Anti-fraud

Category:GUI manual

Anti-Fraud Rules

Anti-fraud rules are accessed via GUI > Alerts > Anti Fraud. Rules combat fraud and attacks, with ongoing additions. Each rule supports custom scripts for actions like firewall rules, besides email alerts. Alerts are archived in Sent Alerts.

Overview

List of Fraud/Watchdog Alerts

Alert Type	Processing	Description
Realtime concurrent calls	Realtime	Monitors concurrent calls per source IP
SIP REGISTER flood/attack	Realtime	Detects REGISTER flooding from single IP
SIP PACKETS flood/attack	Realtime	Detects generic SIP packet floods
Country/Continent destination	Realtime	Calls to specific country/continent (based on first INVITE)
Fraud: sequential	CDR-based	Detects high volume of calls to a destination within a time window
Change CDR country	CDR-based	Source IP geolocation changed between calls
Change REGISTER country	CDR-based	REGISTER source country changed since last success
Billing Watchdog	CDR-based	Billing anomaly detection

Alert Processing Differences

VoIPmonitor processes alerts in two fundamentally different ways, which affects what data is available when the alert triggers:

Realtime Alerts

Processed directly by the sniffer as packets arrive. Triggered immediately based on packet inspection.

Alert	Trigger Condition
Realtime concurrent calls	N+ concurrent calls from same IP
SIP REGISTER flood	N+ REGISTER attempts from same IP in interval
SIP PACKETS flood	N+ SIP packets from same IP in interval
Country/Continent destination	Call to monitored destination (first INVITE)

Characteristics:

Immediate triggering (no database delay)
CDRs not yet available when alert fires
Only IP address available in alert_info (no source port)

CDR-based Alerts

Evaluated by the GUI after CDRs have been stored in the database.

Alert	Trigger Condition
Change CDR country	IP geolocation differs from previous call
Change REGISTER country	REGISTER country differs from last successful registration
RTP alerts	Quality thresholds exceeded (MOS, jitter, loss)
SIP Response alerts	Specific SIP response codes

Characteristics:

Delay between call end and alert (CDR must be written first)
Full CDR data available (including source port, duration, etc.)
Can query database for additional context

Important Limitation: Source Port in Realtime Alerts

Realtime alerts provide the attacker's IP address in alert_info, but do not include the SIP source port.

The source port can be queried from cdr.caller_port, but this has critical limitations:

Delay: CDRs are written after the realtime alert triggers
Port may not exist: Flood attacks are often detected before CDR creation

Recommendation:

For real-time defense: Block by IP address only (accept temporary impact on legitimate traffic from same IP)
For non-real-time blocking: Use CDR-based alerts with database queries

Common Configuration

Options shared across anti-fraud rules:

Option	Description
Enable hyperlinks	Makes email alert titles clickable links to rule definitions
IP include/exclude	Exclude IPs or networks (e.g., `10.0.0.0/8`) or use IP groups
Suppress repeating alerts	Limit alerts to once per X hours to avoid spamming
Numbers include/exclude	Filter source numbers/prefixes
External script	Path to custom script for automated actions

International prefixes configuration:

Setting	Description	Default
International prefixes	Prefixes indicating international calls	`+`, `00`
Min international length	Numbers shorter than this are treated as local	varies
Local numbers are in	Country for classifying international-prefixed calls as local	your country

SIP REGISTER Flood/Attack

Triggers when >= N registration attempts from an IP occur within the set interval.

Mitigation Strategies

When SIP REGISTER floods cause excessive CPU usage or system unresponsiveness:

1. Immediate Blocking via Custom Scripts

Configure a custom script in the SIP REGISTER flood alert rule to automatically block the attacker IP. The alert_info object contains the attacker's IP address.

