To　date,　security　researchers　evaluate　their　solutions　of　mitigating　denial-of-service　(DDoS)　attacks　via　kernel-based　or　kernel-bypassing　testing　tools.　However,　kernel-based　tools　exhibit　poor　scalability　in　attack　traffic　generation　while　kernel-bypassing　tools　result　in　unacceptable　monetary　cost.　We　propose　Excalibur,　a　scalable　and　low-cost　testing　framework　for　DDoS　defense　solutions.　The　key　idea　is　to　leverage　the　programmable　switch　to　perform　testing　tasks　with　Tbps-level　scalability　and　low　cost.　Specifically,　Excalibur　coordinates　both　a　server　and　a　programmable　switch　to　jointly　perform　testing　tasks.　It　realizes　flexible　attack　traffic　generation,　which　requires　a　large　number　of　resources,　in　the　server　while　using　the　switch　to　increase　the　sending　rate　of　attack　traffic　to　Tbps-level.　Our　experiments　on　a　64×100Gbps　Tofino　switch　show　that　Excalibur　achieves　orders-of-magnitude　higher　scalability　and　lower　cost　than　existing　tools.　©　2023　IEEE.