PPPd

pppd is the PPP daemon used on many Unix-like systems to establish and manage Point-to-Point Protocol connections. It implements the PPP protocol, negotiates link parameters with the peer, handles optional authentication, and configures the local network stack for the PPP session, including IPv4 or IPv6 addresses and routes.

Typical usage includes dialing out to an ISP over a telephone line or creating a PPP link over a serial interface, as well as PPP over Ethernet (PPPoE) connections using the rp-pppoe plugin. The daemon can also manage persistent sessions and dial-on-demand connections.

Configuration is primarily done through text files. Global defaults reside in /etc/ppp/options, while per-connection settings are placed in /etc/ppp/peers/ or invoked directly from scripts. Credentials for PAP/CHAP authentication are stored in /etc/ppp/pap-secrets and /etc/ppp/chap-secrets. A connect script or the chat program is used to initialize the physical link, e.g., dialing a modem and answering calls.

During connection, LCP negotiates options, and IPCP configures IPv4 (and IPv6CP for IPv6). The daemon creates a PPP network interface (ppp0, ppp1, etc.) and can configure a default route and DNS servers (via usepeerdns). It supports various options such as defaultroute, mtu, mrru, persist, and maxfail, and it supports plugin modules for extended features.

pppd is commonly integrated with network management tools and is used in both legacy dial-up environments and modern broadband or VPN scenarios. It remains a core component of PPP software stacks on many UNIX-like systems.