Caddy SIP Guardian

A comprehensive Caddy module providing SIP-aware security at Layer 4. Protects your VoIP infrastructure with intelligent rate limiting, attack detection, message validation, and topology hiding.

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Why SIP Guardian?

Traditional SIP security (like fail2ban) parses logs after attacks reach your PBX. SIP Guardian operates at Layer 4, blocking threats before they touch your infrastructure:

Traditional Approach	SIP Guardian
Log parsing delay	Real-time blocking
Regex-based detection	Protocol-aware analysis
Separate fail2ban config	Single Caddyfile
No topology protection	Full B2BUA-lite hiding
Manual IP management	Auto-ban with API control

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Features

🛡️ Core Protection

• Layer 4 SIP Proxying — Handle SIP traffic (UDP/TCP/TLS) before it reaches your PBX
• Intelligent Rate Limiting — Per-method token bucket rate limiting with burst support
• Automatic Banning — Ban IPs that exceed failure thresholds
• Attack Detection — Detect common SIP scanning tools (SIPVicious, friendly-scanner, etc.)
• CIDR Whitelisting — Whitelist trusted networks by IP range
• DNS-aware Whitelisting — Whitelist SIP trunks by hostname or SRV record with auto-refresh
• GeoIP Blocking — Block traffic by country using MaxMind databases

🔍 Extension Enumeration Detection

• Count-based Detection — Block IPs probing too many unique extensions
• Sequential Pattern Detection — Detect numeric extension scanning (100, 101, 102...)
• Rapid-fire Detection — Catch high-speed enumeration attempts
• Configurable Exemptions — Whitelist common extensions like voicemail

✅ SIP Message Validation

• RFC 3261 Compliance — Enforce required headers and message structure
• Injection Prevention — Block NULL bytes and binary injection attacks
• Content-Length Validation — Detect body/header mismatches
• Multiple Modes — Permissive, strict, or paranoid validation

🔒 Topology Hiding (B2BUA-lite)

• Via Header Rewriting — Hide internal proxy chain
• Contact Header Rewriting — Mask internal IP addresses
• Sensitive Header Stripping — Remove P-Asserted-Identity, Server, etc.
• Call-ID Anonymization — Prevent dialog correlation attacks
• Private IP Masking — Automatically hide RFC 1918 addresses

📊 Observability

• Prometheus Metrics — Comprehensive metrics for monitoring
• Webhook Notifications — Real-time alerts for security events
• SQLite Persistence — Durable ban storage across restarts
• Admin API — RESTful API for management and stats

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Architecture

                              Internet
                                  │
                                  ▼
┌──────────────────────────────────────────────────────────────────┐
│                    Caddy SIP Guardian (Layer 4)                   │
│  ┌────────────────────────────────────────────────────────────┐  │
│  │  1. SIP Matcher         ─ Identifies SIP traffic           │  │
│  │  2. Ban Check           ─ Reject banned IPs                │  │
│  │  3. Whitelist Check     ─ Skip checks for trusted IPs      │  │
│  │  4. GeoIP Check         ─ Block by country                 │  │
│  │  5. Validation          ─ RFC 3261 compliance              │  │
│  │  6. Pattern Detection   ─ Scanner fingerprinting           │  │
│  │  7. Enumeration Check   ─ Extension scanning detection     │  │
│  │  8. Rate Limiting       ─ Per-method token buckets         │  │
│  │  9. Topology Hiding     ─ Header rewriting (optional)      │  │
│  └────────────────────────────────────────────────────────────┘  │
└──────────────────────────────────────────────────────────────────┘
                                  │
                                  ▼
┌──────────────────────────────────────────────────────────────────┐
│              FreePBX / Asterisk / Kamailio                        │
│       (Protected from scanners, enumeration, topology leaks)      │
└──────────────────────────────────────────────────────────────────┘

