Using the SMB/CIFS Protocol on Non-Microsoft Clients: Samba

SMB/CIFS is a protocol that is highly entrenched in the Microsoft world of operating systems and networking products. It makes sense, then, that there would be a way in which these workstations and servers could be integrated into a Unix/Linux environment without a lot of difficulty. The answer to this problem is called Samba, which is a set of products that provide for SMB conversations between SMB servers, such as Microsoft Windows operating systems, and those that do not use it natively,such as many different variants of Unix or Linux systems. In addition, Samba has been ported to several other popular operating systems, including OpenVMS.

Samba was originally developed by Andrew Tridgell and is now maintained by him and other developers (the "Samba Team") on the Internet.

From this Web site you can choose a mirror Web site for your country, and then you can view the documentation for Samba and download the most recent version. The software is freely distributed under the GNU public license.

You can configure Samba servers on your Unix boxes to enable high-performance machines to beused as file servers on the Windows network. You can still manage the servers using Unix file administration procedures, and yet to the client computers, the shares appear no different than those offered by other Windows-based file servers.

Troubleshooting a Samba server can be made easier by starting the Samba server software using the - d parameter to specify a debug level (from 1 to 100), which will cause more output messages, depending on the level you specify.

You can also use a LAN analyzer to view the sequence of exchange of SMB messages between server and client. Available as a download from the Samba home page is an extension to the tcpdump utility, called tcpdump-smb. You can use this to capture smb packets and decode the header information.

The Common Internet File System (CIFS)

CIFS is intended to be a replacement, or an upgrade, of the SMB protocol. Design considerations include making it platform-independent. While SMB resides in the NetBIOS legacy environment, CIFS runs on top of TCP. Other improvements over SMB include the following:

• Unicode filenames are used. Because the Internet is global, it's helpful to be able to encode char acters that support multiple languages.
• Service is automatically restored after a network disruption.
• CIFS is not proprietary to Microsoft. It is an Open Group standard (X/Open CAE Specification C209). It has also been proposed as an Internet standard to the Internet Engineering Task Force (IETF). This is an improvement over the earlier SMB protocol.
• DNS is used to translate between computer hostnames and IP addresses.
• Both share-level authentication and user-level authentication are supported. With user-level authentication, the user must be authenticated (using a username/password valid on the server) before access is granted.
• CIFS has been optimized to make communications across a slow link—such as a dial-up modem—possible.
• CIFS is basically an enhanced version of SMB and still uses server message block messaging. Better security and the use of DNS are necessary additions if the protocol is to be used on the Internet.

Microsoft Windows Share-Level Security

Earlier versions of Windows operating systems used the FAT (File Allocation Table) and FAT32 (similar to FAT, but for larger disk volumes, as well as other features) file systems. Beginning with Windows NT, the NTFS file system enabled a more secure file system. FAT and FAT32 don't provide the mechanisms to store security attributes, such as access control lists (ACLs), for files or directories, as NTFS does.

The main benefit of using NTFS is that it does allow you to store a lot more information about a file or a directory. When you use the NTFS file system to format a disk, you can apply user-level security permissions on individual files or directories. You can still create file shares using Windows sever operating systems, but NTFS allows you to further define which files/directories a user can access when using the file share. For an environment that requires a high degree of security, the NTFS partition is the choice to make. Additionally, the Windows 2000 and Server 2003 operating systems allow for other features that make NTFS a more secure choice, including the capability to encrypt and decrypt data on-the-fly when storing or retrieving it from disk. You can also choose to compress data on files so that less disk space is used to store files.

For either of these options, just right-click on a folder and select the properties page from the menu that appears. You'll see an Advanced button on the General tab. After clicking that button, you'll see two important check boxes. The first is Compress Contents to Save Disk Space. The second is Encrypt Contents to Secure Data. Select either or both of these check boxes to enable that feature for the folder.

The only reason to format a disk using FAT or FAT32 is if you are going to dual-boot the computer, and one of the earlier operating systems (such as Windows 9x) will be used. This is because Windows 9x systems are not capable of using an NTFS partition. You can create one partition and format it using FAT, and create additional partitions using NTFS for Windows NT and later operating systems, such as Windows 2000, Server 2003, and XP. However, this sort of dual-boot setup should be used only in an environment where security is not an important issue, such as a standalone computer (one not connected to a network). This is because a FAT partition does not let you set file or directory partitions and does not support encryption.

