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Securing Sun Linux Systems: Part II, Network Security

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The second in a two-part series, this article provides recommendations for securing the Sun Linux 5.0 operating system. This part provides specific recommendations for network security. The information in this article applies only to the Sun Linux 5.0 distribution, although some techniques or recommendations may apply to other Linux distributions. This article is ideal for a reader with a beginner to Intermediate level of expertise.
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This article is the second part of a two-part article that provides recommendations for securing the Sun™ Linux 5.0 operating system. This part provides recommendations for network security. Part I provides recommendations for securing local access and file systems. The information in this article applies only to the Sun Linux 5.0 distribution, although some techniques or recommendations might apply to other Linux distributions.

Sun Linux 5.0 is based on the Red Hat 7.2 Linux Distribution and is a flexible, general purpose operating system. To secure a Sun Linux system against unauthorized access and modification requires changes to its default configuration. Although these changes are, in most cases, relatively minor, we strongly recommend that you make these changes to improve the security posture of a system. The changes and recommendations in this article address the majority of methods that intruders use to gain unauthorized or privileged access to Sun Linux systems. You can implement many of the changes during or immediately after system installation.

As with most security strategies, a balance must be achieved between system manageability and security. Some recommendations in this article might not apply to your environment. Removing some services as recommended in this article might negatively impact your ability to manage a system. You must know your system and security requirements before starting. Implementing the changes recommended in this article requires planning, testing, and documentation.

This article contains the following topics:

  • "Securing Network Services"
  • "Turning Kernel Network Parameters"
  • "References and Related Resources"
  • "About the Authors"
  • "Ordering Sun Documents"
  • "Accessing Sun Documentation Online"

Securing Network Services

Secure network services and configure network parameters for better security. The network services a system provides are entry points that can be attacked and exploited to gain access. It is important to understand the default configuration of Sun Linux, including its services and the methods used to disable or protect them. Often, organizations need to use protocols or services that are not secure. Where possible, we provide recommendations for improving the security of those services.

Network services enable distributed computers and their users to communicate, access remote systems and information, transfer files, send electronic mail, print on network printers, and manage remote systems. Multi-user operating systems such as Sun Linux typically provide many network services. These services are either necessary for the operation and management of an application or are essential to the service an application provides.

By default, Sun Linux only runs the portmap, rpc.statd, and sshd services. This default improves the overall security posture of a system by limiting the number of unnecessary services that are started on a system. In general, all network services that are not required to meet a business requirement should be disabled.

Protect services that are offered by a system by using as many layers as feasible. The most secure systems are not necessarily those with the most layers. The layers added must support the practice of defense-in-depth by implementing complementary and mutually reinforcing security controls. The layers must adhere to the principle of proportionality, whereby the costs of the controls are weighed against the value of the assets being protected. In this article, we describe layers at the network level, such as filtering and logging options, available in both xinetd and TCP Wrappers.

Network services can be attacked in many ways. These services might contain programming flaws, use weak or no authentication, transfer sensitive data in unencrypted format, or allow connections from any network host. These weaknesses allow attackers to access entry points and potentially exploit weaknesses in the installation or configuration.

Some simple methods are available to reduce the risk of attacks against systems. In addition to disabling unneeded services and applying all security patches, use security features (for example, encryption, strong authentication, etc.) with network services whenever possible. You can deploy firewalls like iptables on servers and desktops alike where IP forwarding is not required, but network service protection is. Massive deployments and management of firewalls on many systems can be burdensome, so plan appropriately.

Additionally, there are well-regarded open source and commercial tools that add protection. These tools address security concerns by providing the following protection: access control, logging, strong authentication, and privacy through encryption.

The open source toolkit OpenSSH is a suite of tools to replace UNIX® network commands such as telnet, ftp, rlogin, rsh, and rcp. OpenSSH provides strong authentication and privacy through the use of public-key authentication and support of PAM. Also, OpenSSH provides the capability to securely tunnel X Window network communications over an encrypted channel. When built with the TCP Wrappers library, as is the case in Sun Linux, it provides additional access control. It is a very valuable tool because of the unsafe commands it replaces. After deploying OpenSSH, disable the replaced network services in favor of OpenSSH services.

