Issue #6 April 2005

Tips & Tricks

Red Hat's customer service team receives technical support questions from users all over the world. As they are received, Red Hat technicians add the questions and answers to the Red Hat Knowledgebase on a daily basis. Individuals with a login are granted access. Every month, Red Hat Magazine offers a preview into the Red Hat Knowledgebase by highlighting some of the most recent entries.

This month's issue features articles about security, including an inside view of Red Hat Enterprise Linux 4. We realize it is impossible to answer all your security questions with just one magazine issue, but we've tried to answer a few more with this month's Tips & Tricks.

Tips from RHCEs

Creating multiple terminals with screen

Screen is a useful utility that I stumbled across several years ago and continue to use today. It is a terminal multiplexer that acts as a full screen window manager for text-based terminals. Simply put, screen allows you to open multiple terminals on a single connection.

You might wonder how or why this is useful since Red Hat provides six ttys by default. As an example, consider when only one tty is available, as is the case in single user mode. Another good example is when a user connects to a remote system through SSH or over a modem. Rather than making multiple connections, you can use your single connection and run screen to get multiple ttys.

Screen can monitor terminals and display a message if it detects activity. It provides a clipboard type feature that allows you to copy/paste from window to window. It also gives you a buffer allowing you to scroll back through history.

How many of you have started a long process at work like, for instance, a kernel rebuild? If you begin that process within screen, then you the screen, go home, ssh to your office machine, and re-attach to the screen. The process remains running the entire time. You can attach and detach screens at will, but the processes continue to run.

The screen utility is provided by the screen rpm. You can install it on your Red Hat Enterprise Linux system by typing up2date screen. Some important keystrokes to note: Ctrl-A is the control key for screen. You follow Ctrl-A with another key that corresponds to what you want to do. To see a list of all of the key bindings, press Ctrl-A followed by ?. Don't forget to check out the main page for more information.

The next time you need multiple windows over one connection, don't forget about this valuable utility!

How can I verifying which ports are listening?

After configuring network services, it is important to pay attention to which ports are actually listening on the system's network interfaces. Any open ports can be evidence of an intrusion.

There are two basic approaches for listening the ports that are listening on the network. The less reliable approach is to query the network stack by typing commands such as netstat -an or lsof -i. This method is less reliable since these programs do not connect to the machine from the network but rather check to see what is running on the system. For this reason, these applications are frequent targets for replacement by attackers in an attempt to cover their tracks if they open unauthorized network ports.

A more reliable way to check which ports are listening on the network is to use a port scanner such as nmap.

The following command issued from the command line determines which ports are listening for TCP connections from the network:

nmap -sT -O localhost

The output of this command looks like the following:

Starting nmap V. 3.00 ( )
Interesting ports on localhost.localdomain (
(The 1596 ports scanned but not shown below are in state: closed)
Port State Service
22/tcp open ssh
111/tcp open sunrpc
515/tcp open printer
834/tcp open unknown
6000/tcp open X11
Remote OS guesses: Linux Kernel 2.4.0 or Gentoo 1.2 Linux 2.4.19 rc1-rc7)
Nmap run completed -- 1 IP address (1 host up) scanned in 5 seconds

This output shows the system is running portmap due to the presence of the sunrpc service. However, there is also a mystery service on port 834. To check if the port is associated with the official list of known services, type:

cat /etc/services | grep 834

This command returns no output, which indicates that while the port is in the reserved range (meaning 0 through 1023) and requires root access to open, it is not associated with a known service.

Next, check for information about the port using netstat or lsof. To check for port 834 using netstat, use the following command:

netstat -anp | grep 834

The command returns the following output:

tcp   0     0*     LISTEN     653/ypbind

The presence of the open port in netstat is reassuring because a cracker opening a port surreptitiously on a hacked system would likely not allow it to be revealed through this command. Also, the -p option reveals the process id (PID) of the service which opened the port. In this case the open port belongs to ypbind (NIS), which is an RPC service handled in conjunction with the portmap service.

The lsof command reveals similar information since it is also capable of linking open ports to services:

lsof -i | grep 834

Below is the relevant portion of the output for this command:

ypbind 653 0 7u IPv4 1319 TCP *:834 (LISTEN)
ypbind 655 0 7u IPv4 1319 TCP *:834 (LISTEN)
ypbind 656 0 7u IPv4 1319 TCP *:834 (LISTEN)
ypbind 657 0 7u IPv4 1319 TCP *:834 (LISTEN)

These tools reveal a great deal about the status of the services running on a machine. They are flexible and can provide a wealth of information about network services and configuration. Consulting the man pages for lsof, netstat, nmap, and similar services is therefore highly recommended.

How do I secure SSH to disable direct root login?

To disallows direct root login, edit the /etc/ssh/sshd_config file with a text editor and find the following line:

#PermitRootLogin yes

Change the yes to no and remove the comment character at the beginning of the line:

PermitRootLogin no

Restart the sshd service:

service sshd restart

It is also recommended that you also restrict access to your system by limiting users root access with the su command. Refer to additional articles in Red Hat Knowledgebase on how to restrict the use of su command.

What ports and/or websites do I need to allow through my firewall so that the Red Hat Update Agent (up2date) and Red Hat Network (RHN) services are available?

The Red Hat Network (RHN) service is hosted at This is the default configuration and need not be changed unless you are using a Red Hat Network Satellite server.