Another example is in your home, where you don't have such strict security requirements. For example, you might need to use an older software application that will not run under newer Windows operating systems. Even then, if you are connected to the Internet, you should consider the implications of using FAT or FAT32 on a home computer because many hackers regularly scan IP addresses looking for vulnerable systems. If you stay online for extended periods browsing the Internet—or if you're online all the time using a broadband connection such as a cable or DSL modem, then a FAT- based disk is wide open for planting a Trojan horse and other malicious programs. If you use NTFS instead, and set up your user accounts correctly, you can potentially head off this sort of problem. This is because on NTFS partitions you can set permissions for every file or directory on a one-by-one basis.

Single computers and small LANs typically use an out-of-the-box firewall solution, such as a DSL/cable router, which can offer some degree of protection, such as Network Address Translation (NAT). However, by applying permissions on an NTFS formatted disk, you can further enhance your security.

Printing Protocols and Printing Languages

A printer language is not the same as a printer protocol. For example, PostScript and PCL (Printer Control Language) are languages that describe how a document is to be rendered into the final printed product by the printer. When a printer is directly connected to a printer port on a computer, the printer language is important and is used by the software driver to format the information being sent to the printer.

A printer protocol, however, is used to send the formatted job, both data and instructions compiled using the printer language, to the print device. A few protocols will be detailed here that are more specific in their use and implementation; that is, they generally are used for communicating with a printer.

Several protocols are used for network printing. Some are proprietary protocols used by only one computer or network operating system (NOS). Others, such as 1pr/lpd—which was first developed for use on Unix systems—have been implemented in many environments. Data Link Control (DLC) is an IBM protocol that has been adapted for use on many printers although it is not used much today.

Also covered is the newest printing protocol: the Internet Printing Protocol (IPP), which was created by a working committee of the Internet Engineering Task Force (IETF). Development is underway for standards to further define this new protocol. You will find, however, that both Windows 2000/2003/XP and NetWare 6.x already support IPP. Novell even sells its iPrint as a separate product that can be used in a non-NetWare environment.

Using Ipr/lpd and the TCP Stream Protocols

TCP/IP was originally developed for the Unix operating system (OS), of which there are several flavors. Depending on the version of Unix (as well as Linux) running on a workstation or server, you will find that TCP/IP printing falls into one of two major types:

• BSD (Berkeley System Distribution) Spooling System
• SVR4 (System V, Release 4) Printing System

The BSD system uses the 1pr (line printer remote) program to send files to printers. The printers can be connected to a network, or to a computer. Whichever way, the 1pd (line printer daemon) receives these print requests and interacts with the Ipr to send the print job to the printer. The / et c/ p r int cap text file is used to set up characteristics for each printer. The SVR4 Unix system uses the 1p (line printer) program and the 1psched daemon (printer scheduler) to print files. Although the SVR4 system is considered more sophisticated because it has several utility commands for managing the system, the BSD system probably is easier to manage in a networked environment.

When using either of these methods, the actual print commands and data are sent to the printer in the payload section of a TCP/IP packet.

Although all Unix and Linux systems support TCP/IP printing, many support other protocols as well. For example, Red Hat Linux can also be configured to use SMB (Server Message Block) to connect to a Windows server (or a Unix/Linux computer configured to offer printing services using SMB).

The 1pr/lpd protocols work well, but are mostly used by older operating systems.

TCP/IP stream sockets the Unix provide yet another way to use TCP/IP to connect to a printer. Streams are a two-way communication TCP/IP session between the computer (or print server). When using TCP/IP streams, you need to specify a port (also known as a socket in Windows terminology). Thus the address of the networked printer, paired with a port number, provides a unique address so that the data exchange can be accomplished.

Data Link Control Protocol (DLC)

For all practical purposes, DLC is rarely found anymore within today's networks, although it might still be found on legacy systems. This IBM protocol was widely used during the early days of networked printing, as well as for other applications. However, more robust protocols, such as TCP/IP, have made this protocol a less desirable solution for network printing.

