The Domain Name System

Transcription

1 The Domain Name System Mark Handley) UCL Computer Science CS 3035/GZ01

2 Today 1. The Domain Name System (DNS) 2. A Brief Word on DNS Security

3 A name indicates what we seek. An address indicates where it is. A route indicates how we get there. Jon Postel 22/10/2015 3

4 Hostnames vs. IP Addresses Hostnames (e.g. Mnemonic name used by humans Variable length, full alphabet of characters Provide little (if any) information about location Examples: and bbc.co.uk IP addresses Numerical address used by routers Fixed length, binary number (e.g., ) Hierarchical, related to host location 4

5 Looking Up IP Addresses Before the DNS Per-host file named hosts.txt Flat namespace: each line is an IP address and a name SRI (Menlo Park, California) kept the master copy Everyone else downloads regularly But a single server, manually updated, doesn t scale Always a little out of date name collisions! Traffic implosion (lookups and updates) Single point of failure Need a distributed and hierarchical collection of servers

6 DNS is a wide-area distributed database The biggest in the world! Goals: Scalability Decentralized maintenance Robustness Global scope Names mean the same thing everywhere Don t need all of ACID Atomicity Strong consistency Do need: distributed update/query & Performance

7 Default answer to all systems problems: If it doesn t go fast enough, add a cache. If it doesn t scale, add hierarchy. 22/10/2015 7

8 Domain Name System (DNS) Hierarchical name space divided into pieces called zones Zones distributed over a collection of DNS servers Hierarchy of DNS servers Root servers (identity hardwired into other servers) Top-level domain (TLD) servers Authoritative DNS servers To performing translations: Local DNS servers located near clients Resolver software running on clients

9 DNS Namespace Is Hierarchical Root:. Top-level Domains (TLDs): com. uk. edu. ac.uk. cmu.edu. mit.edu. ucl.ac.uk. Hierarchy of servers follows hierarchy of DNS zones Zone is contiguous section of namespace e.g., complete tree, single node, or subtree Set of nameservers answers queries for names within zone Nameservers must store names and links to other servers in tree

10 DNS has many uses Hostname to IP address translation IP address to hostname translation (reverse lookup) Host name aliasing allows other names for a host Can be arbitrarily many aliases Alias host names point to canonical hostname Mail server location Lookup zone s mail server based on zone name Content distribution networks Load balancing among many servers with different IP addresses Complex, hierarchical arrangements are possible

11 DNS Root Nameservers 13 root servers (see h4p:// servers.org) Named A through M Does this scale? E NASA Mt View, CA F Internet SoPware ConsorQum, Palo Alto, CA (and 37 other locaqons) A Verisign, Dulles, VA C Cogent, Herndon, VA (also Los Angeles, NY, Chicago) D U Maryland College Park, MD K RIPE London (plus 16 other locaqons) G US DoD Vienna, VA H ARL Aberdeen, MD J Verisign (21 locaqons) I Autonomica, Stockholm (plus 29 other locaqons) M WIDE Tokyo plus Seoul, Paris, San Francisco B USC- ISI Marina del Rey, CA L ICANN Los Angeles, CA

12 DNS Root Nameservers 13 root servers (see h4p:// servers.org) Named A through M Each server really cluster of servers (some geographically distributed), replication via IP anycast E NASA Mt View, CA F Internet SoPware ConsorQum, Palo Alto, CA (and 37 other locaqons) A Verisign, Dulles, VA C Cogent, Herndon, VA (also Los Angeles, NY, Chicago) D U Maryland College Park, MD K RIPE London (plus 16 other locaqons) G US DoD Vienna, VA H ARL Aberdeen, MD J Verisign (21 locaqons) I Autonomica, Stockholm (plus 29 other locaqons) M WIDE Tokyo plus Seoul, Paris, San Francisco B USC- ISI Marina del Rey, CA L ICANN Los Angeles, CA

13 TLD and Authoritative Servers Top-level domain (TLD) servers Responsible for com, org, net, edu, etc, and all toplevel country domains: uk, fr, ca, jp Network Solutions maintains servers for com TLD Educause for edu TLD Authoritative DNS servers An organization s DNS servers, providing authoritative information for organization s servers Can be maintained by organization or service provider

14 Local Name Servers Do not strictly belong to hierarchy Each ISP (company, university) has one Also called default or caching name server When host makes DNS query, query is sent to its local DNS server Acts as proxy, forwards query into hierarchy Does work for the client