The OpenSSH tool is very powerful and highly configurable. As a result, it is essential that the tool be configured to be as secure as possible. For recommendations on OpenSSH configuration and tuning, refer to the Sun BluePrint Online articles titled "Configuring the Secure Shell Software" and "Integrating the Secure Shell Software."

To secure network services, perform the following tasks:

  • "Enable and Disable Network Services"
  • "Limit Access to Services Managed by xinetd"
  • "Enable TCP Wrappers"
  • "Enable Packet Filtering"
  • "Secure Telnet Connections"
  • "Secure Remote Access Connections"
  • "Secure FTP"
  • "Secure the Remote Procedure Call (RPC) Services"
  • "Disable or Secure automount Services"
  • "Secure the NFS Services"
  • "Secure the sendmail Services"
  • "Configure Name Service Caching"
  • "Secure Print Services"
  • "Display Access Warnings"

Enable and Disable Network Services

Network services are generally started either at boot time by /sbin/init or on demand by the xinetd daemon.

Sun Linux provides the general purpose command chkconfig to enable and disable services that are started by init and xinetd. Using chkconfig is easier and less error prone than editing configuration files.

To enable or disable a service started by init, use the following command.

# chkconfig --level <runlevels> <service> [on | off]

To enable the Apache Web Server to run on levels 3, 4, and 5, use the following command.

# chkconfig --level 345 httpd on

This command configures the httpd service to start at boot time for run levels 3, 4, and 5 only. This command only enables or prevents a service from starting; it does not actually start the service itself. To start the service manually, use the following command.

# /etc/rc.d/init.d/httpd start

Services started from xinetd are managed in a similar way. The exception is that the run level should not be specified, as it is irrelevant to services managed by xinetd. By default, the xinetd service is automatically started at run levels 4 and 5.

To enable Telnet, use the following command.

# chkconfig telnet on

This command enables Telnet and reloads the xinetd daemon, which makes Telnet available immediately.

Limit Access to Services Managed by xinetd

Sun Linux uses the xinetd daemon to control availability to minor network services. All network services are disabled by default. Services that are required to support a business requirement should be enabled as appropriate.

Use the /sbin/chkconfig command to control xinetd services. The format and operations are the same for init services. When listing services using chkconfig, the init services are listed first followed by the services controlled by xinetd.

An ideally secured server often does not run xinetd, because the daemons started in the /etc/xinetd.conf are frequently not needed.

The xinetd daemon provides a number of facilities that are useful for limiting and monitoring access to the services it manages. Access can be limited by network address, time, and other parameters.

To Change a Managed Service

  1. Edit the /etc/xinetd.d/<service> file, where <service> is the name of the network service to be customized.

  2. Use the chkconfig command to switch the service on.

    This command sends a USR2 signal to the xinetd process to reread its configuration files. Note that the signal differs from the signal used to reconfigure inetd.

Address-based access control is provided through two configuration parameters: only_from and no_access. Both parameters specify an access list, which can contain IP addresses and ranges, host names, domain names, and other parameters. Refer to the xinetd.conf(5) manual page for details.

Of the daemons started from the /etc/xinetd.d directory, the Telnet, FTP, and standard r* commands are described in the following sections. In Sun Linux, these services are disabled by default.

For restricted access servers, all connections to services managed by xinetd should be logged. Logging is enabled by default in Sun Linux.

Sun recommends that you review and consider the use of the xinetd options listed in TABLE 1. You can use them to enforce additional policies such as limiting access times. Also, they allow for binding sockets to individual network interfaces.

TABLE 1 xinetd Options

Option

Description

only_from

Specifies a list of host names, IP addresses, and/or IP address ranges that are allowed access to this service. Packets originating from one of the permitted addresses or address ranges are permitted, while all others are denied access to this service. Note that, especially in the case of UDP services, a packet with a spoofed source address is permitted to access the service.

The xinetd daemon filters UDP services as well as TCP services, unlike TCP Wrappers, which only filter the first packet to a UDP server. However, xinetd runs a separate process that acts as filter, which is a relatively inefficient process and should be limited to low-traffic services.

no_access

Specifies a list of host names, IP addresses, and/or IP address ranges that are not allowed access to a service. If a source address matches an entry on both only_from and no_access lists, the most-specific entry takes precedence.

banner_fail

References a file containing contents sent over a TCP data connection that does not meet the access criteria. The connection is closed immediately after the data is sent.

bind

Listens only for connections on a specified interface address. We strongly recommend that you use this capability to configure services to listen only on those interfaces where the service is required. If a service is not required to run on a specific network, do not make it available. For example, consider binding management services to listen for connections only on management networks.


