Open “src/dsdv/model/dsdv-routing-protocol.h” and add  “Ptr routingStream;” in public attributes:

class RoutingProtocol : public Ipv4RoutingProtocol
{
public:
....

virtual void DoDispose ();

 Ptr routingStream;
...

Then in “src/dsdv/model/dsdv-routing-protocol.cc” at line 250 add the new function:

void
RoutingProtocol::PrintRoutingTable (Ptr stream) const
{
 *stream->GetStream () << "Node: " << m_ipv4->GetObject()->GetId () << " Time: " << Simulator::Now ().GetSeconds () << "s ";
 m_routingTable.Print (stream);
}

Create output file by adding these lines in RoutingProtocol::Start() function:

void
RoutingProtocol::Start ()
{
std::ostringstream temp;
temp << "dsdv-routing-tables_" << m_ipv4->GetObject()->GetId() << ".txt";
routingStream = Create(temp.str(), std::ios::out);

Finally call print routing table each time when DSDV tables are exchanged by adding code in  "src/dsdv/model/dsdv-routing-protocol.cc" on line 785:

void RoutingProtocol::SendTriggeredUpdate () {  
   ...
   this->PrintRoutingTable (routingStream);  
   ...
}

You should find .txt files in your NS3 root directory with routing tables.