diff --git a/Makefile.am b/Makefile.am
index a1cef03..265cea1 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -70,6 +70,8 @@ AM_PL_LOG_FLAGS = -Mstrict -w \
 	-I'$(abs_top_srcdir)'/t/lib \
 	-MDevel::Autoflush
 handwritten_tests = \
+	t/is-and-extract-ipv4.pl \
+	t/is-and-extract-ipv6.pl \
 	t/geturl_connectivity.pl \
 	t/geturl_ssl.pl \
 	t/parse_assignments.pl \
diff --git a/configure.ac b/configure.ac
index 94b62b3..6151e88 100644
--- a/configure.ac
+++ b/configure.ac
@@ -49,6 +49,7 @@ m4_foreach_w([_m], [
 # Perl modules required for tests. If these modules are not installed
 # then some tests will fail. Only prints a warning if not installed.
 m4_foreach_w([_m], [
+    B
     Data::Dumper
     File::Spec::Functions
     File::Temp
diff --git a/ddclient.in b/ddclient.in
index 8b8a4a9..1cec317 100755
--- a/ddclient.in
+++ b/ddclient.in
@@ -2273,65 +2273,95 @@ sub get_ip {
     return $ip;
 }
 
+
 ######################################################################
-## is_ipv4() validates if string is valid IPv4 address and only a
-## valid address with no preceding or trailing spaces/characters
-## and no embedded leading zeros.
+## Regex to find IPv4 address. Accepts embedded leading zeros.
+######################################################################
+my $regex_ipv4 = qr/(?:(?<octet>25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?&octet)/;
+
+######################################################################
+## is_ipv4() validates if string is valid IPv4 address with no preceding
+## or trailing spaces/characters, not even line breaks.
 ######################################################################
 sub is_ipv4 {
-    my ($value) = @_;
-    return (length($value // '') != 0) && ($value eq (extract_ipv4($value) // ''));
+    return (shift // '') =~ /\A$regex_ipv4\z/;
 }
 
 ######################################################################
-## extract_ipv4() extracts the first valid IPv4 address from given string.
-## Accepts leading zeros in the address but removes them in returned value
+## extract_ipv4() finds the first valid IPv4 address in the given string,
+## removes embedded leading zeros, and returns the result.
 ######################################################################
 sub extract_ipv4 {
-    (shift // '') =~ /\b((?:(?<octet>25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?&octet))\b/
-        or return undef;
+    (shift // '') =~ /(?:\b|_)($regex_ipv4)(?:\b|_)/ or return undef;
     (my $ip = $1) =~ s/\b0+\B//g; ## remove embedded leading zeros
     return $ip;
 }
 
+######################################################################
+## Regex that matches an IPv6 address. Accepts embedded leading zeros.
+## Accepts IPv4-mapped IPv6 addresses such as 64:ff9b::192.0.2.13.
+######################################################################
+my $regex_ipv6 = qr/
+    # Define some named groups so we can use Perl's recursive subpattern feature for shorthand:
+    (?<g>[0-9A-F]{1,4}){0}  # "g" matches a group of 1 to 4 hex chars
+    (?<g_>(?&g):){0}        # "g_" matches a group of 1 to 4 hex chars followed by a colon
+    (?<_g>:(?&g)){0}        # "_g" matches a colon followed by a group of 1 to 4 hex chars
+    (?<g0>(?&g)?){0}        # "g0" is an optional "g" (matches a group of 0 to 4 hex chars)
+    (?<g0_>(?&g0):){0}      # "g0_" is an optional "g" followed by a colon
+    (?<x>[:.0-9A-Z]){0}     # "x" matches chars that should never come before or after the address
+    (?<ip4>$regex_ipv4){0}  # "ip4" matches an IPv4 address x.x.x.x
+
+    # Now for the regex itself:
+    (?<!(?&x))  # Not preceded by a character that is not allowed to come before the address
+    (?:
+      (?&g_){7}(?&g)     # Exactly 8 groups of 1-4 hex chars
+      | (?&g_){1,7}:     # OR compressed form with the double colon at the end
+      | :(?&_g){1,7}     # OR compressed form with the double colon at the beginning
+      | ::               # OR compressed to just a double colon
+      |                  # OR compressed form with the double colon in the middle:
+        (?=                   # Only consider this case if the string...
+          (?&g0_){2,7}(?&g0)  # ...has 3 to 8 possibly empty groups (at least 3 because there
+                              # will be at least one group before the ::, at least one group
+                              # after the ::, and the :: itself is an empty group)...
+          (?!(?&x))           # ...then ends.
