Chapters 9 and 8 Samba/SMB, Network Security

1. Chapters 9 and 8Samba/SMB, Network Security Professor Rick Han University of Colorado at Boulder rhan@cs.colorado.edu

2. Announcements • HW #5 a possibility • Programming Assignment #3 due May 2 • Lecture slides from last week online after class • In Chapter 8, read all sections. • Next, Samba/SMB, Network Security Prof. Rick Han, University of Colorado at Boulder

3. Recap of Previous Lecture • An example caching policy for an HTTP proxy • Conditional GET with If-Modified-Since header • Proxy returns page from its cache only if that page is not expired and its Last-Modified is more recent than If-Modified-Since date • Otherwise, proxy forwards conditional GET to server, who either replies with • New page, or • Status 340 “Not Modified” • Network Address Translation (NAT) • Outbound: Substitute NAT’s IP address and TCP port for the packet’s source IP and source TCP port • Inbound: Substitute NAT’s IP addr and TCP port for packet’s dest IP and dest TCP port Prof. Rick Han, University of Colorado at Boulder

4. Recap of Previous Lecture (2) • NAT • Static NAT’s map an inbound packet’s dest IP and dest TCP port to a internal host’s fixed IP addr and TCP port • Enables a Web server behind a NAT to serve Web pages to external hosts • Adds security risk • Dynamic NAT’s provide a firewall masquerading capability • In absence of fixed mappings, external hosts can’t make an inbound connection to any internal host • Internal hosts can still make outbound TCP connections Prof. Rick Han, University of Colorado at Boulder

5. Samba/SMB • Server Message Block (SMB) Protocol • File sharing protocol that ships with Microsoft OS’s • Basis for Network Neighborhood • Application-layer protocol over TCP/UDP/IP • Open-source SAMBA Server suite enables other OS’s such as Linux to speak SMB • Enables an MS client to access files on a UNIX server – very useful! Prof. Rick Han, University of Colorado at Boulder

6. Samba/SMB (2) • For historical reasons, SMB first ran across the NETBIOS API, which then ran across various network protocols, e.g. TCP/UDP, IPX, SNA, DECnet, etc. • SMB packets can be framed in NETBIOS packets which are encapsulated by TCP/UDP • NETBIOS over TCP/UDP is called NBT SMB NETBIOS TCP/UDP, IPX, SNA or … Prof. Rick Han, University of Colorado at Boulder

7. Samba/SMB (3) • Newer version of SMB: Windows 2000 now runs SMB natively on top of TCP/UDP • no NETBIOS framing • renamed to Common Internet FileSystem (CIFS) • CIFS actually refers to entire suite of protocols: file/printer-sharing, service announcement, naming, authentication, authorization • Supports older version of SMB too, to maintain compatibility SMB (Windows 2000) TCP/UDP Prof. Rick Han, University of Colorado at Boulder

8. Samba/SMB (4) • NBT creates an abstraction: a virtual LAN, even if actual nodes are distributed over wide area • NBT provides 3 services over a virtual LAN • Naming Service • Datagram Distribution Service • Session Service • NBT Naming Service • Broadcast: “where’s anchor?” “Here I am” • Point-to-point: required to bridge subnets, because broadcasts are typically confined to a subnet • A NETBIOS Name Server (NBNS) provides name-to-IP mappings for a NETBIOS virtual LAN • Also called WINS in MS terminology Prof. Rick Han, University of Colorado at Boulder

9. Samba/SMB (5) • NBT Naming Service (cont.) • Runs on UDP port 137: NETBIOS naming queries are encapsulated in UDP then IP • NBT Datagram Service • Runs over UDP port 138 • Point-to-point and multicast are straightforward within a LAN • Multicast across IP subnets requires a bridging agent: a NETBIOS Datagram Distribution Server (NBDD) • Multicast datagrams are sent to NBDD, which gets list of hosts in multicast group from NBNS, then sends point-to-point to each host • WINS messed up its implementation of NBDD (as of May 2001) – some group members won’t receive multicast Prof. Rick Han, University of Colorado at Boulder

10. Samba/SMB (6) • NBT Session Service • Runs over TCP port 139 • Implements file sharing • Simple sequence of events: • Source X gives NETBIOS name of destination Y to NBT Name Service and gets back IP address of Y • Source X establishes a TCP connection with Y • Source X sends a NETBIOS SESSION SERVICE REQUEST to Y. Y accepts request. • X and Y exchange files via SMB. • SMB packets consist of “0xFF” then the letters “SMB” followed by a command and data • Commands are patterned after DOS I/O commands, and include OPEN, CLOSE, DELETE, etc. Prof. Rick Han, University of Colorado at Boulder