To verify the server your system is configured to use, issue the command:

 up2date-nox --configure

at a terminal. Here is an example of the command output:

0.  debug              No
1.  useRhn             Yes
2.  rhnuuid            578918ea-cb1c-11d8-8f02-814df2323483
3.  isatty             Yes
4.  showAvailablePacka No
5.  networkSetup       Yes
6.  retrieveOnly       No
7.  enableRollbacks    No
8.  noSSLServerURL
9.  pkgSkipList        ['kernel*']
10. storageDir         /var/spool/up2date
11. adminAddress       ['root@localhost']
12. noBootLoader       No
13. serverURL
14. fileSkipList       []
15. versionOverride
16. sslCACert          /usr/share/rhn/RHNS-CA-CERT
17. noReplaceConfig    Yes
18. useNoSSLForPackage No
19. systemIdPath       /etc/sysconfig/rhn/systemid
20. enableProxyAuth    No
21. retrieveSource     No
22. disallowConfChange ['noReboot', 'sslCACert', 'useNoSSLForPackages', 'noSSLSe
23. headerFetchCount   10
24. networkRetries     5
25. pkgsToInstallNotUp ['kernel', 'kernel-modules']
26. enableProxy        No
27. proxyPassword
28. updateUp2date      Yes
29. keepAfterInstall   No
30. proxyUser
31. removeSkipList     ['kernel*']
32. useGPG             Yes
33. gpgKeyRing         /etc/sysconfig/rhn/up2date-keyring.gpg
34. httpProxy
35. headerCacheSize    40
36. forceInstall       No
37. noReboot           No
Enter number of item to edit :

Entry numbers 8 and 13 — noSSLServerURL and serverURL — define what servers up2date uses.

Alternatively, you may also verify the settings by looking at the contents of the file /etc/sysconfig/rhn/up2date and locating the entries for noSSLServerURL and serverURL:


For RHN and up2date to work correctly the firewall must allow connections to:

  • port 80 (http)
  • port 443 (https)

In addition, to be able to access the Red Hat Network (RHN) website, the firewall must allow connections to:

  • port 80 (http)
  • port 443 (https)

How can I prevent a non-root user from shutting down or rebooting the system?

To prevent non-root users from using the shutdown/reboot/halt commands:

  1. In the file /etc/X11/gdm/gdm.conf, change the line that reads:


  2. In the file /etc/inittab, change the line that reads:
    ca::ctrlaltdel:/sbin/shutdown -t3 -r now


    ca::ctrlaltdel:echo <A message indicating rebooting is not possible>
  3. In the directory /etc/security/console.apps/, delete the files reboot, poweroff, and halt.
  4. Remove the file /usr/bin/poweroff.

To remove files, you can use the rm command. Example: rm /usr/bin/poweroff.

How do I configure Sendmail to use SSL encryption for sending/receiving email?

Sendmail can be configured to encrypt email via the secure socket layer (SSL) when sending and receiving messages. This requires the following configuration changes to Sendmail.

First, SSL requires a valid certificate on the server. This can be obtained from a company which digitally signs the certificate or it can be created locally. To generate an SSL certificate, follow these steps:

cd /usr/share/ssl/certs
make sendmail.pem

Enter the desired information for Country Name, State or Province, Organization Name, etc. when prompted. Next, uncomment the following lines in /etc/mail/ by removing the dnl at the beginning of the line:

define(`confSERVER_KEY',`/usr/share/ssl/certs/sendmail.pem') ...
DAEMON_OPTIONS(`Port=smtps, Name=TLSMTA, M=s')

Restart Sendmail with the command:

service sendmail restart

Secure SMTP is now enabled for sending mail over port 465 using SSL. To enable the secure IMAP/POP3 services, enable them with:

chkconfig pop3s on
chkconfig imaps on

Secure POP3 operates over port 995 and secure IMAP uses port 993. The server is now configured to use SSL for sending and receiving email. When setting up the email client, make sure it is configured to use secure connection (SSL) "Always" in both the sending and receiving preferences.

How can I limit who can use SSH based on a list of users?

One advantage of Pluggable Authentication Module (PAM) is it can be used to limit the number of network users who have access to a certain service based on a list. For example, you can limit SSH connections via PAM.

In /etc/pam.d/sshd, add the following line:

auth  required /lib/security/ onerr=fail item=user sense=allow file=/etc/sshd_users

This will allow a user to login via sshd if they are listed in the /etc/sshd_users file. The options specified have the following meanings:

  • onerr=fail—If an error occurs (file specified is not found or an improperly formatted entry is found in the file), fail this test. This will deny the user access via sshd. The other possible option for onerr is succeed.
  • item=user—This states that we are testing or verifying the user's login name.
  • sense=allow—This means that if the user is found in the file specified, this test succeeds. This will allow the user access if all other PAM tests succeed as well. The other possible option for sense is deny.
  • file=/etc/sshd_users—This specifies the file that will contain the list of users (one per line) that are allowed to access sshd.

With that, the /etc/pam.d/sshd will look like:

auth       required service=system-auth
auth       required
auth       required onerr=fail item=user 
                          sense=allow file=/etc/sshd_users
account    required service=system-auth
password   required service=system-auth
session    required service=system-auth
session    required
session    optional

Afterwards, put the valid SSH users in the /etc/sshd_users file. Each username should be on a new line.

What firewall settings do I need to use NTP (Network Time Protocol)?

To use a Network Time Protocol (NTP) client or server on your system you need to have access to port 123 both incoming and outgoing between any client and the server that it is communicating with. The startup script for the NTP service should create the firewall holes on your machine automatically. You will have to ensure that all points along the route between the client and the server will allow traffic on port 123.

This article is protected by the Open Publication License, V1.0 or later. Copyright © 2004 by Red Hat, Inc.