FA-7451 / DNS resolution / Open access

Dns mx null target denotes no mail service · case 01

Dns mx null target denotes no mail service.

Verified by executionVariant 1 · 4 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The faulty implementation uses `bool(preferences)` for the dns-mx-null-target-denotes-no-mail-service decision.

VERIFIED REPAIR

Apply the explicit dns-mx-null-target-denotes-no-mail-service contract, including the tested boundary and negative cases.

Unsuccessful approach: The attempted repair uses `not any(target == "." for p,target in preferences)` and still violates a separate fixture.

Case contract

Bounded offline decision model: dns mx null target denotes no mail service. Inputs are already validated protocol fields; the explicit fixtures define the supported policy, not a complete protocol implementation.

Why this case matters

This deterministic dns resolution model isolates the dns-mx-null-target-denotes-no-mail-service policy at a network component boundary.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import ipaddress
import re
from urllib.parse import urlsplit, urljoin
N = 1
observations = []
def solve(preferences):
    return bool(preferences)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve([(0, '.')]), False)", "check('fixture 2', solve([(10, '.')]), True)", "check('fixture 3', solve([(10, 'mail.example')]), True)", "check('fixture 4', solve([]), True)"]
for _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:
    exec(_line)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 2	`True`	`True`	Passed
fixture 3	`True`	`True`	Passed
fixture 4	`False`	`True`	Failed
fixture 1	`True`	`False`	Failed

SHA-256 / 5e92062b693de721b704b82a7eb21bdd885685434f136082b2318202e4ff4282

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import ipaddress
import re
from urllib.parse import urlsplit, urljoin
N = 1
observations = []
def solve(preferences):
    return not any(target == "." for p,target in preferences)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve([(0, '.')]), False)", "check('fixture 2', solve([(10, '.')]), True)", "check('fixture 3', solve([(10, 'mail.example')]), True)", "check('fixture 4', solve([]), True)"]
for _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:
    exec(_line)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 2	`False`	`True`	Failed
fixture 3	`True`	`True`	Passed
fixture 4	`True`	`True`	Passed
fixture 1	`False`	`False`	Passed

SHA-256 / 6399e3ff4bf813a7d4282a30aff2d995865a6830e862f6104149ceea9f8000d2

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
import ipaddress
import re
from urllib.parse import urlsplit, urljoin
N = 1
observations = []
def solve(preferences):
    return not any(p == 0 and target == "." for p,target in preferences)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve([(0, '.')]), False)", "check('fixture 2', solve([(10, '.')]), True)", "check('fixture 3', solve([(10, 'mail.example')]), True)", "check('fixture 4', solve([]), True)"]
for _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:
    exec(_line)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 2	`True`	`True`	Passed
fixture 3	`True`	`True`	Passed
fixture 4	`True`	`True`	Passed
fixture 1	`False`	`False`	Passed

SHA-256 / 68c9be87c4d3b0e425a43b765fd30c9f00e23dc0ce1f68740807e8a5baacfd59

Verification & scope

Bounded deterministic policy model over validated inputs; not a complete protocol stack or an interoperability claim. The five variants rotate the same explicit fixture set. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:38:12.258331+00:00.

Case digest / b34c799051f755ef49477db1169565cd2eac9a04d87dc6a7f49b5e39ddcb6a25