> > i have a public hostname "a" (not my machine name)on internet and > my > > ip is xx.xx.xx.xx; > > the problem is other people can get my ip through hostname.but if > he > > try to get my hostname through ip he always failed. > > the case above is in my program (i make it) ,what can i do for > this? > > This is not a CentOS issue. You need your ISP to set your "reverse > DNS". > Depending on your ISP, they may not do this for customers at all. > > did you know qmail ,he can achieve this by local software.i guess the process of dns reserve is like this:the remote pc first try to get information from you if failed he try with your ISP.is this right? but i do't know what kind of protocal it ues when remote pc contact you,and i think i just need fellow the protocal make some code and then i can do what qmail can! thanks for any help!