python.sync_dns_lookup

Performance Medium

Detects socket.gethostbyname() or socket.getaddrinfo() in async functions.

Why It Matters

DNS lookups are blocking operations:

Hidden latency — DNS resolution can take hundreds of milliseconds
Event loop blocked — No other tasks can run during the lookup
Resolver failures — If DNS resolvers are slow, everything stalls
No timeout control — System DNS has its own timeout, often too long

In async code, blocking DNS destroys the concurrency benefits you’re trying to achieve.

Example

# ❌ Before
async def resolve_host(hostname):
    return socket.gethostbyname(hostname)

This blocks the event loop for the entire DNS resolution.

# ✅ After
async def resolve_host(hostname):
    loop = asyncio.get_event_loop()
    return await loop.getaddrinfo(hostname, None)

getaddrinfo is async-aware and doesn’t block.

What Unfault Detects

socket.gethostbyname() in async functions
socket.gethostbyname_ex() in async functions
socket.getaddrinfo() (without await) in async functions
socket.gethostbyaddr() in async functions

Auto-Fix

Unfault replaces blocking socket calls with async equivalents.

Best Practices

# Use asyncio's built-in resolver
loop = asyncio.get_event_loop()
result = await loop.getaddrinfo(host, port)

# Or use aiohttp which handles this internally
async with aiohttp.ClientSession() as session:
    async with session.get(url) as response:
        ...  # DNS is async

# For complex DNS needs, use aiodns
import aiodns
resolver = aiodns.DNSResolver()
result = await resolver.query(hostname, 'A')