blastdns 1.4.2

Async Python interface on top of the BlastDNS resolver

BlastDNS

BlastDNS is an ultra-fast DNS resolver written in Rust. Like massdns, it's designed to be faster the more resolvers you give it. Features include built-in caching, and high accuracy even with unreliable DNS servers. For details, see Architecture. BlastDNS is the main DNS library used by BBOT.

There are three ways to use it:

• Rust CLI tool
• Rust library
• Python library

Benchmark

100K DNS lookups against local dnsmasq, with 100 workers:

Library	Language	Time	QPS	Success	Failed	vs dnspython
massdns	C	1.370s	72,998	100,000	0	28.63x
blastdns-cli	Rust	1.654s	60,470	100,000	0	23.72x
blastdns-python	Python	2.485s	40,249	100,000	0	15.79x
dnspython	Python	39.223s	2,550	100,000	0	1.00x

CLI

The CLI mass-resolves hosts using a specified list of resolvers. It outputs to JSON.

# send all results to jq
$ blastdns hosts.txt --rdtype A --resolvers resolvers.txt | jq

# print only the raw IPv4 addresses
$ blastdns hosts.txt --rdtype A --resolvers resolvers.txt | jq '.response.answers[].rdata.A'

# load from stdin
$ cat hosts.txt | blastdns --rdtype A --resolvers resolvers.txt

# skip empty responses (e.g., NXDOMAIN with no answers)
$ blastdns hosts.txt --rdtype A --resolvers resolvers.txt --skip-empty | jq

# skip error responses (e.g., timeouts, connection failures)
$ blastdns hosts.txt --rdtype A --resolvers resolvers.txt --skip-errors | jq

CLI Help

$ blastdns --help
BlastDNS - Ultra-fast DNS Resolver written in Rust

Usage: blastdns [OPTIONS] --resolvers <FILE> [HOSTS_TO_RESOLVE]

Arguments:
  [HOSTS_TO_RESOLVE]  File containing hostnames to resolve (one per line). Reads from stdin if not specified

Options:
      --rdtype <RECORD_TYPE>
          Record type to query (A, AAAA, MX, ...) [default: A]
      --resolvers <FILE>
          File containing DNS nameservers (one per line)
      --threads-per-resolver <THREADS_PER_RESOLVER>
          Worker threads per resolver [default: 2]
      --timeout-ms <TIMEOUT_MS>
          Per-request timeout in milliseconds [default: 1000]
      --retries <RETRIES>
          Retry attempts after a resolver failure [default: 10]
      --purgatory-threshold <PURGATORY_THRESHOLD>
          Consecutive errors before a worker is put into timeout [default: 10]
      --purgatory-sentence-ms <PURGATORY_SENTENCE_MS>
          How many milliseconds a worker stays in timeout [default: 1000]
      --skip-empty
          Don't show responses with no answers
      --skip-errors
          Don't show error responses
      --brief
          Output brief format (hostname, record type, answers only)
      --cache-capacity <CACHE_CAPACITY>
          DNS cache capacity (0 = disabled) [default: 10000]
  -h, --help
          Print help
  -V, --version
          Print version

Example JSON output

BlastDNS outputs to JSON by default:

{
  "host": "microsoft.com",
  "response": {
    "additionals": [],
    "answers": [
      {
        "dns_class": "IN",
        "name_labels": "microsoft.com.",
        "rdata": {
          "A": "13.107.213.41"
        },
        "ttl": 1968
      },
      {
        "dns_class": "IN",
        "name_labels": "microsoft.com.",
        "rdata": {
          "A": "13.107.246.41"
        },
        "ttl": 1968
      }
    ],
    "edns": {
      "flags": {
        "dnssec_ok": false,
        "z": 0
      },
      "max_payload": 1232,
      "options": {
        "options": []
      },
      "rcode_high": 0,
      "version": 0
    },
    "header": {
      "additional_count": 1,
      "answer_count": 2,
      "authentic_data": false,
      "authoritative": false,
      "checking_disabled": false,
      "id": 62150,
      "message_type": "Response",
      "name_server_count": 0,
      "op_code": "Query",
      "query_count": 1,
      "recursion_available": true,
      "recursion_desired": true,
      "response_code": "NoError",
      "truncation": false
    },
    "name_servers": [],
    "queries": [
      {
        "name": "microsoft.com.",
        "query_class": "IN",
        "query_type": "A"
      }
    ],
    "signature": []
  }
}

