1. A method for providing a seed Uniform Resource Locator (URL) of a document within a computer network to a search engine in the computer network, the computer network having a plurality of documents with URLs distributed therein, the search engine having a web-graph in which exists a set of crawled documents, the method comprising: detecting, by a router, from among data packets exchanged in the computer network external to the search engine, a packet containing a request to access a requested document, the router not being aware of which documents are not reachable by the search engine; extracting a requested document URL of the requested document from the packet containing the request; and determining that the requested document URL was not previously requested within a predetermined time interval, and in response, providing the requested document URL as the seed URL to the search engine, so as to cause the search engine to expand the set of crawled documents by adding the requested document to the set of crawled documents.