[dua] add IsDomainUnicast() helper method (#5230)

src/core/backbone_router/leader.cpp

@@ -288,6 +288,11 @@ void Leader::UpdateDomainPrefixConfig(void)
 #endif
 }
 
+bool Leader::IsDomainUnicast(const Ip6::Address &aAddress) const
+{
+    return HasDomainPrefix() && aAddress.PrefixMatch(mDomainPrefix.mPrefix) >= mDomainPrefix.mLength;
+}
+
 } // namespace BackboneRouter
 } // namespace ot
 

src/core/backbone_router/leader.hpp

@@ -152,6 +152,17 @@ public:
      */
     bool HasDomainPrefix(void) const { return (mDomainPrefix.mLength > 0); }
 
+    /**
+     * This method indicates whether or not the address is a Domain Unicast Address.
+     *
+     * @param[in]  aAddress A reference to the address.
+     *
+     * @retval true  @p aAddress is a domain unicast address.
+     * @retval false @p aAddress is not a domain unicast address.
+     *
+     */
+    bool IsDomainUnicast(const Ip6::Address &aAddress) const;
+
 private:
     void UpdateBackboneRouterPrimary(void);
     void UpdateDomainPrefixConfig(void);