Administrators running Dockerized Vaultwarden behind Caddy reverse proxy on Debian frequently encounter a perplexing security failure: Fail2Ban correctly detects failed login attempts, logs show banned IP addresses, yet attackers continue accessing the login page unimpeded. This isn't a filter problem or regex misconfiguration—it's a fundamental firewall architecture mismatch where ban rules land in the wrong netfilter subsystem or iptables chain that Docker's packet routing never traverses. Understanding why modern Debian defaults create this disconnect and implementing topology-specific solutions ensures your self-hosted password manager actually blocks brute-force attacks rather than merely logging them.

This comprehensive follow-up to the ENGINYRING Vaultwarden installation guide explains the Docker networking layer, iptables chain priorities, nftables migration conflicts, and Caddy's real IP forwarding requirements that collectively determine whether Fail2Ban enforcement actually protects your vault. The solution requires aligning your ban action backend (iptables vs nftables) with Docker's active firewall system, targeting the correct chain for your reverse proxy topology (INPUT vs DOCKER-USER vs FORWARD), and ensuring Vaultwarden logs the actual client IP rather than localhost or proxy addresses.

Understanding the Docker Networking and Firewall Architecture

Docker fundamentally alters Linux firewall packet flow by inserting custom iptables chains that intercept traffic before it reaches traditional INPUT or OUTPUT chains. When you publish container ports with -p 8080:80, Docker doesn't simply open a port—it creates NAT rules in the PREROUTING chain that redirect packets destined for the host's port 8080 to the container's internal IP on port 80, then routes those packets through the FORWARD chain rather than INPUT. This architectural decision means firewall rules in the INPUT chain never see traffic destined for containers, rendering traditional Fail2Ban configurations completely ineffective for Dockerized applications.

Docker creates several custom iptables chains in the filter table to manage this packet routing:

  • DOCKER-USER: Explicitly designed as a user-defined rule insertion point processed before all Docker-managed chains, providing administrators a location for custom filtering that Docker won't overwrite during container lifecycle operations
  • DOCKER-FORWARD: First-stage processing determining whether packets unrelated to established connections should proceed to other Docker chains or accept packets part of established connections
  • DOCKER: Contains dynamically-generated rules determining whether packets not part of established connections should be accepted based on port forwarding configuration of running containers
  • DOCKER-ISOLATION-STAGE-1 and DOCKER-ISOLATION-STAGE-2: Implement network isolation between different Docker networks

The critical insight: Docker's packet flow for published container ports follows PREROUTING (NAT) → FORWARD → DOCKER-USER → DOCKER-FORWARD → DOCKER rather than the traditional PREROUTING → INPUT path. Fail2Ban rules inserted into INPUT chain never evaluate packets destined for containers because those packets bypass INPUT entirely. This explains why administrators see "Banned" status in fail2ban-client status vaultwarden but attackers continue reaching the Vaultwarden login page—the ban exists in a chain the traffic never traverses.

The Debian Bookworm nftables Migration Conflict

Debian 12 (Bookworm) introduced nftables as the default firewall backend, with Debian 13 (Trixie) continuing this modernization. Fail2Ban version 1.1.0 and newer automatically detect nftables availability and default to nftables-based banactions when installed on these distributions. However, Docker Engine as of version 25.0.3 still manages firewall rules exclusively through iptables unless explicitly configured otherwise with experimental nftables support that remains incomplete. This creates a devastating mismatch: Fail2Ban writes ban rules into nftables tables while Docker routes all container traffic through iptables chains, resulting in complete enforcement failure.

When Fail2Ban uses banaction = nftables-multiport (the Debian 12+ default), it creates rules in nftables' inet filter table. These rules have zero effect on Docker-managed traffic because Docker's iptables chains in the filter table process packets first, make forwarding decisions, and Docker's NAT rules in the nat table redirect traffic to containers before nftables evaluation occurs. The packet never reaches the nftables ruleset where Fail2Ban placed the ban. Conversely, even if you force Fail2Ban to use iptables actions while nftables is active, you risk conflicts if other system components manage nftables rules expecting certain packet flow.

Compounding this issue, the iptables command on modern Debian systems may actually be iptables-nft, a compatibility shim providing iptables syntax while writing to nftables backend. Running iptables -L might display rules translated from nftables rather than showing actual iptables-legacy rules. To determine your actual firewall backend:

# Check if iptables is actually nftables shim
update-alternatives --display iptables

# View actual iptables-legacy rules
iptables-legacy -L -n -v

# View nftables ruleset
nft list ruleset

# Check Docker's firewall configuration
docker info | grep -i firewall

If Docker shows "iptables" as its firewall backend but your system uses iptables-nft, you're in a hybrid state requiring careful Fail2Ban configuration to target the iptables compatibility layer rather than native nftables actions.

