Transcript Server Technologies II - College of Computing & Informatics
INFO 321 Server Technologies II
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What is Samba?
◊Samba is essentially a TCP/IP file and print server for Microsoft Windows clients • It can support any SMB/CIFS-enabled client ◊SMB/CIFS?
• SMB is Server (or Session) Message Block • Common Internet File System (CIFS) is Windows File Sharing 2
SMB
◊SMB was originally developed to run over NetBIOS (Network Basic Input Output System) LANs in the mid-1980’s • SMB was originally for MS-DOS and PC-DOS • SMB is the file sharing protocol inside CIFS • The inventor of SMB was Dr. Barry Feigenbaum of IBM, who initially called it BAF • Notice the name Samba contains SaMBa 3
SMB
◊As of Windows 2000, the connection to NetBIOS was removed, thankfully • There exists NBT (NetBIOS over TCP/IP) SMB over TCP/IP without NBT is "naked" When running naked, SMB defaults to using TCP port 445 instead of the NBT Session Service port (TCP/139) ◊SMB should work over any transport protocol 4
CIFS
◊CIFS is a network file system plus a set of auxiliary services supported by a bunch of underlying protocols • Most of which are not particularly well documented by Microsoft; you may faint now • CIFS is "Microsoft's way of doing network file sharing“ since about 1996 ◊There is no RFC number for SMB or CIFS • Microsoft's implementations are the de facto CIFS standards 5
So what?
◊ “One of Samba’s big strengths is that you can use it to blend your mix of Windows and Linux machines together without requiring a separate Windows server. Samba includes support for Active Directory, Unicode, new authentication and filename mangling systems, printing support, trust relationships, LDAP integration and loadable RPC modules.” • From The Unofficial Samba HOWTO 6
If that’s not enough
◊ “Samba is software that can be run on a platform other than Microsoft Windows, for example, UNIX, Linux, IBM System 390, OpenVMS, and other operating systems. Samba uses the TCP/IP protocol that is installed on the host server. When correctly configured, it allows that host to interact with a Microsoft Windows client or server as if it is a Windows file and print server.” • From What is Samba?
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Samba Today
◊Samba is on version 3.5.6 (as of January 19, 2011), and is available for the following platforms • UNIX - AIX (IBM UNIX), HP-UX (HP UNIX), Solaris (Sun UNIX) • Linux - Debian, Fedora, RedHat, SuSE • MVS (IBM mainframes) • Novell (NetWare) • VMS (was DEC, now HP OpenVMS) From http://us1.samba.org/samba/ftp/Binary_Packages/ 8
SMB addressing - UNC
◊A Universal Naming Convention (UNC) path might look something like this: • \\ubiqx\cifs\SMB.html ◊...and would parse out like this: • ubiqx = The name of the server • cifs = The directory path • SMB.html = The file name No!
Much of this section adapted from Implementing CIFS 9
SMB addressing - UNC
◊ UNC doesn't provide a protocol specification • That's not because it always assumes SMB • The UNC format can support all sorts of filesharing protocols, but it is up to the underlying OS or application to figure out which one to use ◊ Protocol and transport discovery are handled by trial-and-error, with each possibility tested until something works • You might imagine a system with AppleTalk, NetWare, and SMB all enabled has a lot of work to do 10
UNC
◊The UNC format is handled natively by Microsoft & IBM's family of operating systems: DOS, OS/2, & Windows • Samba's • Even so, smbclient utility can also parse UNC names, but it does so at the application level rather than within the OS and it only ever tries to deal with SMB smbclient must handle both NBT and naked transport, which can be tricky 11
UNC
◊SMB is the server
identifier
field because it will accept addresses in addition to names ◊Recognized server names are: • NetBIOS names • DNS hostnames • IP addresses ◊Going from an IP address to a NetBIOS name involves guesswork 12
Share names
◊ The directory path looks just like a directory path, but there is one small thing that makes it different; that thing is called the "share name“ • A given directory can have one or many share names ◊ The idea is similar to symbolic links (symlinks) in Unix, or shortcuts in Windows • The share is a named pointer--with its own set of attributes--to the object being made available 13
File name
◊File names are straightforward, right? No.