The following example shows a possible configuration for the Telnet service.

# default: on
# description: The telnet server serves telnet sessions; it uses #
    unencrypted username/password pairs for authentication.
service telnet
{
    flags      = REUSE
    socket_type   = stream
    wait      = no
    user      = root
    server     = /usr/sbin/in.telnetd
    log_on_failure += USERID
    disable     = no
    only_from    = 192.168.1.0/24
    bind      = 192.168.1.50
    banner_fail = /etc/telnet_banner
}

In this example, the Telnet service is configured to bind to the system's IP address 192.168.1.50 and to accept connection requests only from systems on the same network, 192.168.1.0/24. In addition, the banner message defined in /etc/telnet_banner is displayed to any user attempting to connect to this service from an IP address not on the 192.168.1.0/24 network.

Enable TCP Wrappers

The TCP Wrappers tool is an open source tool, developed by Wietse Venema, that provides a flexible configuration mechanism for controlling incoming connections based on pattern matching for hostnames, DNS domains, and network addresses. With Sun Linux, this functionality is integrated into the OS and provides protection for most network services, even those started by xinetd. The tool provides a good logging capability and detects DNS hostname discrepancies that can indicate an attack.

TCP Wrappers are enabled by default on Sun Linux. TCP Wrappers are configured into a number of services, which include ssh, portmap, and every service managed by xinetd. A simple TCP Wrapper configuration could have the following configuration in its /etc/hosts.allow and /etc/hosts.deny files.

# cat /etc/hosts.allow
ALL: LOCAL 10.8.10.0/255.255.255.0 10.8.11.0/255.255.255.0
portmap: 10.8.31.100
sshd: 10.0.0.0/255.0.0.0 192.168.0.0/255.255.0.0
# cat /etc/hosts.deny
ALL: ALL

This sample configuration restricts access to all services using TCP Wrappers to the local subnet and two class C IP ranges (10.8.10.0 and 10.8.11.0). In addition, ssh and portmap allow other IP addresses to connect. For more information about TCP Wrapper capabilities, refer to the tcpd(8) manual page.

Sun Linux uses xinetd as a replacement for the venerable inetd managed network services daemon. The xinetd provides facilities that are similar to TCP Wrappers, and is generally much more flexible than inetd. Unlike TCP Wrappers, xinetd can provide protection for UDP services by intercepting all packets and matching them with access control lists before passing them on. Also, xinetd can perform rate limiting and provides many logging options. On Sun Linux, xinetd has support built in for TCP Wrappers. It is not necessary to use the tcpd binary.

Enable Packet Filtering

Linux kernels based on version 2.4 and newer, such as Sun Linux 5.0, come equipped with a built-in packet filter called netfilter, that is also referred to by the name of the command that controls it: iptables. Netfilter supports all the features commonly found in modern IP packet filters, including stateful inspection filtering.

A stateful inspection filter such as netfilter uses knowledge of packets that were previously seen as a factor in its filtering decisions. It is no longer necessary to pass all TCP packets that have the ACK flag set; DNS responses can be matched to requests. It is only necessary to add rules for the initial packet of a session, then the stateful inspection mechanism takes care of the remaining traffic. Rule sets become more concise, and there is less interference between rules for different protocols, which is a common problem with static packet filters.

Netfilter also supports application level gateways (ALGs). These are rules that handle problem protocols such as FTP that negotiate additional connections, which have to be passed by the filter. The ALGs inspect the contents of packets to determine which additional connections to expect. Only a few protocols are supported at this time, however.

Netfilter is an excellent tool for constructing host-based firewalls to protect against attacks from other hosts, not just those outside a perimeter firewall. They provide an additional layer of defense to protect against unneeded services being enabled by mistake, or when other security tools fail and are circumvented. Always practice defense in depth where possible.

More information on the netfilter tool including source code, how-to documents, and tutorials are available at: http://www.netfilter.org/.