# Block using iptables
iptables -A INPUT -s <ATTACKER_IP> -j DROP

# Block using ipset (more efficient for multiple IPs)
ipset add blacklist <ATTACKER_IP>

2. Network Edge Blocking (Recommended for Prevention)

For long-term protection, block at your network edge before traffic reaches VoIPmonitor:

Session Border Controller (SBC): Configure rate limiting and IP blocking
Firewall: Block malicious IPs at the perimeter
Fail2ban: Automatically block IPs after repeated REGISTER failures

3. Reducing REGISTER Noise

Disable REGISTER processing if not needed: sip-register = no in voipmonitor.conf
Filter REGISTER packets using firewall rules (block specific countries or networks)
Use capture rules to exclude known good REGISTER sources from processing

Realtime Concurrent Calls

Tracks source IPs in realtime (not CDR-based) for concurrent calls. Useful against high-channel attacks.

Parameters

Concurrent calls limit: Trigger on international, local, or both exceeding limits
Time period rules: Vary alerts by work/after hours (defined in Groups > TimePeriods)

Change CDR Country

Triggers when CDR IP source changes country/continent since last call.

Parameters

Exclude countries from alert: Whitelist countries to skip

Change REGISTER Country

Triggers when SIP REGISTER username changes country/continent since last successful registration.

Parameters

Exclude countries from alert: Whitelist countries to skip

Detecting User-Agent Changes

There is no built-in alert for detecting changes to a SIP registration's User-Agent (UA) string. However, you can enable database-level tracking of UA changes by configuring the sniffer to create new register state records when the UA changes.

To enable UA change tracking, set sip-register-state-compare-digest_ua = yes (or its alias sip-register-state-compare-ua = yes) in voipmonitor.conf. This forces the sniffer to create a new record in the register state table whenever the UA changes, even if other registration details remain the same. See Sniffer Configuration for details.

Alternative fraud alerts for registration anomalies

Change REGISTER country: Detects when registration source country changes
Change CDR country: Detects when call source country changes

Custom detection

Query the register_state database table to detect UA changes programmatically
Use custom scripts with anti-fraud rules to trigger actions based on database queries

Country/Continent Destination

Triggers on calls to specific country/continent, based on first SIP INVITE (realtime processing).

SIP PACKETS Flood/Attack

Triggers when >= N packets from an IP occur within the set interval.

Fraud: Sequential

Detects a large volume of calls to the same destination number within a short time period. This alert is useful for identifying sequential dialing patterns, such as repeated calls to premium rate numbers or destination abuse.

Parameters

Interval: Time window in seconds to monitor call volume (e.g., 3600 for 1 hour)
Limit: Maximum number of calls allowed within the interval (e.g., 100)
Called number filters: Destination numbers/prefixes to monitor

Configuration for "Any Single Destination"

To alert on call volume to any destination number (not specific numbers), leave the called number field empty. The alert will then detect excessive calling to any single number within the configured time window.

Example Use Cases

Alert when more than 50 calls are made to the same destination within 1 hour
Detect sequential dialing to premium rate numbers
Identify destination number abuse or toll fraud

Custom Script Examples

Custom scripts receive alert data as command-line arguments. The fourth argument ($argv[4] in PHP) contains JSON-encoded alert data.

Logging Passed Arguments

Simple script to log all arguments for debugging:

#!/bin/bash
echo "$(date): $@" >> /tmp/alert_debug.txt

RTP Alert: Store Audio Files

Script to automatically download audio for calls that triggered an RTP alert:

#!/usr/bin/php
<?php
// Configuration
$directory = '/home/alerts/audio';
$date = trim(`date '+%Y-%m-%d'`);
$guiDir = '/var/www/voipmonitor';
$destdir = $directory . '/' . $date;

// Create destination directory
`mkdir -p $destdir`;

// Parse alert data (4th argument contains JSON)
$alert = json_decode($argv[4]);

// Download audio for each CDR in the alert
foreach ($alert->cdr as $cdr) {
    $params = json_encode([
        'task' => 'getVoiceRecording',
        'user' => 'admin',
        'password' => 'admin',
        'params' => ['cdrId' => $cdr]
    ]);
    $command = "php $guiDir/php/api.php > $destdir/file_id_$cdr.pcap";
    exec("echo '$params' | $command", $arr, $val);
}
?>

RTP Alert: Block IP After Threshold

Script to block IPs that exceed a threshold number of incidents:

#!/usr/bin/php
<?php
// Configuration
$Limit = 19;                                              // Block after this many incidents
$blockCommand = "ssh root@pbx -p2112 ipset add blacklist"; // Remote blocking command
$verbose = 1;                                             // 1 = dry-run, 0 = execute

// Parse alert data
$alertsData = json_decode($argv[4]);

// Build comma-separated list of CDR IDs
$cdrIds = implode(',', $alertsData->cdr);

// Query database for caller IPs and incident counts
$query = "SELECT INET_NTOA(sipcallerip) as ip, COUNT(*) as incidents
          FROM voipmonitor.cdr
          WHERE id IN ($cdrIds)
          GROUP BY sipcallerip
          ORDER BY incidents DESC";
$command = "mysql -h MYSQLHOST -u MYSQLUSER -pMYSQLPASS -N -e \"$query\"";
exec($command, $results);

// Process results and block IPs exceeding limit
foreach ($results as $line) {
    list($ip, $count) = preg_split('/\s+/', trim($line));
    if ($count > $Limit) {
        if ($verbose) {
            echo "$ip: $count incidents - would block\n";
        } else {
            exec("$blockCommand $ip", $output, $rc);
            if ($rc != 0) {
                error_log("Failed to block $ip");
            }
        }
    }
}
?>

Concurrent Calls: Block Attacker IP

Script for blocking IPs based on concurrent calls alert. Enable "By caller IP" in alert settings.

#!/usr/bin/php
<?php
// Parse triggered rules (4th argument)
$triggeredRules = json_decode($argv[4]);

// Count triggers per IP address
$IPtriggers = [];
foreach ($triggeredRules as $rule) {
    $ip = $rule->alert_info->ip;
    $IPtriggers[$ip] = ($IPtriggers[$ip] ?? 0) + 1;
}

// Block all IPs that triggered the rule
foreach ($IPtriggers as $ip => $count) {
    echo "Blocking $ip (triggered $count times)\n";
    passthru("iptables -A INPUT -s $ip -j DROP", $ret);
    if ($ret != 0) {
        echo "ERROR: Failed to block $ip\n";
        exit(1);
    }
}
echo "Blocked " . count($IPtriggers) . " IP(s)\n";
?>

AI Summary for RAG

Summary: VoIPmonitor anti-fraud rules for detecting SIP attacks (REGISTER floods, packet floods), concurrent calls abuse, geographic anomalies, and high-volume sequential calling patterns. Covers realtime vs CDR-based alert processing differences, custom script examples for automated IP blocking via iptables/ipset, and mitigation strategies for SIP REGISTER spam.

Keywords: anti-fraud, REGISTER flood, SIP attack, concurrent calls, country change, custom scripts, iptables, ipset, fail2ban, realtime alerts, CDR-based alerts, source port limitation, sequential calls, sequential dialing, destination abuse

Key Questions:

What anti-fraud alerts are available in VoIPmonitor?
How to block SIP REGISTER flood attacks?
What is the difference between realtime and CDR-based alerts?
How to create custom scripts for automated IP blocking?
Why is source port not available in realtime alerts?
How to configure an alert for a large volume of calls to any single destination number?
What is the Fraud: sequential alert and how does it work?

Anti-Fraud Rules

Overview

List of Fraud/Watchdog Alerts

Alert Processing Differences

Realtime Alerts

CDR-based Alerts

Important Limitation: Source Port in Realtime Alerts

Common Configuration

SIP REGISTER Flood/Attack

Mitigation Strategies

1. Immediate Blocking via Custom Scripts

2. Network Edge Blocking (Recommended for Prevention)

3. Reducing REGISTER Noise

Realtime Concurrent Calls

Change CDR Country

Change REGISTER Country

Detecting User-Agent Changes

Country/Continent Destination

SIP PACKETS Flood/Attack

Fraud: Sequential

Custom Script Examples

Logging Passed Arguments

RTP Alert: Store Audio Files

RTP Alert: Block IP After Threshold

Concurrent Calls: Block Attacker IP

See Also

AI Summary for RAG