Reverse Proxy Topology and Chain Selection

Choosing the correct iptables chain for Fail2Ban rules depends entirely on your reverse proxy deployment topology—whether Caddy runs on the host system or inside a Docker container determines which chain sees client traffic first.

Topology A: Caddy on Host, Vaultwarden in Docker

When Caddy runs directly on the host (installed via apt or compiled binary) and proxies to http://127.0.0.1:8080 where Vaultwarden listens, client connections terminate at the host's network interface first. The packet flow follows: Client → Host NIC → INPUT chain → Caddy process → Localhost loopback → Vaultwarden container. In this topology, banning in the INPUT chain effectively blocks client connections before they reach Caddy because the initial TCP handshake on ports 80/443 occurs in the INPUT chain context.

# For host-based Caddy, INPUT chain is correct
# /etc/fail2ban/jail.d/vaultwarden.local
[vaultwarden]
enabled = true
port = http,https
filter = vaultwarden
banaction = iptables-multiport[name=vaultwarden, port="http,https", protocol=tcp]
logpath = /var/lib/docker/volumes/vaultwarden/_data/vaultwarden.log
maxretry = 5
bantime = 1d
findtime = 10m

This configuration uses the default iptables-multiport action which inserts rules into the INPUT chain. Because Caddy handles the incoming connection on the host, these rules intercept traffic at the correct point.

Topology B: Caddy in Docker, Vaultwarden in Docker

When both Caddy and Vaultwarden run as Docker containers (common with Docker Compose orchestration), client packets never enter the INPUT chain for container-destined traffic. Instead, Docker's PREROUTING NAT redirects packets directly to the FORWARD chain, then through DOCKER-USER and DOCKER chains to determine container routing. The packet flow becomes: Client → Host NIC → PREROUTING (DNAT) → FORWARD → DOCKER-USER → DOCKER-FORWARD → DOCKER → Container.

In this topology, Fail2Ban must insert ban rules into DOCKER-USER chain to intercept traffic before Docker's own forwarding rules accept it. The DOCKER-USER chain specifically exists for this purpose—Docker processes it first and never removes user-added rules during container lifecycle operations, unlike the DOCKER chain which Docker dynamically regenerates.

# For Dockerized Caddy, DOCKER-USER chain required
# /etc/fail2ban/jail.d/vaultwarden.local
[vaultwarden]
enabled = true
port = http,https
filter = vaultwarden
# Explicitly target DOCKER-USER chain
banaction = iptables-multiport[name=vaultwarden, chain="DOCKER-USER", port="http,https", protocol=tcp]
logpath = /var/lib/docker/volumes/vaultwarden/_data/vaultwarden.log
maxretry = 5
bantime = 1d
findtime = 10m

The crucial difference: adding chain="DOCKER-USER" parameter to the banaction directs Fail2Ban to insert rules into DOCKER-USER instead of the default INPUT chain.

Topology C: Behind Cloudflare or CDN

When Cloudflare proxy or another CDN sits in front of your VPS, the TCP connection source IP becomes the CDN edge server IP, not the actual client. Layer 3/4 firewall bans cannot block individual clients because from the VPS perspective, all connections originate from legitimate CDN infrastructure. Banning CDN IP addresses breaks your entire site. For CDN-fronted deployments, Fail2Ban must ban at the CDN API level rather than local firewall, or you must implement application-layer blocking in Caddy reading CF-Connecting-IP or X-Forwarded-For headers.

Solving the iptables vs nftables Backend Conflict

The most reliable solution for Dockerized Vaultwarden on Debian 12+ systems: force Fail2Ban to use iptables-based banactions matching Docker's firewall backend rather than attempting to migrate Docker to nftables.

# /etc/fail2ban/jail.local - Global defaults
[DEFAULT]
# Force iptables backend for Docker compatibility
banaction = iptables-multiport
banaction_allports = iptables-allports

# Use systemd journal backend for log reading
backend = systemd

# Basic timing configuration
bantime = 1d
findtime = 10m
maxretry = 5

# Ignore localhost
ignoreip = 127.0.0.1/8 ::1/128

This global configuration ensures all jails default to iptables-based actions regardless of system defaults. The backend = systemd setting tells Fail2Ban to read logs from systemd journal rather than files, which works seamlessly with Docker containers logging to journald.