---

Quick Start

# Clone the repository
git clone https://git.supported.systems/rsp2k/caddy-sip-guardian.git
cd caddy-sip-guardian

# Build the custom Caddy image
make build

# Start the stack
make run

# View logs
make logs

# Run tests (60 tests)
make test

---

Configuration

Minimal Configuration

{
    layer4 {
        udp/:5060 {
            @sip sip
            route @sip {
                sip_guardian {
                    max_failures 5
                    ban_time 1h
                    whitelist 10.0.0.0/8 192.168.0.0/16
                }
                proxy udp/asterisk:5060
            }
        }
    }
}

Full Configuration

Click to expand complete Caddyfile example

{
    layer4 {
        # UDP SIP (standard port)
        udp/:5060 {
            @sip sip {
                methods REGISTER INVITE OPTIONS ACK BYE CANCEL INFO NOTIFY SUBSCRIBE MESSAGE
            }

            route @sip {
                sip_guardian {
                    # Core settings
                    max_failures 5
                    find_time 10m
                    ban_time 1h
                    whitelist 10.0.0.0/8 192.168.0.0/16

                    # GeoIP blocking (optional)
                    geoip_db /etc/caddy/GeoLite2-Country.mmdb
                    blocked_countries CN RU

                    # Per-method rate limiting
                    rate_limit {
                        register 10/s burst 20
                        invite 5/s burst 10
                        options 20/s burst 50
                    }

                    # Extension enumeration detection
                    enumeration {
                        max_extensions 20
                        extension_window 5m
                        sequential_threshold 5
                        rapid_fire_count 10
                        rapid_fire_window 30s
                        ban_time 2h
                        exempt_extensions 100 200 9999
                    }

                    # SIP message validation
                    validation {
                        enabled true
                        mode strict
                        max_message_size 65535
                        ban_on_null_bytes true
                        ban_on_binary_injection true
                    }

                    # Prometheus metrics
                    metrics {
                        enabled true
                    }

                    # Webhook notifications
                    webhooks {
                        url https://hooks.example.com/sip-alerts
                        events ban unban attack enumeration
                    }

                    # SQLite persistence
                    storage {
                        path /var/lib/caddy/sip_guardian.db
                    }
                }

                # Optional: Topology hiding
                sip_topology_hider {
                    proxy_host 203.0.113.1
                    proxy_port 5060
                    upstream udp/192.168.1.100:5060

                    rewrite_via
                    rewrite_contact
                    strip_headers P-Preferred-Identity P-Asserted-Identity Server User-Agent
                    hide_private_ips
                    # anonymize_call_id  # Optional: randomize Call-IDs
                }

                proxy udp/freepbx:5060
            }
        }

        # TCP SIP
        tcp/:5060 {
            @sip sip
            route @sip {
                sip_guardian { ... }
                proxy tcp/freepbx:5060
            }
        }

        # SIP over TLS
        tcp/:5061 {
            @sip tls sni sip.example.com
            route @sip {
                sip_guardian { ... }
                tls
                proxy tcp/freepbx:5060
            }
        }
    }
}

# Admin API
:2020 {
    handle /api/sip-guardian/* {
        sip_guardian_admin
    }

    # Prometheus metrics endpoint
    handle /metrics {
        metrics
    }
}

Environment Variables

Variable	Default	Description
SIP_UPSTREAM_HOST	freepbx	Upstream SIP server hostname
SIP_UPSTREAM_PORT	5060	Upstream SIP port
SIP_GUARDIAN_MAX_FAILURES	5	Failures before ban
SIP_GUARDIAN_FIND_TIME	10m	Time window for counting failures
SIP_GUARDIAN_BAN_TIME	1h	Ban duration

---

Feature Details

Extension Enumeration Detection

Protects against tools like SIPVicious svwar that scan for valid extensions:

enumeration {
    max_extensions 20        # Ban after 20 unique extensions probed
    extension_window 5m      # Within this time window
    sequential_threshold 5   # Ban if 5+ consecutive extensions (100,101,102...)
    rapid_fire_count 10      # Ban if 10+ extensions in rapid_fire_window
    rapid_fire_window 30s
    ban_time 2h              # Enumeration bans last longer
    exempt_extensions 100 200 9999  # Don't count these (voicemail, etc.)
}

Detection Methods:

