparing logs or checking services, which requires the ability to operate on multiple machines in batch.

Two years ago I wrote an article about using the

expect

command for automatic SSH login. Although flexible, it is cumbersome and single‑process blocking, making batch operations difficult.

Using SSH public‑key login simplifies batch command execution across many servers.

SSH Protocol

SSH (Secure Shell) is an encrypted network protocol that provides a secure channel over an insecure network. It is similar to HTTPS, using TCP and asymmetric encryption, but relies on host key fingerprints rather than a certificate authority.

When you first connect to a server, SSH asks you to verify the server’s public‑key fingerprint; after verification the fingerprint is stored in

~/.ssh/known_hosts

and future connections skip this step.

Typical connection steps include:

TCP three‑way handshake

SSH protocol version negotiation

Client‑server public‑key exchange

Encryption algorithm negotiation

Symmetric key authentication

Secure client‑server communication

Below is a capture of the SSH handshake using tcpdump and Wireshark: