ldattach

ldattach [options] discipline device

-d, --detach
    Detach line discipline from device

-h, --help
    Display help and exit

-L, --list
    List available line disciplines

-n, --nohangup
    Prevent detachment on TTY hangup

-s speed, --speed=speed
    Set serial line speed (e.g., 38400)

-V, --version
    Show version information

-y, --yes
    Detach without prompting

ldattach is a utility from the util-linux package that attaches or detaches a TTY line discipline (ldisc) to a serial device, enabling protocols like SLIP or PPP over serial lines. Line disciplines are kernel modules that alter how data is processed on a TTY device, transforming it into a network interface.

Typically run as root, ldattach loads the required kernel module (e.g., slip or cslip) if not present, sets the line discipline on the specified tty (e.g., /dev/ttyS0), and optionally configures the baud rate. After attachment, the interface (e.g., sl0) can be configured with ifconfig or ip for IP networking.

Common use cases include point-to-point serial links for embedded systems or legacy dial-up. By default, the discipline detaches on TTY hangup (e.g., modem disconnect), but this can be disabled. Debugging output is available in some versions.

Modern networking favors Ethernet or PPP daemons like pppd, making ldattach largely legacy, but it's still useful for raw serial protocols or HDLC. Always verify kernel support via -L option.

Requires root privileges.
Legacy tool; prefer pppd(8) for PPP.
May fail if device busy or module unavailable.
Detaches on hangup by default; disrupts normal serial use.
Kernel modules (e.g., slip.ko) must be available.

slip (SLIP), cslip (CSLIP), ppp (async PPP), ppp_sync (sync PPP), n_hdlc (Cisco HDLC)

ldattach -s 38400 slip /dev/ttyS0
ifconfig sl0 192.168.1.1 pointopoint 192.168.1.2 up
To detach: ldattach -d slip /dev/ttyS0

Introduced in early Linux (1990s) for SLIP serial networking. Integrated into util-linux-ng package. Declined in use post-2000s with broadband rise; maintained for compatibility.

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