+        )                     # If the condition is true, then:
+        (?&g_){1,6}           #   1 to 6 non-empty groups before the double colon
+        (?&_g){1,6}           #   1 to 6 non-empty groups after the double colon
+      |                  # OR an IPv4-mapped IPv6 address:
+        (?:                # It starts with a 96-bit prefix represented by:
+          (?&g_){6}          # Exactly 6 groups of 1-4 hex chars with their colons
+          | (?&g_){1,5}:     # OR compressed form with the double colon at the end
+          | ::(?&g_){0,5}    # OR compressed form with the double colon at the beginning
+          |                  # OR compressed form with the double colon in the middle:
+            (?=                 # Only consider this case if the prefix...
+              (?&g0_){3,6}      # ...has 3 to 6 possibly empty groups...
+              (?&ip4)(?!(?&x))  # ...then ends.
+            )                   # If the condition is true, then:
+            (?&g_){1,4}         #   1 to 4 non-empty groups before the double colon
+            (?&_g){1,4}         #   1 to 4 non-empty groups after the double colon
+            :                   #   colon separating the IPv6 part from the IPv4 part
+        )
+        (?&ip4)            # Prefix is followed by an IPv4 address
+    )
+    (?!(?&x))  # Not followed by a character that is not allowed to come after the address
+/xi;
+
 ######################################################################
 ## is_ipv6() validates if string is valid IPv6 address with no preceding
-## or trailing spaces/characters.
+## or trailing spaces/characters, not even line breaks.
 ######################################################################
 sub is_ipv6 {
-    my ($value) = @_;
-    return (length($value // '') != 0) &&
-        ((extract_ipv6($value) // '') eq (($value =~ s/\b0+\B//g) ? $value : $value));
+    return (shift // '') =~ /\A$regex_ipv6\z/;
 }
 
 ######################################################################
-## extract_ipv6() extracts the first valid IPv6 address from the given string.
-## Accepts leading zeros in the address but removes them in returned value.
+## extract_ipv6() finds the first valid IPv6 address in the given string,
+## removes embedded leading zeros, and returns the result.
 ######################################################################
 sub extract_ipv6 {
-    my $content = shift;
-    my $omits;
-    my $ip          = "";
-    my $linenumbers = 0;
-
-    my @values = split('\n', $content);
-    foreach my $val (@values) {
-        next unless $val =~ /((:{0,2}[A-F0-9]{1,4}){0,7}:{1,2}[A-F0-9]{1,4})/i; # invalid char
-        my $parsed = $1;
-
-        # check for at least 7 colons
-        my $count_colon = () = $parsed =~ /:/g;
-        if ($count_colon != 7) {
-            # or one double colon
-            my $count_double_colon = () = $parsed =~ /::/g;
-            if ($count_double_colon != 1) {
-                next
-            }
-        }
-        $parsed =~ s/\b0+\B//g; ## remove embedded leading zeros
-        return $parsed;
-    }
-    return;
+    (shift // '') =~ /($regex_ipv6)/ or return undef;
+    (my $ip = $1) =~ s/\b0+\B//g; ## remove embedded leading zeros
+    return $ip;
 }
 
 ######################################################################
diff --git a/t/is-and-extract-ipv4.pl b/t/is-and-extract-ipv4.pl
new file mode 100644
index 0000000..41517ab
--- /dev/null
+++ b/t/is-and-extract-ipv4.pl
@@ -0,0 +1,83 @@
+use Test::More;
+use B qw(perlstring);
+
+SKIP: { eval { require Test::Warnings; } or skip($@, 1); }
+eval { require 'ddclient'; } or BAIL_OUT($@);
