pppd: Fix enforcing peer IP address (#235)

pppd: Fix enforcing peer IP address (#235)

If peer address is specified and ipcp-accept-remote is not set then peer
address is enforced.

But there is bug in pppd which allows peer to not use supplied address when
it reply with empty IPCP ConfReq. In this case pppd thinks that peer
accepted its idea of remote/peer address even it is not truth.

This issue can be reproduced by running pppd with arguments:

    ./pppd debug local noauth nolock nodetach asyncmap 0 default-asyncmap novj noaccomp nopcomp nodeflate nobsdcomp nomagic noipv6 noipdefault :10.1.0.1 pty "./pppd debug local noauth nolock nodetach asyncmap 0 default-asyncmap novj noaccomp nopcomp nodeflate nobsdcomp nomagic noipv6 nosendip nodefaultroute 10.0.0.1:10.0.0.2 notty"

Which means that first pppd force usage of address 10.1.0.1 for peer and
second pppd (peer) wants to use only address 10.0.0.1 for itself.

First pppd see this communication

    rcvd [IPCP ConfReq id=0x64 <addr 10.0.0.1>]
    sent [IPCP ConfNak id=0x64 <addr 10.1.0.1>]
    rcvd [IPCP ConfReq id=0x65]
    sent [IPCP ConfAck id=0x65]
    local  IP address 10.0.0.2
    remote IP address 10.1.0.1

and thinks that peer (second pppd) accepted its idea of remote/peer
address.

After applying this patch first pppd correctly detects that peer refused
its proposed peer address and therefore close connection.

    rcvd [IPCP ConfReq id=0x64 <addr 10.0.0.1>]
    sent [IPCP ConfNak id=0x64 <addr 10.1.0.1>]
    rcvd [IPCP ConfReq id=0x65]
    sent [IPCP ConfAck id=0x65]
    Peer refused to agree to his IP address
    Connect time 0.0 minutes.
    Sent 1024 bytes, received 1018 bytes.
    sent [IPCP TermReq id=0x3 "Refused his IP address"]

Signed-off-by: Pali Rohár <pali@kernel.org>