diff --git a/configure.ac b/configure.ac
index ab9ef75..75edda5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -78,6 +78,7 @@ m4_foreach_w([_m], [
     File::Spec::Functions
     File::Temp
     List::Util
+    re
   ], [AX_PROG_PERL_MODULES([_m], [],
          [AC_MSG_WARN([some tests will fail due to missing module _m])])])
 
diff --git a/ddclient.in b/ddclient.in
index a645032..4a2c91b 100755
--- a/ddclient.in
+++ b/ddclient.in
@@ -597,7 +597,13 @@ our %variables = (
         'password'            => setv(T_PASSWD,1, 0, undef,                undef),
         'host'                => setv(T_STRING,1, 1, undef,                undef),
 
-        'use'                 => setv(T_USE,   0, 0, 'ip',                 undef),
+        'use' => setv(T_USE, 0, 0, sub {
+            my ($h) = @_;
+            return "'disabled' if '--usev4' or '--usev6' is enabled, otherwise 'ip'"
+                if ($h // '') eq '<usage>';
+            return 'disabled' if opt('usev4', $h) ne 'disabled' || opt('usev6', $h) ne 'disabled';
+            return 'ip';
+        }, undef),
         'usev4'               => setv(T_USEV4, 0, 0, 'disabled',           undef),
         'usev6'               => setv(T_USEV6, 0, 0, 'disabled',           undef),
         'if'                  => setv(T_IF,    0, 0, 'ppp0',               undef),
@@ -1092,7 +1098,7 @@ $variables{'merged'} = {
 };
 
 # This will hold the processed args.
-my %opt = ();
+our %opt;
 my $deprecated_handler = sub { warning("'-$_[0]' is deprecated and does nothing"); };
 $opt{'fw-banlocal'} = $deprecated_handler;
 $opt{'if-skip'} = $deprecated_handler;
@@ -1937,7 +1943,7 @@ sub usage {
     for (@_) {
         if (ref $_) {
             my ($key, $specifier, $arg_usage) = @$_;
-            my $value = default($key);
+            my $value = default($key, '<usage>');
             next unless $arg_usage;
             $usage .= "  $arg_usage";
             if (defined($value) && $value ne '') {
@@ -2220,15 +2226,17 @@ sub split_by_comma {
 }
 sub default {
     my ($v, $h) = @_;
+    my $var;
     if (defined($h) && $config{$h}) {
         my $proto = $protocols{opt('protocol', $v eq 'protocol' ? undef : $h)};
-        my $var = $proto->{variables}{$v} if $proto;
-        return $var->{default} if $var;
+        $var = $proto->{variables}{$v} if $proto;
     }
-    return undef if !defined($variables{'merged'}{$v});
     # TODO: This might grab an arbitrary protocol-specific variable definition, which could cause
     # surprising behavior.
-    return $variables{'merged'}{$v}{'default'};
+    $var //= $variables{'merged'}{$v};
+    return undef if !defined($var);
+    return $var->{'default'}($h) if ref($var->{default}) eq 'CODE';
+    return $var->{'default'};
 }
 sub opt {
     my $v = shift;
diff --git a/t/update_nics.pl b/t/update_nics.pl
index e5d0ee0..5b48061 100644
--- a/t/update_nics.pl
+++ b/t/update_nics.pl
@@ -70,7 +70,6 @@ my @test_cases = (
             desc => "legacy, fresh, $desc",
             cfg => {
                 'protocol' => 'legacy',
-                'use' => 'disabled',
                 %cfg,
             },
             want_update => 1,
@@ -136,7 +135,6 @@ my @test_cases = (
         ipv6 => 1,
         cfg => {
             'protocol' => 'legacy',
-            'use' => 'disabled',
             'usev6' => 'webv6',
         },
         want_update => 1,
@@ -168,7 +166,6 @@ my @test_cases = (
         ipv6 => 1,
         cfg => {
             'protocol' => 'legacy',
-            'use' => 'disabled',
             'usev4' => 'webv4',
             'usev6' => 'webv6',
         },
@@ -209,7 +206,6 @@ my @test_cases = (
             },
             cfg => {
                 'protocol' => 'legacy',
-                'use' => 'disabled',
                 %cfg,
             },
             want_cfg_changes => {
@@ -238,7 +234,6 @@ my @test_cases = (
             },
             cfg => {
                 'protocol' => 'legacy',
-                'use' => 'disabled',
                 %cfg,
             },
             want_cfg_changes => {
@@ -267,7 +262,6 @@ my @test_cases = (
             },
             cfg => {
                 'protocol' => 'legacy',
-                'use' => 'disabled',
                 %cfg,
             },
             want_recap_changes => {
@@ -299,7 +293,6 @@ my @test_cases = (
             },
             cfg => {
                 'protocol' => 'legacy',
-                'use' => 'disabled',
                 %cfg,
             },
             want_update => 1,
@@ -345,7 +338,6 @@ my @test_cases = (
             },
             cfg => {
                 'protocol' => 'legacy',
-                'use' => 'disabled',
                 %cfg,
             },
             want_recap_changes => {
@@ -374,7 +366,6 @@ my @test_cases = (
             },
             cfg => {
                 'protocol' => 'legacy',
-                'use' => 'disabled',
                 %cfg,
             },
             want_update => 1,
diff --git a/t/variable_defaults.pl b/t/variable_defaults.pl
index b1ac203..8d82347 100644
--- a/t/variable_defaults.pl
+++ b/t/variable_defaults.pl
@@ -1,4 +1,5 @@
 use Test::More;
+use re qw(is_regexp);
 SKIP: { eval { require Test::Warnings; } or skip($@, 1); }
 eval { require 'ddclient'; } or BAIL_OUT($@);
 
