There are some lines in basesrc/usr.sbin/pppd/pppd/Makefile version 1.27:
CPPFLAGS+= -DCBCP_SUPPORT -DCHAPMS -DUSE_CRYPT -DMSLANMAN
CPPFLAGS+= -DINET6

The -DINET6 overrides USE_INET6 in /etc/mk.conf (I don't understand
why one would want this), and results in:

cc   -o pppd -nostdlib -L/usr/local/safeplace/usr/lib -R/usr/local/safeplace/usr
/lib /usr/local/safeplace/usr/lib/crt0.o /usr/local/safeplace/usr/lib/crtbegin.o
 auth.o cbcp.o ccp.o chap.o chap_ms.o demand.o eui64.o fsm.o ipcp.o ipv6cp.o ipx
cp.o lcp.o magic.o main.o options.o sys-bsd.o upap.o utils.o -lpcap -lcrypt -lut
il -lgcc -lc -lgcc /usr/local/safeplace/usr/lib/crtend.o
ipv6cp.o: In function `ipv6_demand_conf':
ipv6cp.o(.text+0x9cb): undefined reference to `eui64_ntoa'
ipv6cp.o(.text+0x9fe): undefined reference to `eui64_ntoa'
ipv6cp.o: In function `ipv6cp_up':
ipv6cp.o(.text+0xb24): undefined reference to `eui64_ntoa'
ipv6cp.o(.text+0xb56): undefined reference to `eui64_ntoa'
ipv6cp.o(.text+0xbdc): undefined reference to `eui64_ntoa'
ipv6cp.o(.text+0xc1e): more undefined references to `eui64_ntoa' follow
collect2: ld returned 1 exit status
*** Error code 1

Stop.

Building with the CPPFLAGS+= -DINET6 works OK.  I think
the Makefile should be updated to check for USE_INET6.

Is it common practice to define INET6 etc. unconditionally?

	Sean.