configuring-snort-ids-for-intrusion-detection

What this skill does

# Configuring Snort IDS for Intrusion Detection

## When to Use

- Deploying a network-based intrusion detection system to monitor traffic at key network boundaries
- Writing custom Snort rules to detect organization-specific threats, attack patterns, or policy violations
- Tuning existing rulesets to reduce false positives while maintaining detection coverage
- Integrating Snort alerts with SIEM platforms for centralized security monitoring
- Validating network security controls by generating test traffic and confirming detection

**Do not use** as a replacement for endpoint detection, for monitoring encrypted traffic without TLS inspection, or as the sole security control without complementary defenses.

## Prerequisites

- Snort 3.x installed from source or package manager (`snort --version` to verify)
- Network interface configured for promiscuous mode on a span port or network tap
- DAQ (Data Acquisition Library) installed for packet capture integration
- Registered Snort account for downloading Snort Subscriber (paid) or Community rulesets from snort.org
- PulledPork 3 or similar rule management tool for automated ruleset updates
- Sufficient CPU and memory for inline traffic inspection at line rate

## Workflow

### Step 1: Install and Verify Snort 3

```bash
# Install dependencies (Ubuntu/Debian)
sudo apt install -y build-essential libpcap-dev libpcre3-dev libnet1-dev \
  zlib1g-dev luajit hwloc libdumbnet-dev bison flex libcmocka-dev \
  libnetfilter-queue-dev libmnl-dev autotools-dev libluajit-5.1-dev \
  pkg-config cmake libhwloc-dev liblzma-dev openssl libssl-dev cpputest \
  libsqlite3-dev uuid-dev

# Install DAQ from source
git clone https://github.com/snort3/libdaq.git
cd libdaq && ./bootstrap && ./configure && make && sudo make install

# Install Snort 3
git clone https://github.com/snort3/snort3.git
cd snort3 && ./configure_cmake.sh --prefix=/usr/local
cd build && make -j$(nproc) && sudo make install
sudo ldconfig

# Verify installation
snort -V
```

### Step 2: Configure Network Interfaces

```bash
# Disable offloading features that interfere with packet inspection
sudo ethtool -K eth1 gro off lro off tso off gso off rx off tx off

# Enable promiscuous mode
sudo ip link set eth1 promisc on

# Create systemd service for persistent interface configuration
sudo tee /etc/systemd/system/snort-iface.service << 'EOF'
[Unit]
Description=Configure Snort capture interface
Before=snort.service

[Service]
Type=oneshot
ExecStart=/sbin/ethtool -K eth1 gro off lro off tso off gso off rx off tx off
ExecStart=/sbin/ip link set eth1 promisc on
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target
EOF

sudo systemctl enable snort-iface.service
```

### Step 3: Configure Snort 3 with Lua Configuration

```bash
# Create Snort directory structure
sudo mkdir -p /usr/local/etc/snort/{rules,builtin_rules,lists,appid}
sudo mkdir -p /var/log/snort

# Edit the main Snort configuration
sudo tee /usr/local/etc/snort/snort.lua << 'LUAEOF'
-- Snort 3 Configuration

-- Network variables
HOME_NET = '10.10.0.0/16'
EXTERNAL_NET = '!$HOME_NET'

-- Path variables
RULE_PATH = '/usr/local/etc/snort/rules'
BUILTIN_RULE_PATH = '/usr/local/etc/snort/builtin_rules'

-- Configure DAQ
daq = {
    module_dirs = { '/usr/local/lib/daq' },
    modules = { { name = 'afpacket', variables = { 'buffer_size_mb=256' } } }
}

-- Decoder configuration
normalizer = { tcp = { ips = true } }

-- Stream inspection
stream = { }
stream_tcp = { policy = 'linux', session_timeout = 180 }
stream_udp = { session_timeout = 30 }
stream_icmp = { }

-- HTTP inspection
http_inspect = { }

-- DNS inspection
dns = { }

-- SSL/TLS inspection
ssl = { }

-- SMB inspection
dce_smb = { }

-- File identification and processing
file_id = { rules_file = '/usr/local/etc/snort/file_magic.rules' }

-- Port scan detection
port_scan = {
    protos = 'all',
    scan_types = 'all',
    memcap = 10000000
}

-- Reputation-based filtering
-- reputation = {
--     blacklist = RULE_PATH .. '/blocklist.rules'
-- }

-- IPS rules
ips = {
    enable_builtin_rules = true,
    include = RULE_PATH .. '/snort3-community.rules',
    variables = {
        nets = { HOME_NET = HOME_NET, EXTERNAL_NET = EXTERNAL_NET },
        ports = {
            HTTP_PORTS = '80 8080 8443',
            SSH_PORTS = '22',
            DNS_PORTS = '53'
        }
    }
}

-- Alert output
alert_fast = {
    file = true,
    packet = false,
    limit = 100
}

-- Unified2 output for Barnyard2/SIEM integration
-- alert_unified2 = { limit = 128 }

-- JSON alert output
alert_json = {
    file = true,
    limit = 100,
    fields = 'timestamp pkt_num proto pkt_gen pkt_len dir src_addr src_port dst_addr dst_port service rule action'
}

-- Syslog output
-- alert_syslog = { level = 'info', facility = 'local1' }

LUAEOF
```

