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.