Linux Network Administrator's Guide, Second Edition

NIS Server Security

NIS used to have a major security flaw: it left your password file readable by virtually anyone in the entire Internet, which made for quite a number of possible intruders. As long as an intruder knew your NIS domain name and the address of your server, he could simply send it a request for the passwd.byname map and instantly receive all your system’s encrypted passwords. With a fast password-cracking program like crack and a good dictionary, guessing at least a few of your users’ passwords is rarely a problem.

This is what the securenets option is all about. It simply restricts access to your NIS server to certain hosts, based on their IP addresses or network numbers. The latest version of ypserv implements this feature in two ways. The first relies on a special configuration file called /etc/ypserv.securenets and the second conveniently uses the /etc/hosts.allow and /etc/hosts.deny files we already encountered in Chapter 12.^[77] Thus, to restrict access to hosts from within the Brewery, their network manager would add the following line to hosts.allow:

ypserv: 172.16.2.

This would let all hosts from IP network 172.16.2.0 access the NIS server. To shut out all other hosts, a corresponding entry in hosts.deny would have to read:

ypserv: ALL

IP numbers are not the only way you can specify hosts or networks in hosts.allow and hosts.deny. Please refer to the hosts_access(5) manual page on your system for details. However, be warned that you cannot use host ...