Adding --pppd-keepalive option.

[FinboySlick]

In reference to #92 where lcp-echo-interval would still be useful. Figured if I'm going to ask for a feature, I might as well do some of the work.

[DimitriPapadopoulos]

Would we need to consider other LCP parameters? This works with pppd on Linux but I cannot find anything similar in ppp on FreeBSD. Do you have any clue?

[FinboySlick]

Honestly, my initial approach would have been to enable any pppd option via a generic --pppd-opt= type of parameter, but I wasn't familiar enough with the code/intent to take into account the design and security implications (and it was very late).

As such, this patch is just to solve an immediate problem. That being said, I think there would be value in making openfortivpn 'independent' of other config files. Since the only way to do that as far as pppd is concerned is the command line, a '--pppd-opt=' type of solution is worth considering.

[luzik]

Can we move such a keepalive packet into upper layer ? ICMP broadcast into new interface or something similar ? I believe that tunnel just need any packet to keep tunnel alive.

[DimitriPapadopoulos]

I think FortiClient handles dead peer detection that at the PPP level, see for example https://gitlab.com/openconnect/openconnect/-/commit/e1eac267 - but perhaps in upper layers too.

• Technical Tip: How to modify the LCP Echo timer in L2TP VPN