SPPP(4) BSD Kernel Interfaces Manual SPPP(4)


sppp — point to point protocol network layer for synchronous lines


device sppp


The sppp network layer implements the state machine and the Link Control Protocol (LCP) of the point to point protocol (PPP) as described in RFC 1661. Note that this layer does not provide network interfaces of its own, it is rather intended to be layered on top of drivers providing a synchronous point-to-point connection that wish to run a PPP stack over it. The corresponding network interfaces have to be provided by these hardware drivers.

The sppp layer provides three basic modes of operation. The default mode, with no special flags to be set, is to create the PPP connection (administrative Open event to the LCP layer) as soon as the interface is taken up with the ifconfig(8) command. Taking the interface down again will terminate the LCP layer and thus all other layers on top. The link will also terminate itself as soon as no Network Control Protocol (NCP) is open anymore, indicating that the lower layers are no longer needed.

Setting the link-level flag link0 with ifconfig(8) will cause the respective network interface to go into passive mode. This means, the administrative Open event to the LCP layer will be delayed until after the lower layers signals an Up event (rise of ’’carrier’’). This can be used by lower layers to support a dialin connection where the physical layer is not available immediately at startup, but only after some external event arrives. Receipt of a Down event from the lower layer will not take the interface completely down in this case.

Finally, setting the flag link1 will cause the interface to operate in dial-on-demand mode. This is also only useful if the lower layer supports the notion of a carrier. Upon configuring the respective interface, it will delay the administrative Open event to the LCP layer until either an outbound network packet arrives, or until the lower layer signals an Up event, indicating an inbound connection. As with passive mode, receipt of a Down event (loss of carrier) will not automatically take the interface down, thus it remains available for further connections.

The sppp layer supports the debug interface flag that can be set with ifconfig(8). If this flag is set, the various control protocol packets being exchanged as well as the option negotiation between both ends of the link will be logged at level LOG_DEBUG. This can be helpful to examine configuration problems during the first attempts to set up a new configuration. Without this flag being set, only the major phase transitions will be logged at level LOG_INFO.

It is possible to leave the local interface IP address open for negotiation by setting it to This requires that the remote peer can correctly supply a value for it based on the identity of the caller, or on the remote address supplied by this side. Due to the way the IPCP option negotiation works, this address is being supplied late during the negotiation, which might cause the remote peer to make wrong assumptions.

In a similar spirit the remote address can be set to the magical value 0.0.0.* which means that we do not care what address the remote side will use, as long as it is not This is useful if your ISP has several dial-in servers. You can of course route add something_or_other 0.0.0.* and it will do exactly what you would want it to.

The PAP and CHAP authentication protocols as described in RFC 1334, and RFC 1994 resp., are also implemented. Their parameters are being controlled by the spppcontrol(8) utility.

VJ header compression is implemented, and enabled by default. It can be disabled using spppcontrol(8).


<ifname><ifnum>: <proto> illegal <event> in state <statename>  An event happened that should not happen for the current state the respective control protocol is in. See RFC 1661 for a description of the state automaton.

<ifname><ifnum>: loopback  The state automaton detected a line loopback (that is, it was talking with itself). The interface will be temporarily disabled.

<ifname><ifnum>: up  The LCP layer is running again, after a line loopback had previously been detected.

<ifname><ifnum>: down  The keepalive facility detected the line being unresponsive. Keepalive must be explicitly requested by the lower layers in order to take place.


inet(4), intro(4), ifconfig(8), spppcontrol(8)

W. Simpson, Editor


The Point-to-Point Protocol (PPP) ,
RFC 1661 .

G. McGregor


The PPP Internet Protocol Control Protocol (IPCP) ,
RFC 1332 .

B. Lloyd


W. Simpson ,
PPP Authentication Protocols
RFC 1334 .

W. Simpson


PPP Challenge Handshake Authentication Protocol (CHAP) ,
RFC 1994 .


The original implementation of sppp was written in 1994 at Cronyx Ltd., Moscow by Serge Vakulenko <vak [AT]>. Jörg Wunsch <joerg_wunsch [AT]> rewrote a large part in 1997 in order to fully implement the state machine as described in RFC 1661, so it could also be used for dialup lines. He also wrote this man page. Serge later on wrote a basic implementation for PAP and CHAP, which served as the base for the current implementation, done again by Jörg Wunsch.



Currently, only the IPCP control protocol and ip(4) network protocol is supported. More NCPs should be implemented, as well as other control protocols for authentication and link quality reporting.

Negotiation loop avoidance is not fully implemented. If the negotiation does not converge, this can cause an endless loop.

The various parameters that should be adjustable per RFC 1661 are currently hard-coded into the kernel, and should be made accessible through spppcontrol(8).

Passive mode has not been tested extensively.

Link-level compression protocols should be supported.

BSD May 25, 2008 BSD