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Introduction:
Operational Technology (OT) networks traditionally rely on a central headquarters with wired internet and a public static IP to connect remote field sites. But when your entire application consists of isolated field sites that must communicate directly—without a central hub—carrier private networks and Zero Tunnel Private Network architectures offer a secure, low-latency solution. This article dissects the cellular-based VPN alternatives, hub-and-spoke trade-offs, and encryption requirements for critical infrastructure, drawing from real-world deployments in water, wastewater, and oil & gas sectors.
Learning Objectives:
- Deploy a Zero Tunnel VPN to interconnect remote OT field sites without a central HQ or public static IP.
- Harden cellular router configurations using Linux/Windows commands and encryption standards for critical infrastructure compliance.
- Evaluate latency, cost, and security trade-offs between carrier private networks, Zero Tunnel VPNs, and private radio.
You Should Know:
- Deploying a Zero Tunnel VPN on Linux with WireGuard for OT Field Sites
Zero Tunnel VPNs create direct site-to-site encrypted tunnels without requiring a centralized gateway—unlike traditional hub-and-spoke VPNs. The post highlights that for architectures where spoke-to-spoke communication isn’t needed (e.g., master/controller designs), a hub-and-spoke model is natural. However, when all sites must talk to each other, Zero Tunnel eliminates the extra hop.
Step‑by‑step guide (Linux – Debian/Ubuntu on a gateway or Raspberry Pi at each field site):
1. Install WireGuard on each site’s gateway:
sudo apt update && sudo apt install wireguard resolvconf -y
2. Generate private/public key pair per site:
cd /etc/wireguard umask 077 wg genkey | tee privatekey | wg pubkey > publickey
3. Create configuration file (`/etc/wireguard/wg0.conf`) for Site A:
[bash] Address = 10.0.1.1/24 PrivateKey = <SiteA_private> ListenPort = 51820 PostUp = iptables -A FORWARD -i wg0 -j ACCEPT; iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE PostDown = iptables -D FORWARD -i wg0 -j ACCEPT; iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE [bash] PublicKey = <SiteB_public> AllowedIPs = 10.0.1.2/32, 192.168.2.0/24 Endpoint = <SiteB_public_IP_or_dynamic_DNS>:51820 PersistentKeepalive = 25
Repeat for Site B, swapping addresses and keys. For cellular connections without static IPs, use a dynamic DNS client (e.g., DuckDNS) or a signaling broker.
4. Enable and start:
sudo systemctl enable wg-quick@wg0 sudo systemctl start wg-quick@wg0
5. Test connectivity: `ping 10.0.1.2` from Site A.
Windows alternative (using WireGuard for Windows): Download the client, import the same config format, and use `New-NetFirewallRule -DisplayName “WireGuard” -Direction Inbound -Protocol UDP -LocalPort 51820 -Action Allow` in PowerShell (Admin).
- Configuring Cellular Routers for Carrier Private Networks (IPWAN)
The discussion mentions carrier private networks (e.g., Verizon’s IPWAN) that are still considered “unsafe” for critical infrastructure in Australia, requiring mandatory encryption. Here’s how to harden a typical cellular router (e.g., Sierra Wireless, Cradlepoint, Garderos).
Step‑by‑step guide for router CLI (Linux-based OpenWRT or similar):
1. Set APN for private network:
uci set network.cellular.apn="private.carrier.com" uci set network.cellular.auth="chap" or pap uci set network.cellular.username="ot_router" uci set network.cellular.password="strongpassword" uci commit network
- Enforce IPsec or WireGuard on top of the carrier network (defense in depth):
opkg update && opkg install strongswan strongswan-mod-kernel-libipsec
3. Disable insecure services:
uci set dropbear.@dropbear[bash].Port=22 change SSH port uci set firewall.cfg022dc4.input='DROP' uci commit firewall /etc/init.d/firewall restart
- For Garderos routers (mentioned by Kade M.), use their on‑prem management system instead of cloud:
– Access web UI at `https://
– Navigate to “Management” → “On‑Prem Mode” → Enter local Garderos server IP
– Enable “No Internet Required” for central management
3. Hardening OT Site-to-Site Encryption for Compliance
David Vásquez noted that cellular ZT VPN routes all traffic through a centralized gateway (hub-and-spoke). For critical infrastructure, encryption isn’t optional—even on carrier private networks. Use IPsec with strong ciphers.
Linux commands to set up IPsec tunnel (Libreswan) between two field sites:
1. Install Libreswan:
sudo apt install libreswan -y
2. Edit `/etc/ipsec.conf` on Site A:
conn site-to-site left=10.0.1.1 leftid=@site-a leftsubnet=192.168.1.0/24 right=10.0.1.2 rightid=@site-b rightsubnet=192.168.2.0/24 ike=aes256-sha2;modp2048 esp=aes256-sha2 ikelifetime=8h keylife=1h authby=secret auto=start
3. Set pre-shared key in `/etc/ipsec.secrets`:
@site-a @site-b : PSK "very_long_random_string_here"
4. Restart: `sudo systemctl restart ipsec`
Windows (built‑in VPN): Use `Add-VpnConnection` PowerShell cmdlet with L2TP/IPsec, but third‑party clients (e.g., TheGreenBow) are more reliable for OT.
4. Overcoming Latency with Hub-and-Spoke Architecture
Latency was a non‑issue in most deployments—any connectivity beats a truck roll. But if you need spoke-to-spoke communication, the extra hop through a central gateway can add 20–50ms. Measure and mitigate.