Method	Trigger	Use Case
Count Threshold	20+ unique extensions	Catches slow scanners
Sequential Pattern	5+ consecutive numbers	Detects svwar immediately
Rapid Fire	10+ in 30 seconds	Catches fast automated scans

---

DNS-aware Whitelisting

Whitelist SIP trunks and providers by hostname or SRV record. IPs are automatically resolved and refreshed:

sip_guardian {
    # Static CIDR whitelist (always available)
    whitelist 10.0.0.0/8 192.168.0.0/16

    # DNS-aware whitelist - resolved to IPs automatically
    whitelist_hosts pbx.example.com trunk.sipcarrier.net
    whitelist_srv _sip._udp.provider.com _sip._tcp.carrier.net
    dns_refresh 5m  # How often to refresh DNS lookups (default: 5m)
}

Why DNS-aware whitelisting?

Static IP Whitelisting	DNS-aware Whitelisting
Breaks when provider changes IPs	Auto-updates when IPs change
Must manually track carrier IPs	Just use their SRV record
Fails silently on changes	Logs refresh events

SRV Record Support:

SIP trunks commonly use SRV records for load balancing and failover. SIP Guardian resolves the full chain:

_sip._udp.carrier.com → sip1.carrier.com, sip2.carrier.com → 203.0.113.10, 203.0.113.11

Admin API Endpoints:

Method	Endpoint	Description
GET	/api/sip-guardian/dns-whitelist	List all resolved DNS entries
POST	/api/sip-guardian/dns-whitelist/refresh	Force immediate DNS refresh

---

SIP Message Validation

Enforces RFC 3261 compliance and blocks malformed/malicious packets:

validation {
    enabled true
    mode strict              # permissive, strict, or paranoid
    max_message_size 65535   # Reject oversized messages
    ban_on_null_bytes true   # Immediate ban for NULL byte injection
    ban_on_binary_injection true
    disabled_rules via_invalid_branch  # Skip specific rules
}

Validation Modes:

Mode	Behavior
Permissive	Log violations, only block critical attacks
Strict	Enforce RFC 3261 required headers
Paranoid	Additional heuristics for edge cases

Validation Rules:

Rule	Severity	Action
null_bytes	🔴 CRITICAL	Immediate ban
binary_injection	🔴 CRITICAL	Immediate ban
missing_via	🟠 HIGH	Count toward ban
missing_from	🟠 HIGH	Count toward ban
missing_to	🟠 HIGH	Count toward ban
missing_call_id	🟠 HIGH	Count toward ban
missing_cseq	🟠 HIGH	Count toward ban
content_length_mismatch	🟠 HIGH	Count toward ban
oversized_message	🟠 HIGH	Count toward ban
invalid_request_uri	🟡 MEDIUM	Reject only

---

Topology Hiding

Hide your internal infrastructure from external attackers:

sip_topology_hider {
    # Your public-facing address
    proxy_host 203.0.113.1
    proxy_port 5060

    # Internal PBX (never exposed)
    upstream udp/192.168.1.100:5060

    rewrite_via              # Add proxy Via, remove on response
    rewrite_contact          # Replace internal Contact URIs
    strip_headers P-Preferred-Identity P-Asserted-Identity Server User-Agent
    hide_private_ips         # Replace RFC 1918 addresses
    # anonymize_call_id      # Optional: randomize Call-IDs
}

What It Hides:

Header/Field	Before	After
Via	192.168.1.100:5060	proxy.example.com:5060
Contact	<sip:100@192.168.1.100>	<sip:100@proxy.example.com>
Server	Asterisk PBX 18.x	(removed)
P-Asserted-Identity	<sip:100@internal.lan>	(removed)

Request Flow:

External UA ──────► Caddy ──────► Asterisk
                      │
                      ├─ Adds Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bK...
                      ├─ Rewrites Contact: <sip:proxy.example.com:5060>
                      └─ Strips: Server, P-Asserted-Identity

Response Flow:

Asterisk ──────► Caddy ──────► External UA
                   │
                   ├─ Removes top Via (proxy's)
                   └─ Dialog state routes response correctly