+
+
+my @valid_ipv4 = (
+    "192.168.1.1",
+    "0.0.0.0",
+    "000.000.000.000",
+    "255.255.255.255",
+    "10.0.0.0",
+);
+
+my @invalid_ipv4 = (
+    undef,
+    "",
+    "192.168.1",
+    "0.0.0",
+    "000.000",
+    "256.256.256.256",
+    ".10.0.0.0",
+);
+
+
+subtest "is_ipv4() with valid addresses" => sub {
+    foreach my $ip (@valid_ipv4) {
+        ok(ddclient::is_ipv4($ip), "is_ipv4('$ip')");
+    }
+};
+
+subtest "is_ipv4() with invalid addresses" => sub {
+    foreach my $ip (@invalid_ipv4) {
+        ok(!ddclient::is_ipv4($ip), sprintf("!is_ipv4(%s)", defined($ip) ? "'$ip'" : 'undef'));
+    }
+};
+
+subtest "is_ipv4() with char adjacent to valid address" => sub {
+    foreach my $ch (split(//, '/.,:z @$#&%!^*()_-+'), "\n") {
+        subtest perlstring($ch) => sub {
+            foreach my $ip (@valid_ipv4) {
+                subtest $ip => sub {
+                    my $test = $ch . $ip;  # insert at front
+                    ok(!ddclient::is_ipv4($test), "!is_ipv4('$test')");
+                    $test = $ip . $ch;  # add at end
+                    ok(!ddclient::is_ipv4($test), "!is_ipv4('$test')");
+                    $test = $ch . $ip . $ch; # wrap front and end
+                    ok(!ddclient::is_ipv4($test), "!is_ipv4('$test')");
+                };
+            }
+        };
+    }
+};
+
+subtest "extract_ipv4()" => sub {
+    my @test_cases = (
+        {name => "undef",     text => undef,              want => undef},
+        {name => "empty",     text => "",                 want => undef},
+        {name => "invalid",   text => "1.2.3.256",        want => undef},
+        {name => "two addrs", text => "1.1.1.1\n2.2.2.2", want => "1.1.1.1"},
+        {name => "host+port", text => "1.2.3.4:123",      want => "1.2.3.4"},
+        {name => "zero pad",  text => "001.002.003.004",  want => "1.2.3.4"},
+    );
+    foreach my $tc (@test_cases) {
+        is(ddclient::extract_ipv4($tc->{text}), $tc->{want}, $tc->{name});
+    }
+};
+
+subtest "extract_ipv4() of valid addr with adjacent non-word char" => sub {
+    foreach my $wb (split(//, '/, @$#&%!^*()_-+:'), "\n") {
+        subtest perlstring($wb) => sub {
+            my $test = "";
+            foreach my $ip (@valid_ipv4) {
+                $test = "foo" . $wb . $ip . $wb . "bar"; # wrap front and end
+                $ip =~ s/\b0+\B//g; ## remove embedded leading zeros for testing
+                is(ddclient::extract_ipv4($test), $ip, perlstring($test));
+            }
+        };
+    }
+};
+
+done_testing();
diff --git a/t/is-and-extract-ipv6.pl b/t/is-and-extract-ipv6.pl
new file mode 100644
index 0000000..bb33bc9
--- /dev/null
+++ b/t/is-and-extract-ipv6.pl
@@ -0,0 +1,492 @@
+use Test::More;
+use B qw(perlstring);
+
+SKIP: { eval { require Test::Warnings; } or skip($@, 1); }
+eval { require 'ddclient'; } or BAIL_OUT($@);
+
+
+my @valid_ipv6 = (
+    "::abcd:efAB:CDEF",  # case sensitivity
+    "08:09:0a:0b:0c:0d:0e:0f",  # leading zeros
+
+    # with thanks to http://home.deds.nl/~aeron/regex/valid_ipv6.txt
+    "1111:2222:3333:4444:5555:6666:7777:8888",
+    "1111:2222:3333:4444:5555:6666:7777::",
+    "1111:2222:3333:4444:5555:6666::",
+    "1111:2222:3333:4444:5555::",
+    "1111:2222:3333:4444::",
+    "1111:2222:3333::",
+    "1111:2222::",
+    "1111::",
+    "::",
+    "1111:2222:3333:4444:5555:6666::8888",
+    "1111:2222:3333:4444:5555::8888",
+    "1111:2222:3333:4444::8888",
+    "1111:2222:3333::8888",
+    "1111:2222::8888",
+    "1111::8888",
+    "::8888",
+    "1111:2222:3333:4444:5555::7777:8888",
+    "1111:2222:3333:4444::7777:8888",
+    "1111:2222:3333::7777:8888",
