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limit home 40 Any /etc/ftp.overlimit limit world 120 Any /etc/ftp.overlimit The first example limits the number of users in the home class (which you assigned your local users to) who can be connected at any one time to 40. The world class of users is limited to 120 simultaneous connections to your server. If the maximum number of connections for a group is reached, a connection from the user is refused and a message that you created in /etc/ftp.overlimit is presented to the user. The keyword Any indicates that this limitation is in effect on any day. You could, instead, assign the particular limit definition to apply to a particular day or time of day. For example, to set a limit that applies only on weekends, you could do the following: limit world 140 SaSu /etc/ftp.overlimit You can also create a limit entry based on time rather than days. To indicate a time period from 10 p.m. to 5 a.m. where access could be extended, you would enter the numbers 2200 0500, instead of SaSu, in the example. If conflicts between limit entries occur, the first matching limit takes precedence. Limiting uploading and downloading Downloading is what most people go to FTP sites to do. Typically, the disk area assigned to anonymous users contains documents or software that can be freely downloaded to your computer over the network. Some options, however, enable you to limit how much of the computer’s resources someone can consume while downloading files. The following sections describe some of them. File limits Using the file-limit parameter, you can limit the number of data files that a user can transfer in a given session based on user class. Here are some examples: file-limit total 100 home file-limit in 5 world file-limit out 30 world The first example allows all users in the home class (which we defined as belonging to your domain) to download and/or upload up to 100 files during an FTP session. To place limits on what the outside world can upload to your computer, the world class is limited to 5 incoming file transfers (uploads) and 30 outgoing transfers (downloads) per session. If no class is indicated, the entry applies to all users who don t fall into a class that already has file-limit defined. Data limits Instead of limiting the number of files a user can transfer, you can limit the amount of data that can be transferred by a user. Here are some examples: byte-limit total 102400000 home byte-limit out 51200000 world byte-limit in 25600000 world The byte-limit parameter limits the amount of data transferred during a session to a certain number of bytes. The first line shows that the total amount of data that a user in the home class can transfer during a session (both in and out) is 102400000 bytes (about 100MB). Someone assigned to the world class can take (out) only 51200000 bytes of data during an FTP session (about 50MB). A user in the world class can also upload only 25600000 bytes of data during a session (about 25MB). Note
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Enabling chmod You can set whether or not a user can change the permissions associated with a file or directory using the chmod parameter. You set chmod to either yes (to allow it) or no (to disallow it). Then you can add a comma-separated list of keywords to choose who is assigned that permission. Here is an example: chmod yes guest,class=home chmod no anonymous In this example, all guest users are allowed to use the chmod command in directories to which they are allowed to upload files. An anonymous user, however, is not allowed to use the chmod command to change file and directory permissions. Enabling delete To allow users with Linux ownership permission to delete files, use the delete parameter. Here is an example: delete yes class=home delete no anonymous,guest This example enables deletions for any user in the home class. However, deletions are not allowed for anonymous or guest users who do not belong to that class. Enabling overwrite To allow users to overwrite files that currently exist on the FTP server, use the overwrite parameter. Here is an example: overwrite no anonymous,guest,class=home In this example, no anonymous users, guest users, or users from the class “home” are allowed to overwrite files using the FTP service (regardless of the ownership of the files). Enabling rename To allow a user to rename a file with the FTP service, use the rename parameter. Here is an example: rename no anonymous,guest This prevents anonymous or guest users from renaming files using the FTP service, regardless of file ownership. Enabling umask To enable a user to change the default permission bits assigned to the files and directories the user uploads, use the umask parameter. Here is an example: umask yes anonymous,guest This example enables anonymous and guest users to use umask. Limiting the number of concurrent users You can limit how many users at a time can be logged in to your FTP server based on the class of the user. This is an excellent way of making sure that your local users can access your server, even when there is extraordinary demand from the outside world. (By not defining any limit values, you ensure that connections will not be refused based on there being too many connections.) Here are some examples of limit entries:
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Another useful option with the upload parameter is the class option. The class option lets you set upload permission for a particular class of users. Here is an example of using the class option: upload class=home /var/ftp /incoming/local yes ftp guest 0666 In this example, along with the previous example, any anonymous user could write to the /incoming directory, but only those who belonged to the class called home can write to the /incoming/local directory (within the /var/ftp directory). The permissions in this case are open (0666), and files and directories are created with FTP and guest for the user and group who own the file, respectively. Tip Using upload is not enough to ensure that an anonymous user can upload files to your FTP directories. Red Hat Linux file and directory ownership also applies. For example, if the incoming directory were owned by root, 700 permission would not allow the anonymous user to upload, but 777 permission would. Preventing files or directories from being retrieved Using the noretrieve parameter, you can prevent files with specific names or from specific directories from being retrieved. Here are some examples: noretrieve relative class=world /pub/homestuff noretrieve absolute /etc/hosts /etc/inittab noretrieve core personal money In these examples, relative and absolute are keywords that indicate whether the files and directories indicated are relative to the FTP user s directory (for example, /var/ftp) or an absolute path from the system s root directory. The first line indicates that any users in the class world cannot retrieve files from the /pub/homestuff directory relative to the /var/ftp directory (that is, /var/ftp/pub/homestuff). The second line says that the system s /etc/hosts and /etc/inittab files cannot be retrieved. When no path name is indicated, as in the third line, no files that match the names shown can be downloaded, regardless of where they reside in the file system. Allowing files or directories to be retrieved If you have prevented a group of files from being retrieved using the noretrieve parameter, you can selectively allow certain files and directories to be retrieved using the all-retrieve parameter. Here s an example: all-retrieve relative class=world /pub/homestuff/publicfile.txt In this example, although you restricted download from the /pub/homestuff directory in a previous example, you now allow the download of the /pub/homestuff/publicfile.txt file to the world class of users. Allowing permission capabilities Even after you have allowed a user or class of users permission to upload files to your FTP server, you still have a lot of control over different aspects of writing and changing files. Each of these permissions capabilities can be turned on or off for anonymous, guest, or real users. Or you can turn them on or off for a specific class of users. Below are a few examples. By default, all these services are turned on when a user has upload capabilities (that is, the capability to write to an FTP server). Note Remember that Red Hat Linux ownership permission still applies to files and directories, regardless of how these permission capabilities are set. For example, even if you are allowed to use chmod, you still couldn t use it to change permission on someone else s file that was set to 0600.
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domain and another set for everyone else. Here s how to do that: class home real,guest,anonymous 10.0.0.0/8 class world real,guest,anonymous * In this example, all users who come from a host computer with an address on network number 10.0.0.0 (presumably your local network) are assigned to the “home” class. Users from any other network are assigned to the “world” class. Instead of a network number, you can use a domain name. Note The /8 following the network address is a shorthand for the netmask. It indicates that the eight leftmost bits represent the network. In this example, therefore, the number 10 represents the network number and the next three octets represent individual host addresses. As discussed earlier, user types defined within each class are represented by one of the following keywords: real For users who have a login account on the local computer. guest For users with real login accounts who are designated as guestusers or guestgroups in the /etc/ftpaccess file. anonymous For anyone who logs in under the anonymous user name. With class names assigned, you can assign limitations or permissions to any of those classes. Allowing uploading of incoming files By default, anonymous users are not permitted to upload files to your FTP server. For uploading to occur, you must specifically allow uploading in the /etc/ftpaccess file, using the upload parameter. Also, specific parameters deal with whether guest and anonymous users can delete, overwrite, rename, or change permissions of files and directories. (Remember that allowing public uploading of files to your FTP server can pose a significant security risk.) Opening directories for uploading You need to add information to your /etc/ftpaccess to enable FTP users to upload files to your Red Hat Linux system. Here are two examples of using the upload parameter to permit ftp users to upload files: upload /var/ftp /incoming yes mike sales 0600 upload /var/ftp /testdir yes ftp ftp 0666 These examples enable you to upload files to the /incoming and /testdir directories in the anonymous user s root directory (/var/ftp). The permission the user has to write to the /incoming directory (and its subdirectories) is as the user mike and the group sales. Files are written with 0600 permission (so only mike could read and write the files). In the second example, ftp user and group permission are used to create the file and its read/write permissions are open (0666). Cross-Reference If you don’t remember what permission settings such as 0666 and 0600 mean, refer to the description of access permissions in Chapter 3. These are common options used with the chmod command to change access permissions. Allowing upload permission also allows those same users the right to create directories (within the upload areas). To disallow the creation of directories, add the nodirs keyword at the end of an upload line (after the permission numbers). To have the ability to create directories turned on for an instance where it may be off, add the dirs keyword to the end of the line.
