TCP Wrappers

The TCP Wrappers package (tcp_wrappers) is installed by default and provides host-based access control to network services. The most important component within the package is the /usr/lib/libwrap.a library. In general terms, a TCP-wrapped service is one that has been compiled against the libwrap.a library.

When a connection attempt is made to a TCP-wrapped service, the service first references the host's access files (/etc/hosts.allow and /etc/hosts.deny) to determine whether or not the client is allowed to connect. In most cases, it then uses the syslog daemon (syslogd) to write the name of the requesting client and the requested service to /var/log/secure or /var/log/messages.

If a client is allowed to connect, TCP Wrappers release control of the connection to the requested service and take no further part in the communication between the client and the server.

In addition to access control and logging, TCP Wrappers can execute commands to interact with the client before denying or releasing control of the connection to the requested network service.

Because TCP Wrappers are a valuable addition to any server administrator's arsenal of security tools, most network services within Community Enterprise Linux are linked to the libwrap.a library. Some such applications include /usr/sbin/sshd, /usr/sbin/sendmail, and /usr/sbin/xinetd.

To determine if a network service binary is linked to libwrap.a, type the following command as the root user:

ldd <binary-name> | grep libwrap

Replace <binary-name> with the name of the network service binary.

If the command returns straight to the prompt with no output, then the network service is not linked to libwrap.a.

The following example indicates that /usr/sbin/sshd is linked to libwrap.a:

~]# ldd /usr/sbin/sshd | grep libwrap
        libwrap.so.0 => /usr/lib/libwrap.so.0 (0x00655000)
~]#