The DLC protocol was developed by IBM primarily for use in connecting to mainframe computers as part of its Systems Network Architecture (SNA) specifications. DLC can also be used to establish terminal sessions with AS/400 computers. In addition to the HP Jet Direct card, you will find that other vendors also make network cards for printers that can use DLC. For example, the Brother NC-600X and NC-2010h network cards both can be used for this purpose. However, you'll find that DLC no longer is present in Windows systems starting with Windows) XP.

System Daemons and Services

Windows servers have background processes that perform many functions, called services. Unix systems also have background processes that work in a similar manner that are called daemons. Regardless of what you call them, these processes, which are called background because they do not require interaction with the keyboard but instead execute on the computer waiting to perform some function, can introduce security problems when they are not needed.

You should become familiar with the background processes on any servers in your network and disable those that are not needed. For example, on Unix systems, there are many background daemons associated with the TCP/IP suite of protocols. Some systems might need all of these, whereas some might need just a few or none of them. It might be that you do need these services. It might be that they need to be configured properly prevent their misuse. You should read the dumentation that comes with your Unix or Linux system to determine the capabilities that these daemons provide and disable them on systems that do no need them.

For example, tftp (the trivial ftp transport application) is a stripped-down version of FTP. It is com and usually can be easily implemented in an EPROM. For this reason, it is useful in some devices need to download operating software from a host. However, note that unlike FTP, tftp has no acc control mechanisms. This means that a username and password are not used. Because there is no authentication, this can be a real security problem if it is not configured properly, such that it can used only for its intended purpose.

On Windows servers, you can use two programs that are provided with the Resource Kits to install or run almost any executable program or batch file as a service. These are INSTRV.EXE, which can be used to install an executable, and SRVANY.EXE, which can be used to make other kinds of files into services. On a server that has several users logging in frequently, you might want to make it a regular part of your routine maintenance to review the services running on the machines and disable or remove those that are not installed by the initial operating-system installation or those that did not come from products you have applied to the system.

To do this, you will need to keep an inventory of what runs on each server, but this kind of inventory information can be useful for other purposes, such as when you need to reinstall a server that has been destroyed by a catastrophic failure.

Removing Dead Wood

Every operating system comes with default options installed that you might not be aware of unless you have read the documentation carefully. For example, default user accounts might be created

when you install the OS or later install a product. For example, the GUEST account in Windows operating systems is installed by default. You should always disable or remove this account. The Administrator account is also a vulnerable target because it is present on all Windows Server computers from Windows NT through Server 2003. You cannot delete this account, but you can rename it so that the hacker's job becomes more difficult. Also, you shouldn't use the Administrator account on a regular basis. Instead, create individual administrative user accounts for each system administrator trusted to perform these high-level tasks. Then put them into the Domain Admins group to allow these users to exercise administrator privileges, while maintaining an audit trail of the actual users who performed certain actions. You should always use separate administrative accounts for your domain administrators. Never use the same account for domain administration as is used for regular user functions. Administrators should be logged in with administrative accounts only when performing activities that require elevated permissions. You can also use group policies to further restrict what each user can do.

Regularly review the user accounts that exist on the network. Use the auditing features provided to determine when an account has not been in use for a long period, and if you can find no reason for its existence, disable it. Maybe someone in another department did not notify you when a user was terminated, or maybe an account was created for an expected new employee or contractor who later changed his mind and did not come on board. New accounts such as these are typically created with a simple password and can leave gaping security holes in your network.

Old programs and files that are no longer needed, or the use for which you are not sure, are also easy targets to cause security problems. As a rule of thumb, if it's not needed, back it up to tape and delete it! If a user finds that something she needs is missing, she will tell you!

When installing a new application product for a user, be sure you know the capabilities of the application. Don't install unneeded optional features that will not normally be used. Read the documentation!

Physical Security Measures

Preventing unauthorized access to resources means that you must first prevent unauthorized access the physical components that make up the network. This includes user workstations, servers, netwo cables and devices, and so on. After the network connection leaves your physical area, such as when you connect to an outside Internet provider, you lose control over the physical aspects of the network. At that point, you must rely on other techniques, such as encryption or tunneling, to mainta security. However, the equipment over which you have control should be closely monitored to en that no one is tampering with anything in a manner that might serve to defeat the security policy effect at your site.