+    "1111:2222::7777:8888",
+    "1111::7777:8888",
+    "::7777:8888",
+    "1111:2222:3333:4444::6666:7777:8888",
+    "1111:2222:3333::6666:7777:8888",
+    "1111:2222::6666:7777:8888",
+    "1111::6666:7777:8888",
+    "::6666:7777:8888",
+    "1111:2222:3333::5555:6666:7777:8888",
+    "1111:2222::5555:6666:7777:8888",
+    "1111::5555:6666:7777:8888",
+    "::5555:6666:7777:8888",
+    "1111:2222::4444:5555:6666:7777:8888",
+    "1111::4444:5555:6666:7777:8888",
+    "::4444:5555:6666:7777:8888",
+    "1111::3333:4444:5555:6666:7777:8888",
+    "::3333:4444:5555:6666:7777:8888",
+    "::2222:3333:4444:5555:6666:7777:8888",
+    # IPv4-mapped IPv6 addresses
+    "1111:2222:3333:4444:5555:6666:0.0.0.0",
+    "1111:2222:3333:4444:5555:6666:00.00.00.00",
+    "1111:2222:3333:4444:5555:6666:000.000.000.000",
+    "1111:2222:3333:4444:5555:6666:123.123.123.123",
+    "1111:2222:3333:4444:5555::123.123.123.123",
+    "1111:2222:3333:4444::123.123.123.123",
+    "1111:2222:3333::123.123.123.123",
+    "1111:2222::123.123.123.123",
+    "1111::123.123.123.123",
+    "::123.123.123.123",
+    "1111:2222:3333:4444::6666:123.123.123.123",
+    "1111:2222:3333::6666:123.123.123.123",
+    "1111:2222::6666:123.123.123.123",
+    "1111::6666:123.123.123.123",
+    "::6666:123.123.123.123",
+    "1111:2222:3333::5555:6666:123.123.123.123",
+    "1111:2222::5555:6666:123.123.123.123",
+    "1111::5555:6666:123.123.123.123",
+    "::5555:6666:123.123.123.123",
+    "1111:2222::4444:5555:6666:123.123.123.123",
+    "1111::4444:5555:6666:123.123.123.123",
+    "::4444:5555:6666:123.123.123.123",
+    "1111::3333:4444:5555:6666:123.123.123.123",
+    "::3333:4444:5555:6666:123.123.123.123",
+    "::2222:3333:4444:5555:6666:123.123.123.123",
+);
+
+my @invalid_ipv6 = (
+    # Empty string and bogus text
+    undef,
+    "",
+    "   ",
+    "foobar",
+
+    # Valid IPv6 with extra text before or after
+    "foo2001:DB8:4341:0781:1111:2222:3333:4444",
+    "foo 2001:DB8:4341:0781::4444",
+    "foo 2001:DB8:4341:0781:1111:: bar",
+    "foo2001:DB8:4341:0781::100bar",
+    "2001:DB8:4341:0781::1 bar",
+    "2001:DB8:4341:0781::0001bar",
+    "foo bar 3001:DB8:4341:0781:1111:2222:3333:4444 foo bar",
+    "__3001:DB8:4341:0781::4444",
+    "__3001:DB8:4341:0781:1111::__",
+    "--3001:DB8:4341:0781::100--",
+    "/3001:DB8:4341:0781::1/",
+    "3001:DB8:4341:0781::0001%",
+    "fdb6:1d86:d9bd:1::4444%eth0",
+    "fdb6:1d86:d9bd:1:1111::%ens192",
+    "fdb6:1d86:d9bd:1::100%en0",
+    "fdb6:1d86:d9bd:1::1%eth1.100",
+
+    # With thanks to http://home.deds.nl/~aeron/regex/invalid_ipv6.txt
+    # Invalid data
+    "XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX",
+
+    # Too many components
+    "1111:2222:3333:4444:5555:6666:7777:8888:9999",
+    "1111:2222:3333:4444:5555:6666:7777:8888::",
+    "::2222:3333:4444:5555:6666:7777:8888:9999",
+
+    # Too few components
+    "1111:2222:3333:4444:5555:6666:7777",
+    "1111:2222:3333:4444:5555:6666",
+    "1111:2222:3333:4444:5555",
+    "1111:2222:3333:4444",
+    "1111:2222:3333",
+    "1111:2222",
+    "1111",
+
+    # Missing :
+    "11112222:3333:4444:5555:6666:7777:8888",
+    "1111:22223333:4444:5555:6666:7777:8888",
+    "1111:2222:33334444:5555:6666:7777:8888",
+    "1111:2222:3333:44445555:6666:7777:8888",
+    "1111:2222:3333:4444:55556666:7777:8888",
+    "1111:2222:3333:4444:5555:66667777:8888",
+    "1111:2222:3333:4444:5555:6666:77778888",
+
+    # Missing : intended for ::
+    "1111:2222:3333:4444:5555:6666:7777:8888:",
+    "1111:2222:3333:4444:5555:6666:7777:",
+    "1111:2222:3333:4444:5555:6666:",
+    "1111:2222:3333:4444:5555:",