Commands to test latency across cellular links:
Linux/Windows: Ping with timestamps ping -D 10.0.1.2 Linux ping -S 10.0.1.1 10.0.1.2 Windows, source address MTR (My TraceRoute) - continuous monitoring sudo mtr --report-wide 10.0.1.2 For cellular interface specific testing (Linux) ping -I wwan0 8.8.8.8
To reduce latency in a hub-and-spoke Zero Tunnel:
- Place the central gateway in a cloud region closest to the field sites (e.g., AWS Local Zone).
- Use UDP instead of TCP for VPN transport (WireGuard defaults to UDP).
- Disable any deep packet inspection on the router (carrier‑side).
5. On-Prem Management Alternatives for Cellular Routers
Josh’s previous article stated that internet is likely required for central management, but Garderos offers an on‑prem option. This is critical for air‑gapped or highly regulated OT environments.
Setting up Garderos on‑prem management server (Linux):
- Download Garderos RouterWare Management Center (RMC) from their portal.
2. Install dependencies:
sudo apt install docker-compose postgresql
3. Run the installer:
chmod +x garderos-rmc-installer.bin sudo ./garderos-rmc-installer.bin --mode silent --prefix /opt/garderos
4. Configure local DNS to resolve `rmc.local` to the server IP.
5. On each cellular router, set management URL:
router config set mgmt.server https://rmc.local:443 router config set mgmt.mode onprem
6. For routers already deployed, push via SSH script:
for ip in $(cat router_ips.txt); do ssh root@$ip "uci set garderos.mgmt.url='https://rmc.local:443'; uci commit; /etc/init.d/garderos-mgmt restart"; done
- Integrating AI for Anomaly Detection in OT Traffic
While not explicitly in the post, AI can monitor the Zero Tunnel traffic for unusual patterns—e.g., unexpected Modbus writes or abnormal latency spikes that might indicate reconnaissance or a compromised router.
Tutorial: Lightweight AI with Zeek + Python on a central gateway:
1. Install Zeek (formerly Bro):
sudo apt install zeek export PATH=$PATH:/opt/zeek/bin
2. Capture traffic on the VPN interface:
zeek -i wg0 -C
- Use a Python script to detect anomalies (e.g., sudden increase in unique destination IPs):
import pandas as pd from sklearn.ensemble import IsolationForest Load Zeek conn.log logs = pd.read_csv('conn.log', sep='\t', comment='') features = logs[['orig_bytes', 'resp_bytes', 'duration']].fillna(0) model = IsolationForest(contamination=0.01) anomalies = model.fit_predict(features) if -1 in anomalies: print("ALERT: Anomalous traffic pattern detected in OT VPN")
4. Run every 5 minutes via cron:
/5 /usr/bin/python3 /opt/ot_ai_monitor.py >> /var/log/ot_ai.log
7. Compliance and Training for Critical Infrastructure
Regions like Australia classify carrier private networks as “unsafe” without added encryption. This demands specific training for OT security teams.
Training course outline (self‑study with commands):
- Module 1: Cellular OT architecture (APN, private IP, SIM authentication)
- Module 2: VPN hardening for Zero Tunnel (WireGuard vs IPsec, certificate management)
- Module 3: Auditing router configs (Linux `grep` and `awk` to check for weak ciphers)
grep -r "IKE=" /etc/ipsec.d/ | grep -v "modp2048|modp4096"
- Module 4: Incident response for compromised cellular links
Isolate a router remotely ssh field-router "iptables -A INPUT -j DROP; ip link set wwan0 down"
Certification alignment: NIST SP 800-82, ISA/IEC 62443, and Australia’s SOCI Act.
What Undercode Say:
- Key Takeaway 1: Carrier private networks (IPWAN) are not inherently secure; encryption via VPN is mandatory for critical infrastructure, even when using a private cellular APN.
- Key Takeaway 2: Zero Tunnel VPNs solve the “no HQ, no public IP” problem by enabling direct site-to-site tunnels, but hub-and-spoke remains optimal for master/controller OT architectures where spoke-to-spoke traffic is unnecessary.
Analysis (10 lines): The LinkedIn discussion reveals a practical divide between cellular carriers’ marketed “private networks” and real-world OT security requirements. Josh’s architecture acknowledges that most remote OT deployments (water, wastewater, oil & gas) already use a centralized SCADA master, making hub-and-spoke Zero Tunnel a natural fit—extra latency is rarely a concern compared to the cost of a truck roll. Kade M.’s warning about Australia’s stance is critical: regulatory compliance often demands encryption regardless of the carrier’s claim of a “private” network. David’s comparison of private radio (lower latency, higher capital cost) versus cellular (cheaper, easier coverage) mirrors classic OT trade‑offs. The mention of Garderos on‑prem management is a lifeline for air‑gapped facilities that cannot route management traffic over the internet. From a security perspective, the biggest risk is misconfigured cellular routers with default credentials or weak cipher suites—attackers scanning 4G/5G ranges have compromised OT sites via unencrypted carrier tunnels. Future articles should address SIM‑based authentication (eAP‑AKA) and the emerging role of 5G network slicing for true isolation.
Prediction:
Within three years, Zero Tunnel architectures will become the default for greenfield OT cellular deployments as 5G standalone (SA) enables native network slicing and edge computing. Carriers will shift from selling “private networks” as separate APNs to offering Zero Tunnel as a service with built‑in post‑quantum encryption. However, regulatory bodies (particularly in Five Eyes countries) will mandate on‑prem key management, killing cloud‑managed Zero Tunnel for critical infrastructure. The convergence of AI‑driven anomaly detection and cellular VPN telemetry will allow real‑threat hunting on remote field sites—turning every router into a distributed sensor. Training courses will pivot from basic VPN configuration to “Zero Trust OT Cellular” with automated compliance checks, and legacy deployments that rely on unencrypted carrier private networks will face mandatory retrofit deadlines after a high‑profile breach of a water utility.
▶️ Related Video (82% Match):
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