Debug Logging

BlastDNS uses the standard Rust tracing ecosystem. Enable debug logging by setting the RUST_LOG environment variable:

# Show debug logs from blastdns only
RUST_LOG=blastdns=debug blastdns hosts.txt --rdtype A --resolvers resolvers.txt

# Show debug logs from everything
RUST_LOG=debug blastdns hosts.txt --rdtype A --resolvers resolvers.txt

# Show trace-level logs for detailed internal behavior
RUST_LOG=blastdns=trace blastdns hosts.txt --rdtype A --resolvers resolvers.txt

Valid log levels (from least to most verbose): error, warn, info, debug, trace

Rust API

Installation

# Install CLI tool
cargo install blastdns

# Add library to your project
cargo add blastdns

Usage

use blastdns::{BlastDNSClient, BlastDNSConfig};
use futures::StreamExt;
use hickory_client::proto::rr::RecordType;
use std::time::Duration;

// read DNS resolvers from a file (one per line -> vector of strings)
let resolvers = std::fs::read_to_string("resolvers.txt")
    .expect("Failed to read resolvers file")
    .lines()
    .map(str::to_string)
    .collect::<Vec<String>>();

// create a new blastdns client with default config
let client = BlastDNSClient::new(resolvers).await?;

// or with custom config
let mut config = BlastDNSConfig::default();
config.threads_per_resolver = 5;
config.request_timeout = Duration::from_secs(2);
let client = BlastDNSClient::with_config(resolvers, config).await?;

// resolve: lookup a domain, returns only the rdata strings
let answers = client.resolve("example.com", RecordType::A).await?;
for answer in answers {
    println!("{}", answer);  // e.g., "93.184.216.34"
}

// resolve_full: lookup a domain, returns the full DNS response
let result = client.resolve_full("example.com", RecordType::A).await?;
println!("{}", serde_json::to_string_pretty(&result).unwrap());

// resolve_batch: process many hosts in parallel, returns simplified output
// streams back (host, record_type, Vec<rdata>) tuples as they complete
// automatically filters out errors and empty responses
let wordlist = ["one.example", "two.example", "three.example"];
let mut stream = client.resolve_batch(
    wordlist.into_iter().map(Ok::<_, std::convert::Infallible>),
    RecordType::A,
);
while let Some((host, record_type, answers)) = stream.next().await {
    println!("{} ({}):", host, record_type);
    for answer in answers {
        println!("  {}", answer);  // e.g., "93.184.216.34" for A records
    }
}

// resolve_batch_full: process many hosts with full DNS response structures
// streams back (host, Result<response>) tuples with configurable filtering
let wordlist = ["one.example", "two.example", "three.example"];
let mut stream = client.resolve_batch_full(
    wordlist.into_iter().map(Ok::<_, std::convert::Infallible>),
    RecordType::A,
    false,  // skip_empty: don't filter out empty responses
    false,  // skip_errors: don't filter out errors
);
while let Some((host, outcome)) = stream.next().await {
    match outcome {
        Ok(response) => println!("{}: {} answers", host, response.answers().len()),
        Err(err) => eprintln!("{} failed: {err}", host),
    }
}

// resolve_multi: resolve multiple record types for a single host
// returns only successful results with answers as dict[record_type, Vec<rdata>]
let record_types = vec![RecordType::A, RecordType::AAAA, RecordType::MX];
let results = client.resolve_multi("example.com", record_types).await?;
for (record_type, answers) in results {
    println!("{}: {} answers", record_type, answers.len());
    for answer in answers {
        println!("  {}", answer);
    }
}

// resolve_multi_full: resolve multiple record types with full responses
// returns all results (success and failure) as dict[record_type, Result<response>]
let record_types = vec![RecordType::A, RecordType::AAAA, RecordType::MX];
let results = client.resolve_multi_full("example.com", record_types).await?;
for (record_type, result) in results {
    match result {
        Ok(response) => println!("{}: {} answers", record_type, response.answers().len()),
        Err(err) => eprintln!("{} failed: {err}", record_type),
    }
}

