[stunnel-users] Stunnel 5.44 server side 'exec = pppd' runs second child 'pppd' process after reconnection. Bug?

[Eric Eberhard]

Sounds good.  The inetd will handle the dropping of connections just fine
... but the below sounds like it would help even more.  E

-----Original Message-----
From: stunnel-users [mailto:stunnel-users-bounces at stunnel.org] On Behalf Of
Peter Pentchev
Sent: Monday, May 13, 2019 2:16 PM
To: Martin Got <martingot at protonmail.com>
Cc: stunnel-users at stunnel.org
Subject: Re: [stunnel-users] Stunnel 5.44 server side 'exec = pppd' runs
second child 'pppd' process after reconnection. Bug?

On Mon, May 13, 2019 at 04:07:57PM +0000, Martin Got wrote:
> I'm trying to make stunnel wrapped ppp connection to achieve bidirectional
data transfer over stunnel like shown below.
> 
> Stunnel client   --connect-->    Stunnel server
> pppd client       --connect-->    pppd server
> 10.0.1.2             <--data-->     10.0.1.1
> 
> OpenBSD 6.4amd64 with Stunnel 5.44 server works till stunnel retries 'exec
= pppd' section once stunnel client is reconnected. Also 'exec = pppd'
section retries after a short network related communication lag also.
Previous 'pppd' child instance haven't killed by stunnel 5.44 before new
instance started.
> 
> So second 'pppd' process started and runs simultaneously with the first
'pppd' and link down. Restarting Stunnel server can clear child 'pppd'
processes. So newly reestablished 'pppd' link between 10.0.1.1 <--> 10.0.1.2
endpoints works till next interconnection.
> 
> Does stunnel server have an option to start only one instance in 'exec'
section or what should be done to fix this?

Unfortunately, stunnel was not designed for your use case; it was designed
to allow many different clients to make use of different instances of the
same server program, one instance per client.

You might try to do something else: if you really only have a single PPP
client and you can really guarantee that anything that connects to stunnel
must reuse the same connection, you could write a separate program that
listens on a TCP port or on a local Unix socket, waits for the first
connection, starts an instance of pppd, and then, from that point on, sends
all the traffic from this instance to all the connections that it has
received, even if a new connection appears before the old one has been torn
down. Then you can point stunnel to the local Unix socket of that service,
and anything that connects to stunnel will establish a new connection to
your service, but it will not matter, since your service will be written in
such a way as to send all the traffic to all the connections. So the service
should do something like:

1. Create a local Unix socket and start listening on it.

2. Wait for the first connection.

3. Start an instance of pppd (at most one instance shall be running at
   any point in time)

4. Keep waiting for new connections; accept any and treat them all in
   the same way.

5. If any data comes from a remote connection, send it to pppd.

6. If any data comes from pppd, send it to *all* the remote connections.

7. If any of the remote connections is closed, well, so be it, good
   riddance, another one will come along presently (or maybe another one
   has already come along and has started talking to pppd through this
   service).

8. If the pppd child process dies, close all the remote connections and
   go back to step 2.

Then you shall only have a problem if there are two "real" connections from
different places at the same time - but if I understand your setup
correctly, this should not really happen, since it is only you (your laptop
or something like that) that connects to the PPP server.

If you want, I could try to whip up some simple implementation of such a
pseudo-multiplexer.

G'luck,
Peter

--
Peter Pentchev  roam@{ringlet.net,debian.org,FreeBSD.org} pp at storpool.com
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