Available Network Services

Risks To Services

Network services can pose many risks for Linux systems. Below is a list of some of the primary issues:

• Denial of Service Attacks (DoS) - By flooding a service with requests, a denial of service attack can render a system unusable as it tries to log and answer each request.

• Script Vulnerability Attacks - If a server is using scripts to execute server-side actions, as Web servers commonly do, a cracker can attack improperly written scripts. These script vulnerability attacks can lead to a buffer overflow condition or allow the attacker to alter files on the system.
• Buffer Overflow Attacks - Services that connect to ports numbered 0 through 1023 must run as an administrative user. If the application has an exploitable buffer overflow, an attacker could gain access to the system as the user running the daemon. Because exploitable buffer overflows exist, crackers use automated tools to identify systems with vulnerabilities, and once they have gained access, they use automated rootkits to maintain their access to the system.

The threat of buffer overflow vulnerabilities is mitigated in Community Enterprise Linux by ExecShield, an executable memory segmentation and protection technology supported by x86-compatible uni- and multi-processor kernels. ExecShield reduces the risk of buffer overflow by separating virtual memory into executable and non-executable segments. Any program code that tries to execute outside of the executable segment (such as malicious code injected from a buffer overflow exploit) triggers a segmentation fault and terminates.

Execshield also includes support for No eXecute (NX) technology on AMD64 platforms and eXecute Disable (XD) technology on Itanium and Intel 64 systems. These technologies work in conjunction with ExecShield to prevent malicious code from running in the executable portion of virtual memory with a granularity of 4KB of executable code, lowering the risk of attack from stealthy buffer overflow exploits.

To limit exposure to attacks over the network, all services that are unused should be turned off.

Identifying and Configuring Services

To enhance security, most network services installed with Community Enterprise Linux are turned off by default. There are, however, some notable exceptions:

• cupsd - The default print server for Community Enterprise Linux.

• lpd - An alternative print server.
• xinetd - A super server that controls connections to a range of subordinate servers, such as gssftp and telnet.
• sendmail - The Sendmail Mail Transport Agent (MTA) is enabled by default, but only listens for connections from the localhost.
• sshd - The OpenSSH server, which is a secure replacement for Telnet.

When determining whether to leave these services running, it is best to use common sense and err on the side of caution. For example, if a printer is not available, do not leave cupsd running. The same is true for portmap. If you do not mount NFSv3 volumes or use NIS (the ypbind service), then portmap should be disabled.

Community Enterprise Linux ships with three programs designed to switch services on or off. They are the Services Configuration Tool (system-config-services), ntsysv, and chkconfig. For information on using these tools, refer to Controlling Access to Services.

Figure 46.3. Services Configuration Tool

If unsure of the purpose for a particular service, the Services Configuration Tool has a description field, illustrated in Figure 46.3, "Services Configuration Tool", that provides additional information.

Checking which network services are available to start at boot time is only part of the story. You should also check which ports are open and listening. Refer to "Verifying Which Ports Are Listening" for more information.

Insecure Services

Potentially, any network service is insecure. This is why turning off unused services is so important. Exploits for services are routinely revealed and patched, making it very important to regularly update packages associated with any network service. Refer to "Security Updates" for more information.

Some network protocols are inherently more insecure than others. These include any services that:

• Transmit Usernames and Passwords Over a Network Unencrypted - Many older protocols, such as Telnet and FTP, do not encrypt the authentication session and should be avoided whenever possible.

• Transmit Sensitive Data Over a Network Unencrypted - Many protocols transmit data over the network unencrypted. These protocols include Telnet, FTP, HTTP, and SMTP. Many network file systems, such as NFS and SMB, also transmit information over the network unencrypted. It is the user's responsibility when using these protocols to limit what type of data is transmitted.

  Remote memory dump services, like netdump, transmit the contents of memory over the network unencrypted. Memory dumps can contain passwords or, even worse, database entries and other sensitive information.

  Other services like finger and rwhod reveal information about users of the system.

Examples of inherently insecure services include rlogin, rsh, telnet, and vsftpd.

All remote login and shell programs (rlogin, rsh, and telnet) should be avoided in favor of SSH. Refer to "Security Enhanced Communication Tools" for more information about sshd.

FTP is not as inherently dangerous to the security of the system as remote shells, but FTP servers must be carefully configured and monitored to avoid problems. Refer to "Securing FTP" for more information about securing FTP servers.

Services that should be carefully implemented and behind a firewall include:

• finger

• authd (this was called identd in previous Community Enterprise Linux releases.)
• netdump
• netdump-server
• nfs
• rwhod
• sendmail
• smb (Samba)
• yppasswdd
• ypserv
• ypxfrd

More information on securing network services is available in "Server Security".

The next section discusses tools available to set up a simple firewall.