@@ -23,14 +24,78 @@ for my $tc (@test_cases) {
     if ($tc->{def}{required}) {
         is($tc->{def}{default}, undef, "'$tc->{desc}' (required) has no default");
     } else {
-        local %ddclient::variables = (merged => {var => $tc->{def}});
+        # Preserve all existing variables in $variables{merged} so that variables with dynamic
+        # defaults can reference them.
+        local %ddclient::variables = (merged => {
+            %{$ddclient::variables{merged}},
+            'var for test' => $tc->{def},
+        });
+        # Variables with dynamic defaults will need their own unit tests, but we can still check the
+        # clean-slate hostless default.
+        local %ddclient::config;
+        local %ddclient::opt;
+        local %ddclient::globals;
         my $norm;
-        my $default = ddclient::default('var');
+        my $default = ddclient::default('var for test');
         diag("'$tc->{desc}' default: " . ($default // '<undefined>'));
-        is($default, $tc->{def}{default}, "'$tc->{desc}' default() return value matches default");
+        is($default, $tc->{def}{default}, "'$tc->{desc}' default() return value matches default")
+            if ref($tc->{def}{default}) ne 'CODE';
         my $valid = eval { $norm = ddclient::check_value($default, $tc->{def}); 1; } or diag($@);
         ok($valid, "'$tc->{desc}' (optional) has a valid default");
         is($norm, $default, "'$tc->{desc}' default normalizes to itself") if $valid;
     }
 }
+
+my @use_test_cases = (
+    {
+        desc => 'clean slate hostless default',
+        want => 'ip',
+    },
+    {
+        desc => 'usage string',
+        host => '<usage>',
+        want => qr/disabled.*ip|ip.*disabled/,
+    },
+    {
+        desc => 'usev4 disables use by default',
+        host => 'host',
+        cfg => {usev4 => 'webv4'},
+        want => 'disabled',
+    },
+    {
+        desc => 'usev6 disables use by default',
+        host => 'host',
+        cfg => {usev4 => 'webv4'},
+        want => 'disabled',
+    },
+    {
+        desc => 'explicitly setting use re-enables it',
+        host => 'host',
+        cfg => {use => 'web', usev4 => 'webv4'},
+        want => 'web',
+    },
+);
+for my $tc (@use_test_cases) {
+    my $desc = "'use' dynamic default: $tc->{desc}";
+    local %ddclient::protocols =
+        (protocol => {variables => $ddclient::variables{'protocol-common-defaults'}});
+    local %ddclient::variables = (merged => {
+        'protocol' => $ddclient::variables{'merged'}{'protocol'},
+        'use' => $ddclient::variables{'protocol-common-defaults'}{'use'},
+        'usev4' => $ddclient::variables{'merged'}{'usev4'},
+        'usev6' => $ddclient::variables{'merged'}{'usev6'},
+    });
+    local %ddclient::config = (host => {protocol => 'protocol', %{$tc->{cfg} // {}}});
+    local %ddclient::opt;
+    local %ddclient::globals;
+
+    my $got = ddclient::opt('use', $tc->{host});
+
+    if (is_regexp($tc->{want})) {
+        like($got, $tc->{want}, $desc);
+    } else {
+        is($got, $tc->{want}, $desc);
+    }
+}
+
 done_testing();