Editing Point-to-Point Protocol daemon (section)

{{Infobox Software
| name                   = '''Paul's PPP'''
| logo                   = 
| screenshot             = 
| caption                = 
| author                 = Paul Mackerras, based on earlier work by Drew Perkins, Brad Clements, Karl Fox, Greg Christy, and Brad Parker
| developer              = 
| released               = 
| latest release version = 2.5.0
| latest release date    = {{Release date and age|2023|04|04}}
| latest preview version = 
| latest preview date    = 
| operating system       = [[Linux]], [[Solaris (operating system)|Solaris]]
| platform               = 
| language               = 
| genre                  = [[Internet]]
| license                = [[BSD]]-like licenses. Not all of them apply to all parts of pppd
| website                = {{URL|https://ppp.samba.org}}
| repo                   = {{URL|https://github.com/ppp-project/ppp}}
}}

'''Point-to-Point Protocol daemon''' '''(PPPD)''' is the [[daemon (computing)|daemon]] that implements [[Point-to-Point Protocol]] (PPP). PPP is used to manage network connections between two nodes on [[Unix-like operating system]]s. It is configured using [[command-line argument]]s and [[configuration file]]s.

While it has initially been used to manage only [[dial-up Internet access|dial-up access]], it is also used to manage [[broadband]] connections such as [[Digital subscriber line|DSL]], if [[Point-to-Point Protocol over Ethernet]] (PPPoE) or [[Point-to-Point Protocol over ATM]] (PPPoA) is used.

The role of pppd is managing PPP session establishment and session termination. 
During session establishment, pppd has the role of:
* Looped link detection: PPP detects looped links using [[Magic number (programming)|magic numbers]]. When PPPD sends PPP LCP messages, these messages include a magic number. If a line is looped, the node receives an LCP message with its own magic number, instead of getting a message with the peer's magic number.
*Automatic self configuration: Using [[Link Control Protocol]] it has to negotiate protocol features like Address-and-Control-Field-Compression (ACFC), escaped characters, and the compression, encryption (like [[Microsoft Point-to-Point Encryption|MPPE]]) and authentication methods to use.
*Access control and authentication: Using protocols like [[Challenge-handshake authentication protocol]] (CHAP) or [[Password authentication protocol]] (PAP) it has to provide and check authentication data.
*Layer 3 configuration: If using [[Internet Protocol Control Protocol]] (IPCP), it will negotiate or determine [[Internet Protocol|IP]] parameters such as the [[IP address]]es, the [[maximum transmission unit]], and [[name server]] addresses. Some versions may also support [[Internetwork Packet Exchange Control Protocol]] (IPXCP) and [[AppleTalk Control Protocol]] (ATCP) for routing [[Internetwork Packet Exchange|IPX]] or [[AppleTalk]] over the link.
*After negotiation is complete, it has to set up the required network interfaces and routes, so that the connection is run by the [[Kernel (operating system)|kernel]].

pppd terminates a PPP link when:
* too many frames with invalid [[frame check sequence]] (FCS) field have been received
* the link is considered "idle" (if configured)
* another program or the peer requests link termination.

Some newer versions of pppd are also capable of handling [[Dial-on-demand routing]], where pppd sets up a virtual network, captures the packages it receives and establishes a PPP connection and forwards the captured and not-yet transmitted packages over the link.