This time. Digging into. Networking. Protocols. Naming DNS & DHCP
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- Emory Carroll
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1 This time Digging into Networking Protocols Naming DNS & DHCP
2 Naming IP addresses allow global connectivity But they re pretty useless for humans! Can t be expected to pick their own IP address Can t be expected to remember another s IP address DHCP : Setting IP addresses DNS : Mapping a memorable name to a routable IP address
3 DHCP Dynamic Host Configuration Protocol New host DHCP server
4 DHCP Dynamic Host Configuration Protocol Doesn t have an IP address yet (can t set src addr) New host DHCP server
5 DHCP Dynamic Host Configuration Protocol Doesn t have an IP address yet (can t set src addr) New host DHCP server Doesn t know who to ask for one
6 DHCP Dynamic Host Configuration Protocol Doesn t have an IP address yet (can t set src addr) New host DHCP server Doesn t know who to ask for one Solution: Discover one on the local subnet
7 DHCP Dynamic Host Configuration Protocol Doesn t have an IP address yet (can t set src addr) New host DHCP discover (L2 broadcast) DHCP server Doesn t know who to ask for one Solution: Discover one on the local subnet
8 DHCP Dynamic Host Configuration Protocol Doesn t have an IP address yet (can t set src addr) New host DHCP discover (L2 broadcast) DHCP server Doesn t know who to ask for one DHCP offer Solution: Discover one on the local subnet
9 DHCP Dynamic Host Configuration Protocol Doesn t have an IP address yet (can t set src addr) Doesn t know who to ask for one Solution: Discover one on the local subnet New host DHCP discover (L2 broadcast) DHCP offer DHCP server offer includes: IP address, DNS server, gateway router, and duration of this offer ( lease time)
10 DHCP Dynamic Host Configuration Protocol Doesn t have an IP address yet (can t set src addr) Doesn t know who to ask for one Solution: Discover one on the local subnet New host DHCP discover (L2 broadcast) DHCP offer DHCP request (L2 broadcast) DHCP server offer includes: IP address, DNS server, gateway router, and duration of this offer ( lease time)
11 DHCP Dynamic Host Configuration Protocol Doesn t have an IP address yet (can t set src addr) Doesn t know who to ask for one Solution: Discover one on the local subnet New host DHCP discover (L2 broadcast) DHCP offer DHCP request (L2 broadcast) DHCP server offer includes: IP address, DNS server, gateway router, and duration of this offer ( lease time) request asks for the offered IP address
12 DHCP Dynamic Host Configuration Protocol Doesn t have an IP address yet (can t set src addr) Doesn t know who to ask for one Solution: Discover one on the local subnet New host DHCP discover (L2 broadcast) DHCP offer DHCP request (L2 broadcast) DHCP ACK DHCP server offer includes: IP address, DNS server, gateway router, and duration of this offer ( lease time) request asks for the offered IP address
13 DHCP attacks Requests are broadcast: attackers on the same subnet can hear new host s request Race the actual DHCP server to replace: DNS server - Redirect any of a host s lookups ( what IP address should I use when trying to connect to google.com? ) to a machine of the attacker s choice Gateway - The gateway is where the host sends all of its outgoing traffic (so that the host doesn t have to figure out routes himself) - Modify the gateway to intercept all of a user s traffic - Then relay it to the gateway (MITM) - How could the user detect this?
14 Hostnames & IP addresses gold:~ dml$ ping google.com PING google.com ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=52 time= ms 64 bytes from : icmp_seq=1 ttl=52 time=6.304 ms 64 bytes from : icmp_seq=2 ttl=52 time=5.186 ms 64 bytes from : icmp_seq=3 ttl=52 time= ms
15 Hostnames & IP addresses gold:~ dml$ ping google.com PING google.com ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=52 time= ms 64 bytes from : icmp_seq=1 ttl=52 time=6.304 ms 64 bytes from : icmp_seq=2 ttl=52 time=5.186 ms 64 bytes from : icmp_seq=3 ttl=52 time= ms
16 Hostnames & IP addresses gold:~ dml$ ping google.com PING google.com ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=52 time= ms 64 bytes from : icmp_seq=1 ttl=52 time=6.304 ms 64 bytes from : icmp_seq=2 ttl=52 time=5.186 ms 64 bytes from : icmp_seq=3 ttl=52 time= ms
17 Hostnames & IP addresses gold:~ dml$ ping google.com PING google.com ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=52 time= ms 64 bytes from : icmp_seq=1 ttl=52 time=6.304 ms 64 bytes from : icmp_seq=2 ttl=52 time=5.186 ms 64 bytes from : icmp_seq=3 ttl=52 time= ms google.com is easy to remember, but not routable is routable Name resolution: The process of mapping from one to the other
