DoQ-to-UDP State Reduction and Source-Port Oracle

Summary

This report covers the client-triggered DoQ forwarding path in:

• dnsproxy v0.81.2 (adguard/dnsproxy:v0.81.2)
• AdGuard Home v0.107.74 (adguard/adguardhome:latest, image version label v0.107.74)

The issue was reproduced on 2026-04-25 with the products configured through
their documented DoQ listener and plain UDP upstream surfaces. The scope is the
internal backend UDP hop created when a DoQ query is forwarded to a udp://
upstream.

On that path, the backend DNS ID is not preserved as an independent source of
entropy. For both products, the backend observer saw dns_id=0 for every
sampled client-triggered query on the tested path. Repeated reruns then showed
the same txid=0 behavior and the same positive source-port oracle on every
sampled run. A separate quoted-port ICMP oracle distinguished the correct
backend UDP source port from a wrong one with a stable, client-visible behavior
change.

Attached evidence:

• dnsproxy oracle path on v0.81.2: attachments/artifacts/g03/20260425T141500Z-g03-v0812/summary.txt
• dnsproxy v0.81.2 repeatability: attachments/artifacts/g03/repeatability-v0812.md
• dnsproxy steering follow-up on v0.81.2: attachments/artifacts/g04/20260425T141900Z-g04-v0812/summary.txt
• AdGuard Home oracle path: attachments/artifacts/g05/20260425T113000Z-g05/summary.txt

Root Cause Analysis

The observable behavior is consistent across both products:

1. A DoQ client query is accepted on the frontend listener.
2. The query is forwarded over a backend UDP leg.
3. On that backend leg, the forwarded DNS ID collapses to 0 on the
   client-triggered path instead of remaining a fresh per-query variable.
4. The backend UDP source port is still allocated per query.
5. When an ICMP error quotes the actual backend source port, the forwarding path
   flips behavior in a way that does not occur for a wrong quoted port.

That combination removes txid from the backend tuple on the tested path and
leaves the UDP source port as the main remaining variable. In practical terms,
the backend hop stops behaving like a fresh (txid, source-port) pair per
forwarded query and instead becomes a one-variable state exposure.

For dnsproxy, the correct quoted port does more than produce a failure signal:
it can push resolution away from the primary UDP upstream and into the fallback
upstream. For AdGuard Home, the same condition produces a fast SERVFAIL.

Reproduce

Prerequisites:

• Docker and Docker Compose
• OpenSSL
• build the lab helper image used by the attached harness and observer

The attached reproducer bundle contains only the files needed for this report:

• scripts: attachments/scripts/
• helper image build files: attachments/docker/unbound-doq-attacker/
• compose files: attachments/docker-compose.g03.yml,
  attachments/docker-compose.g04.yml, attachments/docker-compose.g05.yml
• shipped evidence: attachments/artifacts/...

Build the helper image first:

1. cd attachments
2. docker build -t unbound-doq-attacker:latest -f docker/unbound-doq-attacker/Dockerfile docker/unbound-doq-attacker

To rerun dnsproxy:

1. cd attachments
2. bash scripts/repro-g03-dnsproxy-oracle.sh
3. Inspect artifacts/g03/<RUN_ID>/summary.txt
4. Inspect artifacts/g03/<RUN_ID>/entropy-backend.jsonl,
   txid_correct-backend.jsonl, and port_correct-backend.jsonl

To rerun the dnsproxy fallback-steering case:

1. cd attachments
2. bash scripts/repro-g04-dnsproxy-steering.sh
3. Inspect artifacts/g04/<RUN_ID>/summary.txt
4. Inspect steering_correct-main.jsonl and steering_correct-fallback.jsonl

To rerun AdGuard Home:

1. cd attachments
2. bash scripts/repro-g05-adguardhome-oracle.sh
3. Inspect artifacts/g05/<RUN_ID>/summary.txt
4. Inspect entropy-backend.jsonl, txid_correct-backend.jsonl, and
   port_correct-backend.jsonl

The attached evidence includes fresh dnsproxy v0.81.2 reruns, one official-
profile AdGuard Home run, and the minimal reproducer bundle used by both.

Impact

For both products, the tested DoQ-to-UDP path is no longer a full
(txid, source-port) search surface:

• dnsproxy: four of four sampled runs showed txid=0 on the backend hop and
  a positive source-port oracle on v0.81.2. The remaining unknown is
  port_only. Median wrong/correct port latency was 327.99 ms / 40.93 ms.
• AdGuard Home: four of four sampled runs showed txid=0 on the backend hop
  and a positive source-port oracle. The aggregate again classifies the
  remaining unknown as port_only. Median wrong/correct port latency was
  319.14 ms / 37.02 ms.

Product-specific effects:

• dnsproxy: a correct port guess produced an empty client-visible answer on
  the base oracle path, and in the fallback profile it steered all eight tested
  queries away from the main upstream and into the fallback upstream.
• AdGuard Home: a correct port guess produced fast SERVFAIL and an extra
  backend query.

This is the security-relevant point. On the tested official profiles, the
backend hop no longer forces an off-path attacker to deal with two fresh random
fields per forwarded DNS race. The DNS ID is already known: it is
deterministically 0 on the client-triggered DoQ-to-UDP path. The only
remaining backend tuple variable is the UDP source port, and the attached
evidence shows a repeatable oracle for that remaining variable.

That places the path in the same threat-model class as oracle-assisted DNS
forgery work such as SAD DNS and TUdoor: the attack first uses an oracle to
learn or validate the tuple state that protects an off-path response race, and
only then attempts the forged response. This report stops short of a forgery
demo, but the evidence already shows the crucial precondition on the tested
backend hop: the tuple is not high-entropy anymore. It has been reduced from
(txid, source-port) to source-port only.

Attachments
attachments.zip