An exhaustive list of netfilter and iptables related material is available at: http://www.linuxguruz.com/iptables/.

Secure Telnet Connections

Telnet is a user-interactive service for accessing remote systems on a network. Unfortunately, this service provides little in the way of security. By default, the only authentication information required is user name and password. Neither of these items are encrypted while in transit, and are, therefore, vulnerable to a variety of attacks including: man in the middle attack, session hijacking, and network sniffing. Tools implementing the Secure Shell protocol can serve as an effective replacement and are strongly recommended.

By default, the Telnet service is disabled in Sun Linux. If this service must be used, then consider using strong authentication mechanisms such as Kerberos, One-time passwords, tokens, or other methods. In addition, consider restricting access to the Telnet service using xinetd filtering, TCP Wrappers, or host-based firewalls. That way, the risks associated with running a nonsecure service are reduced by limiting who can access it.

Host-based firewalls and TCP Wrappers limit the hosts that may connect to a system. By restricting access to services based on IP addresses, a system can limit its exposure to network attacks. Note that firewalls do not prevent sniffing of Telnet connections.

CAUTION

One-time passwords and filtering access do not protect the contents of a session from being disclosed through network sniffing. In addition, be aware that these alternatives typically do not protect a session against being hijacked by a malicious user. The malicious user, in effect, can take over a session from an authorized user.

Secure Remote Access Connections

Access control and accountability are critical to the security of a system. Access control should involve strong authentication for system access, while accountability information should provide tracking data relative to system changes.

The standard r* commands (rsh, rlogin, and rcp) break both of these recommendations, because most implementations of r* commands involve zones of trust. Within a zone of trust, all systems are trusted and no additional authentication is required. Hence, an intruder need only gain access to one server to gain access to all servers.

The default authentication mechanism of the r* commands uses the hostname or IP address of a system and a user ID for authentication. No additional authentication is required. Considering the ease with which an IP address and user ID can be stolen or misused, this default is clearly not a secure mechanism. We recommend that you do not use the r* commands in this manner, and that you do not allow servers to offer a service in this manner.

Where possible, the use of the r* commands should be replaced with a more secure alternative such as Secure Shell. There might be times when this is not possible. In those cases, it is recommended that the host-based authentication mechanism used by the rlogin command be disabled.

To Disable Host-Based Authentication

  • Comment out the pam_rhosts_auth.so entry in /etc/pam.d/rlogin file as follows:

    #%PAM-1.0
    # For root login to succeed here with pam_securetty, "rlogin" must be
    # listed in /etc/securetty.
    auth    required /lib/security/pam_nologin.so
    auth    required /lib/security/pam_securetty.so
    auth    required /lib/security/pam_env.so
    #auth    sufficient  /lib/security/pam_rhosts_auth.so
    auth    required /lib/security/pam_stack.so service=system-auth
    account  required /lib/security/pam_stack.so service=system-auth
    password  required /lib/security/pam_stack.so service=system-auth
    session  required /lib/security/pam_stack.so service=system-auth

    Shown in bold text, this change forces rlogin to prompt for a password. The rsh protocol does not allow for password authentication, and should therefore always be disabled. Note that the pound sign (#) is added to the line.

Secure FTP

The ftp daemon has many of the same security issues as the telnet daemon. By default, all authentication information transmitted over a network is in clear-text, in much the same fashion as the Telnet protocol. This exposes the FTP protocol to many of the same attack scenarios as Telnet, including man in the middle, session hijacking, and network sniffing. For these reasons, consider alternatives to FTP when FTP transport functionality is required.

OpenSSH provides a secure alternative to FTP, providing most of the functionality of command-line based FTP access.

The Washington University FTP (wu-ftp) service is provided in Sun Linux. This implementation of the FTP service supports several security enhancements not commonly found in its predecessors. In particular, the wu-ftp service supports:

  • Enhanced access control using the /etc/ftpaccess file. Use this file to restrict access to specific users and restrict access based on group membership, source address or network, and user classes.

  • Extended logging capabilities that permit you to log connection events, individual FTP commands, individual file transfers, upload or download, and security rule violations as defined by the FTP configuration.

  • Explicit definition of the control and data ports used by the FTP server. This capability is very useful for controlling FTP access in firewall environments.