18 Terminology = domain name is a subdomain of cs.umd.edu Domain names can map to a set of IP addresses gold:~ dml$ dig google.com ; <<>> DiG P1 <<>> google.com ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ;; flags: qr rd ra; QUERY: 1, ANSWER: 11, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION: ;google.com. IN A ;; ANSWER SECTION: google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A We ll understand this more in a bit; for now, note that google.com is mapped to many IP addresses
19 Terminology = domain name is a subdomain of cs.umd.edu Domain names can map to a set of IP addresses gold:~ dml$ dig google.com ; <<>> DiG P1 <<>> google.com ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ;; flags: qr rd ra; QUERY: 1, ANSWER: 11, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION: ;google.com. IN A ;; ANSWER SECTION: google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A google.com. 105 IN A We ll understand this more in a bit; for now, note that google.com is mapped to many IP addresses
20 Terminology zone = a portion of the DNS namespace, divided up for administrative reasons Think of it like a collection of hostname/ip address pairs that happen to be lumped together - mail.google.com, dev.google.com, Subdomains do not need to be in the same zone Allows the owner of one zone (umd.edu) to delegate responsibility to another (cs.umd.edu)
21 Namespace hierarchy. Zones edu com net umd.edu duke.edu cs.umd.edu
22 Terminology Nameserver = A piece of code that answers queries of the form What is the IP address for foo.bar.com? Every zone must run 2 nameservers Several very common nameserver implementations: BIND, PowerDNS (more popular in Europe) Authoritative nameserver : Every zone has to maintain a file that maps IP addresses and hostnames ( is ) One of the name servers in the zone has the master copy of this file. It is the authority on the mapping.
23 Terminology Resolver - while name servers answer queries, resolvers ask queries. Every OS has a resolver. Typically small and pretty dumb. All it typically does it forward the query to a local Recursive nameserver - a nameserver which will do the heavy lifting, issuing queries on behalf of the client resolver until an authoritative answer returns. Prevalence There is almost always a local (private) recursive name server But very rare for name servers to support recursive queries otherwise
24 Terminology Record (or resource record ) = usually think of it as a mapping between hostname and IP address But more generally, it can map virtually anything to virtually anything Many record types: (A)ddress records (IP <-> hostname) Mail server (MX, mail exchanger) SOA (start of authority, to delineate different zones) Others for DNSSEC to be able to share keys Records are the unit of information
25 Terminology Nameservers within a zone must be able to give: Authoritative answers (A) for hostnames in that zone The umd.edu zone s nameservers must be able to tell us what the IP address for umd.edu is A record: umd.edu = is a valid IP address for umd.edu Pointers to name servers (NS) who host zones in its subdomains The umd.edu zone s nameservers must be able to tell us what the name and IP address of the cs.umd.edu zone s nameservers NS record: cs.umd.edu = ipa01.cs.umd.edu. Ask ipa01.cs.umd.edu for all cs.umd.edu subdomains
26 Terminology Nameservers within a zone must be able to give: Authoritative answers (A) for hostnames in that zone The umd.edu zone s nameservers must be able to tell us what the IP address for umd.edu is A record: umd.edu = is a valid IP address for umd.edu Pointers to name servers (NS) who host zones in its subdomains The umd.edu zone s nameservers must be able to tell us what the name and IP address of the cs.umd.edu zone s nameservers NS record: cs.umd.edu = ipa01.cs.umd.edu. Ask ipa01.cs.umd.edu for all cs.umd.edu subdomains
27 DNS Domain Name Service at a very high level Requesting host What is an IP address for cs.umd.edu?
28 DNS Domain Name Service at a very high level Local nameserver (recursive) Requesting host What is an IP address for cs.umd.edu?
29 DNS Domain Name Service at a very high level Local nameserver (recursive) 1 Requesting host What is an IP address for cs.umd.edu?
30 DNS Domain Name Service at a very high level Root DNS server. Local nameserver (recursive) 1 Requesting host What is an IP address for cs.umd.edu?
31 DNS Domain Name Service at a very high level 2 Root DNS server. Local nameserver (recursive) 1 Requesting host What is an IP address for cs.umd.edu?
32 DNS Domain Name Service at a very high level 2 3 Root DNS server. Local nameserver (recursive) 1 Requesting host What is an IP address for cs.umd.edu?
33 DNS Domain Name Service at a very high level Local nameserver (recursive) NS Root DNS server. TLD DNS server (.edu ) Requesting host What is an IP address for cs.umd.edu?
34 DNS Domain Name Service at a very high level 2 3 NS Root DNS server. Local nameserver (recursive) 1 4 TLD DNS server (.edu ) Requesting host What is an IP address for cs.umd.edu?
35 DNS Domain Name Service at a very high level 2 3 NS Root DNS server. Local nameserver (recursive) TLD DNS server (.edu ) Requesting host What is an IP address for cs.umd.edu?
