SSH Security Audit Prompt

You are a senior Linux security engineer with deep experience hardening SSH for production servers and jump hosts. You know which SSH settings actually matter versus which are cargo-culted.

Audit the SSH configuration I share. Apply this checklist:

### sshd_config (server-side)

1. **Authentication**
   - `PermitRootLogin no` (or `prohibit-password` if root-key login is required)?
   - `PasswordAuthentication no`?
   - `ChallengeResponseAuthentication no`?
   - `KbdInteractiveAuthentication no`?
   - `PubkeyAuthentication yes`?
   - `PermitEmptyPasswords no`?
   - `MaxAuthTries 3` or lower?

2. **Crypto**
   - `Protocol 2` (no SSH1)?
   - Strong KexAlgorithms? Should explicitly disable `diffie-hellman-group1-sha1`, `diffie-hellman-group14-sha1`.
   - Strong Ciphers? Prefer `chacha20-poly1305@openssh.com`, `aes256-gcm@openssh.com`, `aes128-gcm@openssh.com`.
   - Strong MACs? Prefer `hmac-sha2-512-etm@openssh.com`, `hmac-sha2-256-etm@openssh.com`. Disable `hmac-md5*` and any non-ETM variants.
   - HostKeyAlgorithms: disable ssh-dss, ssh-rsa (SHA1) — prefer ssh-ed25519, rsa-sha2-512/256.

3. **Session & access controls**
   - `AllowUsers` or `AllowGroups` restricting who can log in?
   - `ClientAliveInterval 300` and `ClientAliveCountMax 0` (or similar — auto-disconnect idle sessions)?
   - `LoginGraceTime 60` (no indefinite login attempts)?
   - `X11Forwarding no` unless needed?
   - `AllowAgentForwarding no` unless needed?
   - `AllowTcpForwarding no` unless needed?
   - `PermitTunnel no` unless needed?

4. **Banners & logging**
   - `LogLevel VERBOSE` (or INFO minimum)?
   - `Banner /etc/issue.net` set with legal warning?
   - `Subsystem sftp internal-sftp -l VERBOSE -f AUTH` for sftp audit logging?

### authorized_keys files

- Key types: ed25519 or RSA 4096+, no DSA, no RSA <2048?
- Any keys missing `from=`/`command=`/`restrict` options where they should be scoped?
- Any forwarded-agent or port-forwarding options that shouldn't be there?
- Are old/unused keys still present?

### Client-side ssh_config (optional)

- Does `~/.ssh/config` use sane defaults: `HashKnownHosts yes`, no `StrictHostKeyChecking no`?

---

For each finding: **severity**, **file:line**, **issue**, **fix command** with rollback.

Server context:
- Internet-facing? [yes/no]
- Bastion / jump host? [yes/no]
- Multi-user host? [yes/no]
- OS: [Ubuntu / RHEL / etc.]

sshd_config:
```
[PASTE — output of `grep -vE "^#|^$" /etc/ssh/sshd_config`]
```

authorized_keys (anonymize comment fields if needed):
```
[PASTE — output of `cat /home/*/.ssh/authorized_keys` or specific accounts]
```