what s in a name? taking a deeper look at the domain name system mike boylan penn state mac admins conference
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1 what s in a name? taking a deeper look at the domain name system mike boylan penn state mac admins conference
2 whoami work for robert morris university, pittsburgh, pa primarily mac and voip on twitter
3 the rundown dns in a nutshell common dns scenarios troubleshooting dns dns in os x server
4 dns in a nutshell architecture and concepts, explained
5 dns is the phonebook of the internet
6 dns is the phonebook of the internet at the most basic level, it turns names into numbers and numbers into names
7 user types apple.com into browser
8 apple.com translated to
9 networking stuff happens...
10 apple.com loads in the browser
11 apple.com translated to
12 how?
13 so much stuff! queries resolvers nameservers resolution from root caching zones and records delegations forwarding
14 queries
15 queries at what address is apple.com located? two main types of queries...
16 recursive query
17 recursive query usually made by a client host (your mac) asking dns server for an answer or... please kindly return an error if you don t know
18 iterative query
19 iterative query usually made by dns servers return the best answer you have or... please refer me to an authoritative source
20 simple illustration
21 rmu nameserver
22 rmu nameserver
23 rmu nameserver
24 walking the tree
25 rmu server root server com server example.com server
26 rmu server root server com server example.com server recursive query for host.example.com
27 rmu server root server com server example.com server not in cache or zones, iteratively asks root server
28 rmu server root server com server example.com server responds with referral to.com tld name server
29 rmu server root server com server example.com server iteratively asks.com name server
30 rmu server root server com server example.com server responds with referral to example.com name server
31 rmu server root server com server example.com server iteratively asks example.com name server
32 rmu server root server com server example.com server responds with ip address
33 rmu server root server com server example.com server responds to client query with ip address
34 yes, all dns servers are mac minis
35 loljk
36 dns follows a client/server model
37 dns follows a client/server model resolvers are clients who issue queries nameservers are servers (really?) with dns info* *zone data and more...
38 zones
39 zones domain name space = tree of domain names tree subdivided into zones, starting with root may consist of 1 domain, or many and subdomains delegation info for subzones contain resource records
40 from wikipedia
41 records
42 records different types used for different purposes most common: A, AAAA, CNAME, MX, NS, PTR, SOA, SPF, SRV, TXT
43 nameservers
44 nameservers zones holding dns record information cache query results (TTL defines how long) authoritative delegation info for subdomains forwarding info for unauthoritative queries
45 primary hosts the zone file(s) can host multiple zones notifies slaves of changes primary secondary (slave) transfers the zone file(s) can have multiple slaves transfers are over tcp 53 secondary (slave)
46 the roots
47
48 the roots <a-m>.root-servers.net.* geographically dispersed around the world publish root server zone file lists authoritative dns servers for all TLDs *not actually just 13 root name servers
49
50 delegations and forwarding
51 delegation used to separate management of part of a zone organization maintains all data in that subdomain ex: mac.rmu.edu is delegated to mike boylan
52 forwarding generally used for off-site queries also used to send queries upstream ex: mac.rmu.edu nameserver configured to forward non-authoritative queries to rmu.edu nameserver
53 question
54 question is host.example.com a fqdn?
55 question is host.example.com a fqdn? technically not... host.example.com. is... notice the trailing dot. or represents dns root
56 fqdn unambiguous domain name specifies its position in the dns tree absolutely carries a trailing period sometimes called rooted domain max length of 256 bytes
57 anatomy of an fqdn
58 host.example.com.
59 host.example.com. read right to left... root pointer top level doman (tld) second-level domain [subdomain(s)] host or resource name
60 snd esrever
61 reverse dns
62 reverse dns turns numbers into names uses a special zone called the.in-addr.arpa zone uses PTR records read right to left apple.com. = in-addr.arpa = apple.com.
63 reverse dns absolutely critical for os x server services used for rudimentary spam filtering one half of a forward-confirmed reverse dns entry FCrDNS: what s the name for this ip? what s the ip for this name? do they match? woohoo!
64 reverse dns outermost dns servers should have catch-all reverse zones for RFC1918 local network addresses prisoner.iana.org, blackhole-(1-2).iana.org designed to catch leaked queries, but often become overloaded
65 common scenarios AD DNS, Round-Robin DNS, & Split-Horizon DNS
66 active directory dns
67 why is using active directory dns such a popular choice?