36 DNS Domain Name Service at a very high level 2 3 NS Root DNS server. Local nameserver (recursive) 4 5 NS TLD DNS server (.edu ) 1 Requesting host Authoritative DNS server ( umd.edu ) What is an IP address for cs.umd.edu?
37 DNS Domain Name Service at a very high level 2 3 NS Root DNS server. Local nameserver (recursive) 1 Requesting host NS TLD DNS server (.edu ) Authoritative DNS server ( umd.edu ) What is an IP address for cs.umd.edu?
38 DNS Domain Name Service at a very high level 2 3 NS Root DNS server. Local nameserver (recursive) 1 Requesting host NS TLD DNS server (.edu ) Authoritative DNS server ( umd.edu ) What is an IP address for cs.umd.edu?
39 DNS Domain Name Service at a very high level 2 3 NS Root DNS server. Local nameserver (recursive) 1 Requesting host NS TLD DNS server (.edu ) A Authoritative DNS server ( umd.edu ) What is an IP address for cs.umd.edu? cs.umd.edu
40 DNS Domain Name Service at a very high level 2 3 NS Root DNS server. Local nameserver (recursive) 1 8 Requesting host NS TLD DNS server (.edu ) A Authoritative DNS server ( umd.edu ) What is an IP address for cs.umd.edu? cs.umd.edu
41 DNS Domain Name Service at a very high level 2 3 NS Root DNS server. Local nameserver (recursive) 1 8 Requesting host NS 9 TLD DNS server (.edu ) A Authoritative DNS server ( umd.edu ) What is an IP address for cs.umd.edu? cs.umd.edu
42 DNS Domain Name Service at a very high level Local nameserver (recursive) 1 8 Requesting host NS Root DNS server. 7 6 NS 9 TLD DNS server (.edu ) A Caching responses is critical to DNS s success Every response (3,5,7,8) has a time-to-live (TTL). TTLs should be reasonably Authoritative DNS server ( umd.edu ) long (days), but some are minutes. What is an IP address for cs.umd.edu? cs.umd.edu
43 How do they know these IP addresses? Local DNS server: host learned this via DHCP A parent knows its children: part of the registration process Root nameserver: hardcoded into the local DNS server (and every DNS server) 13 root servers (logically): A-root, B-root,, M-root These IP addresses change very infrequently UMD runs D-root. - IP address changed beginning of 2013!! - For the most part, the change-over went alright, but Lots of weird things happened ask me some time.
44 Caching Central to DNS s success Also central to attacks Cache poisoning : filling a victim s cache with false information
45 Queries Local nameserver (recursive) 1 8 Requesting host NS Root DNS server. 7 6 NS 9 TLD DNS server (.edu ) A Authoritative DNS server ( umd.edu ) Every query (2,4,6) has the same request in it ( what is the IP address for cs.umd.edu? ) But different: - dst IP (port = 53) - query ID hat is an IP address for cs.umd.edu? cs.umd.edu
46 What s in a response? Many things, but for the attacks we re concerned with A record: gives the authoritative response for the IP address of this hostname NS record: describes this is the name of the nameserver who should know more about how to answer this query than I do Often also contains glue records (IP addresses of those name servers to avoid chicken and egg problems) Resolver will generally cache all of this information
47 Query IDs 2 3 The local resolver has a lot of incoming/outgoing queries at any point in time. Local nameserver (recursive) To determine which response maps to which queries, it uses a query ID Query ID: 16-bit field in the DNS header Requester sets it to whatever it wants Responder must provide the same value in its response
48 Query IDs Local nameserver (recursive) The local resolver has a lot of incoming/outgoing queries at any point in time. To determine which response maps to which queries, it uses a query ID Query ID: 16-bit field in the DNS header Requester sets it to whatever it wants Responder must provide the same value in its response How would you implement query IDs at a resolver?
49 Query IDs used to increment Local nameserver (recursive) Global query ID value Map outstanding query ID to local state of who to respond to (the client) Basically: new Packet(queryID++)
50 Query IDs used to increment Local nameserver (recursive) Global query ID value Map outstanding query ID to local state of who to respond to (the client) Basically: new Packet(queryID++) How would you attack this?