◊Windows is case insensitive • But UNIX and Linux are case sensitive ◊Other problem areas include • filename lengths • valid characters • file access permissions • the end-of-line delimiter in text files 14
SMB URL
◊Fortunately, there such a thing as an SMB URL • It fits into the general URI syntax and can be used to specify files, directories, and other SMB-shared stuff • It is intended as a more portable, and more complete way to specify SMB paths at the application level 15
Reach out
◊… and touch an SMB server ◊To do this we need two steps to identify who we’re talking to, and how • Server identifier interpretation • Transport discovery 16
Server identifier interpretation
◊Try a URL like this • smb://server/ • Assuming the server’s name is ‘server’ ◊To resolve the name server, try ◊It could be an IPv4 address • That’s easy; and IPv6 addresses should be in brackets, smb://[fe80::240:f4ff:fe1f:8243]/ 17
Server identifier interpretation
◊It could be a NetBIOS Name • Look for un-escaped dots in the NetBIOS name • smb://my%2Enode.scope/ has a NetBIOS Name of ‘my.node’ and Scope ID of ‘scope’ • Use a NBT Name Query to get the server’s IP address ◊It could be a DNS name • Try DNS to get the IP address 18
Port number please?
◊Now we need to divine the port number • Port 139 is for NBT • Port 445 is for raw TCP • Which one applies?
◊And if other security protocols are being used, higher port numbers could apply ◊So this is another mess to figure out, if you weren’t given it 19
Transport discovery
◊Try running naked • Open a TCP connection to port 445 on the server, but do not send an NBT SESSION REQUEST--just start sending SMB messages and see if that works ◊Try NBT transport ◊Try reverse-mapping a NetBIOS Name 20
Can we do something?
◊All that was just to establish a connection to an SMB server ◊Now we can look at SMB commands ◊They may contain a header, parameters, and data Image from http://www.ubiqx.org/cifs/figures/smb-03.html
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SMB Header
◊The SMB header typically has eight 4-Byte lines • SMB_HEADER { • PROTOCOL = "\xffSMB" (4B) • COMMAND =
SMB Header
• FLAGS2 =
SMB Header
• TID =
SMB Header
• UID =
SMB Header
• MID =
SMB Message Parameters
◊SMB_PARAMETERS { ◊WordCount =
SMB Message Parameters
◊Each SMB message type (species?) has a different record structure that is carried in the Words block ◊Think of that structure as representing the parameters passed to a function (the function identified by the SMB command code listed in the header) 28
SMB Message Data
◊SMB_DATA { ◊ByteCount =
Parameters vs Data blocks
◊So if the Parameters block and the Data block look alike, it’s not just you • “…any functional distinction that may have separated the Parameter and Data blocks has been blurred” - Implementing CIFS 30
AndX messages
◊SMB can form a linked list of related messages using a format called AndX messages ◊The AndX message has two fields in a 4 Byte line • AndXCommand (1 B) • (reserved) (1 B) • AndXOffset (2 B) 31
Figure from http://www.ubiqx.org/cifs/figures/smb-05.html
AndX messages
◊The AndXCommand field provides the SMB command code for the the list (
not next
the current one) AndX block in • The AndXOffset value in each AndX parameter block gives the offset (relative to the start of the SMB) of the next AndX block; AndXOffset of the last AndX block has a value of zero (0) 32
AndX messages
◊The AndXOffset contains the byte index, relative to the start of the SMB header, of that next AndX block--think of it as a pointer • Since the AndXOffset value is independent of the SMB_PARAMETERS.WordCount and SMB_DATA.ByteCount values, it is possible to provide padding between the AndX blocks 33
Start the conversation
◊After establishing the transport protocol, the SMB client and server can exchange messages ◊The first one is always NEGOTIATE PROTOCOL REQUEST/RESPONSE, which does just that ◊It’s followed by SESSION SETUP REQUEST • Didn’t we already do that? 34
Transport vs user sessions
◊No, we established the transport layer session; now it’s time to establish a user logon session ◊In the second message in the handout, we have • AccountName = "GUEST“ ◊This is to log on as the GUEST user 35
Find a tree
◊Once a user logon is done, often the next command is to find the shared directory tree • TREE CONNECT does this ◊Recall the tree concept identifies shared folders 36
SMB Connection
◊After transport disconnect, all resources are released, not too surprisingly Figure from http://www.ubiqx.org/cifs/figures/smb-06.html
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Ok, enough SMB!
