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11
INFO 321Server Technologies II
2
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
3
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
4
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
5
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
6
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
7
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?
8
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/
9
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
Much of this section adapted from Implementing CIFS
No!
10
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
11
UNC◊ The UNC format is handled natively by
Microsoft & IBM's family of operating systems: DOS, OS/2, & Windows• Samba's 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
• Even so, smbclient must handle both NBT and naked transport, which can be tricky
12
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
13
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
14
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
15
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
16
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
17
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]/
18
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
19
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
20
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
21
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
22
SMB Header
◊ The SMB header typically has eight 4-Byte lines• SMB_HEADER { • PROTOCOL = "\xffSMB" (4B)• COMMAND = <SMB Command code (1
B)> The CIFS Technical Reference lists these
• STATUS = <Status code> (4B)• FLAGS = <Old flags> (1 B)
23
SMB Header• FLAGS2 = <New flags> (2 B)• EXTRA = <Sometimes used for
additional data> (12 B)The PidHigh subfield is used to
accommodate systems that have 32-bit Process IDs
The 8-byte Signature subfield is for SMB message signing, as in a digital signature
When not in use, these fields must be filled with zeros
24
SMB Header• TID = <Tree ID> (2B)
The SMB used to open a share is called a "Tree Connect“; the TID field is used to identify connections to shares once they have been established
• PID = <Process ID> (2B) The "Process ID" is set by the client, and is
intended as an identifier for the process sending the SMB request
The most important thing to note regarding the PID is that file locking and access modes are maintained relative to the value in this field
25
SMB Header
• UID = <User ID> (2B) The "User ID" is also known as a VUID
(Virtual User ID)It is assigned by the server after the user
logs in, and is valid until the user logs offIt does not need to be the user's actual User
ID on the server system
26
SMB Header
• MID = <Multiplex ID> (2B) }The "Multiplex ID“ is used by the client to
keep track of multiple outstanding requestsThe server must echo back the MID and the
PID provided in the client requestThe client can use those values to make
sure that the reply is matched up to the correct request
27
SMB Message Parameters
◊ SMB_PARAMETERS { ◊ WordCount = <Number of words in
the Words array> (1 B)◊ Words[WordCount] = <SMB
parameters; varies with SMB command> (2*WordCount B) }• The SMB_PARAMETERS.Words block is
limited in length to 510 bytes
28
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)
29
SMB Message Data
◊ SMB_DATA { ◊ ByteCount = <Number of bytes in
the Bytes field> (2 B)◊ Bytes[ByteCount] = <Contents
varies with SMB command> (ByteCount B) } • SMB_DATA.Bytes may be as much as
65,535 bytes in length
30
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
31
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)
32
AndX messages
◊ The AndXCommand field provides the SMB command code for the next AndX block in the list (not the current one)• 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)
Figure from http://www.ubiqx.org/cifs/figures/smb-05.html
33
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
34
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?
35
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
36
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
37
SMB Connection
◊ After transport disconnect, all resources are released, not too surprisingly
Figure from http://www.ubiqx.org/cifs/figures/smb-06.html
38
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
39
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
40
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
41
Samba processes◊ Samba is implemented in Linux mostly
via two processes (daemons, to be precise):• The daemon smbd provides file sharing,
network printing, authentication and authorization
• The daemon nmbd provides name resolution and service announcement
◊ Samba runs as a normal user process, binding to the CIFS sockets
42
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
43
Installing Samba◊ Then install 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
44
Simple smb.conf file
• # Global Parameters• [global]• workgroup = MIDEARTH• security = SHARE• [Plans]• path = /plans• read only = Yes• guest ok = Yes
45
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
46
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
47
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)
48
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)
49
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
50
Really minimal smb.conf file
[global]workgroup = WKGnetbios name = MYNAME
[share1]path = /tmp
[share2]path = /my shared foldercomment = Some random files
51
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
52
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
53
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
54
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
55
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
56
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
57
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
58
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
59
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
60
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
61
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
62
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
63
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
64
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
65
Sources◊ The Unofficial Samba HOWTO from
oregontechsupport.com◊ 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
66
Sources
◊ CIFS Technical Reference by SNIA (2002)
◊ Nemeth, Snyder, Hein. (2007) Linux Administration Handbook. Prentice Hall, ISBN 0131480049
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