51 Cache poisoning Local nameserver (recursive) Bad guy
52 Cache poisoning Local nameserver (recursive) Bad guy
53 Cache poisoning Authoritative DNS server ( bank.com ) Local nameserver (recursive) Bad guy
54 Cache poisoning Authoritative DNS server ( bank.com ) Local nameserver (recursive) Bad guy
55 Cache poisoning Authoritative DNS server ( bank.com ) Local nameserver (recursive) 16322: A = Bad guy
56 Cache poisoning Authoritative DNS server ( bank.com ) Local nameserver (recursive) 16322: A = Bad guy
57 Cache poisoning Authoritative DNS server ( bank.com ) Local nameserver (recursive) Will cache = and ignore authority s answer 16322: A = Bad guy
58 Cache poisoning How do you guess this? Authoritative DNS server ( bank.com ) Local nameserver (recursive) Will cache = and ignore authority s answer 16322: A = Bad guy
59 Cache poisoning How do you guess this? Local nameserver (recursive) Authoritative DNS server ( bank.com ) Will cache = and ignore authority s answer 16322: A = Bad guy
60 Cache poisoning How do you guess this? Local nameserver (recursive) Authoritative DNS server ( bank.com ) Will cache = and ignore authority s answer 16322: A = Bad guy
61 Cache poisoning How do you guess this? Local nameserver (recursive) Authoritative DNS server ( bank.com ) Will cache = and ignore authority s answer 16322: A = Bad guy Next is likely 16322
62 Details of getting the attack to work Must guess query ID: ask for it, and go from there Partial fix: randomize query IDs Problem: small space Attack: issue a Lot of query IDs Must guess source port number Typically constant for a given server (often always 53) The answer must not already be in the cache It will avoid issuing a query in the first place
63 Cache poisoning Can we do more harm than a single record? com. TLD Local nameserver (recursive) Bad guy
64 Cache poisoning Can we do more harm than a single record? com. TLD Local nameserver (recursive) Bad guy
65 Cache poisoning Can we do more harm than a single record? com. TLD Local nameserver (recursive) Bad guy
66 Cache poisoning Can we do more harm than a single record? com. TLD Local nameserver (recursive) Bad guy Next is likely 16322
67 Cache poisoning Can we do more harm than a single record? com. TLD Local nameserver (recursive) Bad guy somethingnotcached.bank.com Next is likely 16322
68 Cache poisoning Can we do more harm than a single record? com. TLD Local nameserver (recursive) Bad guy somethingnotcached.bank.com Next is likely 16322
69 Cache poisoning Can we do more harm than a single record? com. TLD Local nameserver (recursive) : NS bank.com = ns.bank.com A ns.bank.com = Bad guy somethingnotcached.bank.com Next is likely 16322
70 Cache poisoning Can we do more harm than a single record? Local nameserver (recursive) com. TLD : NS bank.com = ns.bank.com A ns.bank.com = Bad guy somethingnotcached.bank.com Next is likely 16322
71 Cache poisoning Can we do more harm than a single record? Local nameserver (recursive) com. TLD Will cache the person to ask for ALL bank.com queries is : NS bank.com = ns.bank.com A ns.bank.com = somethingnotcached.bank.com Bad guy Next is likely
72 Solutions? Randomizing query ID? Not sufficient alone: only 16 bits of entropy Randomize source port, as well There s no reason for it stay constant Gets us another 16 bits of entropy DNSSEC?
73 DNSSEC Root DNS server.
74 DNSSEC Ask.edu Root DNS server..edu s public key = PKedu (Plus. s sig of this zone-key binding)
75 DNSSEC Ask.edu Root DNS server..edu s public key = PKedu (Plus. s sig of this zone-key binding) TLD DNS server (.edu )
76 DNSSEC Ask.edu Root DNS server..edu s public key = PKedu (Plus. s sig of this zone-key binding) Ask umd.edu umd.edu s public key = PKumd (Plus edu s sig of this zone-key binding) TLD DNS server (.edu )
77 DNSSEC Ask.edu Root DNS server..edu s public key = PKedu (Plus. s sig of this zone-key binding) Ask umd.edu umd.edu s public key = PKumd (Plus edu s sig of this zone-key binding) TLD DNS server (.edu ) Authoritative DNS server ( umd.edu )
78 DNSSEC Ask.edu Root DNS server..edu s public key = PKedu (Plus. s sig of this zone-key binding) Ask umd.edu umd.edu s public key = PKumd (Plus edu s sig of this zone-key binding) TLD DNS server (.edu ) IN A (Plus umd.edu s signature of the answer Authoritative DNS server ( umd.edu )
79 DNSSEC Ask.edu Root DNS server..edu s public key = PKedu (Plus. s sig of this zone-key binding) Ask umd.edu umd.edu s public key = PKumd (Plus edu s sig of this zone-key binding) TLD DNS server (.edu ) Only the authoritative answer is signed IN A (Plus umd.edu s signature of the answer Authoritative DNS server ( umd.edu )
80 Properties of DNSSEC If everyone has deployed it, and if you know the root s keys, then prevents spoofed responses Very similar to PKIs in this sense But unlike PKIs, we still want authenticity despite the fact that not everyone has deployed DNSSEC What if someone replies back without DNSSEC? Ignore = secure but you can t connect to a lot of hosts Accept = can connect but insecure Back to our notion of incremental deployment DNSSEC is not all that useful incrementally
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