diff --git a/Makefile.am b/Makefile.am
index a9d782b..4f0f916 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -80,6 +80,7 @@ handwritten_tests = \
 	t/protocol_dyndns2.pl \
 	t/skip.pl \
 	t/ssl-validate.pl \
+	t/update_nics.pl \
 	t/use_web.pl \
 	t/variable_defaults.pl \
 	t/write_recap.pl
diff --git a/configure.ac b/configure.ac
index 5ca6da7..ab9ef75 100644
--- a/configure.ac
+++ b/configure.ac
@@ -77,6 +77,7 @@ m4_foreach_w([_m], [
     B
     File::Spec::Functions
     File::Temp
+    List::Util
   ], [AX_PROG_PERL_MODULES([_m], [],
          [AC_MSG_WARN([some tests will fail due to missing module _m])])])
 
diff --git a/t/update_nics.pl b/t/update_nics.pl
new file mode 100644
index 0000000..e5d0ee0
--- /dev/null
+++ b/t/update_nics.pl
@@ -0,0 +1,463 @@
+use Test::More;
+use File::Temp;
+use List::Util qw(max);
+eval { require ddclient::Test::Fake::HTTPD; } or plan(skip_all => $@);
+SKIP: { eval { require Test::Warnings; } or skip($@, 1); }
+eval { require 'ddclient'; } or BAIL_OUT($@);
+my $ipv6_supported = eval {
+    require IO::Socket::IP;
+    my $ipv6_socket = IO::Socket::IP->new(
+        Domain => 'PF_INET6',
+        LocalHost => '::1',
+        Listen => 1,
+    );
+    defined($ipv6_socket);
+};
+my $http_daemon_supports_ipv6 = eval {
+    require HTTP::Daemon;
+    HTTP::Daemon->VERSION(6.12);
+};
+
+sub run_httpd {
+    my ($ipv) = @_;
+    return undef if $ipv eq '6' && (!$ipv6_supported || !$http_daemon_supports_ipv6);
+    my $httpd = ddclient::Test::Fake::HTTPD->new(
+        host => $ipv eq '4' ? '127.0.0.1' : '::1',
+        daemon_args => {V6Only => 1},
+    );
+    my $ip = $ipv eq '4' ? '192.0.2.1' : '2001:db8::1';
+    $httpd->run(sub { return [200, ['content-type' => 'text/plain; charset=utf-8'], [$ip]]; });
+    diag("started IPv$ipv HTTP server running at " . $httpd->endpoint());
+    return $httpd;
+}
+my %httpd = (
+    '4' => run_httpd('4'),
+    '6' => run_httpd('6'),
+);
+local %ddclient::builtinweb = (
+    v4 => {url => "" . $httpd{'4'}->endpoint()},
+    defined($httpd{'6'}) ? (v6 => {url => "" . $httpd{'6'}->endpoint()}) : (),
+);
+
+local $ddclient::globals{debug} = 1;
+local $ddclient::globals{verbose} = 1;
+local $ddclient::now = 1000;
+our @updates;
+local %ddclient::protocols = (
+    # The `legacy` protocol reads the legacy `wantip` property and sets the legacy `ip` and `status`
+    # properties.  (Modern protocol implementations read `wantipv4` and `wantipv6` and set `ipv4`,
+    # `ipv6`, `status-ipv4`, and `status-ipv6`.)  It always succeeds.