Locking the Door

As silly as it might seem, the simple door lock is an often-overlooked security device. You wouldn't leave your front door at home unlocked all the time, would you? The servers in your network that hold valuable or sensitive data should not be sitting out on a desktop or in an unlocked room where anyone can access them. Routers, hubs, switches, and other devices should be similarly protected. Wiring closets and computer rooms should have a lock on them or be protected by some sort of mon itoring on a 24-hour basis. If you have a round-the-clock operations staff, you might not need to lock the computer room. But if that staff consists of only one person during any particular period, get a lock for the door! Ideally, access to these secure areas will be tracked and logged, such as through employee badge readers. With very sensitive systems, you may even want to go as far as securing physical access through biometric authorization systems. Biometric systems, though still in their infancy, can help ensure that someone cannot gain access to a secure area simply by borrowing or stealing a physical token.

Backup media, such as tapes or writable CDs, should be treated the same as live data. Don't back up a server or your own personal workstation and then leave the tape cartridge or CD lying on the desk or in an unlocked drawer.

Uninterruptible Power Supply (UPS)

Keeping data secure can mean keeping it out of the hands of those who are not permitted to view it. It also can mean keeping the data safe from corruption. As more and more business-critical information is being committed to electronic form, it is important to take steps to be sure that it is not unintentionally compromised. A good UPS will p for itself the first time you have to spend daysreconstructing a database or reinstalling programs that become unusable due to a power outage or another problem of this sort.

Most computer operating systems have features that will work with a UPS so that the UPS can perform an orderly shutdown when it detects that power has been lost. If you are using a battery-backu UPS that has only a limited supply of power, an orderly shutdown can save a lot of problems when compared to a system crash.

Disposing of Hardware and Media in a Secure Manner

When you upgrade your network and bring in new workstations or servers, it is a generous thing to give employees, or an organization such as a school, your old equipment if it is still usable. However, you should establish a policy which dictates that all hard disks are to be erased and, when appropriate, a legal copy of the operating system reinstalled on it. If you leave important information on a computer you give away, don't be surprised when you see it again.

There is also the legal aspect to this. If you give away an old computer system, do you have the legal right to keep the software packages and install them on a new system? Probably not, unless you have a site license or another license that allows you to do so. For that reason, do not give away a computer that has applications installed on it unless you intend to give away the software packages also.

Disposing of used floppy disks, backup tapes, and tape cartridges also poses a potential security threat. It is better to destroy these information carriers than to give them away without being absolutely sure that you have purged them of any recoverable information. A bulk magnetic eraser can be a good security tool to use before disposing of this kind of stuff.

The Two Sides of Security

Locking the computer-room door is a preventive measure intended to keep out those who have no business being there. Preventive measures should be taken for software access mechanisms also. However, no matter how good you are at putting into place the access control mechanisms to protect resources, there is always going to be someone who will try, and possibly succeed, in breaking through. For this reason, you also must be able to keep audit trails of events on the network so that you can determine whether someone is trying to break your security, or whether indeed they have done so.

Before the Fact: Controlling Access

Controlling access to the network is done by several common mechanisms:

• User accounts and passwords

• Physical identifiers

• Resource protections

In many operating systems, the concept of a resource owner is important in this scheme. For example, OpenVMS and Windows 2000/Server 2003/XP keep track of the user who creates a resource, such as a file. That owner is able to change the protections applied to the file and can grant others the permissions needed to use the file. To a lesser degree, the same can be said for Unix/Linux operating systems.

Identifying Users

In a homogeneous network in which only one user account and password is required for access to permitted resources throughout the network, system management is not usually a complicated matter. Windows 2000/XP and Server 2003 allow for the creation of areas of control, called domains, that operate as security boundaries. Users in a domain can be granted access to resources on any computer, either server or workstation, that the network administrator wants to give them. In addition, trust relationships can be established between domains when administrators cooperate, making it possible for the user to still use only one username and password to connect to resources throughout the network. From Windows 2000 and more recent Windows versions, group policies can be used to further control access to important resources

Enforcing Passwords Security

It might not seem like an important thing to mention at this point, but you need to enforce a policy that makes users choose good passwords. And when you do that, you must decide what makes a good password for your environment. Simply put, a good password is one that is hard to guess. When you consider that a standard password-cracking technique used by hackers is to simply try every word in a dictionary, you can begin to understand that luck doesn't have a lot to do with penetrating a network. It comes mostly from lax security that allows doors that are easy to open.