15 DNS in Operation Most queries and responses are UDP datagrams Two types of queries: Recursive: Client NS server may ask other servers if it doesn t know the answer Answer: A Iterative: Client NS server will reply with what it does know Referral:.edu NS

16 Local NS Does Clients Work Root NS TLD NS 1. Client s resolver makes recursive query to local NS 2. Local NS processing: Local NS Clients Authorita9ve NS Local NS sends iterative queries to other NS s or finds answer in cache 3. Local NS responds with answer to client s request

17 Recursive vs. Iterative Queries Recursive query Less burden on client More burden on nameserver has to return answer to query Iterative query More burden on client Less burden on nameserver simply refers query to another server Most root and TLD servers will not answer Local name server answers recursive query 17

18 DNS is a distributed database storing resource records RR includes: (name, type, value, time-to-live) Type = A (address) name is hostname value is IP address Type = NS (name server) name is domain (e.g. cs.ucl.ac.uk) value is hostname of authoritative name server for this domain Type = CNAME name is an alias for some canonical (real) name e.g. is really cms.cs.ucl.ac.uk value is canonical name Type = MX (mail exchange) value is name of mail server associated with domain name pref field discriminates between multiple MX records 18

19 Example: Recursive DNS Lookup Client. (root) authority edu.: NS no.: NS uk.: NS Contact for edu. edu. authority scholarly.edu.: NS pedanqc.edu.: NS Contact for scholarly.edu. Local NS. (root): NS edu.: NS scholarly.edu.: NS scholarly.edu. authority A imap.scholarly.edu.: A A

20 Example: Recursive Query, Step 1 Glue record

21 Example: Recursive Query, Step 2 Glue record

22 Example: Recursive Query, Step 3

23 DNS Caching Performing all these queries takes time And all this before actual communication takes place e.g., one-second latency before starting Web download Caching can greatly reduce overhead The top-level servers very rarely change Popular sites (e.g., news.bbc.co.uk) visited often Local DNS server often has the information cached How DNS caching works DNS servers cache responses to queries Responses include a time-to-live (TTL) field Server deletes cached entry after TTL expires

24 Reverse Mapping (IP to Hostname) How do we go the other direction, from an IP address to the corresponding hostname? Why do we care to? Troubleshooting, security, spam IP address already has natural quad hierarchy: But: IP address has most significant hierarchy element on the left, while has it on the right Idea: reverse the quads = , and look that up in the DNS Under what top-level domain? Convention: in- addr.arpa So lookup is for in-addr.arpa

25 Inserting Resource Records into DNS Example: just created startup FooBar Get a block of address space from ISP, say /25 Register foobar.com at Network Solutions (say) Provide registrar with names and IP addresses of your authoritative name server (primary and secondary) Registrar inserts RR pairs into the com TLD server: (foobar.com, dns1.foobar.com, NS) (dns1.foobar.com, , A) Put in your (authoritative) server dns1.foobar.com: Type A record for Type MX record for foobar.com

26 Setting Up foobar.com (cont d) In addition, need to provide reverse PTR bindings e.g., dns1.foobar.com Normally, these would go in in- addr.arpa Problem: you can t run the name server for that domain. Why not? Because your block is /25, not /24 And whoever has /25 won t be happy with you owning their PTR records Solution: ISP runs it for you, but it s more of a headache to keep it up-to-date.

27 DNS protocol operation Most queries and responses via UDP, server port 53 Source port UDP length Query ID Source IP DesQnaQon IP Dest port UDP cksum Q A T R R R opcode A C D A Z rcode IP header UDP header DNS payload

28 DNS Server State UDP socket listening on port 53 Client UDP length 11 UDP cksum QopcoATRR R de A C D A Z rcod e TLD NS Client UDP length 22 UDP cksum QopcoATRR R de A C D A Z rcod e Local NS TLD NS Local NS at least needs to keep state associating Query ID which query (if any)

29 DNS Server State UDP socket listening on port 53 Client UDP length 11 UDP cksum QopcoATRR R de A C D A Z rcod e UDP length UDP cksum UDP length UDP cksum Q R opcoatrr de C Z rcod A D A QopcoATRR e R de A C D A Z rcod e TLD NS Client UDP length 22 UDP cksum QopcoATRR R de A C D A Z rcod e Local NS UDP length 53 QopcoATRR UDP R de cksum A C D A QopcoATRR de C Z rcod R A D A e UDP length UDP cksum Z rcod e TLD NS Local NS at least needs to keep state associating Query ID which query (if any)

30 DNS Resource Record (RR) in Detail type: determines the meaning of rdata class: always IN (Internet) rdata: data associated with the RR name (variable length) type class 4l rdlength rdata (variable length)