---

Admin API

Endpoints

Method	Endpoint	Description
GET	/api/sip-guardian/bans	List all banned IPs
GET	/api/sip-guardian/stats	View statistics
POST	/api/sip-guardian/ban/{ip}	Manually ban an IP
DELETE	/api/sip-guardian/unban/{ip}	Remove IP from ban list
GET	/api/sip-guardian/enumeration/stats	Enumeration statistics
GET	/api/sip-guardian/validation/stats	Validation statistics

Examples

List Banned IPs

curl http://localhost:2020/api/sip-guardian/bans

{
  "bans": [
    {
      "ip": "185.224.128.0",
      "reason": "scanner_detected",
      "banned_at": "2024-01-15T10:30:00Z",
      "expires_at": "2024-01-15T11:30:00Z"
    }
  ],
  "total": 1
}

View Stats

curl http://localhost:2020/api/sip-guardian/stats

{
  "total_requests": 15234,
  "blocked_requests": 423,
  "active_bans": 12,
  "enumeration_detections": 5,
  "validation_failures": 89,
  "rate_limited": 156
}

Manually Ban IP

curl -X POST http://localhost:2020/api/sip-guardian/ban/192.168.1.100 \
    -H "Content-Type: application/json" \
    -d '{"reason": "manual_ban", "duration": "24h"}'

Unban IP

curl -X DELETE http://localhost:2020/api/sip-guardian/unban/192.168.1.100

---

Prometheus Metrics

# Core metrics
sip_guardian_requests_total{method="REGISTER"}
sip_guardian_blocked_total{reason="banned"}
sip_guardian_active_bans

# Rate limiting
sip_guardian_rate_limited_total{method="INVITE"}

# Enumeration detection
sip_guardian_enumeration_detections_total{reason="sequential_pattern"}
sip_guardian_enumeration_tracked_ips

# Validation
sip_guardian_validation_violations_total{rule="missing_via"}
sip_guardian_validation_results_total{result="valid"}
sip_guardian_message_size_bytes

# GeoIP
sip_guardian_geoip_blocked_total{country="CN"}

---

Integration Examples

FreePBX / Asterisk

{
    layer4 {
        udp/:5060 {
            @sip sip
            route @sip {
                sip_guardian {
                    max_failures 3
                    ban_time 24h
                    whitelist 10.0.0.0/8
                }
                proxy udp/freepbx:5060
            }
        }
    }
}

Kamailio (as SBC)

{
    layer4 {
        udp/:5060 {
            @sip sip
            route @sip {
                sip_guardian {
                    max_failures 5
                    ban_time 1h
                }
                sip_topology_hider {
                    proxy_host sbc.example.com
                    proxy_port 5060
                    upstream udp/kamailio:5060
                    rewrite_via
                    rewrite_contact
                }
                proxy udp/kamailio:5060
            }
        }
    }
}

High Availability Setup

{
    layer4 {
        udp/:5060 {
            @sip sip
            route @sip {
                sip_guardian {
                    storage {
                        # Shared storage for HA
                        path /shared/sip_guardian.db
                    }
                }
                # Load balance across PBX cluster
                proxy udp/pbx1:5060 udp/pbx2:5060 udp/pbx3:5060 {
                    lb_policy round_robin
                    health_check interval=30s
                }
            }
        }
    }
}

---

Troubleshooting

Common Issues

Legitimate users getting banned

Symptoms: Users report being unable to connect; they appear in ban list.

Solutions:

1. Add their network to the whitelist:

   whitelist 10.0.0.0/8 192.168.0.0/16 YOUR.NETWORK.0.0/16
   

2. Increase max_failures threshold
3. Check if their client sends malformed packets (validation logs)
4. Temporarily unban: curl -X DELETE http://localhost:2020/api/sip-guardian/unban/IP

SIP traffic not being matched

Symptoms: Traffic passes through but SIP Guardian doesn't process it.

Solutions:

1. Verify the matcher syntax:

   @sip sip {
       methods REGISTER INVITE OPTIONS
   }
   

2. Check Caddy logs for matcher errors
3. Ensure traffic is arriving on the correct port (UDP vs TCP)
4. Test with tcpdump -i any port 5060 to verify traffic flow

Topology hiding breaks calls

Symptoms: Calls connect but audio is one-way or missing.

