resolve: add DNSConfiguration field to indicate DNSSEC support

The varlink API is currently missing the "DNSSEC supported" field, which
is required for to re-implement the existing resolvectl status output
with varlink instead of dbus.

Add this field to DNSConfiguration.

src/resolve/resolved-manager.c

@@ -2055,6 +2055,7 @@ static int dns_configuration_json_append(
                 Set *negative_trust_anchors,
                 Set *dns_scopes,
                 DnssecMode dnssec_mode,
+                bool dnssec_supported,
                 DnsOverTlsMode dns_over_tls_mode,
                 ResolveSupport llmnr_support,
                 ResolveSupport mdns_support,
@@ -2138,6 +2139,7 @@ static int dns_configuration_json_append(
                         SD_JSON_BUILD_PAIR_CONDITION(!set_isempty(negative_trust_anchors),
                                                      "negativeTrustAnchors",
                                                      JSON_BUILD_STRING_SET(negative_trust_anchors)),
+                        JSON_BUILD_PAIR_CONDITION_BOOLEAN(dnssec_mode >= 0, "dnssecSupported", dnssec_supported),
                         JSON_BUILD_PAIR_STRING_NON_EMPTY("dnssec", dnssec_mode_to_string(dnssec_mode)),
                         JSON_BUILD_PAIR_STRING_NON_EMPTY("dnsOverTLS", dns_over_tls_mode_to_string(dns_over_tls_mode)),
                         JSON_BUILD_PAIR_STRING_NON_EMPTY("llmnr", resolve_support_to_string(llmnr_support)),
@@ -2169,6 +2171,7 @@ static int global_dns_configuration_json_append(Manager *m, sd_json_variant **co
                         m->trust_anchor.negative_by_name,
                         scopes,
                         manager_get_dnssec_mode(m),
+                        manager_dnssec_supported(m),
                         manager_get_dns_over_tls_mode(m),
                         m->llmnr_support,
                         m->mdns_support,
@@ -2225,6 +2228,7 @@ static int link_dns_configuration_json_append(Link *l, sd_json_variant **configu
                         l->dnssec_negative_trust_anchors,
                         scopes,
                         link_get_dnssec_mode(l),
+                        link_dnssec_supported(l),
                         link_get_dns_over_tls_mode(l),
                         link_get_llmnr_support(l),
                         link_get_mdns_support(l),
@@ -2255,6 +2259,7 @@ static int delegate_dns_configuration_json_append(DnsDelegate *d, sd_json_varian
                         /* negative_trust_anchors= */ NULL,
                         scopes,
                         /* dnssec_mode= */ _DNSSEC_MODE_INVALID,
+                        /* dnssec_supported= */ false,
                         /* dns_over_tls_mode= */ _DNS_OVER_TLS_MODE_INVALID,
                         /* llmnr_support= */ _RESOLVE_SUPPORT_INVALID,
                         /* mdns_support= */ _RESOLVE_SUPPORT_INVALID,

src/shared/varlink-io.systemd.Resolve.c

@@ -274,6 +274,8 @@ SD_VARLINK_DEFINE_STRUCT_TYPE(
                 SD_VARLINK_DEFINE_FIELD(negativeTrustAnchors, SD_VARLINK_STRING, SD_VARLINK_ARRAY|SD_VARLINK_NULLABLE),
                 SD_VARLINK_FIELD_COMMENT("DNSSEC mode."),
                 SD_VARLINK_DEFINE_FIELD(dnssec, SD_VARLINK_STRING, SD_VARLINK_NULLABLE),
+                SD_VARLINK_FIELD_COMMENT("Indicates if the current DNS server supports DNSSEC. Always false if DNSSEC mode is \"no\"."),
+                SD_VARLINK_DEFINE_FIELD(dnssecSupported, SD_VARLINK_BOOL, SD_VARLINK_NULLABLE),
                 SD_VARLINK_FIELD_COMMENT("DNSOverTLS mode."),
                 SD_VARLINK_DEFINE_FIELD_BY_TYPE(dnsOverTLS, DNSOverTLSMode, SD_VARLINK_NULLABLE),
                 SD_VARLINK_FIELD_COMMENT("LLMNR support."),