Configuring Vaultwarden Logging for Real Client IPs

Fail2Ban effectiveness depends on Vaultwarden logging actual client IP addresses rather than localhost (127.0.0.1) or the proxy container IP. When Vaultwarden logs show IP: 127.0.0.1 for failed logins, your regex matches and bans localhost, accomplishing nothing.

Vaultwarden reads client IP from the X-Real-IP header by default, falling back to X-Forwarded-For if X-Real-IP is absent. Caddy automatically sets X-Forwarded-For but you should explicitly configure X-Real-IP for clarity:

# Caddyfile configuration
vault.yourdomain.com {
    reverse_proxy vaultwarden:80 {
        # Explicitly set real IP header
        header_up X-Real-IP {remote_host}
        
        # Caddy sets X-Forwarded-For automatically
        # but explicit configuration documents intent
        header_up X-Forwarded-For {remote_host}
        header_up X-Forwarded-Proto {scheme}
    }
}

For Cloudflare-proxied deployments, configure Vaultwarden's IP_HEADER environment variable to read Cloudflare's header:

# docker-compose.yml for Vaultwarden behind Cloudflare
services:
  vaultwarden:
    image: vaultwarden/server:latest
    environment:
      # Tell Vaultwarden to trust CF-Connecting-IP
      IP_HEADER: "CF-Connecting-IP"
    volumes:
      - ./vw-data:/data
    ports:
      - "8080:80"

After configuration, test by intentionally failing a login and checking Vaultwarden's log file (/data/vaultwarden.log inside container) for an entry showing your actual public IP rather than 127.0.0.1 or a container IP.

Complete Working Configuration for Docker + Caddy

This complete configuration combines all elements for reliable Fail2Ban enforcement with Dockerized Vaultwarden behind Caddy on Debian 12+.

Step 1: Vaultwarden Docker Compose with Logging

# docker-compose.yml
version: '3'

services:
  vaultwarden:
    image: vaultwarden/server:latest
    container_name: vaultwarden
    restart: unless-stopped
    environment:
      # Enable file logging (required for Fail2Ban)
      LOG_FILE: "/data/vaultwarden.log"
      LOG_LEVEL: "info"
      # Disable new user registration after initial setup
      SIGNUPS_ALLOWED: "false"
      # Set domain for proper links in emails
      DOMAIN: "https://vault.yourdomain.com"
    volumes:
      - ./vw-data:/data
    ports:
      - "127.0.0.1:8080:80"
    networks:
      - vaultwarden_net

  caddy:
    image: caddy:latest
    container_name: caddy
    restart: unless-stopped
    ports:
      - "80:80"
      - "443:443"
    volumes:
      - ./Caddyfile:/etc/caddy/Caddyfile
      - ./caddy_data:/data
      - ./caddy_config:/config
    networks:
      - vaultwarden_net

networks:
  vaultwarden_net:
    driver: bridge

Step 2: Caddyfile with Real IP Headers

# Caddyfile
vault.yourdomain.com {
    # Automatic HTTPS with Let's Encrypt
    
    reverse_proxy vaultwarden:80 {
        # Pass real client IP to Vaultwarden
        header_up X-Real-IP {remote_host}
        header_up X-Forwarded-For {remote_host}
        header_up X-Forwarded-Proto {scheme}
        
        # WebSocket support for notifications
        header_up Connection {>Connection}
        header_up Upgrade {>Upgrade}
    }
    
    # Security headers
    header {
        # Enable HSTS
        Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
        # Prevent clickjacking
        X-Frame-Options "SAMEORIGIN"
        # Prevent MIME sniffing
        X-Content-Type-Options "nosniff"
        # Referrer policy
        Referrer-Policy "same-origin"
    }
    
    # Logging for debugging
    log {
        output file /var/log/caddy/vault-access.log
    }
}

Step 3: Fail2Ban Filter for Vaultwarden

# /etc/fail2ban/filter.d/vaultwarden.local
[INCLUDES]
before = common.conf

[Definition]
# Match failed login attempts
failregex = ^.*Username or password is incorrect\. Try again\. IP: \. Username:.*$
            ^.*Invalid admin token\. IP: .*$

ignoreregex =

[Init]
datepattern = ^%%Y-%%m-%%d %%H:%%M:%%S

This filter combines both web vault and admin panel failed attempts into one pattern. The <ADDR> placeholder (or <HOST> on older Fail2Ban versions) extracts the IP address from the log line.

