When you call socket.getaddrinfo, you are essentially asking the operating system to translate a hostname and service into a list of address information structures. This function is not just a simple hostname resolver; it is a powerful abstraction that handles a wide variety of cases, including IPv4, IPv6, and even service name resolution.
Behind the scenes, getaddrinfo will query the system’s name resolver libraries to interpret the parameters you pass in. This usually involves consulting the DNS servers configured on your machine, but it can also look up entries from local configuration files like /etc/hosts. It is a layered process that aims to be both robust and versatile.
The result you get back from getaddrinfo is a list of tuples, each describing one possible way to connect to the requested host and service. Each tuple contains the address family (AF_INET or AF_INET6), socket type (SOCK_STREAM, SOCK_DGRAM, etc.), protocol, canonical name, and the actual socket address – typically a tuple of hostname and port.
One subtlety that’s often overlooked is that getaddrinfo does its best to return results in the order of the system’s preferences and reachability considerations. For example, if your system prefers IPv6 over IPv4 and both are available for the host requested, you’ll probably see the IPv6 addresses first. This ordering could affect how your application handles connection attempts.
Consider this example to see how getaddrinfo works:
import socket
addrinfos = socket.getaddrinfo('example.com', 'http')
for family, socktype, proto, canonname, sockaddr in addrinfos:
print(f"Family: {family}, Type: {socktype}, Protocol: {proto}, Address: {sockaddr}")
Here, specifying ‘http’ as the service name automatically maps to port 80. That’s because getaddrinfo understands service names from the system’s service database, typically /etc/services. You can also pass numerical port values as strings or integers, and the function will handle the conversion seamlessly.
The flexibility of getaddrinfo is what allows programmers to write network code that is agnostic of IP versions and transport protocols. You don’t need to hard-code assumptions about IPv4 addresses or TCP sockets; instead, you parse the returned info and use the parameters that match your needs.
However, this flexibility introduces complexity. The function’s numerous parameters determine not only the addresses returned but also the order and filtering of results. To control this, you need to understand each parameter’s role, which brings us to the design principles underlying these parameters.
Anker USB C Hub, 7-in-1 Multi-Port USB Adapter for Laptops, 4K@60Hz USB C to HDMI Splitter, 85W Max Power Delivery, 3xUSBA & C 3.0 Data Ports, SD/TF Card, for Type C Devices (Charger Not Included)
$25.99 (as of December 13, 2025 16:09 GMT +00:00 - More infoProduct prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on [relevant Amazon Site(s), as applicable] at the time of purchase will apply to the purchase of this product.)Design principles behind socket.getaddrinfo parameters
The family parameter lets you specify the desired address family—commonly AF_INET for IPv4 or AF_INET6 for IPv6. Using AF_UNSPEC signals that you are open to any family. This design explicitly separates concerns: by specifying the family, you guide getaddrinfo to filter the results, avoiding the need to filter them manually later.
Similarly, the socktype parameter filters by the communication semantics you intend: SOCK_STREAM for reliable byte streams (TCP), SOCK_DGRAM for unreliable datagrams (UDP), or even SOCK_RAW if you need raw access. This selective filtering means getaddrinfo can directly answer the question: “Give me addresses suitable for TCP, please,” without additional logic in your code.
The protocol parameter refines this further when the distinction matters. Often, it’s set to zero, letting the system choose the default protocol for the given socket type. However, specifying it explicitly is important when multiple protocols might share the same socket semantics.
One of the subtler design choices is the flags parameter, which modifies the behavior rather than narrowing the result set. For example, AI_PASSIVE tells the function to prepare addresses suitable for binding a server socket, often returning wildcard addresses (e.g., 0.0.0.0 for IPv4). This clarifies the intent behind the request—are you looking to connect or to bind?
Flags like AI_CANONNAME instruct getaddrinfo to perform additional work, such as retrieving the canonical name of the host. This separates basic resolution from more expensive queries, allowing you to optimize for performance or completeness.
The separation between input parameters that filter (family, socktype, protocol) and those that alter behavior (flags) is designed to make the API predictable and composable. Each parameter has a single, well-defined role, which will allow you to combine them to express precisely what you want.
Here’s an example that demonstrates the use of these parameters to query IPv6 TCP addresses for the HTTPS service:
import socket
addrinfos = socket.getaddrinfo(
"example.com",
"https",
family=socket.AF_INET6,
socktype=socket.SOCK_STREAM,
proto=0,
flags=socket.AI_CANONNAME,
)
for family, socktype, proto, canonname, sockaddr in addrinfos:
print(f"Family: {family}, Type: {socktype}, Protocol: {proto}")
print(f"Canonical name: {canonname}")
print(f"Socket address: {sockaddr}n")
By explicitly requesting AF_INET6 and SOCK_STREAM, the results are already filtered for IPv6 TCP sockets. Adding AI_CANONNAME retrieves the canonical hostname, which can be useful for logging or further DNS validation.
Another key principle is that all parameters are optional, with sensible defaults. If you omit the family, it indicates no preference; if you omit socktype or protocol, their defaults let the system supply all matching entries. This design supports progressive disclosure—start broad, then narrow as your application demands become clearer.
Because getaddrinfo integrates name and service resolution, it avoids duplication of logic commonly found when you separately resolve hostnames and map services to ports. This reduces bugs and makes your network code easier to maintain.