11. Samba/SMB (7) • SMB • Several dialects of SMB, so there is always a negotiation phase to make sure SMB client speaks the same dialect as SMB server • Network Neighborhood is supported by a “Browsing” Service • Browsing is organized in terms of IP subnets and Workgroups. • A "Workgroup" is a set of NBT nodes on an IP subnet that shares the same Workgroup name. • On each subnet, the Workgroup members hold an "election," which involves sending group datagrams via the NBT Datagram Service. • A Domain Master Browser enables browsing across subnets Prof. Rick Han, University of Colorado at Boulder

12. Samba/SMB (8) • CIFS • Removes NETBIOS/NBT • Also, replaces NETBIOS services with standard-based services • Example: NBNS is replaced with Dynamic DNS • SAMBA • Racing to stay compatible with latest MS twist on CIFS, e.g. Windows 2000 • See www.samba.org for more info Prof. Rick Han, University of Colorado at Boulder

13. Network Security • Classic properties of secure systems: • Confidentiality • Encrypt message so only sender and receiver can understand it. • Authentication • Both sender and receiver need to verify the identity of the other party in a communication: are you really who you claim to be? • Authorization • Does a party with a verified identity have permission to access (r/w/x/…) information? Gets into access control policies. Prof. Rick Han, University of Colorado at Boulder

14. Network Security (2) • Classic properties of secure systems: (cont.) • Integrity • During a communication, can both sender and receiver detect whether a message has been altered? • Non-Repudiation • Originator of a communication can’t deny later that the communication never took place • Availability • Guaranteeing access to legitimate users. Prevention of Denial-of-Service (DOS) attacks. Prof. Rick Han, University of Colorado at Boulder

15. Encryption Decryption Encryption Decryption Cryptography plaintext ciphertext plaintext • Encryption algorithm also called a cipher • Cryptography has evolved so that modern encryption and decryption use secret keys • Only have to protect the keys! => Key distribution problem • Cryptographic algorithms can be openly published plaintext ciphertext plaintext Key KA Key KB Prof. Rick Han, University of Colorado at Boulder

16. Cryptography (2) • Cryptography throughout history: • Julius Caesar cipher: replaced each character by a character cyclically shifted to the left. Weakness? • Easy to attack by looking at frequency of characters • Mary Queen of Scots: put to death for treason after Queen Elizabeth’s I’s spymaster cracked her encryption code • WWII: Allies break German Enigma code and Japanese naval code • Enigma code machine (right) Prof. Rick Han, University of Colorado at Boulder

17. Cryptography (3) • Cryptanalysis – Type of attacks: • Brute force: try every key • Ciphertext-only attack: • Attacker knows ciphertext of several messages encrypted with same key (but doesn’t know plaintext). • Possible to recover plaintext (also possible to deduce key) by looking at frequency of ciphertext letters • Known-plaintext attack: • Attackers observes pairs of plaintext/ciphertext encrypted with same key. • Possible to deduce key and/or devise algorithm to decrypt ciphertext. Prof. Rick Han, University of Colorado at Boulder

18. Cryptography (4) • Cryptanalysis – Type of attacks: • Chosen-plaintext attack: • Attacker can choose the plaintext and look at the paired ciphertext. • Attacker has more control than known-plaintext attack and may be able to gain more info about key • Adaptive Chosen-Plaintext attack: • Attacker chooses a series of plaintexts, basing the next plaintext on the result of previous encryption • Differential cryptanalysis – very powerful attacking tool • But DES is resistant to it • Cryptanalysis attacks often exploit the redundancy of natural language • Lossless compression before encryption removes redundancy Prof. Rick Han, University of Colorado at Boulder

19. Cryptography (5) • Symmetric or Secret-Key Cryptography • Both sender and receiver keys are the same: KA=KB • Data Encryption Standard (DES) • Encodes plaintext in 64-bit chunks using a 64-bit key (56 bits + 8 bits parity) • Uses permutation or transposition of characters: • abcd  dbac • Was cracked in 1997 • Triple-DES: put the output of DES back as input into DES again, loop again Prof. Rick Han, University of Colorado at Boulder

20. Cryptography (6) • Public-Key Cryptography • Host who wants data sent to it advertises a public encryption key Kpublic • Decryption algorithm has the property that only a private key Kprivate can decrypt the ciphertext • Based on the difficulty of factoring the product of two prime #’s • Even though attacker knows the public key Kpublic and the encryption algorithm, the attacker still does not know the private key Kprivate • Example: RSA encryption algorithm Prof. Rick Han, University of Colorado at Boulder