Botnets are an important security problem on the Internet. They continuously evolve their structure, protocols and attacks. This survey analyzes and compares the most important efforts done in the network-based detection area. It accomplishes four tasks: first, the comparison of previous surveys and the proposal of four new dimensions to analyze their classification schemes. Second, a new classification and comparison of network-based botnet detection proposals, that includes the definition of twenty desired properties of every botnet detection paper. Third, an extensive comparison between the most representative detection proposals. Fourth, the description of the most important problems and highlights in the area. We conclude that the area has achieved great advances so far, but there are still many open problems.