> I'm trying to make arns. I get many errors to compile :-(.

I don't know what arns is, but...

> >cc -c arns.c
[buncha warnings deleted]
> >cc -c pf.c
> >cc -O  -c ddp.c
[more buncha warnings deleted]
> >cc -O  -c aarp.c
> >cc -o arns arns.o pf.o ddp.o aarp.o
> >arns.o: Undefined symbol `_crypt' referenced from text segment
                             ^^^^^^
> >*** Error code 1
> >
> >Stop.

Looks like you need to add -lcrypt to the linking option like before.

ken