Subject: PPPd's selection of interfaces
To: None <,>
From: Lucio de Re <>
List: tech-net
Date: 09/21/1998 20:15:28
'Scuse the multiple posting, I'm not sure which list is more 

It took me a while to figure what had seemingly gone wrong with NAT and 
filtering on a client site, although it had been staring me in the 
face: on startup two "pppd" instances had somehow reversed positions 
and the nett result was that a site intended to be within the network 
found strict filtering rules applied to it, while the rest of the 
Internet was granted the type of access I permitted such sites as are 
intended to be "within" the network.

Sad and hard to spot, off the cuff.  Even harder to fix remotely, as I 
had to sever my connection to alter the situation (I then had the 
second instance of pppd started only after the first connection had 
been fully established).

There are a few possibilities here: the more logical one seems to me 
for pppd to grab the next available interface as early as possible in 
its operation so that one has a deterministic interface assignment.
Perhaps even better might be to specify the interface in the options, 
something too obvious to have been overlooked: am I not reading the MAN 
pages properly?

Alternatively, it may be acceptable for pppd to drop into background 
(on demand, if necessary) only once connection is established.  

This last approach is too fraught with problems (such as delaying 
system start up excessively) to be recommendable, but may be 
permissible under certain conditions.

If some work needs to be done to the pppd code, I'll happily do it, but 
I'd like to find out from users which approach makes the most practical