== One consequence of Linux's dynamic network device naming

I knew that Linux did [[dynamic network device naming
NetworkInterfaceNaming]], but one consequence of that only really sunk
in recently.  Dynamic naming is relatively predictable for interfaces
that are always on (especially when [[various UbuntuEthernetNaming]]
[[distributions RHELEthernetNaming]] work hard to make them very
persistent), but where it can really surprise you (and your software)
is when the interfaces are dynamic. Like, say, PPP interfaces.

Just like Ethernet interfaces, the first active PPP connection gets
_ppp0_, the second one gets _ppp1_, and so on. But unlike Ethernets,
PPP connections can routinely come and go, and as a result it may not
be clear to you (or your software) what PPP device is what connection.
Also, it is easier to commit the old mistake of trying to bring up a
connection that's configured as _ppp1_ when you don't have a _ppp0_.

(Scripts run by _pppd_ itself get told the right network device, which
can be both useful and potentially confusing.)

Unfortunately it looks like you cannot rename PPP devices, because it
seems that you can only rename network devices that aren't active and
when a PPP device isn't active, it doesn't exist at all.  So I can't
have _pppdialup0_ and _pppdsl0_ instead of, well, whatever _pppN_ name
they get right now.

One can half-fix this with software; _pppd_ has a concept of a logical
link name, and contrary to what the manual page appears to claim, it
*does* pass this information to the _ip-up_ script as the environment
variable _$LINKNAME_. So your local link-up script can save the mapping
between actual PPP device (the first argument) and the logical link name
somewhere, and then other things can read it.