Tutorial For Lpi Exam 201: Part 5

Topic 209: File Sharing Servers


Brad Huntting and David Mertz
Professional Neophytes
July, 2005

Welcome to "File Sharing Servers", the fifth of eight tutorials designed to prepare you for LPI exam 201. In this tutorial you will learn how to use a Linux system as a networked file server, using any of several protocols supported by Linux

Before You Start

About this series

The Linux Professional Institute (LPI) certifies Linux system administrators at junior and intermediate levels. There are two exams at each certification level. This series of eight tutorials helps you prepare for the first of the two LPI intermediate level system administrator exams--LPI exam 201. A companion series of tutorials is available for the other intermediate level exam--LPI exam 202. Both exam 201 and exam 202 are required for intermediate level certification. Intermediate level certification is also known as certification level 2.

Each exam covers several or topics and each topic has a weight. The weight indicate the relative importance of each topic. Very roughly, expect more questions on the exam for topics with higher weight. The topics and their weights for LPI exam 201 are:

Topic 201: Linux Kernel (5) Topic 202: System Startup (5) Topic 203: Filesystems (10) Topic 204: Hardware (8) Topic 209: File Sharing Servers (8) Topic 211: System Maintenance (4) Topic 213: System Customization and Automation (3) Topic 214: Troubleshooting (6)

About this tutorial

Welcome to "File Sharing Servers", the fifth of eight tutorials designed to prepare you for LPI exam 201. In this tutorial you will learn how to use a Linux system as a networked file server, using any of several protocols supported by Linux.

The current LPI guidelines for the specific Topic 209 exam cover NFS and Samba. A system administrator designing a server configuration, however, should also keep in mind whether FTP, SCP/SSH, HTTP, or other protocols might, in fact, fit their specific requirements.

Prerequisites

To get the most from this tutorial, you should already have a basic knowledge of Linux and a working Linux system on which you can practice the commands covered in this tutorial.

About file servers

One of the most significant uses for Linux, particularly in a server context, is providing shared files to client systems. In fact, in a general way, serving files is probably most of what all networking is used for. This tutorial--and in fact, this series of tutorials--will not address peer-to-peer file sharing servers such as BitTorrent. Rather, we will look only at older client-server arrangements: a central server that provides disk stores for multiple clients. Even when clients upload files, those are always stored and served by the server, rather than in a decentralized fashion.

Protocols widely used for file serving include HTTP (the WWW), TFTP (trivial file transfer protocol), FTP (file transfer protocol), SCP (secure copy; a specialized use of SSH), RCP (remote copy; generally deprecated), NFS (network file system), and Samba (server message block). HTTP and SSH will be discussed in the tutorials for LPI exam 202, as will security issues around FTP. TFTP and RCP are special purpose or deprecated, and will not be addressed in these tutorials.

This tutorial looks at NFS and Samba in some detail, and briefly describes FTP. NFS and Samba are network file sharing protocols that allow mostly transparent access to remote filesystems. FTP might require a custom FTP client program, although many desktop environments or tools (on Linux or otherwise) hide the details of this negotiation, and effectively present the same user interface as an NFS or Samba mounted drive.

Network File System

Using NFS on a client

If the server is properly configured, and the client has appropriate permissions, mounting a remote filesystem with NFS requires only the mount command:

mount -t nfs my.nfs.server.com:/path/on/server /path/on/client

or a suitable entry in /etc/fstab:

my.nfs.server.com:/path/on/server /path/on/client nfs rw,soft 0 0

The soft option tells the kernel to send an IO error (EIO) to user processes in the event of network difficulties. The default hard option will cause processes to hang while the nfs server is unreachable.

In addition, the helper programs rpc.lockd, rpc.statd, and rpc.quotad may be run on client and/or server.

Configuring an NFS server (part one)

An NFS server requires 3 distinct programs, as well as 3 optional programs.