+    "1111:2222:3333:4444:",
+    "1111:2222:3333:",
+    "1111:2222:",
+    "1111:",
+    ":",
+    ":8888",
+    ":7777:8888",
+    ":6666:7777:8888",
+    ":5555:6666:7777:8888",
+    ":4444:5555:6666:7777:8888",
+    ":3333:4444:5555:6666:7777:8888",
+    ":2222:3333:4444:5555:6666:7777:8888",
+    ":1111:2222:3333:4444:5555:6666:7777:8888",
+
+    # :::
+    ":::2222:3333:4444:5555:6666:7777:8888",
+    "1111:::3333:4444:5555:6666:7777:8888",
+    "1111:2222:::4444:5555:6666:7777:8888",
+    "1111:2222:3333:::5555:6666:7777:8888",
+    "1111:2222:3333:4444:::6666:7777:8888",
+    "1111:2222:3333:4444:5555:::7777:8888",
+    "1111:2222:3333:4444:5555:6666:::8888",
+    "1111:2222:3333:4444:5555:6666:7777:::",
+
+    # Double ::
+    "::2222::4444:5555:6666:7777:8888",
+    "::2222:3333::5555:6666:7777:8888",
+    "::2222:3333:4444::6666:7777:8888",
+    "::2222:3333:4444:5555::7777:8888",
+    "::2222:3333:4444:5555:7777::8888",
+    "::2222:3333:4444:5555:7777:8888::",
+
+    "1111::3333::5555:6666:7777:8888",
+    "1111::3333:4444::6666:7777:8888",
+    "1111::3333:4444:5555::7777:8888",
+    "1111::3333:4444:5555:6666::8888",
+    "1111::3333:4444:5555:6666:7777::",
+
+    "1111:2222::4444::6666:7777:8888",
+    "1111:2222::4444:5555::7777:8888",
+    "1111:2222::4444:5555:6666::8888",
+    "1111:2222::4444:5555:6666:7777::",
+
+    "1111:2222:3333::5555::7777:8888",
+    "1111:2222:3333::5555:6666::8888",
+    "1111:2222:3333::5555:6666:7777::",
+
+    "1111:2222:3333:4444::6666::8888",
+    "1111:2222:3333:4444::6666:7777::",
+
+    "1111:2222:3333:4444:5555::7777::",
+
+    # Invalid data
+    "XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:1.2.3.4",
+    "1111:2222:3333:4444:5555:6666:256.256.256.256",
+
+    # Too many components
+    "1111:2222:3333:4444:5555:6666:7777:8888:1.2.3",
+    "1111:2222:3333:4444:5555:6666:7777:1.2.3.4",
+    "1111:2222:3333:4444:5555:6666::1.2.3.4",
+    "::2222:3333:4444:5555:6666:7777:1.2.3.4",
+    "1111:2222:3333:4444:5555:6666:1.2.3.4.5",
+
+    # Too few components
+    "1111:2222:3333:4444:5555:1.2.3.4",
+    "1111:2222:3333:4444:1.2.3.4",
+    "1111:2222:3333:1.2.3.4",
+    "1111:2222:1.2.3.4",
+    "1111:1.2.3.4",
+    "1.2.3.4",
+
+    # Missing :
+    "11112222:3333:4444:5555:6666:1.2.3.4",
+    "1111:22223333:4444:5555:6666:1.2.3.4",
+    "1111:2222:33334444:5555:6666:1.2.3.4",
+    "1111:2222:3333:44445555:6666:1.2.3.4",
+    "1111:2222:3333:4444:55556666:1.2.3.4",
+    "1111:2222:3333:4444:5555:66661.2.3.4",
+
+    # Missing .
+    "1111:2222:3333:4444:5555:6666:255255.255.255",
+    "1111:2222:3333:4444:5555:6666:255.255255.255",
+    "1111:2222:3333:4444:5555:6666:255.255.255255",
+
+    # Missing : intended for ::
+    ":1.2.3.4",
+    ":6666:1.2.3.4",
+    ":5555:6666:1.2.3.4",
+    ":4444:5555:6666:1.2.3.4",
+    ":3333:4444:5555:6666:1.2.3.4",
+    ":2222:3333:4444:5555:6666:1.2.3.4",
+    ":1111:2222:3333:4444:5555:6666:1.2.3.4",
+
+    # :::
+    ":::2222:3333:4444:5555:6666:1.2.3.4",
+    "1111:::3333:4444:5555:6666:1.2.3.4",
+    "1111:2222:::4444:5555:6666:1.2.3.4",
+    "1111:2222:3333:::5555:6666:1.2.3.4",
+    "1111:2222:3333:4444:::6666:1.2.3.4",
+    "1111:2222:3333:4444:5555:::1.2.3.4",
+
+    # Double ::
+    "::2222::4444:5555:6666:1.2.3.4",
+    "::2222:3333::5555:6666:1.2.3.4",
+    "::2222:3333:4444::6666:1.2.3.4",
+    "::2222:3333:4444:5555::1.2.3.4",
+
+    "1111::3333::5555:6666:1.2.3.4",
+    "1111::3333:4444::6666:1.2.3.4",
+    "1111::3333:4444:5555::1.2.3.4",
+
+    "1111:2222::4444::6666:1.2.3.4",
+    "1111:2222::4444:5555::1.2.3.4",
+
+    "1111:2222:3333::5555::1.2.3.4",
+
+    # Missing parts
+    "::.",
