how to pass parms to rc.custom?

In the supplied rc.custom.gz, there is this (at line 4):

if [ "$IPADDR" != "" ]
then ... do stuff
fi

How would I set IPADDR to something such that the stuff is done?    Is it 
simply a matter of setting it in 1/settings.s?      That's what I think I 
deduce from looking at
2/etc/rc.S
I have been a bit unwilling to put "permanent" settings in 1/settings.s 
since I notice that buildit completely rewrites it (1/settings.s) every time 
I buildit   (why does it do that?     doesn't trust me?   I guess that's 
wise ...)    I suppose I can edit the settings.s on the diskette, which is 
the way I am supposed to do it according to the FAQ.     But I wonder if 
there's some place on the build directory where I can safely store settings 
like this and have them included in every diskette I build without having to 
think about it.

John