Step 4: Fail2Ban Jail Configuration

# /etc/fail2ban/jail.d/vaultwarden.local
[vaultwarden]
enabled = true

# Read logs from Docker volume
# Adjust path to match your docker-compose volume mount
logpath = /var/lib/docker/volumes/vaultwarden_vw-data/_data/vaultwarden.log

# Alternative: use Docker logs via journald
# backend = systemd
# journalmatch = _SYSTEMD_UNIT=docker.service CONTAINER_NAME=vaultwarden

# Target DOCKER-USER chain for Dockerized reverse proxy
banaction = iptables-multiport[name=vaultwarden, chain="DOCKER-USER", port="http,https", protocol=tcp]

# Ban all ports to prevent any access
# banaction = iptables-allports[name=vaultwarden, chain="DOCKER-USER"]

# Timing configuration
findtime = 10m
bantime = 24h
maxretry = 5

# Filter
filter = vaultwarden

# Ports to monitor
port = http,https

Critical parameters explained:

  • logpath: Must point to the actual log file location on the host filesystem, not the container path. Docker volumes typically mount to /var/lib/docker/volumes/
  • chain="DOCKER-USER": Essential for Dockerized reverse proxy topology. Omitting this parameter defaults to INPUT chain which Docker bypasses
  • banaction: Uses iptables explicitly rather than nftables to match Docker's firewall backend
  • findtime: Time window for counting failures. 10 minutes allows legitimate users several attempts while catching rapid brute force
  • bantime: Duration of ban. 24 hours discourages attackers without permanent blocks that might catch dynamic IPs reassigned to legitimate users
  • maxretry: Number of failures before ban. 5 provides reasonable tolerance for typos while stopping brute force quickly

Step 5: Apply Configuration and Test

# Restart Fail2Ban to load new configuration
sudo systemctl restart fail2ban

# Verify jail is active
sudo fail2ban-client status
sudo fail2ban-client status vaultwarden

# Test with intentional failed login
# From a different machine or incognito browser, attempt login with wrong password 6 times

# Check if IP was banned
sudo fail2ban-client status vaultwarden

# Verify iptables rule was created in DOCKER-USER chain
sudo iptables -L DOCKER-USER -n -v

# Watch for bans in real-time
sudo tail -f /var/log/fail2ban.log

# Test that banned IP actually cannot connect
# From banned IP, attempt to curl the site - should timeout or be rejected

# Unban for testing
sudo fail2ban-client set vaultwarden unbanip YOUR.IP.ADDRESS

Successful configuration shows the banned IP in the jail status output, an iptables rule in DOCKER-USER chain with non-zero packet and byte counters matching the banned IP, and actual connection failures from the banned address.

Troubleshooting Common Issues

Issue: Ban Shows But IP Can Still Connect

Symptoms: fail2ban-client status vaultwarden shows IP in banned list, but that IP can still access the site.

Diagnosis:

# Check which chain Fail2Ban used
sudo iptables -L INPUT -n -v | grep BANNED.IP
sudo iptables -L DOCKER-USER -n -v | grep BANNED.IP
sudo iptables -L FORWARD -n -v | grep BANNED.IP

# Check if nftables has rules but iptables doesn't
sudo nft list ruleset | grep BANNED.IP

# Verify Docker is using iptables not nftables
docker info | grep -i firewall

Solution: If rules appear in INPUT but your reverse proxy is Dockerized, change jail configuration to use chain="DOCKER-USER". If rules appear in nftables but Docker uses iptables, change jail configuration to use banaction = iptables-multiport instead of nftables-based action.

Issue: Fail2Ban Bans Localhost (127.0.0.1)

Symptoms: All bans show IP: 127.0.0.1 in Fail2Ban log. Vaultwarden log shows IP: 127.0.0.1 for all failed attempts.

Diagnosis: Caddy not forwarding real client IP to Vaultwarden. Check Caddyfile for missing header_up X-Real-IP directive.

Solution: Add explicit header forwarding in Caddyfile as shown in configuration above. Restart Caddy, test failed login, verify Vaultwarden log now shows actual client IP.

Issue: Fail2Ban Can't Read Log File

Symptoms: /var/log/fail2ban.log shows "Unable to open file" or permission denied errors.

