springboot 2.7.10
spring-cloud 3.1.5 

executing POST http://xxx/a/b/c 

Caused by: java.net.UnknownHostException: xxx 

org.springframework.cloud.openfeign.FeignClient  UnknownHostException

spring-cloud-openfeign-core-3.1.5.jar

xxx 是服务名 ，上注册中心查看对应的服务是否有 相应的 实例

spring.cloud.nacos.discovery.namespace  
注意该配置 
查看 注册中心  对应的namespace   是否有服务实例 


设置正确之后 ，调用ok 


import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestParam;

@FeignClient(name = "xxx")
public interface XxxApi {

    String PREFIX = ApiConstants.RPC_PREFIX + "/a";

    @PostMapping(value = PREFIX + "/b")
    Result a(@RequestParam(value = "param1", required = false) String param1);
}