expose already existing possibility to change the DNS servers to the DnsStub API

rutil/dns/AresDns.cxx

@@ -450,6 +450,12 @@ AresDns::internalInit(const std::vector<GenericIPAddress>& additionalNameservers
    }
 }
 
+void
+AresDns::changeNameServers(const std::vector<GenericIPAddress>& additional)
+{
+   mAdditionalNameservers = additional;
+}
+
 bool AresDns::checkDnsChange()
 {
    // We must return 'true' if there are changes in the list of DNS servers

rutil/dns/AresDns.hxx

@@ -37,6 +37,7 @@ class AresDns : public ExternalDns, public FdSetIOObserver
       int internalInit(const std::vector<GenericIPAddress>& additionalNameservers,
                        AfterSocketCreationFuncPtr socketfunc, unsigned int features=0, ares_channeldata** channel = 0, int timeout=0, int tries=0);
 
+      virtual void changeNameServers(const std::vector<GenericIPAddress>& additional);
       virtual bool checkDnsChange();
 
       virtual unsigned int getTimeTillNextProcessMS();

rutil/dns/DnsStub.cxx

@@ -303,6 +303,17 @@ DnsStub::skipDNSQuestion(const unsigned char *aptr,
    return aptr;
 }
 
+bool
+DnsStub::changeNameServers(const NameserverList& additional)
+{
+    if(mDnsProvider)
+    {
+        mDnsProvider->changeNameServers(additional);
+        return mDnsProvider->checkDnsChange();
+    }
+    return false;
+}
+
 bool
 DnsStub::checkDnsChange()
 {

rutil/dns/DnsStub.hxx

@@ -182,6 +182,7 @@ class DnsStub : public ExternalDnsHandler
       void setDnsCacheTTL(int ttl);
       void setDnsCacheSize(int size);
       void reloadDnsServers();
+      bool changeNameServers(const NameserverList& additional);
       bool checkDnsChange();
       bool supportedType(int);
 

rutil/dns/ExternalDns.hxx

@@ -44,6 +44,7 @@ class ExternalDns
       // reinitializes with the same additionalNameservers and AfterSocketCreationFuncPtr as last call to init
       virtual int init(int dnsTimeout = 0, int dnsTries = 0, unsigned int features = 0) = 0;  
 
+      virtual void changeNameServers(const std::vector<GenericIPAddress>& additional) = 0;
       //returns 'true' only is there are changes in the DNS server list
       virtual bool checkDnsChange() = 0;