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Compared to the voice service over PSTN, people look forward to a competent voice over IP (VoIP) service. The introduction of the SIP protocol has a profound impact on the VoIP world. Exploiting more SIP proxy servers can raise the service availability. However, how to assure the efficiency and availability of the service by such an improvement becomes a challenging issue. Based on above, we propose a probing-based name resolution scheme to achieve high availability and load balance for the SIP-based VoIP service. The idea is based on intercepting the prerequisite name resolution process in a typical client-server application in the IP network. We tag the probing mechanism onto the open source project domain name relay daemon (DNRD) to become a domain name resolution based load balancer (DN-LB). By our scheme, all SIP request messages from the SIP clients can be fairly distributed to a failure-proof SIP proxy server in the server farm without the necessity of using any additional costly intermediate network devices and changing the standard SIP architecture. Therefore, DN-LB can increase the service reliability and fault tolerance with a low cost for the SIP-based VoIP service.