Socket Families

This article clarifies some aspects of sockets in Python, a topic that can be confusing, especially for advanced use cases. Let’s start with the basics. A socket is an endpoint for two-way communication. Network sockets are defined by three parameters:

1. IP Address
2. Port
3. Protocol

Servers typically bind a socket to a port and listen for incoming connections. Clients then connect to the server’s hostname:port to initiate communication.

Python’s socket class provides access to these low-level features, but its usage can be initially confusing.

The Python socket class mirrors the Linux socket class. You create a new socket using socket.socket(), which accepts three main parameters: socket family, socket type, and socket protocol.

Possible values for these parameters are defined as constants in the class, categorized as AF_XXXX (address families) and PF_XXXX (protocol families). Here’s a quick overview of address families:

• AF_INET, AF_INET6, AF_UNIX: Used for communication over TCP or UDP.
• AF_PACKET: Used for accessing raw network card data (e.g., for custom protocol implementation). Requires root privileges or CAP_NET_RAW capabilities and operates at OSI layer 2.

Socket type parameters include:

• SOCK_STREAM: Connection-based protocol (connection persists) that terminates when either party closes it or an error occurs. Typically used with TCP.
• SOCK_DGRAM: Single datagram transmission. A datagram is sent, a response is received, and the connection closes. Typically used with UDP.
• SOCK_RAW: Accesses raw packets, including the link-level header (but not the physical layer header). Operates at a lower level than SOCK_STREAM and SOCK_DGRAM.
• SOCK_RDM, SOCK_SEQPACKET: Less commonly used.

The protocol parameter can usually be omitted for standard TCP or UDP. Otherwise, you can specify the protocol (e.g., IPPROTO_IP).

If you use AF_PACKET, you bind to an interface (e.g., wlan0) rather than an IP address, as you’re operating at OSI layer 2. You also can’t use sendto() (which requires a destination address); use send() instead.

Regarding connect() and bind(), it can be confusing to know when to use each. bind() is usually called on the server side, and connect() on the client side, but this is not mandatory. bind() starts listening on a port, while connect() initiates a connection to the other party. For example, if two clients are randomly sending UDP datagrams to each other, both need to call bind() and then connect() when they want to transmit. In persistent communication scenarios, one side can listen (bind()) and the other connect (connect()), and then reuse that connection.

Socket connections are bidirectional, allowing simultaneous transmission and reception. Advanced options like TTL can be set using setsockopt().

Working with lower-level sockets requires more knowledge and skill than using higher-level libraries. So congratulation to be in the elite team 🙂