+    legacy => {
+        update => sub {
+            for my $h (@_) {
+                push(@updates, [@_]);
+                $ddclient::config{$h}{status} = 'good';
+                $ddclient::config{$h}{ip} = delete($ddclient::config{$h}{wantip});
+                $ddclient::config{$h}{mtime} = $ddclient::now;
+            }
+        },
+        variables => {
+            %{$ddclient::variables{'protocol-common-defaults'}},
+        },
+    },
+);
+
+my @test_cases = (
+    map({
+        my %cfg = %{delete($_->{cfg})};
+        my $desc = join(' ', map("$_=$cfg{$_}", keys(%cfg)));
+        {
+            desc => "legacy, fresh, $desc",
+            cfg => {
+                'protocol' => 'legacy',
+                'use' => 'disabled',
+                %cfg,
+            },
+            want_update => 1,
+            want_recap_changes => {
+                'atime' => $ddclient::now,
+                'ip' => '192.0.2.1',
+                'mtime' => $ddclient::now,
+                'status' => 'good',
+                'warned-min-error-interval' => 0,
+                'warned-min-interval' => 0,
+                'wtime' => 0,
+            },
+            want_cfg_changes => {
+                'atime' => $ddclient::now,
+                'ip' => '192.0.2.1',
+                'mtime' => $ddclient::now,
+                'status' => 'good',
+                'status-ipv4' => undef,
+                'status-ipv6' => undef,
+                'wantipv4' => '192.0.2.1',
+                'wantipv6' => undef,
+                'warned-min-error-interval' => 0,
+                'warned-min-interval' => 0,
+                'wtime' => 0,
+            },
+            %$_,
+        };
+    } {cfg => {use => 'web'}}, {cfg => {usev4 => 'webv4'}}),
+    {
+        desc => 'legacy, fresh, use=web (IPv6)',
+        ipv6 => 1,
+        cfg => {
+            'protocol' => 'legacy',
+            'use' => 'web',
+            'web' => 'v6',
+        },
+        want_update => 1,
+        want_recap_changes => {
+            'atime' => $ddclient::now,
+            'ip' => '2001:db8::1',
+            'mtime' => $ddclient::now,
+            'status' => 'good',
+            'warned-min-error-interval' => 0,
+            'warned-min-interval' => 0,
+            'wtime' => 0,
+        },
+        want_cfg_changes => {
+            'atime' => $ddclient::now,
+            'ip' => '2001:db8::1',
+            'mtime' => $ddclient::now,
+            'status' => 'good',
+            'status-ipv4' => undef,
+            'status-ipv6' => undef,
+            'wantipv4' => undef,
+            'wantipv6' => '2001:db8::1',
+            'warned-min-error-interval' => 0,
+            'warned-min-interval' => 0,
+            'wtime' => 0,
+        },
+    },
+    {
+        desc => 'legacy, fresh, usev6=webv6',
+        ipv6 => 1,
+        cfg => {
+            'protocol' => 'legacy',
+            'use' => 'disabled',
+            'usev6' => 'webv6',
+        },
+        want_update => 1,
+        want_recap_changes => {
+            'atime' => $ddclient::now,
+            'ip' => '2001:db8::1',
+            'mtime' => $ddclient::now,
+            'status' => 'good',
+            'warned-min-error-interval' => 0,
+            'warned-min-interval' => 0,
+            'wtime' => 0,
+        },
+        want_cfg_changes => {
+            'atime' => $ddclient::now,
+            'ip' => '2001:db8::1',
+            'mtime' => $ddclient::now,
+            'status' => 'good',
+            'status-ipv4' => undef,
+            'status-ipv6' => undef,
+            'wantipv4' => undef,
+            'wantipv6' => '2001:db8::1',
+            'warned-min-error-interval' => 0,
+            'warned-min-interval' => 0,
+            'wtime' => 0,
+        },
+    },
+    {
+        desc => 'legacy, fresh, usev4=webv4 usev6=webv6',
+        ipv6 => 1,
+        cfg => {
+            'protocol' => 'legacy',
+            'use' => 'disabled',
+            'usev4' => 'webv4',
+            'usev6' => 'webv6',
+        },
+        want_update => 1,
+        want_recap_changes => {
+            'atime' => $ddclient::now,
+            'ip' => '192.0.2.1',
+            'mtime' => $ddclient::now,
+            'status' => 'good',
+            'warned-min-error-interval' => 0,
+            'warned-min-interval' => 0,
+            'wtime' => 0,
+        },
+        want_cfg_changes => {
+            'atime' => $ddclient::now,
+            'ip' => '192.0.2.1',
+            'mtime' => $ddclient::now,
+            'status' => 'good',
+            'status-ipv4' => undef,
+            'status-ipv6' => undef,
+            'wantipv4' => '192.0.2.1',
+            'wantipv6' => '2001:db8::1',
+            'warned-min-error-interval' => 0,
+            'warned-min-interval' => 0,
+            'wtime' => 0,
+        },
+    },
+    map({
+        my %cfg = %{delete($_->{cfg})};
+        my $desc = join(' ', map("$_=$cfg{$_}", keys(%cfg)));
+        {
+            desc => "legacy, no change, not yet time, $desc",
+            recap => {