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That was the full greeting. To shorten the greetings that a user sees, you can set the greeting option to brief or terse in the /etc/ftpaccess file as follows: greeting terse The terse option causes much shorter output to be displayed before the login prompt. Here is an example: $ ftp maple Connected to pine. 220 FTP server ready. Name (maple:mike): Changing the FTP server hostname Normally, your system s hostname is displayed when someone connects to your FTP service. If you want to assign a special hostname that applies only to connections to your FTP server, you can change that name in the /etc/ftpaccess file. Here is an example of the line you would enter: hostname ftppine The next time an FTP user connects, that user will see the hostname as ftppine. Adding a message before login If you want to have a message appear after a user connects to your FTP service, but before the login prompt, you can do that with the banner option in the /etc/ftpaccess file. First, you need to create a text file that contains the message you want to print. Next, add a banner line pointing to that file (relative to your system root, not the FTP relative root). For example: banner /etc/ftpbanner.msg With this example, the next time a user connects to the FTP service, the contents of the /etc/ftpbanner.msg file are displayed after the connection message and before the login prompt appears. Controlling FTP access Just because you open your computer to public access doesn t mean that you have to let everyone abuse your computing resources. In the /etc/ftpaccess file, you can define exactly which directories are readable and writable. You can limit the number of users who can access your FTP server at a time. You can also set permissions or limits to your resources for individuals or groups. The following sections describe some of your options. Creating user classes By setting up user classes, you create a method of assigning access to different resources to different groups of users. Any user who does not fall into a class will not have access to your FTP server. That s why the default class includes all users with the following line: class all real,guest,anonymous * The first thing you need to do is decide which groups of users you want to treat separately. An effective technique is to define your local users as one class and all other users as another class. The class keyword lets you assign classes based on network address and types of user within that address (that is, real, guest, or anonymous). For example, you may want to assign one set of permissions for all users within your Internet
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Adding helpful information The FTP service on your Red Hat Linux computer is set up to automatically display messages and to alert the user of the existence of README files at different times in a user s navigation of your site. When FTP is installed, however, none of these files exist. So to help the navigation of your site, you need to create these files. Creating README files A README file is a standard method of providing information about the contents of a software package or, in this case, the contents of an FTP directory. Your FTP server is set up, by default, to alert users to the existence of README files when they either log in or change into a directory that contains a README file. Here are the entries in the /etc/ftpaccess file that make this happen: readme README* login readme README* cwd=* The asterisk at the end of README indicates that any files that begin with the word README are matched (such as README.info, README-help, or README.txt). The README file is usually in plain text, so anyone can read it. You can create one using any text editor. Add a README file to the /var/ftp directory (to describe the contents of the FTP server) or to any directory where you want to describe the contents to the user who enters. Users who enter a directory containing a README file will see a message that tells them that the README file is there. They have to open the file themselves to see the contents. Creating message files If you want to make sure that the users see a message when they log in or enter a particular directory, you can create welcome.msg and .message files. You can create those files using any text editor. Two entries in the /etc/ftpaccess file define how these message files are set up to be read: message /welcome.msg login message .message .cwd=* You can create a welcome.msg in the FTP user s root directory (probably /var/ftp for anonymous users). It should be a plain-text message. It will appear when the user first logs in to your FTP server. In each directory, you can create a .message file. When users enter that directory, the contents of the .message file are displayed. As with the welcome.msg file, the .message file should be in plain text. Changing FTP login greetings When users log in to your FTP server, they see several standard greetings. You can change what greetings they see and what those greetings include by adding information to your /etc/ftpaccess file. Changing the initial greeting If you don’t change anything, a user who logs in to your FTP server won’t see that much information. The default greeting when a user logs into your FTP server looks something like this: $ ftp maple Connected to maple. 220 maple FTP server (Version wu-2.6.1(1) Mon Dec 25 09:21:53 EDT 2000) ready. Name (maple:mike):
