This looks interesting, so I've installed it on my dev machine. A number of points occur to me:

1) Although the default config file specifies:

# initialize fut_drop to default action of log and drop
init_fut_drop   :   1
[download]

... this doesn't cause the creation of the relevant DROP and LOG rules in the fut-drop-log chain.

Changing the initial declaration of $default_drop in the code, to set it to 1 instead of leaving it undef, fixed that:

my $default_drop = 1;   # use our drop or one speced by user

(2) Each invocation of the code (even without the above change) causes an additional RETURN rule to be appended to the fut chain.

(3) Having futzed (no pun intended) about with the ssh pattern in deep_match in the config (to cope with my installation), I've successfully blocked on invalid username login attempts. However, despite the spurn_time being left at default (1800s), the block does not appear to time out.

(4) FUT's logging of its SKIP messages via syslog (including the whole log line that it has just failed to perform a deep_match on), coupled with the quick_match scanning, triggers a loop. The loop continues until the length of the repeatedly prepended SKIP messages exceeds the syslog line length and the string that quick_match originally spotted disappears off the end eg:

(Update: lines of syslog output snipped - it was my syslog.conf at fault here)

Hope this is useful feedback.

Update: This is based on using FUT with Perl 5.8.1 on an old RedHat 7.3 system

S.