SSH

OpenSSH is the standard client for encrypted remote shells, and also a general-purpose transport: file transfer (sftp, rsync), git, and port forwarding all ride on it. For network debugging, the verbose mode and the tunneling features are diagnostic tools in their own right.

Availability

Cygwin and native Windows OpenSSH keep separate configuration. Cygwin reads ~/.ssh inside the Cygwin home; native Windows reads C:\Users\<you>\.ssh. Keys set up in one are not seen by the other.

Debugging

Verbose mode walks through each phase of the connection: TCP connect, host key exchange, which keys are offered, which authentication methods fail. Add up to -vvv for more detail:

ssh -v username@host

Config

Put per-host settings in ~/.ssh/config so connections are reproducible and short to type:

Host jumpbox
  HostName bastion.example.com
  User boris
  IdentityFile ~/.ssh/id_ed25519

Host internal-db
  HostName 10.0.12.5
  User boris
  ProxyJump jumpbox

With the above, ssh internal-db transparently hops through the bastion.

Port Forwarding

• Local forward: expose a remote-only service on your own machine. Here, a database reachable only from the jump host appears on localhost:5432:

ssh -L 5432:db.internal:5432 username@jumpbox

• Dynamic forward: turn the SSH connection into a SOCKS proxy on localhost:1080, then point tools like curl at socks5h://localhost:1080:

ssh -D 1080 username@jumpbox

Key Generation

• Preferred modern key type (ed25519), with extra KDF rounds on the passphrase:

ssh-keygen -o -a 100 -t ed25519 -f ~/.ssh/id_ed25519 -C "example-email@gmail.com"

• RSA fallback for old servers that do not support ed25519:

ssh-keygen -t rsa -b 4096 -f ~/.ssh/id_rsa -C "example-email@gmail.com"