When an NFS client mounts an NFS file system, it contacts the following server daemons, most of which must run stand alone (as opposed to being started from inetd): portmap (sometimes named portmapper or rpc.bind), rpc.mountd (sometimes mounted), and rpc.nfsd (sometimes nfsd).

In addition, there are three optional helper programs rpc.lockd, rpc.statd, and rpc.quotad which respectively provide global locking, accelerate the lstat family of syscalls (used by ls -l, etc), and provide support for quotas.

Configuring an NFS server (part two)

All three NFS related servers use "TCPwrappers" (i.e. tcpd) for access control, and hence may require entries in /etc/host.allow.

Neither nfsd nor portmap normally require any configuration beyond /etc/hosts.allow.

The configuration file for mountd is (indirectly) /etc/exports. It says which filesystems can be mounted by which clients. Under the Linux implementation of NFS, /etc/exports is not directly parsed by mountd. Instead, the exportfs -a command parses /etc/exports and writes the result to /var/lib/nfs/xtab where mountd can read it. There are other flags to exportfs which allow these two files to be desynchronized. That is, you may temporarily add or remove exported directories without modifying the semi-permanent records in /etc/exports.

Administrators of other Unix-like servers should note that the syntax of the Linux /etc/exports file differs significantly from that of SunOS or BSD.

Configuring /etc/hosts.allow and /etc/hosts.deny

The configuration file /etc/hosts.allow describes hosts that are allowed to connect to a Linux system. This configuration is not specific to NFS, but a system needs to be permitted to connect in the first place to use and NFS server. Similarly, /etc/hosts.deny is a list of hosts prohibited from connecting.

Slightly unintuitively, first allowed hosts are searched, then denied hosts, but anything left unmatched is granted access. This does not mean that the login mechanisms of individual servers are not still operative, but a cautious administrator might deny anything not explicitly permitted (a little paranoia is good) by using:

# /etc/hosts.deny
ALL:ALL EXCEPT localhost:DENY

With an /etc/hosts.deny set to deny everything (except connections from LOCALHOST), only those connections explicitly permitted will be allowed. For example:

#/etc/hosts.allow
# Allow localhost and intra-net domain to use all servers
ALL : 127.0.0.1, 192.168.
# Let everyone ssh here except 216.73.92.* and .microsoft.com
sshd: ALL EXCEPT 216.73.92. .microsoft.com : ALLOW
# Let users in the *.example.net domain ftp in
ftpd: .example.net

Configuring /etc/exports

Here's a sample /etc/export file:

# sample /etc/exports file
/               master(rw) trusty(rw,no_root_squash)
/projects       proj*.local.domain(rw)
/usr            *.local.domain(ro) @trusted(rw)
/home/joe       pc001(rw,all_squash,anonuid=150,anongid=100)
/pub            (ro,insecure,all_squash)

Normally, root (uid 0) on the client is treated as nobody (uid 65534) on the server; this is called root squashing as it protects files owned by root (and not group/other writable) from being altered by NFS clients. The no_root_squash tag disables this behavior, and allows the root user on trusty full access to the / partition. This can be useful for installing and configuring software.

The /usr partition will be read only for all hosts except those in the "trusted" netgroup.

When /home/joe is mounted by pc001, all remote users (regardless of uid/gid) will be treated as if they have uid=150, gid=100. This is useful if the remote NFS client is a single user workstation or does not support different users (e.g. DOS).

Normally, Linux (and other Unix-like operating systems) reserves the use of TCP and UDP ports 1-1023 (so called secure ports) for use by processes running as root. To ensure that the root user has initiated a remote NFS mount, the NFS server normally requires remote clients to use "secure ports" when mounting NFS filesystems. This convention, however, is not honored by some operating systems (notably Windows). In such cases, the insecure option allows the NFS client to use any TCP/UDP port. This is usually required when serving Windows clients.