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Copy the sample ftponly file to /etc and make it executable as follows: # cp /usr/share/doc/wu-ftpd*/examples/ftponly /etc/ # chmod 755 /etc/ftponly 5. Add the line /etc/ftponly to the /etc/shells file. 6. Create directories needed for the guest user s home directory to have everything FTP needs to work properly. To do this, I just copied everything from the /var/ftp directory structure (as root user) to my guest user s home directory (/home/mike). After that, I changed group assignment of /home/mike/pub to mike: # cp -r /var/ftp/* /home/mike # chgrp mike /home/mike/pub 7. Add the necessary definitions to /etc/ftpaccess. At the very least, you need to define the user as a guest login. For example, to define mike as a guest login, add the following to /etc/ftpaccess: guestuser mike Cross-Reference With just guestuser mike defined, the user mike has /home/mike as his root directory and he can get files (but not put them on the server). Also, he can browse the directory structure below /home/mike only. To add to or change permissions, see the section on controlling FTP access later in this chapter. At this point, the guest user can use the ftp command to connect to the server and have access to only those files and directories that are under the user s restricted directory. Setting Up FTP Directories, Message Files, and Greetings As an FTP site administrator, you are responsible for setting up the directory structure used on your site. You also have an opportunity to make the navigation of your site easier for users by providing a variety of message files and README files on your site and to change the greetings on your FTP server. The following sections describe these features. Creating the FTP directory structure Because you are providing a limited view of the file system to those who use your FTP service, you must provide everything they need within the root file system that they can access (typically, /var/ftp). How to set up the minimal directory structure for FTP is described in the section on setting up guest users. A viable directory structure is provided in the /var/ftp directory when you install the FTP service in Red Hat Linux (specifically, the anonftp package). After that, most of what you want to share with the public can be arranged in the /pub directory structure that you create. For directories containing information that is restricted to certain classes of user, you may want to create a different root directory (such as /local). Specific directories can be included or excluded within the directory structure using a variety of parameters in the /etc/ftpaccess file. You can choose which directories allow upload and download. You can also set whether users can rename, delete, overwrite, or change permissions of files. See the sections on controlling FTP access for further information on setting up user directories and files.
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Instead of having /var/ftp as their root directory, however, real users have / as their root directory and their regular home directories (such as /home/user, where user is the user name) as their current directory when they log in. Unlike the anonymous user, the real user has no special restrictions with regard to the file system. Whatever they can access when they log in through a regular login prompt can be accessed when they log in through FTP. No special configuration needs to be done in the /etc/ftpaccess file to allow a user with a real account to connect to the server through FTP. However, if by some chance a user or a group fell into a guestuser or guestgroup definition you set up, you could redefine that user or group as real again by setting a realuser or realgroup value for the user or group, respectively. Tip To prevent any real user from using the FTP service, you can simply add the person s user name to the /etc/ftpusers list. By default, these users are excluded because they are on this list: root, bin, daemon, adm, lp, sync, shutdown, halt, mail, news, uucp, operator, games, and nobody. Most of those users are administrative accounts that could be exploited. In the /etc/ftpaccess file, all administrative accounts except the ftp user and group are denied access to the FTP service by default. Guest users A guest user is sort of halfway between a real user and an anonymous user. You can assign any name as a guest user. However, the guest user is limited to a restricted area of the file system, typically the user s home directory. Using guest user accounts is a great way to give specific users permission to add files to and remove them from a specific part of your file system but not allow them to do much else. This is useful, for example, if you want to give users a place to set up their own Web pages. In general terms, an FTP guest user will have an account set up in the Linux /etc/passwd file, then that user will be defined as either a guestuser or guestgroup in the /etc/ftpaccess file. The following is an example of how to set up an FTP guest user account: 1. Add the user account as you would normally. For example, to add a user named mike with a home directory of /home/mike, type the following (as root user): # useradd mike 2. Add a password for mike using the passwd command (entering it twice, as prompted): # passwd mike Changing password for user mike New UNIX password:******** Retype new UNIX password: ******** 3. Edit the user s account information in /etc/passwd so that the user s root directory is changed to the restricted directory (for example, the user s /home directory). Here is an example of how that new entry would look for the user named mike: mike:x:501:501:guest acct:/home/mike/./pub:/etc/ftponly The dot (.) after /home/mike makes the /home/mike directory on the FTP server the root directory for mike. When mike logs in, his current directory is /home/mike/pub. The /etc/ftponly value prevents a shell from starting up (such as /bin/bash) if mike were to log in from a regular Linux prompt. 4.