31 DNS Protocol Message Query and reply messages have identical format Question section: query for name server Answer section: RRs answering the question Authority section: RRs that point to an authoritative NS Additional section: glue RRs Header QuesQon secqon Answer secqon Authority secqon AddiQonal secqon RR RR RR RR RR RR

32 DNS Protocol Header Query ID: 16-bit identifier shared between query, reply Flags word QR: query (0) or response (1) opcode: standard query (0) AA: authoritative answer TC: truncation RD: Recursion desired RA: Recursion available Z: (reserved and zeroed) rcode: response code; ok (0) Q R Query ID A T R R opcode A C D A Z rcode qdcount 1 ancount 0 nscount arcount qdcount: number of question entries (QEs) in message ancount: number of RRs in the answer section nscount: number of RRs in the authority section arcount: number of RRs in the additional section

33 All problems in computer science can be solved by another level of indirection... Except for the problem of too many layers of indirection. David Wheeler 22/10/

34 DNS Load Balancing Big companies want to load balance requests across many servers or datacentres. Can reply with lots of IP addresses in one A record. Only gets you so far. DNS is not required to be globally consistent! Give different answers depending on who asks. Ugly hack, but very widely used. Essentially DNS is the internet s indirection infrastructure. 22/10/

35 Today 1. The Domain Name System (DNS) 2. A Brief Word on DNS Security

36 Open recursive servers DNS servers should not recurse except for local clients. used to not be a problem. got misused DNS amplification attack Attacker sends small query to DNS server: Spoofs source address of request to be that of intended victim DNS server recurses, builds big response packet, sends it to victim repeat from many bots, thousands of times per second 22/10/

37 Implications of Subverting DNS 1. Redirect victim s web traffic to rogue servers 2. Redirect victim s to rogue servers (MX records in DNS)

38 Security Problem #1: Coffee Shop As you sip your latte and surf the Web, how does your laptop find google.com? Answer: it asks the local DNS nameserver Which is run by the coffee shop or their contractor And can return to you any answer they please Including a man in the middle site that forwards your query to Google, gets the reply to forward back to you, yet can change anything they wish in either direction How can you know you re getting correct data?

39 Security Problem #1: Coffee Shop As you sip your latte and surf the Web, how does your laptop find google.com? Answer: it asks the local DNS nameserver Which is run by the coffee shop or their contractor And can return to you any answer they please Including a man in the middle site that forwards your query to Google, gets the reply to forward back to you, yet can change anything they wish in either direction How can you know you re getting correct data?

40 Security Problem #1: Coffee Shop As you sip your latte and surf the Web, how does your laptop find google.com? Answer: it asks the local DNS nameserver Which is run by the coffee shop or their contractor And can return to you any answer they please Including a man in the middle site that forwards your query How to can Google, you know gets you re the reply getting to forward correct back data? to you, yet can change anything they wish in either direction Today, you can t (though if site is HTTPS, that helps). One day, hopefully: DNSSEC extensions to DNS How can you know you re getting correct data?

41 Security Problem #2: Cache Poisoning Suppose you are evil and you control the name server for foobar.com. You receive a request to resolve and reply: ;; QUESTION SECTION:! ; IN A!! ;; ANSWER SECTION:! IN A !! ;; AUTHORITY SECTION:! foobar.com. 600 IN NS dns1.foobar.com.! foobar.com. 600 IN NS google.com.!! ;; ADDITIONAL SECTION:! google.com. 5 IN A !! Evidence of the asack disappears 5 seconds later! A foobar.com machine, not google.com

42 DNS Cache Poisoning (cont d) OK, but how do you get the victim to look up in the first place? Perhaps you connect to their mail server and send HELO Which their mail server then looks up to see if it corresponds to your source address (anti-spam measure) Note, with compromised name server we can also lie about PTR records (address name mapping) e.g., for = in- addr.arpa return google.com (or whitehouse.gov, or whatever) If our ISP lets us manage those records as we see fit, or we happen to directly manage them

43 (Partial) Fix: Bailiwick Checking DNS resolver ignores all RRs not in or under the same zone as the question Widely deployed since ca Other attacks remain (e.g., Kaminsky poisoning) ;; QUESTION SECTION:! ; IN A!! ;; ANSWER SECTION:! IN A !! ;; AUTHORITY SECTION:! foobar.com. 600 IN NS dns1.foobar.com.! foobar.com. 600 IN NS google.com.!! ;; ADDITIONAL SECTION:! google.com. 5 IN A !!