MockBlastDNSClient for Testing

MockBlastDNSClient implements the DnsResolver trait and provides a drop-in replacement that returns fabricated DNS responses without making real network requests.

use blastdns::{MockBlastDNSClient, DnsResolver};
use hickory_client::proto::rr::RecordType;
use std::collections::HashMap;

// Create a mock client
let mut mock_client = MockBlastDNSClient::new();

// Configure mock responses
let responses = HashMap::from([
    (
        "example.com".to_string(),
        HashMap::from([
            ("A".to_string(), vec!["93.184.216.34".to_string()]),
            ("AAAA".to_string(), vec!["2606:2800:220:1:248:1893:25c8:1946".to_string()]),
        ]),
    ),
]);

// Hosts that should return NXDOMAIN
let nxdomains = vec!["notfound.example.com".to_string()];

mock_client.mock_dns(responses, nxdomains);

// Use like any DnsResolver
let answers = mock_client.resolve("example.com".to_string(), RecordType::A).await?;
assert_eq!(answers, vec!["93.184.216.34"]);

// NXDOMAIN hosts return empty responses
let answers = mock_client.resolve("notfound.example.com".to_string(), RecordType::A).await?;
assert_eq!(answers.len(), 0);

MockBlastDNSClient supports all DnsResolver methods including resolve, resolve_full, resolve_batch, resolve_batch_full, resolve_multi, and resolve_multi_full.

Python API

The blastdns Python package is a thin wrapper around the Rust library.

Installation

# Using pip
pip install blastdns

# Using uv
uv add blastdns

# Using poetry
poetry add blastdns

Development Setup

# install python dependencies
uv sync
# build and install the rust->python bindings
uv run maturin develop
# run tests
uv run pytest

Usage

To use it in Python, you can use the Client class:

import asyncio
from blastdns import Client, ClientConfig, DNSResult, DNSError


async def main():
    resolvers = ["1.1.1.1:53"]
    client = Client(resolvers, ClientConfig(threads_per_resolver=4, request_timeout_ms=1500))

    # resolve: lookup a single host, returns only rdata strings
    answers = await client.resolve("example.com", "A")
    for answer in answers:
        print(f"  {answer}")  # e.g., "93.184.216.34"

    # resolve_full: lookup a single host, returns full DNS response as Pydantic model
    result = await client.resolve_full("example.com", "AAAA")
    print(f"Host: {result.host}")
    print(f"Response code: {result.response.header.response_code}")
    for answer in result.response.answers:
        print(f"  {answer.name_labels}: {answer.rdata}")

    # resolve_batch: simplified batch resolution with minimal output
    # returns only (host, record_type, list[rdata]) - no full DNS response structures
    # automatically filters out errors and empty responses
    hosts = ["example.com", "google.com", "github.com"]
    async for host, rdtype, answers in client.resolve_batch(hosts, "A"):
        print(f"{host} ({rdtype}):")
        for answer in answers:
            print(f"  {answer}")  # e.g., "93.184.216.34" for A records

    # resolve_batch_full: process many hosts in parallel with full responses
    # streams results back as they complete
    hosts = ["one.example.com", "two.example.com", "three.example.com"]
    async for host, result in client.resolve_batch_full(hosts, "A"):
        if isinstance(result, DNSError):
            print(f"{host} failed: {result.error}")
        else:
            print(f"{host}: {len(result.response.answers)} answers")

    # resolve_multi: resolve multiple record types for a single host in parallel
    # returns only successful results with answers
    record_types = ["A", "AAAA", "MX"]
    results = await client.resolve_multi("example.com", record_types)
    for record_type, answers in results.items():
        print(f"{record_type}: {answers}")

    # resolve_multi_full: resolve multiple record types with full response data
    record_types = ["A", "AAAA", "MX"]
    results = await client.resolve_multi_full("example.com", record_types)
    for record_type, result in results.items():
        if isinstance(result, DNSError):
            print(f"{record_type} failed: {result.error}")
        else:
            print(f"{record_type}: {len(result.response.answers)} answers")


asyncio.run(main())

Python API Methods

• Client.resolve(host, record_type=None) -> list[str]: Lookup a single hostname, returning only rdata strings. Defaults to A records. Returns a list of strings (e.g., ["93.184.216.34"] for A records). Perfect for simple use cases where you just need the record data without the full DNS response structure.

• Client.resolve_full(host, record_type=None) -> DNSResult: Lookup a single hostname, returning the full DNS response. Defaults to A records. Returns a Pydantic DNSResult model with typed fields for easy access to headers, queries, answers, etc.

• Client.resolve_batch(hosts, record_type=None): Simplified batch resolution that returns only the essential data. Takes an iterable of hostnames and streams back (host, record_type, answers) tuples where answers is a list of rdata strings (e.g., ["93.184.216.34"] for A records, ["10 aspmx.l.google.com."] for MX records). Automatically filters out errors and empty responses. Perfect for processing large lists of hosts efficiently.

• Client.resolve_batch_full(hosts, record_type=None, skip_empty=False, skip_errors=False): Resolve many hosts in parallel with full DNS responses. Takes an iterable of hostnames and streams back (host, result) tuples as results complete. Each result is either a DNSResult or DNSError Pydantic model. Set skip_empty=True to filter out successful responses with no answers. Set skip_errors=True to filter out error responses.

• Client.resolve_multi(host, record_types) -> dict[str, list[str]]: Resolve multiple record types for a single hostname in parallel, returning only successful results with answers. Takes a list of record type strings (e.g., ["A", "AAAA", "MX"]) and returns a dictionary mapping record types to lists of rdata strings. Only includes record types that resolved successfully and have answers.

• Client.resolve_multi_full(host, record_types) -> dict[str, DNSResultOrError]: Resolve multiple record types for a single hostname in parallel, returning full DNS responses. Takes a list of record type strings and returns a dictionary keyed by record type. Each value is either a DNSResult (success) or DNSError (failure) Pydantic model. Includes all record types, even those that failed or had no answers.

MockClient for Testing

MockClient provides a drop-in replacement for Client that returns fabricated DNS responses without making real network requests. It implements the same interface as Client and is useful for testing code that depends on DNS lookups.

import pytest
from blastdns import MockClient, DNSResult


@pytest.fixture
def mock_client():
    """Create a mock client with pre-configured test data."""
    client = MockClient()
    client.mock_dns({
        "example.com": {
            "A": ["93.184.216.34"],
            "AAAA": ["2606:2800:220:1:248:1893:25c8:1946"],
            "MX": ["10 aspmx.l.google.com.", "20 alt1.aspmx.l.google.com."],
        },
        "cname.example.com": {
            "CNAME": ["example.com."]
        },
        "_NXDOMAIN": ["notfound.example.com"],  # hosts that return NXDOMAIN
    })
    return client


@pytest.mark.asyncio
async def test_my_function(mock_client):
    # resolve() returns simple rdata strings
    answers = await mock_client.resolve("example.com", "A")
    assert answers == ["93.184.216.34"]

    # resolve_full() returns full DNS response structure
    result = await mock_client.resolve_full("example.com", "A")
    assert isinstance(result, DNSResult)
    assert len(result.response.answers) == 1

    # NXDOMAIN hosts return empty responses (not errors)
    answers = await mock_client.resolve("notfound.example.com", "A")
    assert len(answers) == 0

    # resolve_batch() works with all mocked hosts
    async for host, rdtype, answers in mock_client.resolve_batch(["example.com"], "A"):
        print(f"{host}: {answers}")  # ["93.184.216.34"]

    # resolve_multi() resolves multiple record types in parallel
    results = await mock_client.resolve_multi("example.com", ["A", "AAAA", "MX"])
    assert len(results) == 3
    assert results["MX"] == ["10 aspmx.l.google.com.", "20 alt1.aspmx.l.google.com."]