+                'atime' => $ddclient::now - ddclient::opt('min-interval'),
+                'ip' => '192.0.2.1',
+                'mtime' => $ddclient::now - ddclient::opt('min-interval'),
+                'status' => 'good',
+            },
+            cfg => {
+                'protocol' => 'legacy',
+                'use' => 'disabled',
+                %cfg,
+            },
+            want_cfg_changes => {
+                'status-ipv4' => undef,
+                'status-ipv6' => undef,
+                'wantip' => '192.0.2.1',
+                'wantipv4' => '192.0.2.1',
+                'wantipv6' => undef,
+            },
+            %$_,
+        };
+    }
+        {cfg => {use => 'web'}},
+        {cfg => {usev4 => 'webv4'},
+         want_recap_changes_TODO => 'usev4 and usev6 should check status from legacy protocols'}),
+    map({
+        my %cfg = %{delete($_->{cfg})};
+        my $desc = join(' ', map("$_=$cfg{$_}", keys(%cfg)));
+        {
+            desc => "legacy, min-interval elapsed but no change, $desc",
+            recap => {
+                'atime' => $ddclient::now - ddclient::opt('min-interval') - 1,
+                'ip' => '192.0.2.1',
+                'mtime' => $ddclient::now - ddclient::opt('min-interval') - 1,
+                'status' => 'good',
+            },
+            cfg => {
+                'protocol' => 'legacy',
+                'use' => 'disabled',
+                %cfg,
+            },
+            want_cfg_changes => {
+                'status-ipv4' => undef,
+                'status-ipv6' => undef,
+                'wantip' => '192.0.2.1',
+                'wantipv4' => '192.0.2.1',
+                'wantipv6' => undef,
+            },
+            %$_,
+        };
+    }
+        {cfg => {use => 'web'}},
+        {cfg => {usev4 => 'webv4'},
+         want_recap_changes_TODO => 'usev4 and usev6 should check status from legacy protocols'}),
+    map({
+        my %cfg = %{delete($_->{cfg})};
+        my $desc = join(' ', map("$_=$cfg{$_}", keys(%cfg)));
+        {
+            desc => "legacy, needs update, not yet time, $desc",
+            recap => {
+                'atime' => $ddclient::now - ddclient::opt('min-interval'),
+                'ip' => '192.0.2.2',
+                'mtime' => $ddclient::now - ddclient::opt('min-interval'),
+                'status' => 'good',
+            },
+            cfg => {
+                'protocol' => 'legacy',
+                'use' => 'disabled',
+                %cfg,
+            },
+            want_recap_changes => {
+                'warned-min-interval' => $ddclient::now,
+            },
+            want_cfg_changes => {
+                'status-ipv4' => undef,
+                'status-ipv6' => undef,
+                'wantip' => '192.0.2.1',
+                'wantipv4' => '192.0.2.1',
+                'wantipv6' => undef,
+            },
+            %$_,
+        };
+    }
+        {cfg => {use => 'web'}},
+        {cfg => {usev4 => 'webv4'},
+         want_recap_changes_TODO => 'usev4 and usev6 should check status from legacy protocols'}),
+    map({
+        my %cfg = %{delete($_->{cfg})};
+        my $desc = join(' ', map("$_=$cfg{$_}", keys(%cfg)));
+        {
+            desc => "legacy, min-interval elapsed, needs update, $desc",
+            recap => {
+                'atime' => $ddclient::now - ddclient::opt('min-interval') - 1,
+                'ip' => '192.0.2.2',
+                'mtime' => $ddclient::now - ddclient::opt('min-interval') - 1,
+                'status' => 'good',
+            },
+            cfg => {
+                'protocol' => 'legacy',
+                'use' => 'disabled',
+                %cfg,
+            },
+            want_update => 1,
+            want_recap_changes => {
+                'atime' => $ddclient::now,
+                'ip' => '192.0.2.1',
+                'mtime' => $ddclient::now,
+                'warned-min-error-interval' => 0,
+                'warned-min-interval' => 0,
+                'wtime' => 0,
+            },
+            want_cfg_changes => {
+                'atime' => $ddclient::now,
+                'ip' => '192.0.2.1',
+                'mtime' => $ddclient::now,
+                'status-ipv4' => undef,
+                'status-ipv6' => undef,
+                'wantipv4' => '192.0.2.1',
+                'wantipv6' => undef,
+                'warned-min-error-interval' => 0,
+                'warned-min-interval' => 0,
+                'wtime' => 0,
+            },
+            %$_,
+        };
+    }
+        {cfg => {use => 'web'}},
+        {cfg => {usev4 => 'webv4'},
+         want_update_TODO => 'usev4 and usev6 should check status from legacy protocols',
+         want_cfg_changes_TODO => 'usev4 and usev6 should check status from legacy protocols',
+         want_recap_changes_TODO => 'usev4 and usev6 should check status from legacy protocols'}),
+    map({
+        my %cfg = %{delete($_->{cfg})};
+        my $desc = join(' ', map("$_=$cfg{$_}", keys(%cfg)));
+        {
