DNS configuration lifetime validation

DNS configuration lifetime needs to be at least same as the router lifetime
So it can not expire before the router is timing out.
this prevents DNS information to expire between RA messages causing inconsintent
configuration in the Wi-SUN network

"The value of Lifetime SHOULD by default be at least
3 * MaxRtrAdvInterval, where MaxRtrAdvInterval is the
maximum RA interval as defined in [RFC4861]"

Router lifetime should be
Default: 3 * MaxRtrAdvInterval

Author: Mika

source/Common_Protocols/icmpv6.c

@@ -886,6 +886,11 @@ static buffer_t *icmpv6_ra_handler(buffer_t *buf)
             uint8_t dns_search_list_len = length - 8; // Length includes type and length
             //Cut Padding
             dns_search_list_len = icmpv6_dns_search_list_remove_pad(dns_search_list, dns_search_list_len);
+
+            // validate lifetime to be at least same amount as default route lifetime
+            if (dns_lifetime > 0 && dns_lifetime < router_lifetime) {
+                dns_lifetime = router_lifetime;
+            }
             //tr_info("DNS Search List: %s Lifetime: %lu", trace_array(dns_search_list, dns_search_list_len), (unsigned long) dns_lifetime);
             // Add DNS server to DNS information storage.
             net_dns_server_search_list_set(cur->id, buf->src_sa.address, dns_search_list, dns_search_list_len, dns_lifetime);
@@ -899,6 +904,12 @@ static buffer_t *icmpv6_ra_handler(buffer_t *buf)
             }
             uint8_t dns_count = (dns_length - 1) / 2;
             uint32_t dns_lifetime = common_read_32_bit(dptr + 2); // 2 x reserved
+
+            // validate lifetime to be at least same amount as default route lifetime
+            if (dns_lifetime > 0 && dns_lifetime < router_lifetime) {
+                dns_lifetime = router_lifetime;
+            }
+
             for (int n = 0; n < dns_count; n++) {
                 uint8_t *dns_srv_addr = dptr + 6 + n * 16;
                 //tr_info("DNS Server: %s Lifetime: %lu", trace_ipv6(dns_srv_addr), (unsigned long) dns_lifetime);