NFS utilities

nfsstat displays a time series of NFS related statistics (client and/or server) regarding the local machine similar to iostat and vmstat.

The showmount command queries mountd and shows which clients are currently mounting filesystems. As NFS is a stateless protocol, and the mountd daemon is queried infrequently, the output of showmount can become inaccurate. Unfortunately, there is not really any way to force showmount to become accurate. However, where it is inaccurate, showmount almost always errs in showing stale mounts rather than omitting active mounts (i.e. relatively harmlessly).

In this context, "stateless" means that the nfsd daemons that serve up the actual file data have no memory of which files are open, nor even which clients have which partitions mounted. Each request (readblock, writeblock, etc) contains all the information needed to complete it (partition id provided by mountd, inode number, block number(s), read/write/etc, data). The HTTP protocol is similar in this respect. An upside of statelessness if the server reboots, the clients will notice only a brief period of interrupted access.

Server Message Block

Samba server configuration

The Samba server smbd provides file and print services (largely for Windows clients). While it can be started from inetd, it is typically run as a stand alone daemon smbd -D. nmbd is the netbios nameserver (or WINS server). It too can be run from inetd, but is more typically run as a stand alone daemon nmbd -D. Samba can function as a server in a Windows WORKGROUP, as well as Primary Domain Controller.

The configuration file for both smbd and nmbd is /etc/samba/smb.conf. Copious configuration parameters are described in the smb.conf man page. The lmhosts file is used to map NetBios names to IP addresses. It's format is similar to (but not identical to) the /etc/hosts file.

There are several excellent HOWTOs on the subject of Samba configuration as well as several books. This section touches on the basic ideas with pointers to more complete documentation.

Setting up a home-directory file share

The following smb.conf snippet allows users to access their (local) home directories from remote Samba clients:

[homes]
   comment = Home Directories
   browseable = no

This is usually included in the default smb.conf file.

Setting up a print share with CUPS

Of the numerous Unix printing systems, CUPS is the least antiquated and probably the currently most popular. Depending on your distribution, CUPS may be enabled in the default smb.conf. Here is a simple example of a CUPS print share:

[global]
  load printers = yes
  printing = cups
  printcap name = cups

[printers]
  comment = All Printers
  path = /var/spool/samba
  browseable = no
  public = yes
  guest ok = yes
  writable = no
  printable = yes
  printer admin = root

[print$]
  comment = Printer Drivers
  path = /etc/samba/drivers
  browseable = yes
  guest ok = no
  read only = yes
  write list = root

CUPS can provide ppd (Postscript printer description) files and Windows drivers for clients, which, when setup properly, allow remote users to take advantage of the full range of a printers features (color versus black-and-white, resolution, paper tray select, double vs single sided printing, etc). Traditional Unix printing systems are quite cumbersome by comparison. Consult the cupsaddsmb man page for more information.

Authentication

Samba (unlike NFS) requires individual users to authenticate with the server. As with any network authentication service, care should be taken to insure that passwords are never passed over the network unencrypted. See the section on "encrypt passwords" in the smb.conf man page for details.

There are a variety of mechanisms Samba can use to authenticate remote users (clients). By their nature most of these are incompatible with the standard Unix password hash. The notable exception is when passwords are passed over the wire in the clear unencrypted, which is almost always a bad idea.

Assuming you encrypt passwords on the wire, smbpasswd will usually be used to setup users with an initial Samba password. The "Unix password sync" option allows smbpasswd to change Unix passwords whenever users change their Samba password.

Alternatively, the pam_smb module when configured can authenticate Linux users using the Samba database directly. As if that's not enough choices, LDAP can be used to authenticate Samba and/or Linux users.

Debugging Samba

When configuring a Samba server, the testparm (also called smbtestparm) command can be quite useful. It will parse the smb.conf file and report any problems.

The nmblookup command does for Samba what nslookup does for DNS; it queries the NetBios directory. See the nmblookup man page for more details.