68 auto updating of ip addresses mapped to names in a dhcp environment
69 active directory dns & os x (server) make sure your AD admin makes the reverse zone os x clients unfortunately do not support secure dynamic updates
70 round-robin dns
71 one name, multiple addresses
72 ex: apple.com resolves to:
73 views or split-horizon dns
74 what you see depends on who you are
75 allows you to use the same dns server(s) to provide different results based on source address
76 subnet 1 query: hello.example.com subnet 2 query: hello.example.com nameservers result: result: xx.xx.xx.xx
77 troubleshooting dns dns doesn t have to be a four letter word
78 The strangest problems often turn out to be misconfigured DNS. DNS is critical to so many subsystems, often in obscure ways, that a problem with DNS can mask itself as other problems. This goes for a client that can t reach its DNS servers, as well as a host with invalid DNS data describing it, or a client trying to reach a host with invalid DNS data. Tim Limoncelli Time Management For System Administrators - O'Reilly Books
79 ISPs are evil
80 ISPs are evil often resolve unresolvable domains to ad pages Did you mean? -- No, I didn t, but thanks? can cause login delays & various other problems fortunately, can usually opt-out
81 dns command line tools in os x
82 dns command line tools in os x nslookup host dig dscacheutil scutil changeip (Server only)
83 nslookup is its own resolver format: nslookup <name> or <ip address>
84 boylan-mbp:~ boylan$ nslookup apple.com Server: Address: #53 Non-authoritative answer: Name: apple.com Address: Name: apple.com Address: Name: apple.com Address:
85 boylan-mbp:~ boylan$ nslookup Server: Address: #53 Non-authoritative answer: in-addr.arpa name = apple.com.
86 host is its own resolver format: host <name> or <ip address>
87 boylan-mbp:~ boylan$ host apple.com apple.com has address apple.com has address apple.com has address
88 boylan-mbp:~ boylan$ host in-addr.arpa domain name pointer apple.com.
89 dig is its own resolver simple format ip: dig <name> <type> simple format name: dig -x <ip address> specific server: useful options: +trace, +short, no name or ip
90 boylan-mbp:~ boylan$ dig apple.com ; <<>> DiG P1 <<>> apple.com ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION: ;apple.com. IN A ;; ANSWER SECTION: apple.com IN A apple.com IN A apple.com IN A
91 boylan-mbp:~ boylan$ apple.com ; <<>> DiG P1 apple.com ; (1 server found) ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION: ;apple.com. IN A ;; ANSWER SECTION: apple.com. 753 IN A apple.com. 753 IN A apple.com. 753 IN A
92 boylan-mbp:~ boylan$ dig _ldap._tcp.ad.example.edu SRV ; <<>> DiG P1 <<>> _ldap._tcp.ad.example.edu SRV ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 2, ADDITIONAL: 3 ;; QUESTION SECTION: ;_ldap._tcp.ad.example.edu. IN SRV ;; ANSWER SECTION: _ldap._tcp.ad.example.edu. 111 IN SRV dc01.ad.example.edu. _ldap._tcp.ad.example.edu. 111 IN SRV dc02.ad.example.edu. _ldap._tcp.ad.example.edu. 111 IN SRV dc03.ad.example.edu.
93 boylan-mbp:~ boylan$ dig _kerberos._tcp.ad.example.edu SRV ; <<>> DiG P1 <<>> _kerberos._tcp.ad.example.edu SRV ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 2, ADDITIONAL: 3 ;; QUESTION SECTION: ;_kerberos._tcp.ad.example.edu. IN SRV ;; ANSWER SECTION: _kerberos._tcp.ad.example.edu. 600 _kerberos._tcp.ad.example.edu. 600 _kerberos._tcp.ad.example.edu. 600 IN SRV dc-02.ad.example.edu. IN SRV dc-03.ad.example.edu. IN SRV dc-04.ad.example.edu.
94 dscacheutil uses osx host name/address resolution and dns query routing mechanisms format: dscacheutil -q host -a name <name> dscacheutil -q host -a ip_address <ip>
95 boylan-mbp:~ boylan$ dscacheutil -q host -a name apple.com name: apple.com ip_address: ip_address: ip_address:
96 boylan-mbp:~ boylan$ dscacheutil -q host -a ip_address name: st11p01ww-apple.apple.com alias: in-addr.arpa apple.com ip_address:
97 scutil used to get or set your hostname, computer name, and/or local host name. format: scutil --get [HostName ComputerName LocalHostName] scutil --set [HostName ComputerName LocalHostName] <name>
98 boylan-mbp:~ boylan$ scutil --get ComputerName boylan-mbp
99 changeip (server only) used to check for FCrDNS for hostname used to need invoked after a name or IP change most common usage: sudo changeip -checkhostname
100 $ sudo changeip -checkhostname Password: Primary address = Current HostName DNS HostName = host.example.com = host.example.com The names match. There is nothing to change. dirserv:success = "success"
101 inconsistent results
102 inconsistent results Apple will not confirm, but there still seems to be a tight integration between directory services caching and dns resolution
103 clearing the dns cache
104 clearing the dns cache sudo dscacheutil -flushcache sudo killall -HUP mdnsresponder
105 in system.log after HUPing mdnsresponder... mdnsresponder[55]: SIGHUP: Purge cache
106 is that really still necessary?
107 is that really still necessary? fun fact: Apple HUPs mdnsresponder multiple times during Server.app initial setup
108
109 speaking of Server.app...
110 dns in os x server from server.app setup to dns service
111
112
113
114
115
116
117
118 okay, real demo time...
119 resources additional information
120 ISBN
121 resources bb aspx
122 really geeky resources
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