◊Aren’t you glad someone wrote Samba to do all this stuff for you? Who, you ask?
• Aussie Andrew Tridgell reverse engineered SMB and started Samba in 1992 ◊Samba exists only on the non-Windows side; a Windows system only needs to be configured for ‘Microsoft networking’ Much of this section is from (Nemeth, 2007) and the Samba3-HOWTO 38
Samba = Unix CIFS server
◊Samba can provide these services • File sharing • Network printing • Authentication and authorization • Name resolution • Service announcement Browse file servers or printers 39
Samba services
◊Samba can also • Do basic Windows primary domain controller functions • With LDAP, serve as a backup domain controller • Support some other Windows functions such as domain logins, roaming user profiles, and CIFS print spooling 40
Samba processes
◊Samba is implemented in Linux mostly via two processes (daemons, to be precise): • The daemon authorization • The daemon smbd nmbd provides file sharing, network printing, authentication and provides name resolution and service announcement ◊Samba runs as a normal user process, binding to the CIFS sockets 41
Installing Samba
◊Most Linux distributions include Samba, but you can download the latest from samba.org, and install it • root# rpm -Uvh samba-3.0.20-1.i386.rpm
◊ Create a mount point for shared files • root# mkdir /plans • root# chmod 755 /plans From Samba ByExample 42
Installing Samba
◊Then i nstall the smb.conf
file (which we’ll get to soon) ◊ Verify that the /etc/hosts file contains the following entry: • 192.168.1.1 server ◊ Start samba • root# chkconfig smb on • root# /etc/rc.d/init.d/smb restart 43
Simple smb.conf file
• # Global Parameters • [global] • workgroup = MIDEARTH • security = SHARE • [Plans] • path = /plans • read only = Yes • guest ok = Yes 44
From the PC side
◊Make sure the PC is in the workgroup (e.g. MIDEARTH) ◊Make sure its IP address is in the same range as the Samba system (192.168.1.x), and they have the same netmask (255.255.255.0) ◊Find the samba server in the Network Neighborhood 45
Is Samba alive?
◊Check samba status with • root# smbclient -L localhost -U% • The -U% option gives no user name and password ◊And prove it can handle a login • root# smbclient -L server -Uroot%password ◊That should establish basic file sharing 46
Samba configuration file
◊Like most Unix-y apps, it has a ‘.conf’ file which governs how it runs • /etc/smb.conf or /etc/samba/smb.conf or maybe /usr/local/samba/lib/smb.conf
◊When Samba is running, it checks the conf file every few seconds, so changes will take effect quite quickly (no reboot needed) 47
Config file sections
◊Each section in the smb.conf file (“stanza”) represents either a share or a meta-service on the Samba server • The [global] stanza contains settings that apply to the whole Samba server ◊Samba supports a number of meta services, each of which serves its own purpose From (Samba3-HOWTO) 48
Config file meta-services
• The [homes] stanza is a meta-service that causes Samba to provide a personal home share for each user • The [printers] stanza is a meta-service that establishes print queue support, and species the location of the intermediate spool directory into which print jobs are received from Windows clients prior to being dispatched to the UNIX/Linux print spooler 49
Really minimal smb.conf file
[global] workgroup = WKG netbios name = MYNAME [share1] path = /tmp [share2] path = /my shared folder comment = Some random files 50
Security issues
◊File sharing is naturally a security hole ◊To control which clients can access Samba shared resources, look for the hosts allow clause in the smb.conf file • It should contain only the IP addresses or IP address ranges you trust to have access ◊Turn on password encryption • encrypt passwords = true 51
Security issues
◊Password encryption makes Samba keep a hash of the password for each user • Recall Unix and Windows keep different user login information ◊To set this password, use smbpasswd • sudo smbpasswd –a username ◊Users can then change their password • smbpasswd –r smbservername –U username 52
Security issues
◊Block the Samba server from outside your organization • Samba uses encryption only for password authentication, not for data transport!