+    "::..",
+    "::...",
+    "::1...",
+    "::1.2..",
+    "::1.2.3.",
+    "::.2..",
+    "::.2.3.",
+    "::.2.3.4",
+    "::..3.",
+    "::..3.4",
+    "::...4",
+
+    # Extra : in front
+    ":1111:2222:3333:4444:5555:6666:7777::",
+    ":1111:2222:3333:4444:5555:6666::",
+    ":1111:2222:3333:4444:5555::",
+    ":1111:2222:3333:4444::",
+    ":1111:2222:3333::",
+    ":1111:2222::",
+    ":1111::",
+    ":::",
+    ":1111:2222:3333:4444:5555:6666::8888",
+    ":1111:2222:3333:4444:5555::8888",
+    ":1111:2222:3333:4444::8888",
+    ":1111:2222:3333::8888",
+    ":1111:2222::8888",
+    ":1111::8888",
+    ":::8888",
+    ":1111:2222:3333:4444:5555::7777:8888",
+    ":1111:2222:3333:4444::7777:8888",
+    ":1111:2222:3333::7777:8888",
+    ":1111:2222::7777:8888",
+    ":1111::7777:8888",
+    ":::7777:8888",
+    ":1111:2222:3333:4444::6666:7777:8888",
+    ":1111:2222:3333::6666:7777:8888",
+    ":1111:2222::6666:7777:8888",
+    ":1111::6666:7777:8888",
+    ":::6666:7777:8888",
+    ":1111:2222:3333::5555:6666:7777:8888",
+    ":1111:2222::5555:6666:7777:8888",
+    ":1111::5555:6666:7777:8888",
+    ":::5555:6666:7777:8888",
+    ":1111:2222::4444:5555:6666:7777:8888",
+    ":1111::4444:5555:6666:7777:8888",
+    ":::4444:5555:6666:7777:8888",
+    ":1111::3333:4444:5555:6666:7777:8888",
+    ":::3333:4444:5555:6666:7777:8888",
+    ":::2222:3333:4444:5555:6666:7777:8888",
+    ":1111:2222:3333:4444:5555:6666:1.2.3.4",
+    ":1111:2222:3333:4444:5555::1.2.3.4",
+    ":1111:2222:3333:4444::1.2.3.4",
+    ":1111:2222:3333::1.2.3.4",
+    ":1111:2222::1.2.3.4",
+    ":1111::1.2.3.4",
+    ":::1.2.3.4",
+    ":1111:2222:3333:4444::6666:1.2.3.4",
+    ":1111:2222:3333::6666:1.2.3.4",
+    ":1111:2222::6666:1.2.3.4",
+    ":1111::6666:1.2.3.4",
+    ":::6666:1.2.3.4",
+    ":1111:2222:3333::5555:6666:1.2.3.4",
+    ":1111:2222::5555:6666:1.2.3.4",
+    ":1111::5555:6666:1.2.3.4",
+    ":::5555:6666:1.2.3.4",
+    ":1111:2222::4444:5555:6666:1.2.3.4",
+    ":1111::4444:5555:6666:1.2.3.4",
+    ":::4444:5555:6666:1.2.3.4",
+    ":1111::3333:4444:5555:6666:1.2.3.4",
+    ":::3333:4444:5555:6666:1.2.3.4",
+    ":::2222:3333:4444:5555:6666:1.2.3.4",
+
+    # Extra : at end
+    "1111:2222:3333:4444:5555:6666:7777:::",
+    "1111:2222:3333:4444:5555:6666:::",
+    "1111:2222:3333:4444:5555:::",
+    "1111:2222:3333:4444:::",
+    "1111:2222:3333:::",
+    "1111:2222:::",
+    "1111:::",
+    ":::",
+    "1111:2222:3333:4444:5555:6666::8888:",
+    "1111:2222:3333:4444:5555::8888:",
+    "1111:2222:3333:4444::8888:",
+    "1111:2222:3333::8888:",
+    "1111:2222::8888:",
+    "1111::8888:",
+    "::8888:",
+    "1111:2222:3333:4444:5555::7777:8888:",
+    "1111:2222:3333:4444::7777:8888:",
+    "1111:2222:3333::7777:8888:",
+    "1111:2222::7777:8888:",
+    "1111::7777:8888:",
+    "::7777:8888:",
+    "1111:2222:3333:4444::6666:7777:8888:",
+    "1111:2222:3333::6666:7777:8888:",
+    "1111:2222::6666:7777:8888:",
+    "1111::6666:7777:8888:",
+    "::6666:7777:8888:",
+    "1111:2222:3333::5555:6666:7777:8888:",
+    "1111:2222::5555:6666:7777:8888:",
+    "1111::5555:6666:7777:8888:",
+    "::5555:6666:7777:8888:",
+    "1111:2222::4444:5555:6666:7777:8888:",
+    "1111::4444:5555:6666:7777:8888:",
+    "::4444:5555:6666:7777:8888:",
+    "1111::3333:4444:5555:6666:7777:8888:",
+    "::3333:4444:5555:6666:7777:8888:",
+    "::2222:3333:4444:5555:6666:7777:8888:",
+);
+
+my @if_samples = (
+    # Sample output from:
+    #   ip -6 -o addr show dev <interface> scope global
+    # This seems to be consistent accross platforms. The last line is from Ubuntu of a static
+    # assigned IPv6.