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The anonymous FTP user Because most visitors to your FTP site from the Internet will not have an individual login account to your computer, the anonymous user name is used on public FTP sites. With the anonymous user name, anyone who can reach the FTP site from the network can log in to the server and have minimal permission for its use. Typically, minimal means that an anonymous user can only copy files from (and not write files to) the FTP server and that only selected directories are even visible to the anonymous user. Here is an example of a login session from an anonymous user: $ ftp maple Connected to maple. 220 maple FTP server (Version wu-2.6.1-18) Mon Nov 26 09:21:53 EDT 2000) ready. Name (pine:mike): anonymous 331 Guest login ok, send your complete e-mail address as password. Password: ********* 230 Guest login ok, access restrictions apply. Remote system type is UNIX. Using binary mode to transfer files. ftp> ftp> bye 221-You have transferred 0 bytes in 0 files. 221-Total traffic for this session was 313 bytes in 0 transfers. 221-Thank you for using the FTP service on maple.trees. 221 Goodbye. After connecting to the server, I typed anonymous at the login prompt. The server says that anonymous is okay and to enter my e-mail address as the password. (The password doesn t appear; I ve used asterisks here to represent the password.) The server accepted my password as valid. If it had been invalid, I could have continued, but the server would have warned me to type a valid e-mail address next time. Because this is an illustration, I just ended the session by typing bye and I was logged off. While I was logged on to the FTP server, I had access to all files and directories that the server allowed to the anonymous user. That would probably include only a restricted area of the server s system (the /var/ftp directory, by default, on Red Hat Linux systems). There were probably restrictions on what I could change, delete, or create on the server as an anonymous user as well. One of the great advantages of anonymous FTP is that it can easily be automated. Instead of using the ftp command, users can simply type the FTP address of the server into a Web browser. Users don t even need to know that they are being logged in as an anonymous user when they visit the FTP site. The list of files and directories requested simply appears. Tip Assuming you have installed the anonftp software package and started the FTP service, anonymous users are given access to your FTP server by default. This assumes that your firewall is not blocking access to the service and that “disable = yes” was changed to disable = no in the file /etc/xinetd.d/wu-ftpd. To prevent anonymous users from accessing your FTP server, add the following line to your /etc/ftpaccess file: defaultserver private Real users The users who have valid login accounts to your computer can also access the computer via FTP. These users can have any user name that the administrator defines. To use FTP, they can simply type their user names and passwords to the FTP prompts. At that point, they will be logged in to the FTP server. Cross-Reference See Chapter 11 for information on setting up regular real user accounts.
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delete no anonymous This prevents anonymous user name from deleting files or directories. overwrite no anonymous This prevents anonymous user name from overwriting existing files or directories. rename no anonymous This prevents anonymous user name from renaming any files or directories. log transfers anonymous, guest, real inbound, outbound This logs file transfers for the anonymous user, guest user, and any real users (that is, those who have their own user accounts on the Linux system). Both uploads (inbound) and downloads (outbound) transfers are logged. shutdown /etc/shutmsg This checks the /etc/shutmsg file to see if the server is about to be shut down. If it is, your FTP server sends a message to current FTP users, warning them that the server is about to go down. It also denies new FTP connections and disconnects current users at a specified time prior to shutdown. (By default, the /etc/shutmsg file does not exist.) See the section on setting up FTP shutdowns later in this chapter for information on how to set up a shutdown file for scheduled FTP shutdowns. passwd-check rfc822 warn This checks that passwords for anonymous logins are rfc822-compliant addresses. In other words, the FTP server asks for any valid e-mail address as the password for the anonymous login. If the address is not compliant (that is, is not in the form user@host.domain), the server will “warn” the user but still allow the user to log in. FTP Root Directory For a user who logs in as an anonymous user, the /var/ftp directory is assigned as the user s root directory. In other words, the anonymous user could not cd above the /var/ftp directory (or even know what files exist outside the /var/ftp directory structure). Within the /var/ftp directory are those directories and files necessary to make FTP work properly, without your having to access other files in the file system. The /bin directory contains executable commands that FTP may need (such as compress, ls, and gzip). The /etc directory contains passwd and group entries. The /lib directory holds shared object libraries needed by FTP. Finally, the /pub directory is available for placing the files that you want to be generally available to anonymous users. Note Previous to Red Hat Linux 7, the location of the FTP home directory was /home/ftp. Because administrators often shared /home directories across a group of computers so users could have access to their files from different computers, it was not useful to share the FTP home directory in this way. The FTP home directory is now /var/ftp. The home directory for your Web server (www directory) has also been moved to /var. Creating FTP Users The different types of users who can use the FTP services from your server include the anonymous user name, any of the real users who have been added to your computer (in /etc/passwd), and any special guest accounts that you set up. The following sections describe each of these types of users and how to set them up.
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