◊Hence might want to use firewall to block • UDP ports 137-139 • TCP ports 137, 139, and 445 53
What are my settings?
◊To find the current Samba settings use the command • testparm –v ◊This will include all your smb.conf settings, and other defaults • Only override defaults for a reason – otherwise leave them out of smb.conf
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Encoding compatibility
◊Try echo $LANG ◊Samba uses UTF-8 character set ◊If you’re using anything else, set the character sets in Samba to be the same as your system using, e.g.
• unix charset = ISO8859-15 • display charset = ISO8859-15 55
Removing NetBIOS support
◊If all Windows clients are at least Win2k, have DNS provided outside of Samba, and you don’t need Network Neighborhood browsing in Samba, you can remove NetBIOS support [global] disable netbios = yes name resolve order = host 56
Removing NetBIOS support
◊Without NetBIOS support, you don’t need to run nmbd ; so remove it from Samba startup in /etc/init.d
◊To connect to a Samba server without NetBIOS support, use the full DNS name of the host • \\server.example.com.\myshare • The extra dot tells Windows to use DNS 57
Authentication
◊Windows uses a challenge/response approach to respond to an authentication request • If you have the same Samba username and password as in Windows, then this will be invisible in the background ◊For larger scale systems, can use NIS, LDAP or Windows authen. services 58
Authentication
◊To merge authentication services between Linux and Windows, make Samba act as a Primary Domain Controller ◊Or can use Nate Yocom’s open source pGina (GINA = Graphical Identification aNd Authentication) to handle almost any authentication method 59
Basic file sharing in smb.conf
◊If everyone has a home directory, the [homes] stanza can share them • [homes] • comment = Home directories • browseable = no • valid users = %S • writeable = yes • guest ok = no 60
Basic file sharing
◊This will allow any user to access their home Linux directory from any Windows system ◊Can you see another user’s home directory?
• Samba uses Linux file permissions through CIFS, so usually the answer would be yes • The valid users line prevents it though 61
Group shares
◊A group can be defined by access control lists (ACLs), but that’s a bit messy ◊Easier to define a pseudo user, and have Samba pretend people are in that group • E.g. create a Linux ‘eng’ group, and matching Samba user and group of the same name • The pseudo user is the owner of the directory 62
Group shares
• [eng] • valid users = @eng • force user = eng • force group = eng • path = /home/eng • nt acl support = no • create mask = 0660 • force create mask = 0660 63
Group shares
• security mask = 0000 • directory mask = 2770 • force directory mask = 2770 • directory security mask = 0000 More info on these masks circa p. 324 of HOWTO • browseable = no • writeable = yes • guest ok = no 64
Sources
◊ The Unofficial Samba HOWTO oregontechsupport.com
from ◊Vernooij, Terpstra, Carter. (May 27, 2009) Samba3-HOWTO from samba.org
◊ Samba3-ByExample from samba.org
◊ What is Samba? from samba.org
◊ Implementing CIFS , by Christopher R. Hertel 65
Sources
◊ CIFS Technical Reference by SNIA (2002) ◊Nemeth, Snyder, Hein. (2007)
Linux Administration Handbook
. Prentice Hall, ISBN 0131480049 66