There are many services which a site may wish to provide for its users, some of which may be external. There are a variety of security reasons to attempt to isolate services onto dedicated host computers. The services which a site may provide will, in most cases, have different levels of access needs and models of trust.
Name Servers (DNS and NIS (+)) - The Internet uses the Domain Name System (DNS) to perform address resolution for host and network names. The Network Information Service (NIS) and NIS+ are not used on the global Internet, but are subject to the same risks as a DNS server.
Name-to-address resolution is critical to the secure operation of any network. An attacker who can successfully control or impersonate a DNS server can re-route traffic to subvert security protections. For example, routine traffic can be diverted to a compromised system to be monitored; or, users can be tricked into providing authentication secrets.
An organization should create well known, protected sites to act as secondary name servers and protect their DNS masters from denial of service attacks using filtering routers. Traditionally, DNS has had no security capabilities.
In particular, the information returned from a query could not be checked for modification or verified that it had come from the name server in question. Work has been done to incorporate digital signatures into the protocol which, when deployed, will allow the integrity of the information to be cryptographically verified.
Password.Key Servers (NIS(+) and KDC) - Password and key servers generally protect their vital information (i.e., the passwords and keys) with encryption algorithms. However, even a one-way encrypted password can be determined by a dictionary attack (wherein common words are encrypted to see if they match the stored encryption).
It is therefore necessary to ensure that these servers are not accessible by hosts which do not plan to use them for the service, and even those hosts should only be able to access the service (i.e., general services, such as Telnet and FTP, should not be allowed by anyone other than administrators).
Authentication/Proxy Servers (SOCKS, FWTK) - A proxy server provides a number of security enhancements. It allows sites to concentrate services through a specific host to allow monitoring, hiding of internal structure, etc. This funneling of services creates an attractive target for a potential intruder.
The type of protection required for a proxy server depends greatly on the proxy protocol in use and the services being proxied. The general rule of limiting access only to those hosts which need the services, and limiting access by those hosts to only those services, is a good starting point.
Electronic Mail - Electronic mail (email) systems have long been a source for intruder break-ins because email protocols are among the oldest and most widely deployed services. Also, by it's very nature, an email server requires access to the outside world; most email servers accept input from any source.
An email server generally consists of two parts: a receiving/sending agent and a processing agent. Since email is delivered to all users, and is usually private, the processing agent typically requires system (root) privileges to deliver the mail.
Most email implementations perform both portions of the service, which means the receiving agent also has system privileges. This opens several security holes which this document will not describe. There are some implementations available which allow a separation of the two agents.
Such implementations are generally considered more secure, but still require careful installation to avoid creating a security problem.
World Wide Web (WWW) - The Web is growing in popularity exponentially because of its ease of use and the powerful ability to concentrate information services. Most WWW servers accept some type of direction and action from the persons accessing their services.
The most common example is taking a request from a remote user and passing the provided information to a program running on the server to process the request. Some of these programs are not written with security in mind and can create security holes.
If a Web server is available to the Internet community, it is especially important that confidential information not be co-located on the same host as that server. In fact, it is recommended that the server have a dedicated host which is not "trusted" by other internal hosts.
Many sites may want to co-locate FTP service with their WWW service. But this should only occur for anon-ftp servers that only provide information (ftp-get). Anon-ftp puts, in combination with WWW, might be dangerous (e.g., they could result in modifications to the information your site is publishing to the web) and in themselves make the security considerations for each service different.
File Transfer Protocol (FTP, TFTP) - FTP and TFTP both allow users to receive and send electronic files in a point-to-point manner. However, FTP requires authentication while TFTP requires none. For this reason, TFTP should be avoided as much as possible.
Improperly configured FTP servers can allow intruders to copy, replace and delete files at will, anywhere on a host, so it is very important to configure this service correctly. Access to encrypted passwords and proprietary data, and the introduction of Trojan horses are just a few of the potential security holes that can occur when the service is configured incorrectly.
FTP servers should reside on their own host. Some sites choose to co-locate FTP with a Web server, since the two protocols share common security considerations However, the practice isn't recommended, especially when the FTP service allows the deposit of files (see section on WWW above).
Services offered internally to your site should not be co-located with services offered externally. Each should have its own host. TFTP does not support the same range of functions as FTP, and has no security whatsoever.
This service should only be considered for internal use, and then it should be configured in a restricted way so that the server only has access to a set of predetermined files (instead of every world-readable file on the system).
Probably the most common usage of TFTP is for downloading router configuration files to a router. TFTP should reside on its own host, and should not be installed on hosts supporting external FTP or Web access.
NFS - The Network File Service allows hosts to share common disks. NFS is frequently used by diskless hosts who depend on a disk server for all of their storage needs. Unfortunately, NFS has no built-in security. It is therefore necessary that the NFS server be accessible only by those hosts which are using it for service.
This is achieved by specifying which hosts the file system is being exported to and in what manner (e.g., read-only, read-write, etc.). Filesystems should not be exported to any hosts outside the local network since this will require that the NFS service be accessible externally. Ideally, external access to NFS service should be stopped by a firewall.