  • Containment using the chroot(1M) facility. This facility allows a service to be configured to run in a contained area, such as a user's home directory, thereby limiting the file system objects a user can access.

For more information on these capabilities and other configuration options provided by this service, refer to ftpaccess(5).

Sun Linux includes a fully configured set of directories for anonymous access. A default /etc/ftpusers file is included as well. In this file, specify all accounts not allowed to use the incoming FTP service. At a minimum, include all system accounts (for example, games, news, uucp, and so forth) in addition to the root account.

Frequently, intruders use FTP with these accounts to gain unauthorized access. Commonly, root access to a server over Telnet or Secure Shell is disabled while root FTP access is not. This configuration provides a backdoor for intruders who might modify a system's configuration by uploading modified configuration files, thereby accessing a system remotely.

The second security feature of the in.ftpd daemon is the ability of the daemon to log IP addresses of all connections and commands issued to the ftp daemon through the syslog service. Logging is enabled with the -l option. Commands issued to the ftp daemon are logged when the -d option is used. By logging FTP connection requests and commands to a log server for parsing, you can track and resolve unauthorized access attempts.

CAUTION

Using the -d option has the potential for logging passwords that are erroneously entered at the login prompt. It is important that the log files be sufficiently protected to prevent the disclosure of this sensitive information.

The wu-ftp daemon can change the banner message shown before login. Use this daemon to hide from potential attackers the version of the FTP server used.

To Change Banner Messages for Incoming FTP Connections

  1. Review the /etc/ftpaccess file to determine if the following entry is present:

    banner=/etc/ftpd/banner.msg
  2. If the entry is not present, add it.

  3. Replace the contents of the /etc/banner.msg file with a line similar to the following, as appropriate for your environment:

    Authorized Use Only

Secure the Remote Procedure Call (RPC) Services

The Remote Procedure Call (RPC) mechanism provides a way for network services to communicate and make procedure calls on remote systems. When a new RPC service is started, it registers with portmap, the central RPC service agent. The portmap maintains a table of RPC services (listed by program number) and the network addresses on which they listen for clients to connect. A client first communicates with the portmap service to determine the network address it must use to contact an RPC service. You can list current RPC services by using the rpcinfo command, which communicates with the portmap service.

CAUTION

Typically, the RPC services available in most Linux distributions are not secure. This situation is not true of other operating systems, such as the Solaris OE, that implement protocols such as AUTH_DH and RPCSEC_GSS and have the ability to restrict RPC usage to a local system using RPC that is implemented over TLI. For more information on AUTH_DH, RPCSEC_GSS, and TLI, refer to the Solaris documentation available at http://docs.sun.com/. Given that the Sun Linux operating system does not support these capabilities, unless RPC-based services are required to support a business function, such as accessing or sharing files via NFS, then disable them.

To Disable the RPC portmap Service

  • Use the following commands together:

    # /sbin/chkconfig -level 0123456 portmap off
    # /etc/rc.d/init.d/portmap stop

    The first command prevents the RPC portmap service from starting at boot time and the second command stops any currently running instances of the portmap service.

    The second command illustrates how you can disable a service without the need for a reboot. You could omit the second command, but then a system reboot would be required to completely disable the service.

    NOTE

    This approach to disabling services applies to any services managed by the chkconfig command. Use this command to either allow or prevent a service from starting at boot time. It does not start or stop services on a running system.

    CAUTION

    Be aware that stopping the portmap service does not directly prevent RPC services from running on a system. In particular, if an RPC service is configured to listen on a fixed port, such as through xinetd, then it can still be accessed. Be sure to review the contents of the /etc/xinetd.d directory for services with type parameters that include RPC. For these services, disable or restrict access to them.

Disable or Secure automount Services

The automount service automates the process of mounting file systems and devices without requiring that users have administrative privileges on a system. This process is accomplished using the kernel level autofs service with the automountd system daemon. File systems mounted by this service are automatically unmounted after a predefined period of inactivity.

In Sun Linux, the automount service is capable of mounting not only NFS shares but also removable media such as CD-ROMs and diskettes. Typically, the automount service is used heavily in environments that use naming services such as NIS or LDAP, because remote file system information used by this service can be easily stored in these directories. It can be used on a standalone system as well, where there might be a requirement for automatically mounting local removable media.

As with any service, if the automount service is not required to support business functions, it should be disabled or removed.

To Disable autofs

  • Use the following command:

    # /sbin/chkconfig -levels 0123456 autofs off

To Remove autofs

Use the following command:

# rpm -e autofs

If the automount service is required, then take steps to ensure that its configuration is as secure as possible. First, ensure that the automount service is configured to obtain its information from the correct naming service (for example, files, NIS, LDAP, etc.). Next, ensure that the file systems listed in the automount configuration are mounted appropriately.

For example, the /etc/auto.misc file can be configured to mount various forms of removable media. If this file is configured improperly, a user could mount a diskette, formatted as a Linux (ext2) file system, that contains set-user-ID binaries. These binaries could then be run, giving the user added privileges and possibly allowing the entire system to be breached. To prevent this situation, ensure that only valid devices are listed and that mount options are added, where appropriate, to the automount service configuration files such as /etc/auto.master and /etc/auto.misc.

In the next example, the keywords nosuid and nodev are added to the /etc/auto.misc file for the cd and floppy resources. This action instructs the system to ignore set-user-ID or set-group-ID bits set on any files mounted on the devices, and to not interpret any character or block special devices stored on them as well. This change is made because the Sun Linux NFS implementation does not currently support NFSSEC. Consequently, the NFS services are not able to use strong authentication or encryption such as is provided by Kerberos.

cd -fstype=iso9660,ro,nosuid,nodev :/dev/cdrom
floppy -fstype=auto,nosuid,nodev :/dev/fd0

NOTE

These options are part of the standard configuration provided by Sun Linux. For other mount options, refer to the mount(8) manual page.

CAUTION

Some mount options are specific to certainly file system types. Always be sure to use the correct options for the file systems that you are using.

Secure the NFS Services

A Sun Linux system can be an NFS server, an NFS client, both, or neither. From a security perspective, the best option is to neither provide NFS services nor accept them from any other systems as long as they are not required.

If both NFS server and client services are not required, they can be uninstalled by removing the nfs-utils package as follows:

# rpm -e nfs-utils

If either the NFS client or server services are needed, then this package should not be disabled because both the client and server software is contained in the same package.

By default, the NFS client service is enabled, whereas the server service is disabled.

To Disable the NFS Client Service

  • Use the following commands:

    # chkconfig --level 0123456 nfslock off
    # /etc/rc.d/init.d/nfslock stop

    The first command prevents the NFS client service from starting at boot time. The second command stops the NFS client service if it is already running on a system.

    Frequently, business requirements require an NFS server. If this is the case, ensure that the NFS server configuration is as secure as possible.

To Improve Security of the NFS Server Configuration

  1. Explicitly list hosts allowed access to NFS server directories.

  2. Do not open access to all systems.

  3. Export only the lowest directory necessary.

  4. Never share more data than is needed to meet the business requirement.

  5. Export read-only wherever possible. In addition, consider the use of other flags such as secure and root_squash.

    For more information on these options, refer to the exports(5) and mount(8) manual pages. The NFS server and various mechanisms available to secure it encompass more material than can be covered here.

Secure the sendmail Services

The sendmail daemon forwards and receives mail from other systems. You should use centralized mail servers to receive mail instead of using local servers. However, allow local systems to generate outgoing mail and forward it to other servers.

Ideally, a more secure Mail Transport Agent (MTA) should be used instead of the MTA bundled with Sun Linux. The sendmail daemon bundled with Sun Linux has been subject to denial of service, buffer overflow, and misconfiguration attacks. Alternative MTAs with smaller and more robust code are available. These other MTAs are more security conscious and if configured properly, compromise the security of the server less than sendmail.

If sendmail must be used as the Mail Transfer Agent (MTA), then refer to recommendations made at SendMail Consortium, in the Sendmail O'Reilly publications, and through the SunSolve OnLineSM service. There are a wide variety of sendmail versions in use, and there are differences in the associated sendmail.cf configuration files. Because of this, a sample sendmail.cf file is not included with this article.

If sendmail is not required or only outgoing sendmail is required, we recommend that you remove, disable, or enable only outgoing sendmail. The following sections provide instructions and recommendations.