Solutions:

1. This is usually an RTP/media issue, not SIP signaling
2. Ensure RTP ports are forwarded through your firewall
3. Check if hide_private_ips is replacing IPs in SDP body
4. Verify NAT traversal settings on your PBX

High memory usage

Symptoms: Caddy process memory grows over time.

Solutions:

1. Dialog/transaction state cleanup runs automatically, but check TTL settings
2. Reduce extension_window to clean up enumeration tracking faster
3. Check for memory leaks with go tool pprof
4. Ensure cleanup goroutines are running (check logs)

GeoIP database not loading

Symptoms: Country blocking not working; errors about mmdb file.

Solutions:

1. Download the database:

   wget https://download.maxmind.com/app/geoip_download?edition_id=GeoLite2-Country&suffix=tar.gz
   

2. Verify path in config matches actual file location
3. Check file permissions (Caddy needs read access)
4. Ensure you're using GeoLite2-Country, not City database

Debug Mode

Enable verbose logging:

{
    debug
    layer4 {
        # ... your config
    }
}

Check logs:

docker logs caddy-sip-guardian 2>&1 | grep -E "(sip_guardian|topology)"

---

Building from Source

# Using xcaddy
xcaddy build \
    --with github.com/mholt/caddy-l4 \
    --with git.supported.systems/rsp2k/caddy-sip-guardian

# Or with local development
xcaddy build \
    --with github.com/mholt/caddy-l4 \
    --with git.supported.systems/rsp2k/caddy-sip-guardian=/path/to/local/module

Running Tests

# Run all unit tests (60 tests)
make test

# Run with verbose output
docker run --rm -v $(pwd):/app -w /app golang:1.25 go test -v ./...

# Test specific feature
go test -v -run TestEnumeration ./...
go test -v -run TestValidation ./...
go test -v -run TestTopology ./...

---

Changelog

v0.3.0 (2024-12)

• ✨ Topology Hiding — B2BUA-lite functionality for hiding internal infrastructure
• ✨ SIP Message Parsing — Full RFC 3261 compliant parser with header manipulation
• ✨ Dialog State Management — Stateful response routing for topology hiding
• 🐛 Fixed PRNG overflow in branch generation

v0.2.0 (2024-12)

• ✨ SIP Message Validation — RFC 3261 compliance checking
• ✨ Injection Detection — NULL byte and binary injection blocking
• ✨ Validation Modes — Permissive, strict, and paranoid modes

v0.1.0 (2024-12)

• ✨ Extension Enumeration Detection — Sequential pattern and rapid-fire detection
• ✨ Per-method Rate Limiting — Token bucket with configurable burst
• ✨ GeoIP Blocking — Country-based blocking with MaxMind
• ✨ Prometheus Metrics — Comprehensive observability
• ✨ Webhook Notifications — Real-time security alerts
• ✨ SQLite Persistence — Durable ban storage

---

Security Considerations

Consideration	Recommendation
Whitelist internal networks	Always add your LAN to prevent self-blocking
Start permissive	Begin with permissive validation, tighten after monitoring
Monitor false positives	Watch metrics for legitimate traffic being blocked
Keep GeoIP updated	Refresh MaxMind databases monthly
Use webhooks	Configure alerts for immediate security notification
Protect admin API	Don't expose :2020 to the internet

---

Detected Attack Patterns

SIP Guardian automatically detects:

Pattern	Detection Method
SIPVicious	User-Agent fingerprinting
friendly-scanner	User-Agent fingerprinting
sipcli / sip-scan	User-Agent fingerprinting
Sequential scanning	Extension pattern analysis
NULL byte injection	Binary content inspection
Malformed packets	RFC 3261 validation

---

License

MIT — see LICENSE for details.

Contributing

Contributions welcome! Please:

1. Fork the repository
2. Create a feature branch
3. Add tests for new functionality
4. Submit a pull request

Related Projects

• caddy-l4 — Layer 4 proxy for Caddy
• Caddy — The HTTP/2 web server with automatic HTTPS
• SIPVicious — SIP security testing tools (for testing your setup)