+    ["ip -6 -o addr show dev <interface> scope global", <<'EOF'],
+2: ens160    inet6 fdb6:1d86:d9bd:1::8214/128 scope global dynamic noprefixroute \       valid_lft 63197sec preferred_lft 63197sec
+2: ens160    inet6 2001:DB8:4341:0781::8214/128 scope global dynamic noprefixroute \       valid_lft 63197sec preferred_lft 63197sec
+2: ens160    inet6 2001:DB8:4341:0781:89b9:4b1c:186c:a0c7/64 scope global temporary dynamic \       valid_lft 85954sec preferred_lft 21767sec
+2: ens160    inet6 fdb6:1d86:d9bd:1:89b9:4b1c:186c:a0c7/64 scope global temporary dynamic \       valid_lft 85954sec preferred_lft 21767sec
+2: ens160    inet6 fdb6:1d86:d9bd:1:34a6:c329:c52e:8ba6/64 scope global temporary deprecated dynamic \       valid_lft 85954sec preferred_lft 0sec
+2: ens160    inet6 fdb6:1d86:d9bd:1:b417:fe35:166b:4816/64 scope global dynamic mngtmpaddr noprefixroute \       valid_lft 85954sec preferred_lft 85954sec
+2: ens160    inet6 2001:DB8:4341:0781:34a6:c329:c52e:8ba6/64 scope global temporary deprecated dynamic \       valid_lft 85954sec preferred_lft 0sec
+2: ens160    inet6 2001:DB8:4341:0781:f911:a224:7e69:d22/64 scope global dynamic mngtmpaddr noprefixroute \       valid_lft 85954sec preferred_lft 85954sec
+2: ens160    inet6 2001:DB8:4341:0781::100/128 scope global noprefixroute \       valid_lft forever preferred_lft forever
+EOF
+    # Sample output from MacOS:
+    #   ifconfig <interface> | grep -w "inet6"
+    # (Yes, there is a tab at start of each line.) The last two lines are with a manually
+    # configured static GUA.
+    ["MacOS: ifconfig <interface> | grep -w \"inet6\"", <<'EOF'],
+	inet6 fe80::1419:abd0:5943:8bbb%en0 prefixlen 64 secured scopeid 0xa
+	inet6 fdb6:1d86:d9bd:1:142c:8e9e:de48:843e prefixlen 64 autoconf secured
+	inet6 fdb6:1d86:d9bd:1:7447:cf67:edbd:cea4 prefixlen 64 autoconf temporary
+	inet6 fdb6:1d86:d9bd:1::c5b3 prefixlen 64 dynamic
+	inet6 2001:DB8:4341:0781:141d:66b9:2ba1:b67d prefixlen 64 autoconf secured
+	inet6 2001:DB8:4341:0781:64e1:b68f:e8af:5d6e prefixlen 64 autoconf temporary
+	inet6 fe80::1419:abd0:5943:8bbb%en0 prefixlen 64 secured scopeid 0xa
+	inet6 2001:DB8:4341:0781::101 prefixlen 64
+EOF
+    ["RHEL: ifconfig <interface> | grep -w \"inet6\"", <<'EOF'],
+        inet6 2001:DB8:4341:0781::dc14  prefixlen 128  scopeid 0x0<global>
+        inet6 fe80::cd48:4a58:3b0f:4d30  prefixlen 64  scopeid 0x20<link>
+        inet6 2001:DB8:4341:0781:e720:3aec:a936:36d4  prefixlen 64  scopeid 0x0<global>
+        inet6 fdb6:1d86:d9bd:1:9c16:8cbf:ae33:f1cc  prefixlen 64  scopeid 0x0<global>
+        inet6 fdb6:1d86:d9bd:1::dc14  prefixlen 128  scopeid 0x0<global>
+EOF
+    ["Ubuntu: ifconfig <interface> | grep -w \"inet6\"", <<'EOF'],
+        inet6 fdb6:1d86:d9bd:1:34a6:c329:c52e:8ba6  prefixlen 64  scopeid 0x0<global>
+        inet6 fdb6:1d86:d9bd:1:89b9:4b1c:186c:a0c7  prefixlen 64  scopeid 0x0<global>
+        inet6 fdb6:1d86:d9bd:1::8214  prefixlen 128  scopeid 0x0<global>
+        inet6 fdb6:1d86:d9bd:1:b417:fe35:166b:4816  prefixlen 64  scopeid 0x0<global>
+        inet6 fe80::5b31:fc63:d353:da68  prefixlen 64  scopeid 0x20<link>
+        inet6 2001:DB8:4341:0781::8214  prefixlen 128  scopeid 0x0<global>
+        inet6 2001:DB8:4341:0781:34a6:c329:c52e:8ba6  prefixlen 64  scopeid 0x0<global>
+        inet6 2001:DB8:4341:0781:89b9:4b1c:186c:a0c7  prefixlen 64  scopeid 0x0<global>
