I think this problem should be fixed in squid-2.5.9nb10 package, but
please note:

- squid-2.5.9nb10 simply back out applying  squid-2.5.STABLE9's
  transparent_port.patch.

- With squid and IP Filter 4.1 (and lator) combination, transparent
  proxy using NAT device has been never working; squid dosen't handle
  IP Filter 4.1's NAT related ioctl(2) properly yet.

- NetBSD 2.0 and 3.0 release branch were missing <netnet/ipl.h> header
  file and NAT related ioctl(2) isn't usable other than base system
  commands, ipnat(8).  NetBSD current solved this problem and pullup
  request has processed 3.0 branch, not yet 2.0 branch.

- This problem dosen't exist with older IP Filter; NetBSD 1.6 branch
  dosen't have this problem.

After NetBSD 2.0 branch has <netinet/ipl.h> header, I'll commit real
fix for this problem (and report this bug to squid-bugs, too).

Best regards.

-- 
Takahiro Kambe <taca@back-street.net>