This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
In a small file with 25 ftp: links the link checker found one serious error (404) and 18 bogeys (400) apparently caused by its HTTP HEAD check method. In other words, the ftp: check worked only for 7 files.
Yes, there seems to be 2 bugs related to this: validator.w3.org seems to need passive FTP, that's why it produces the "FTP error code 425" bogeys. Passive is now the default in the CVS version of checklink (and while I was at it, also the validator). So, the online version should be corrected when 0.6.2 is released. But this exposes another bug in libwww-perl's LWP::Protocol::ftp module; it tries to close an opened FTP data connection before/without reading anything when the HEAD method is used. I've posted a patch upstream that should fix this, no responses so far. A message with the subject "[PATCH (revised)] FTP HEAD: ABOR needed" should appear in <http://archive.develooper.com/libwww@perl.org/> soonish. Leaving open so I remember to make the necessary changes when/if the patch is applied upstream.
Some more info: the libwww-perl author reports that he has applied my patch; it should be included in the next libwww-perl release. Still leaving open, pending for new upstream release so I can document it.
My patch is applied in libwww-perl 5.70 and later, and the documentation recommends that or later. But for various other reasons, we cannot upgrade the libwww-perl version on validator.w3.org yet, so that particular instance will still have this issue. So, kind of fixed, and probably the online version will also be updated when the next version (5.77?) of libwww-perl is out.