best way to force a single packet to be sent via a specific interface

[Lowell Gilbert]

Aryeh Friedman <aryeh.friedman at gmail.com> writes:

> I have both ethernet (the default route and such) and ppp done via
> "ppp -ddial" the man page for ppp(8) says tun0 will not have an IP
> assigned until a single packet is sent on it... what is the best way
> to have one packet sent on it (ping only always ifaces on multicast,
> traceroute seems to have no way to say exit after the first hop and/or
> after one packet is sent, tcpdump refuses a "-c 0" arg saying it is
> illegal)

I can't parse what you said about ping.  
I seem to recall doing that (with '-I' back when I was doing demand
dialing with ppp(8).