Mdns Discovery

mDNS Discovery

Discover services and devices on a local network using mDNS (Multicast DNS) / Zeroconf. Zero configuration.

Python Library

pip install zeroconf

Publishing a Service

from zeroconf import Zeroconf, ServiceInfo
import socket

zeroconf = Zeroconf()

info = ServiceInfo(
    type_="_myservice._tcp.local.",
    name="MyDevice._myservice._tcp.local.",
    addresses=[socket.inet_aton("192.168.1.100")],
    port=8000,
    properties={"version": "1", "name": "My Device"},
)

zeroconf.register_service(info)

Browsing for Services

from zeroconf import Zeroconf, ServiceBrowser, ServiceStateChange

def on_change(zeroconf, service_type, name, state_change):
    if state_change == ServiceStateChange.Added:
        info = zeroconf.get_service_info(service_type, name)
        if info:
            device_id = info.properties.get(b"device_id", b"").decode()
            host = socket.inet_ntoa(info.addresses[0])
            port = info.port
            print(f"Found: {name} @ {host}:{port}")

zeroconf = Zeroconf()
browser = ServiceBrowser(zeroconf, "_myservice._tcp.local.", [on_change])

TXT Record Properties

Attach metadata to your service advertisement:

properties={
    "device_id": "uuid-here",
    "device_name": "My Laptop",
    "device_type": "windows",
}

Retrieve on the browsing side:

val = info.properties.get(b"device_id", b"").decode()

Key Patterns

Local IP Helper

def get_local_ip() -> str:
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    try:
        s.connect(("8.8.8.8", 80))
        return s.getsockname()[0]
    finally:
        s.close()

Service Name Convention

Services use the format _<name>._tcp.local.:

• _instanttransfer._tcp.local.
• _http._tcp.local.
• _ssh._tcp.local.

Pitfalls

• Same network required — mDNS does not route across subnets
• Firewall — Windows Firewall may block mDNS (UDP 5353)
• VPNs — may interfere with mDNS discovery
• Properties are bytes — keys/values must be under 255 bytes each