+            desc => "legacy, previous failed update, not yet time to retry, $desc",
+            recap => {
+                'atime' => $ddclient::now - ddclient::opt('min-error-interval'),
+                'ip' => '192.0.2.2',
+                'mtime' => $ddclient::now - max(ddclient::opt('min-error-interval'),
+                                                ddclient::opt('min-interval')) - 1,
+                'status' => 'failed',
+            },
+            cfg => {
+                'protocol' => 'legacy',
+                'use' => 'disabled',
+                %cfg,
+            },
+            want_recap_changes => {
+                'warned-min-error-interval' => $ddclient::now,
+            },
+            want_cfg_changes => {
+                'status-ipv4' => undef,
+                'status-ipv6' => undef,
+                'wantip' => '192.0.2.1',
+                'wantipv4' => '192.0.2.1',
+                'wantipv6' => undef,
+            },
+            %$_,
+        };
+    } {cfg => {use => 'web'}}, {cfg => {usev4 => 'webv4'}}),
+    map({
+        my %cfg = %{delete($_->{cfg})};
+        my $desc = join(' ', map("$_=$cfg{$_}", keys(%cfg)));
+        {
+            desc => "legacy, previous failed update, time to retry, $desc",
+            recap => {
+                'atime' => $ddclient::now - ddclient::opt('min-error-interval') - 1,
+                'ip' => '192.0.2.2',
+                'mtime' => $ddclient::now - ddclient::opt('min-error-interval') - 2,
+                'status' => 'failed',
+            },
+            cfg => {
+                'protocol' => 'legacy',
+                'use' => 'disabled',
+                %cfg,
+            },
+            want_update => 1,
+            want_recap_changes => {
+                'atime' => $ddclient::now,
+                'ip' => '192.0.2.1',
+                'mtime' => $ddclient::now,
+                'status' => 'good',
+                'warned-min-error-interval' => 0,
+                'warned-min-interval' => 0,
+                'wtime' => 0,
+            },
+            want_cfg_changes => {
+                'atime' => $ddclient::now,
+                'ip' => '192.0.2.1',
+                'mtime' => $ddclient::now,
+                'status' => 'good',
+                'status-ipv4' => undef,
+                'status-ipv6' => undef,
+                'wantipv4' => '192.0.2.1',
+                'wantipv6' => undef,
+                'warned-min-error-interval' => 0,
+                'warned-min-interval' => 0,
+                'wtime' => 0,
+            },
+            %$_,
+        };
+    } {cfg => {use => 'web'}}, {cfg => {usev4 => 'webv4'}}),
+);
+
+for my $tc (@test_cases) {
+    SKIP: {
+        skip("IPv6 not supported on this system", 1) if $tc->{ipv6} && !$ipv6_supported;
+        skip("HTTP::Daemon too old for IPv6 support", 1)
+            if $tc->{ipv6} && !$http_daemon_supports_ipv6;
+        subtest($tc->{desc} => sub {
+            local $ddclient::_l = ddclient::pushlogctx($tc->{desc});
+            # Copy %{$tc->{recap}} so that updates to $recap{$h} don't update %{$tc->{recap}}.
+            local %ddclient::recap = (host => {%{$tc->{recap} // {}}});
+            my $cachef = File::Temp->new();
+            # $cachef is an object that stringifies to a filename.
+            local $ddclient::globals{cache} = "$cachef";
+            my %cfg = (
+                %{$tc->{recap} // {}},  # Simulate a previous update.
+                web => 'v4',
+                webv4 => 'v4',
+                webv6 => 'v6',
+                %{$tc->{cfg} // {}},
+            );
+            # Copy %cfg so that updates to $config{$h} don't update %cfg.
+            local %ddclient::config = (host => {%cfg});
+            local @updates;
+
+            ddclient::update_nics();
+
+            TODO: {
+                local $TODO = $tc->{want_update_TODO};
+                is_deeply(\@updates, [(['host']) x ($tc->{want_update} ? 1 : 0)],
+                          'got expected update');
+            }
+            my %want_recap = (host => {
+                %{$tc->{recap} // {}},
+                %{$tc->{want_recap_changes} // {}},
+            });
+            TODO: {
+                local $TODO = $tc->{want_recap_changes_TODO};
+                is_deeply(\%ddclient::recap, \%want_recap, 'recap matches')
+                    or diag(ddclient::repr(Values => [\%ddclient::recap, \%want_recap],
+                                           Names => ['*got', '*want']));
+            }
+            my %want_cfg = (host => {
+                update => $tc->{want_update} ? 1 : 0,
+                %cfg,
+                %{$tc->{want_cfg_changes} // {}},
+            });
+            TODO: {
+                local $TODO = $tc->{want_cfg_changes_TODO};
+                is_deeply(\%ddclient::config, \%want_cfg, 'config matches')
+                    or diag(ddclient::repr(Values => [\%ddclient::config, \%want_cfg],
+                                           Names => ['*got', '*want']));
+            }
+        });
+    }
+}
+
+done_testing();