Diagnosis:

# Check log file exists and permissions
ls -la /var/lib/docker/volumes/vaultwarden_vw-data/_data/vaultwarden.log

# Verify Fail2Ban can read it
sudo -u fail2ban cat /var/lib/docker/volumes/vaultwarden_vw-data/_data/vaultwarden.log | tail

Solution: Ensure Fail2Ban user has read permission on Docker volume directory. Add Fail2Ban user to docker group: sudo usermod -aG docker fail2ban, then restart Fail2Ban service.

Issue: IPv6 Addresses Not Banned

Symptoms: IPv4 addresses banned successfully, but IPv6 addresses bypass bans.

Solution: Add IPv6 banaction:

# /etc/fail2ban/jail.d/vaultwarden.local
[vaultwarden]
enabled = true
logpath = /var/lib/docker/volumes/vaultwarden_vw-data/_data/vaultwarden.log
filter = vaultwarden
port = http,https
findtime = 10m
bantime = 24h
maxretry = 5

# IPv4 and IPv6 ban actions
banaction = iptables-multiport[name=vaultwarden, chain="DOCKER-USER", port="http,https", protocol=tcp]
            ip6tables-multiport[name=vaultwarden, chain="DOCKER-USER", port="http,https", protocol=tcp]

Advanced Configuration Options

Progressive Ban Times

Implement exponentially increasing ban durations for repeat offenders:

# /etc/fail2ban/jail.d/vaultwarden.local
[vaultwarden]
enabled = true
logpath = /var/lib/docker/volumes/vaultwarden_vw-data/_data/vaultwarden.log
filter = vaultwarden
port = http,https
findtime = 10m
maxretry = 5

# Progressive ban configuration
bantime = 1h
bantime.increment = true
bantime.factor = 2
bantime.maxtime = 30d
bantime.rndtime = 30m

# First ban: 1 hour
# Second ban: 2 hours
# Third ban: 4 hours
# Continues doubling up to 30 days maximum

Recidive Jail for Persistent Attackers

Create a separate jail monitoring Fail2Ban's own log to catch IPs banned multiple times across different jails:

# /etc/fail2ban/jail.d/recidive.local
[recidive]
enabled = true
logpath = /var/log/fail2ban.log
banaction = iptables-allports[name=recidive, chain="DOCKER-USER"]
findtime = 7d
bantime = 30d
maxretry = 3

# Bans IPs that get banned 3+ times in 7 days for 30 days on all ports

Email Notifications on Ban

# /etc/fail2ban/jail.local
[DEFAULT]
# SMTP configuration
destemail = admin@yourdomain.com
sender = fail2ban@yourdomain.com
mta = sendmail

# Action with email notification
action = %(action_mwl)s

# action_mwl = ban + send email with whois data and log lines

Monitoring and Maintenance

Regular monitoring ensures Fail2Ban continues protecting your Vaultwarden instance effectively:

# Daily checks
# View currently banned IPs across all jails
sudo fail2ban-client status | grep "Jail list" | sed 's/.*://; s/,//g' | xargs -n1 sudo fail2ban-client status

# Check ban statistics
sudo fail2ban-client status vaultwarden

# View recent bans in Fail2Ban log
sudo grep "Ban" /var/log/fail2ban.log | tail -20

# Verify iptables DOCKER-USER chain has current bans
sudo iptables -L DOCKER-USER -n -v | grep DROP

# Check iptables counters to ensure rules are matching packets
# Non-zero packet/byte counts indicate rules are actively filtering

Set up automated monitoring with a simple cron script:

#!/bin/bash
# /usr/local/bin/fail2ban-report.sh
# Daily Fail2Ban summary

echo "Fail2Ban Status Report - $(date)"
echo "======================================"
echo

for jail in $(fail2ban-client status | grep "Jail list" | sed 's/.*://; s/,//g'); do
    echo "Jail: $jail"
    fail2ban-client status "$jail" | grep "Currently banned"
    echo
done

# Add to cron: 0 9 * * * /usr/local/bin/fail2ban-report.sh | mail -s "Daily Fail2Ban Report" admin@yourdomain.com

Performance Considerations

Fail2Ban introduces minimal performance overhead for typical self-hosted Vaultwarden deployments handling hundreds of daily authentication attempts. However, high-traffic scenarios warrant optimization:

  • Log file size management: Implement log rotation to prevent enormous files that slow Fail2Ban's regex matching. Configure logrotate for Vaultwarden logs with daily rotation and 7-day retention
  • Filter regex optimization: Use anchored patterns (^ and $) to reduce backtracking in regex engine. The provided filters already implement this best practice
  • Backend selection: systemd backend performs better than file polling for Docker containers logging to journald. However, direct file backend with pyinotify provides lowest latency for file-based logging
  • Database journal: Fail2Ban maintains a SQLite database tracking bans. Periodic cleanup prevents bloat: configure dbpurgeage = 30d in jail.local to purge ban history older than 30 days

Security Hardening Beyond Fail2Ban

Fail2Ban provides reactive defense against brute-force attacks but should complement, not replace, proactive security measures. While Fail2Ban blocks attackers after detecting malicious activity, comprehensive VPS security requires multiple defensive layers. For a complete hardening approach covering SSH configuration, kernel parameters, mandatory access controls, and advanced security practices, consult our ultimate VPS security guide covering everything from basic hardening to elite sysadmin techniques.

  • Strong master passwords: Enforce minimum complexity requirements for vault master passwords. Vaultwarden cannot enforce this directly, but client-side education proves critical
  • Two-factor authentication: Enable 2FA for all Vaultwarden accounts. Even successful credential guessing cannot access vaults without the second factor
  • Admin page protection: Disable admin panel when not needed: ADMIN_TOKEN="". When enabled, use a cryptographically strong random token and consider IP allowlisting in Caddy
  • Rate limiting at reverse proxy: Caddy can implement request rate limiting before traffic reaches Vaultwarden, providing defense-in-depth
  • Geographic restrictions: If your users concentrate in specific countries, block traffic from unexpected regions using GeoIP filtering in iptables or Caddy
  • Regular updates: Keep Vaultwarden, Caddy, Fail2Ban, and the host OS updated with security patches. Subscribe to Vaultwarden's GitHub release notifications
  • Intrusion detection: Beyond reactive banning, implement proactive intrusion detection to identify compromised systems, monitor suspicious processes, and detect backdoors before they cause damage. Our guide on detecting intruders in your VPS server provides detailed procedures for comprehensive security monitoring

How ENGINYRING VPS Supports Self-Hosted Security

ENGINYRING VPS infrastructure provides the dedicated resources, network isolation, and administrative control necessary for implementing robust Fail2Ban protection with Dockerized applications. Unlike shared hosting where firewall access remains restricted, VPS plans deliver full root access enabling custom iptables chains, Fail2Ban configuration, and Docker networking architecture required for secure self-hosted password management.

The consistent network environment and clean IP reputation ensure legitimate users experience reliable Vaultwarden access while Fail2Ban effectively blocks attackers without false positives from shared IP ranges or oversold resources causing intermittent connectivity failures that might trigger bans. Snapshot capabilities enable testing Fail2Ban configurations safely—create a snapshot before modifying firewall rules, test thoroughly, and restore instantly if configuration errors accidentally block legitimate access.

For questions about VPS configuration or infrastructure optimization for self-hosted applications, review the Terms of Service and contact technical support for guidance on network-level security implementation.

Conclusion

Fail2Ban with Dockerized Vaultwarden behind Caddy requires understanding the packet flow through Docker's iptables chains, aligning Fail2Ban's firewall backend with Docker's active system, targeting the correct chain for your reverse proxy topology, and ensuring Vaultwarden logs actual client IPs rather than proxy addresses. The core issues—nftables/iptables mismatches, INPUT versus DOCKER-USER chain selection, and missing real IP headers—each independently prevent bans from blocking attackers despite Fail2Ban reporting successful enforcement.

Implementing the complete configuration provided in this guide resolves all three failure modes simultaneously: forcing iptables-based banactions matches Docker's firewall backend, specifying chain="DOCKER-USER" targets the correct packet path, and configuring header_up X-Real-IP in Caddy ensures Vaultwarden logs meaningful IP addresses. After applying these changes, Fail2Ban transforms from a false sense of security into genuine brute-force protection, immediately blocking attackers after maxretry failed attempts while maintaining reliable access for legitimate users. Test thoroughly, monitor regularly, and combine Fail2Ban with defense-in-depth security practices for production-grade password vault protection.

Source & Attribution

This article is based on original data belonging to ENGINYRING.COM blog. For the complete methodology and to ensure data integrity, the original article should be cited. The canonical source is available at: Fixing Fail2Ban with Dockerized Vaultwarden Behind Caddy: Why Bans Don't Block and How to Solve It.