Key Features:

• Supports all Client methods: resolve, resolve_full, resolve_batch, resolve_batch_full, resolve_multi, resolve_multi_full
• Returns the same data structures as Client for drop-in compatibility
• NXDOMAIN hosts (specified in _NXDOMAIN list) return empty responses, not errors
• Unmocked hosts also return empty responses
• Auto-formats PTR queries (IP addresses → reverse DNS format) just like the real client

Response Models

The *_full() methods return Pydantic V2 models for type safety and IDE autocomplete:

• DNSResult: Successful DNS response with host and response fields
• DNSError: Failed DNS lookup with an error field
• Response: DNS message with header, queries, answers, name_servers, etc.

The base methods (resolve, resolve_batch, resolve_multi) return simple Python types (lists, dicts, strings) for convenience when you don't need the full response structure.

ClientConfig exposes the knobs shown above (threads_per_resolver, request_timeout_ms, max_retries, purgatory_threshold, purgatory_sentence_ms) and validates them before handing them to the Rust core.

Architecture

BlastDNS is built on top of hickory-dns, but only makes use of the low-level Client API, not the Resolver API.

Beneath the hood of the BlastDNSClient, each resolver gets its own ResolverWorker tasks, with a configurable number of workers per resolver (default: 2, configurable via BlastDNSConfig.threads_per_resolver).

When a user calls BlastDNSClient::resolve, a new WorkItem is created which contains the request (host + rdtype) and a oneshot channel to hold the result. This WorkItem is put into a crossfire MPMC queue, to be picked up by the first available ResolverWorker. Workers are spawned lazily when the first request is made.

Caching

BlastDNS includes an optional TTL-aware cache using an LRU eviction policy. The cache is enabled by default with a capacity of 10,000 entries and can be configured or disabled entirely:

• Only positive responses with answers are cached (no errors, NXDOMAIN, or empty responses)
• Cache entries automatically expire based on DNS record TTLs (clamped to configurable min/max bounds)
• Expired entries are removed on access
• Thread-safe with minimal lock contention

Configure via BlastDNSConfig:

• cache_capacity: Number of entries (default: 10000, set to 0 to disable)
• cache_min_ttl: Minimum TTL (default: 10 seconds)
• cache_max_ttl: Maximum TTL (default: 1 day)

Retry Logic and Fault Tolerance

BlastDNS handles unreliable resolvers through a multi-layered retry system:

Client-Level Retries: When a query fails with a retryable error (network timeouts, connection failures), the client automatically retries up to max_retries times (default: 10). Each retry creates a fresh WorkItem and sends it back to the shared queue, where it can be picked up by any available worker—not necessarily the same resolver. This means retries naturally route around problematic resolvers.

Purgatory System: Each worker tracks consecutive errors. After hitting purgatory_threshold failures (default: 10), the worker enters "purgatory"—it sleeps for purgatory_sentence milliseconds (default: 1000ms) before resuming work. This temporarily sidelines struggling resolvers without removing them entirely, allowing the system to self-heal if resolver issues are transient.

Non-Retryable Errors: Configuration errors (invalid hostnames) and system errors (queue closed) fail immediately without retry, preventing wasted work on queries that can't succeed.

This architecture ensures maximum accuracy even with a mixed pool of reliable and unreliable DNS servers, as queries naturally migrate toward responsive resolvers while problematic ones throttle themselves.

Testing

BlastDNS has two types of tests:

Unit Tests (No DNS Server Required)

Unit tests use MockBlastDNSClient (Rust) or MockClient (Python) and run without any external dependencies:

# Rust unit tests
cargo test

# Python unit tests
uv run pytest

Integration Tests (Require DNS Server)

Integration tests verify real DNS resolution against a local dnsmasq server running on 127.0.0.1:5353 and [::1]:5353.

Install dnsmasq:

sudo apt install dnsmasq

Start the test DNS server:

sudo ./scripts/start-test-dns.sh

Run integration tests:

# Rust integration tests (marked with #[ignore])
cargo test -- --ignored

# Python integration tests with real DNS
uv run pytest -k "not mock"

When done, stop the test DNS server:

./scripts/stop-test-dns.sh

Linting

Rust

# Run clippy for lints
cargo clippy --all-targets --all-features

# Run rustfmt for formatting
cargo fmt --all

Python

# Run ruff for lints
uv run ruff check --fix

# Run ruff for formatting
uv run ruff format