Samba client configuration

The smbclient command provides FTP-like access to a Samba file share. Transparent access to SMB file shares is trickier; see the smbmount man page or the sharity package for more info.

File Transfer Protocol

About FTP

FTP is an old and widely used network protocol. FTP is normally run over two separate ports, 20 and 21. Port 21 is used as a control stream (transmitting login information and commands) while port 20 is used as the data stream over which actual file content is transmitted.

Generally, FTP is not considered a very secure protocol in the sense that in its default mode of operation, control information--i.e. login passwords--are transmitted in the clear. For that matter, data streams are also unencrypted, but FTP shares that feature with NFS and Samba (for secure data channels, SSH/SCP is a better choice). It is possible to layer FTP's control port over SSH, hence protecting control information.

Traditional FTP clients provide their own shell environment over which to transmit control commands and configure connections. Sometimes GUI frontends are used to provide friendlier interfaces to FTP transfers. However, nowadays, many non-dedicated tools incorporate FTP--everything from file managers to text editors are often happy to work with files served by an FTP server.

Anonymous FTP

For what FTP is most often used for, its security usually does not matter. Probably most often, FTP servers are used for "anonymous FTP"--that is, data that is available to the world at large, and hence does not require security. By convention, a username of anonymous is configured to allow access, and an identifying password (often an email address) is requested but not verified. Sometimes a username/password is required, but such a combination is provided without any deep user authentication (e.g. people who want to volunteer for a project).

Most web browsers and many file managers and tools support FTP servers transparently. Often these tools will use an FTP URL to request a file (or also to upload a file to a server). For example, the command line tool wget will retreive files from FTP servers using e.g.:

$ wget ftp://example.net/pub/somefile
$ wget ftp://user:passwd@example.net/pub/somefile

File managers will often "mount" an FTP server in a manner that is essentially identical to a local filesystem, or NFS or Samba drive (this does not, however, use the mount and /etc/fstab system; and such pseudo-partititions are usually named by their URL).

Choices of FTP servers

Given the age and ubiquity of FTP a bewildering number of implementations are available, and installed with various Linux distributions. Configuring the FTP server you decide to use will require a visit to the documentation accompanying the particular server.

Some popular Linux FTP servers include wu-ftpd, vsftpd, ProFTPd, BSD ftpd, and TUX FTP. There are many less used ones as well. In most every case, the configuration of a server will live in a file like /etc/FOOftpd.conf (for an appropriate value of "FOO"). I am fond of vsftpd, which is both fast and avoids known security glitches (the "vs" stands for "very secure").

A sample FTPd configuration file

Given the wealth of servers, configuration syntaxes will differ. But a few concepts taken from /etc/vsftpd.conf illustrate the types of options other servers provide. For vsftpd each option takes the form "option=value", with the usual hash marks for comment lines. Most other FTPd configuration files are similar.

anonymous_enable: Controls whether anonymous logins are permitted. anon_world_readable_only: When enabled, anonymous users will only be

allowed to download world-readable files.

* chroot_local_user: If enabled, local users will be placed in a

chroot() jail in their home directory after login.

* pasv_enable: Should the server use the "passive FTP" style in which

clients initiate ports (helps with firewalls at clients).

ssl_enable: If enabled, vsftpd will support SSL secure connections. tcp_wrappers: If enabled incoming connections will be fed through

access control (i.e. /etc/hosts.allow and /etc/hosts.deny.

Launching an FTP server

In the simplest case, you may start an FTP server the same way you might launch any daemon, e.g.:

% sudo vsftpd

At this point the server will listen for incoming connections, according the rules configured in its configuration file. You may also launch an FTP server from an "network super-server" such as inetd or xinetd. The LPI 202 tutorials will discuss these super-servers.

Launching a daemon individually, even if in appropiate startup scripts--either for a particular runlevel or in /etc/rcS.d/--gives you finer control over the behavior of an FTP server.