### Step 4: Download and Configure Rulesets

```bash
# Download Snort 3 Community Rules
wget https://www.snort.org/downloads/community/snort3-community-rules.tar.gz
tar xzf snort3-community-rules.tar.gz
sudo cp snort3-community-rules/snort3-community.rules /usr/local/etc/snort/rules/

# Install PulledPork 3 for automated rule management
git clone https://github.com/shirkdog/pulledpork3.git
cd pulledpork3
sudo python3 setup.py install

# Configure PulledPork
sudo tee /usr/local/etc/pulledpork3/pulledpork.conf << 'EOF'
registered_ruleset = true
oinkcode = <YOUR_OINK_CODE>
snort_path = /usr/local/bin/snort
local_rules = /usr/local/etc/snort/rules/local.rules
sorule_path = /usr/local/etc/snort/so_rules/
snort_version = 3.0.0.0
blocklist_path = /usr/local/etc/snort/lists/
pid_path = /var/run/snort.pid
ips_policy = balanced
EOF

# Run PulledPork to fetch and process rules
sudo pulledpork3 -c /usr/local/etc/pulledpork3/pulledpork.conf
```

### Step 5: Write Custom Detection Rules

```bash
# Create local rules file
sudo tee /usr/local/etc/snort/rules/local.rules << 'EOF'
# Detect reverse shell on common ports
alert tcp $HOME_NET any -> $EXTERNAL_NET 4444 (
    msg:"LOCAL Possible Reverse Shell on port 4444";
    flow:established,to_server;
    content:"/bin/sh"; nocase;
    sid:1000001; rev:1;
    classtype:trojan-activity;
    priority:1;
)

# Detect Mimikatz execution indicators over SMB
alert tcp any any -> $HOME_NET 445 (
    msg:"LOCAL Mimikatz Lateral Movement via SMB";
    flow:established,to_server;
    content:"|FF|SMB";
    content:"mimikatz"; nocase; distance:0;
    sid:1000002; rev:1;
    classtype:trojan-activity;
    priority:1;
)

# Detect DNS tunneling (high-entropy long subdomain queries)
alert udp $HOME_NET any -> any 53 (
    msg:"LOCAL Possible DNS Tunneling - Long Query Name";
    content:"|01 00|"; offset:2; depth:2;
    byte_test:1,>,50,12;
    sid:1000003; rev:1;
    classtype:policy-violation;
    priority:2;
)

# Detect cleartext password transmission via FTP
alert tcp $HOME_NET any -> any 21 (
    msg:"LOCAL FTP Cleartext Password Detected";
    flow:established,to_server;
    content:"PASS "; depth:5;
    sid:1000004; rev:1;
    classtype:policy-violation;
    priority:2;
)

# Detect potential port scan (SYN flood pattern)
alert tcp $EXTERNAL_NET any -> $HOME_NET any (
    msg:"LOCAL Possible Port Scan SYN Flood";
    flow:stateless;
    flags:S,12;
    threshold:type both, track by_src, count 100, seconds 10;
    sid:1000005; rev:1;
    classtype:attempted-recon;
    priority:2;
)
EOF
```

### Step 6: Validate Configuration and Run

```bash
# Validate configuration
snort -c /usr/local/etc/snort/snort.lua --daq-dir /usr/local/lib/daq -T

# Run Snort in IDS mode on the capture interface
sudo snort -c /usr/local/etc/snort/snort.lua --daq-dir /usr/local/lib/daq \
  -i eth1 -l /var/log/snort -D

# Test rules against a PCAP file
snort -c /usr/local/etc/snort/snort.lua --daq-dir /usr/local/lib/daq \
  -r test_traffic.pcap -l /var/log/snort/test/ -A fast

# Create systemd service for production deployment
sudo tee /etc/systemd/system/snort.service << 'EOF'
[Unit]
Description=Snort 3 IDS
After=network.target snort-iface.service

[Se