While user access to administrative controls is an important issue for
system administrators within an organization, keeping tabs on which
network services are active is of paramount importance to anyone who
administers and operates a Linux system.
Many services under Red Hat Enterprise Linux behave as network servers. If a network
service is running on a machine, then a server application called a
daemon is listening for connections on one or
more network ports. Each of these servers should be treated as potential
avenue of attack.
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
bring a system to a screeching halt 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 mount an attack on 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
which 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.
Note
The threat of buffer overflow vulnerabilities is mitigated in
Red Hat 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® EM64T
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.
For more information about ExecShield and NX or XD technologies,
refer to the whitepaper entitled New Security
Enhancements in Red Hat Enterprise Linux v.3, Update 3,
available at the following URL:
To enhance security, most network services installed with Red Hat Enterprise Linux are
turned off by default. There are, however, some notable exceptions:
cupsd — The default print server for Red Hat Enterprise Linux.
lpd — An alternate print server.
xinetd — A super server that controls
connections to a host of subordinate servers, such as
vsftpd and telnet.
sendmail — The Sendmail mail
transport agent 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.
Red Hat 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 the chapter titled
Controlling Access to Services in the
Red Hat Enterprise Linux System Administration Guide.
Figure 4-3. Services Configuration Tool
If unsure of the purpose for a particular service, the
Services Configuration Tool has a description field,
illustrated in Figure 4-3, that may be of some
use.
But checking which network services are available to start at
boot time is not enough. Good system administrators should also check
which ports are open and listening. Refer to Section 5.8 Verifying Which Ports Are Listening for more on this subject.
Potentially, any network service is insecure. This is why turning
unused services off is so important. Exploits for services are
revealed and patched routinely, making it very important to keep
packages associated with any network service updated. Refer to Chapter 3 Security Updates for more information about this issue.
Some network protocols are inherently more insecure than others. These
include any services which do the following things:
Pass 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.
Pass Sensitive Data Over a Network
Unencrypted — Many protocols pass data over the
network unencrypted. These protocols include Telnet, FTP, HTTP,
and SMTP. Many network file systems, such as NFS and SMB, also
pass information over the network unencrypted. It is the user's
responsibility when using these protocols to limit what type of
data is transmitted.
Also, remote memory dump services, like
netdump, pass 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 includes the following:
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 Section 5.6 Securing FTP for
more information on securing FTP servers.
Services which should be carefully implemented and behind a
firewall include:
