>> Therefore, I have to tell NetBSD that it should route all packets
>> originating from the IP of ISP #1 through the gateway of ISP #1 and
>> so on (some kind of routing by source address).

I wanted just that (for almost that application).  I wrote a
source-routing pseudo-interface to do this.

It currently exists only for 1.4T, as far as I know.  When I scrape
together the round tuits, I'm going to roll it forward to something
more modern, probably -current.  If you want its present (1.4T) state,
look under ftp.rodents.montreal.qc.ca:/mouse/source-tree/postpatches/ -
specifically, sys/dev/pseudo/if_srt.[ch] for the pseudo-device driver
and usr.sbin/srtconfig/* for the configuration utility.

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