To Remove or Disable sendmail

If no sendmail functionality is required to support a business need, remove or disable it. Base your decision on the need to provide outgoing mail from a system. If this capability is needed, then do not remove the sendmail software.

  • To remove the sendmail software, use the following command:

    # rpm -e sendmail sendmail-cf sendmail-doc
  • To disable the sendmail service, use the following commands:

    # chkconfig --level 0123456 sendmail off
    # /etc/rc.d/init.d/sendmail stop

    The first command prevents the sendmail service from starting at boot time. The second command stops the sendmail service if it is already running on a system.

To Allow Only Outgoing sendmail

The sendmail daemon is not needed for email delivery to other systems. All messages that can be are immediately delivered. Messages that cannot be immediately delivered are queued for future delivery. The sendmail daemon, if running, retries to deliver these messages again.

  • To enable only outgoing sendmail, use a cron job to start sendmail every hour to process undelivered messages.

    The following cron entry starts sendmail every hour to flush the mail queue:

    0 * * * * /usr/lib/sendmail –q

Configure Name Service Caching

The name service cache daemon nscd provides caching for name service requests. It exists to provide a performance boost to pending requests and to reduce name service network traffic. By default, the nscd package is installed, but the nscd daemon is disabled. This section applies only if you need to enable the nscd daemon.

The nscd daemon maintains cache entries for databases such as passwd, group, and hosts. For security reasons, it does not cache the shadow password file. All name service requests made through system library calls are routed to nscd.

Because caching name service data makes spoofing attacks easier, we recommend that you modify the configuration of nscd to cache as little data as possible. This task is accomplished by setting the positive ttl to zero in the /etc/nscd.conf file for the name service requests deemed vulnerable to spoofing attacks. In particular, modify the configuration so that passwd and group have positive and negative ttl values of zero.

NOTE

There might be a performance impact on systems that use name services intensively.

Use the nscd -g option to view the current nscd configuration on a server. This option is a helpful resource when tuning nscd.

Secure Print Services

When a Sun Linux system is installed, the line printing package is installed, but needs to be configured. By default, the lpd daemon is not started.

To Secure Print Services

  1. To ensure that line printer services are not started, disable the package at startup by using the following command.

    # /sbin/chkconfig --level 0123456 lpd off
  2. If printing services are not required for the system, remove the package by using the following command:

    # rpm -e LPRng

Display Access Warnings

Displaying access warnings allows companies to pursue full legal recourse if a system is accessed or compromised by unauthorized users. These warnings contain messages about inappropriate and unauthorized use of a system. They generally warn users that their sessions and accounts might be monitored for illegal or inappropriate use. Consult your legal counsel for requirements and verbiage.

The contents of the /etc/issue file are displayed for local and serial access. The contents of /etc/issue.network are displayed for incoming Telnet connections. Also, you can use the message of the day /etc/motd file.

The following is an example warning.

This system is for the use of authorized users only.
Individuals using this computer system without authority, or in
excess of their authority, are subject to having all of their
activities on this system monitored and recorded by system
personnel.


In the course of monitoring individuals improperly using this
system or in the course of system maintenance, the activities
of authorized users may also be monitored.


Anyone using this system expressly consents to such monitoring
and is advised that if such monitoring reveals possible
evidence of criminal activity, system personnel may provide the
evidence of such monitoring to law enforcement officials.

If sendmail service must be configured to run on a system, then consider modifying the banner message that is displayed to clients connecting to a service on TCP port 25. By default, the banner includes host name and version information that might be used by attackers and automated vulnerability scanners. Changing the banner text does not impact the ability of the service to function.

To Change the Banner Message

  1. Add the following line to the /etc/mail/sendmail.mc file and rebuild the configuration file:

    define(´confSMTP_LOGIN_MSG', ´´Mail Server Ready'')dnl
  2. Place the line after the include directive in this file, and before any FEATURE lines.

    If you modify the contents in /etc/sendmail.cf file, which is generated from the /etc/mail/sendmail.mc file, all changes are lost at the next generation cycle.

  3. To generate the /etc/sendmail.cf from the /etc/mail/sendmail.mc file, do the following:

    # m4 /etc/mail/sendmail.mc > /tmp/sendmail.cf
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