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

Peter Pentchev roam at ringlet.net
Mon May 13 23:16:01 CEST 2019

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
>             <--data-->
> 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 <--> 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

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.


Peter Pentchev  roam@{ringlet.net,debian.org,FreeBSD.org} pp at storpool.com
PGP key:        http://people.FreeBSD.org/~roam/roam.key.asc
Key fingerprint 2EE7 A7A5 17FC 124C F115  C354 651E EFB0 2527 DF13
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