ldattach

ldattach [OPTIONS]

    The line discipline to attach. This can be specified as a numeric ID or a symbolic name (e.g., N_PPP, N_SLIP, N_TTY).


    The path to the serial device or pseudo-terminal (e.g., /dev/ttyS0, /dev/pts/1).

-h, --help
    Display a help message and exit.

-V, --version
    Display version information and exit.

ldattach is a Linux command-line utility used to attach a specified line discipline to a serial port or pseudo-terminal device. Line disciplines are kernel modules that modify the way data is processed between a user application and a terminal device driver. They define how data is buffered, interpreted, and handled, allowing for specialized communication protocols over serial lines.

Common line disciplines include N_PPP (for Point-to-Point Protocol), N_SLIP (for Serial Line Internet Protocol), and N_TTY (the default terminal line discipline). By attaching a specific line discipline, ldattach prepares the serial device for use by applications that expect that particular protocol. For instance, attaching N_PPP to a serial port before running a PPP daemon allows the kernel to handle the low-level PPP framing.

This command is crucial for setting up various serial-based network connections or for interacting with specialized hardware connected via serial ports. It typically requires superuser (root) privileges to modify the line discipline of a device.

Attaching a line discipline requires root privileges. The specified line discipline must be available and loaded in the kernel. Attaching the wrong line discipline can make the device unusable until reset. Only one line discipline can be active on a device at a time. Be cautious when using this command on devices currently in use.

In the Linux kernel, a line discipline is a layer of code that sits between the character device driver (e.g., for a serial port) and the higher-level applications. It processes data flow, handles specific protocols, and manages terminal control sequences. For example, the N_TTY line discipline handles character echoing, line editing, and signal generation for standard interactive terminals, while N_PPP encapsulates and decapsulates PPP frames over a serial link.

ldattach is primarily used for:

• Setting up PPP (Point-to-Point Protocol) connections over a modem or serial cable, typically in conjunction with the pppd daemon.
• Configuring SLIP (Serial Line Internet Protocol) connections.
• Enabling specific kernel-level protocol handling for specialized hardware connected via serial, such as IrDA (Infrared Data Association) devices using the N_IRDA line discipline.
• Debugging or testing scenarios where direct manipulation of a serial device's line discipline is required.

ldattach is part of the widely used util-linux project, a collection of essential Linux utilities. Its functionality has been fundamental in Unix-like systems for managing serial communications, adapting over time with new kernel line discipline implementations. It provides a direct interface to the TIOCSETD or TIOCSETD ioctls, which have long been part of the Unix terminal interface.

stty(1), pppd(8), slattach(8), agetty(8), setserial(8)