Enforcing Good Passwords

When deciding how passwords are to be constructed, there are a few guidelines you can follow:

• Use more than one word. Multiple words "glued" together make a pattern of characters t is much harder for a simple password-cracking program to guess. Don't use words that natur go together. For example, Atlantabraves is not a good choice; Atlantayoko is a better choice. Never use a name of a celebrity or a popular institution.

• Use nonalphabetic characters somewhere in the password. This can be numeric ch acters or punctuation characters, provided that the operating system you use will permit the

• In Unix and Windows NT/2000/Server 2003 Server editions as well as Windows XP, passwords are case sensitive. If you use both upper- and lowercase characters in a p word, you can confound many password-guessing applications. Do not, however, substitute numeric characters that resemble alphabetic characters. One of the easiest things a password- hacker application can do is to substitute the letter "0" for zero. Don't fall for that one!

• Don't make passwords too difficult to memorize. The last thing you want is to have frustrated users writing down passwords so that they will be able to remember them. If you this happening, it's time to re-educate the employee. There are many methods in use today t can be used to provide "one password" for all applications on the network. You should inves gate these types of applications and, if appropriate for your network, justify the cost versus t cost of a network intrusion. Many of these systems involve smart cards and PIN numbers. Again, although this may be an expensive up-front cost, justify it by the value of your data.

• Use password history restrictions if the operating system permits it. This means the operating system keeps track of a limited number of passwords that the user has previou used and will not allow them to be reused within a certain time frame. A common practice is change your password when forced to do so and then to change it back to a value that you like and can easily remember.

Be sure that you do not create user accounts and assign them a password that never gets changed by the user. Most operating systems will allow you to set a password to be expired on its first use so that when a new user logs in the first time, he will be ruired to change his password.

Sometimes it is important to have a password that makes no sense whatsoever. In a highly secure environment this can make sense, in that you want something that is hard to guess. However, remember that when something is difficult to remember it usually gets written down somewhere, which can defeat the purpose of a password altogether. Unix has a command, passwd, that can be used to computer-generate a password for a user. For example, the command

passwordusername

displays a list of potential passwords that are generally difficult to guess. The user can select one from this list to use if he is having a difficult time thinking one up. The only problem with this method is in getting the user to memorize the password.

Password Policies

No user account, including one used by an administrator (or root for the Unix/Linux community), should ever be allowed to keep the same password for an extended period. A good idea for passwords is to require that they be changed every 30-60 days, depending on the level of security you need at your site. You also should enforce a minimum length for passwords. Most operating systems willallow you to specify this value so that users cannot change their password to one that is shorter than the size you require.

On Unix systems, you can set the password minimum length by specifying it in a field in the file /etc/default/passwd.

On Novell NetWare servers, you can enforce a minimum password length by modifying the object properties of the template object used to create a user account, or by modifying the properties of an individual user object for a particular user.

Depending on the particular operating system, you can enforce other restrictions on passwords or user accounts to enhance security on the network. Some of the capabilities you might find include these:

• Password expirations—A password should not be used indefinitely.
• Password history lists—This feature prevents a password from being reused within a specified period.
• Account lockouts—When a hacker is trying to use the brute-force method to guess a password for an account, you should be able to lock out the account automatically after a specified number of attempts within a specific time frame.

PasswordGrabbers

About the oldest trick known to those who would want to break into another user's account is the use of a program that imitates the operating system's own logon procedure. This kind of program generally is executed by someone who logs in using his own account on another's workstation. He thenruns a program that does nothing but wait until the unsuspecting user tries to log in. The program prompts for a username and password, mimicking the operating system in every respect. However, instead of logging the user on to the system, which the program is unable to do, it simply stores the password in a file and then generates a phony error message.

If the user is not too concerned about security, he will probably never know that he has been fooled. The user might think he has entered his password incorrectly and try again. The second time it will succeed because it is the operating system that is prompting the user this second time. The password grabber program has already done its job and it disappears.

The user who began this fraud simply retrieves the file, thus getting the password, and then can freely log in as that user and cause many problems when it comes to tracking down the real person who is abusing security. Because the perpetrator is now using someone else's username and password, he is difficult to catch.