+        inet6 2001:DB8:4341:0781:f911:a224:7e69:d22  prefixlen 64  scopeid 0x0<global>
+EOF
+    ["Busybox: ifconfig <interface> | grep -w \"inet6\"", <<'EOF'],
+          inet6 addr: fe80::4362:31ff:fe08:61b4/64 Scope:Link
+          inet6 addr: 2001:DB8:4341:0781:ed44:eb63:b070:212f/128 Scope:Global
+EOF
+);
+
+
+subtest "is_ipv6() with valid addresses" => sub {
+    foreach my $ip (@valid_ipv6) {
+        ok(ddclient::is_ipv6($ip), "is_ipv6('$ip')");
+    }
+};
+
+subtest "is_ipv6() with invalid addresses" => sub {
+    foreach my $ip (@invalid_ipv6) {
+        ok(!ddclient::is_ipv6($ip), sprintf("!is_ipv6(%s)", defined($ip) ? "'$ip'" : 'undef'));
+    }
+};
+
+subtest "is_ipv6() with char adjacent to valid address" => sub {
+    foreach my $ch (split(//, '/.,:z @$#&%!^*()_-+'), "\n") {
+        subtest perlstring($ch) => sub {
+            foreach my $ip (@valid_ipv6) {
+                subtest $ip => sub {
+                    my $test = $ch . $ip;  # insert at front
+                    ok(!ddclient::is_ipv6($test), "!is_ipv6('$test')");
+                    $test = $ip . $ch;  # add at end
+                    ok(!ddclient::is_ipv6($test), "!is_ipv6('$test')");
+                    $test = $ch . $ip . $ch; # wrap front and end
+                    ok(!ddclient::is_ipv6($test), "!is_ipv6('$test')");
+                };
+            }
+        };
+    }
+};
+
+subtest "extract_ipv6()" => sub {
+    my @test_cases = (
+        {name => "undef",            text => undef,               want => undef},
+        {name => "empty",            text => "",                  want => undef},
+        {name => "invalid",          text => "::12345",           want => undef},
+        {name => "two addrs",        text => "::1\n::2",          want => "::1"},
+        {name => "zone index",       text => "fe80::1%0",         want => "fe80::1"},
+        {name => "url host+port",    text => "[::1]:123",         want => "::1"},
+        {name => "url host+zi+port", text => "[fe80::1%250]:123", want => "fe80::1"},
+        {name => "zero pad",         text => "::0001",            want => "::1"},
+    );
+    foreach my $tc (@test_cases) {
+        is(ddclient::extract_ipv6($tc->{text}), $tc->{want}, $tc->{name});
+    }
+};
+
+subtest "extract_ipv6() of valid addr with adjacent non-word char" => sub {
+    foreach my $wb (split(//, '/, @$#&%!^*()_-+'), "\n") {
+        subtest perlstring($wb) => sub {
+            my $test = "";
+            foreach my $ip (@valid_ipv6) {
+                $test = "foo" . $wb . $ip . $wb . "bar"; # wrap front and end
+                $ip =~ s/\b0+\B//g; ## remove embedded leading zeros for testing
+                is(ddclient::extract_ipv6($test), $ip, perlstring($test));
+            }
+        };
+    }
+};
+
+subtest "interface config samples" => sub {
+    for my $sample (@if_samples) {
+        my ($name, $text) = @$sample;
+        subtest $name => sub {
+            my $ip = ddclient::extract_ipv6($text);
+            ok(ddclient::is_ipv6($ip), "extract_ipv6(\$text) returns an IPv6 address");
+            foreach my $line (split(/\n/, $text)) {
+                my $ip = ddclient::extract_ipv6($line);
+                ok(ddclient::is_ipv6($ip),
+                   sprintf("extract_ipv6(%s) returns an IPv6 address", perlstring($line)));
+            }
+        }
+    }
+};
+
+done_testing();