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TCP_wrappers write a message into your system logs every time a connection is requested, whether it is granted access or not. These entries in the logs above are reason enough to have TCP_wrappers installed on your system. The messages are written through the standard syslog facility and by default go to the same place as mail transactions. On my Linux distribution, Caldera Network Desktop based on Red Hat Linux, the default has been changed so that the messages are written to the same log file as other daemons (LOG_DAEMON facility).
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Option fields let administrators easily change the log facility and priority level for a rule by using the severity directive.
In the following example, connections to the SSH daemon from any host in the example.com domain are logged to the the default authpriv facility (because no facility value is specified) with a priority of emerg:
sshd : .example.com : severity emerg |
It is also possible to specify a facility using the severity option. The following example logs any SSH connection attempts by hosts from the example.com domain to the local0 facility with a priority of alert:
sshd : .example.com : severity local0.alert |
In any event, when someone accesses my machine via telnet, a message like this is placed in the /var/log/messages:
Apr 9 17:24:58 ads in.telnetd[15339]: connect from somewhere.else.com
If the connection was refused, the message would read:
Apr 9 17:25:15 ads in.telnetd[15604]: refused connect from someother.place.comIf I want to see all the telnet attempts in my log, I can simply type the command:
grep telnetd /var/log/messagesTCP_wrappers can give me even more information through the use of “booby traps”. TCP_wrappers can be configured to run shell commands when certain services are requested. Let's assume I have reason to suspect someone at nasty.badguy.com is trying to use the Trivial FTP program (TFTP) to steal my password file. In my /etc/hosts.deny file, I can put the following line (this example is straight from the hosts_access(5) man page that comes with TCP_wrappers):
in.tftpd : nasty.bad-guy.com : ( /usr/sbin/safe_finger -l @%h |\ /bin/mail -s %d->%h root) &Access to TFTP is denied to all users from nasty.badguy.com. In addition, the command:
safe_finger @nasty.badguy.comis run, and the results are piped into a mail message sent to the root user with the subject line:
in.tftpd->nasty.bad-guy.comsafe_finger is a command provided along with the TCP_wrappers that strips out any “bad” characters, like control sequences and data overruns. Running safe_finger @hostname generates a list of everyone currently logged into that system. The strings %h and %d are called expansions, and tcpd replaces them with the corresponding text for the host name and daemon process, respectively. Other expansions include %a for the client Internet address and %u for the client user name.
Now, this isn't a perfect solution, since our cracker friend may have disabled his finger service or altered it to give false information; however, this example does show us the power of the TCP_wrappers program.
Configuring and using tcpwrappers
Logging Denied Connections
The logging of attempted (and failed) logins is done using the syslog facility.By examining /etc/syslog.conf, you'll see
# The authpriv log file should be restricted access; these # messages shouldn't go to terminals or publically-readable # files. authpriv.*;remoteauth.* /var/log/secure.logThis line will cause tcpwrappers to log any attempted logins to /var/log/secure.log.Another option, is to add a line as follows:
*.info /var/log/allmessagesThis will log any and all notifications of actions on your system. You can then use one of the common log sifting programs to notify you of any unusual activity. This will be covered in a future article on Stepwise - Editor
Note that any time you make changes to syslog.conf, you'll need to notify the syslogd process by sending it a -HUP signal, as shown above with inetd.
It is also worth repeating that you should never use tabs in the /etc/syslog.conf file, only spaces.
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Last modified: March 12, 2019
