[#3781] Append zero to generated v6 FQDNs

/doc/sphinx/arm/dhcp6-srv.rst
    Updated the arm

/src/lib/dhcpsrv/d2_client_mgr.cc
    D2ClientMgr::generateFqdn() - modified to append zero
    to names that end in hyphen.

/src/lib/dhcpsrv/tests/d2_client_unittest.cc
    TEST_F(D2ClientMgrParamsTest, generateFqdn) - updated test

Author: tmarkwalder

changelog_unreleased/3781-kea-dhcpv6-synthesized-hostnames-invalid-for-ddns-if-last-quartet-is-zero

@@ -0,0 +1,5 @@
+[bug]		tmark
+	When generating FQDNs from IP addresses for DDNS,
+	kea-dhcp6 will now append a zero to the end of
+	prefixes that would otherwise end in a hyphen.
+	(Gitlab #3781)

doc/sphinx/arm/dhcp6-srv.rst

@@ -3758,6 +3758,10 @@ qualifying suffix is "example.com", and the default value is used for
 
 ``myhost-3001-1--70E.example.com.``
 
+As of Kea 2.7.8, generated prefixes that would have ended in a hyphen
+will have a zero appended to the end. For example, the generated prefix
+for the address "2001:db8:1::" would be "myhost-2001-db8-1--0".
+
 .. _dhcp6-host-name-sanitization:
 
 Sanitizing Client FQDN Names

src/lib/dhcpsrv/d2_client_mgr.cc

@@ -173,6 +173,10 @@ D2ClientMgr::generateFqdn(const asiolink::IOAddress& address,
     std::replace(hostname.begin(), hostname.end(),
                  (address.isV4() ? '.' : ':'), '-');
 
+    if (*(hostname.rbegin()) == '-') {
+        hostname.append("0");
+    }
+
     std::ostringstream gen_name;
     gen_name << ddns_params.getGeneratedPrefix() << "-" << hostname;
     return (qualifyName(gen_name.str(), ddns_params, trailing_dot));

src/lib/dhcpsrv/tests/d2_client_unittest.cc

@@ -745,6 +745,11 @@ TEST_F(D2ClientMgrParamsTest, generateFqdn) {
     EXPECT_EQ("prefix-2001-db8--2.suffix.com.",
               mgr.generateFqdn(v6address, *ddns_params_, do_dot));
 
+    // Verify that it appends a zero when v6 address ends in colon.
+    asiolink::IOAddress v6address2("2001:db8::");
+    EXPECT_EQ("prefix-2001-db8--0.suffix.com.",
+              mgr.generateFqdn(v6address2, *ddns_params_, do_dot));
+
     // Create a disabled config.
     subnet_->setDdnsSendUpdates(false);