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CS 5523 Lecture 27:
DNS Implementation
] Questions on Laboratory 4
] Practical issues in name resolution
] Name resolution from the viewpoint of the user
] Representation of naming information in DNS
] Resolver library and low-level DNS queries
Practical issues in name resolution:
] Name resolution is needed in most communication because
users usually specify hosts by name rather than IP address.
] /etc/resolv.conf specifies IP addresses of the name
servers on most Unix systems.
] A single name may require many inquiries to be resolved.
] Local shared caching can considerably reduce network traffic.
] The name servers must be relatively reliable, or the network
won’t work.
DNS queries:
] host resolution - resolves host names into IP addresses
] mail host location - resolves domain names into IP addresses of
mail hosts
] reverse resolution - returns a host name given an IP address
] host information - given a host name, return information about
the host
] well-known services - return a list of services given a hostname
Give an example of a use of each of these types of queries.
Typical name resolution in a user program:
user
program
resolver
library
local
resolver
function call function return
configuration
files
udp
local cache
foreign
name
server
cache lookup
udp
local host application
The /etc/resolv.config file for medusa
domain cs.utsa.edu
search cs.utsa.edu atm.utsa.edu utsa.edu
nameserver 129.115.11.
nameserver 129.115.102.
nameserver 128.83.139.
] domain – indicates the local default domain name for resolving
short names
] search – list to search for hostname lookup
] nameserver – foreign name servers to be queried in listed order
DNS databases:
] A domain name identifies a node.
] Each information at each node consists of resource records.
] Each resource record (RR) has:
• owner (domain name)
• type (A, CNAME, HINFO, MX, NS, PTR, SOA)
• class (IN = internet)
• TTL
• RDATA (describes the information above)
Types defined in arpa/nameser.h :
typedef enum __ns_type { ns_t_a = 1, /* Host address / ns_t_ns = 2, / Authoritative server / ns_t_md = 3, / Mail destination / ns_t_mf = 4, / Mail forwarded / ns_t_cname = 5, / Canonical name / ns_t_soa = 6, / Start of authority zone / ns_t_mb = 7, / Mailbox domain name / ns_t_mg = 8, / Mail group member / ns_t_mr = 9, / Mail rename name / ns_t_null = 10, / Null resource record / ns_t_wks = 11, / Well known service / ns_t_ptr = 12, / Domain name pointer / ns_t_hinfo = 13, / Host information / ns_t_minfo = 14, / Mailbox information / ns_t_mx = 15, / Mail routing information / ns_t_txt = 16, / Text strings / ns_t_rp = 17, / Responsible person / ns_t_afsdb = 18, / AFS cell database / ns_t_x25 = 19, / X_25 calling address / ns_t_isdn = 20, / ISDN calling address / ns_t_rt = 21, / Router / ns_t_nsap = 22, / NSAP address / ns_t_rt = 21, / Router / ns_t_nsap = 22, / NSAP address / ns_t_nsap_ptr = 23, / Reverse NSAP lookup / / (deprecated) */
ns_t_sig = 24, /* Security signature / ns_t_key = 25, / Security key / ns_t_px = 26, / X.400 mail mapping / ns_t_gpos = 27, / Geographical position / / (withdrawn) / ns_t_aaaa = 28, / Ip6 Address / ns_t_loc = 29, / Location Information / ns_t_nxt = 30, / Next domain (security) / ns_t_eid = 31, / Endpoint identifier / ns_t_nimloc = 32,/ Nimrod Locator / ns_t_srv = 33, / Server Selection / ns_t_atma = 34, / ATM Address / ns_t_naptr = 35, / Naming Authority PoinTeR */
/* Query type values not in resource records / ns_t_ixfr = 251, / Incremental zone transfer / ns_t_axfr = 252, / Transfer zone of authority / ns_t_mailb = 253,/ Transfer mailbox records / ns_t_maila = 254,/ Transfer mail agent records / ns_t_any = 255, / Wildcard match */ ns_t_max = 65536 } ns_type;
Class field definitions in arpa/nameser.h :
- Values for class field / typedef enum __ns_class { ns_c_in = 1, / Internet. / / Class 2 unallocated/unsupported. / ns_c_chaos = 3, / MIT Chaos-net. / ns_c_hs = 4, / MIT Hesiod. / / Query class values which do not appear in resource records / ns_c_none = 254, / for prereq. sections in update requests / ns_c_any = 255, / Wildcard match. */ ns_c_max = 65536 } ns_class;
Zone partitioning of the DNS name space:
] zone - contains attribute data for names in domain minus the
sub-domains administrated by lower-level authorities:
Example: UTSA has a name server for utsa.edu, but cs.utsa.edu
names are resolved by the CS Division server
] at least two name servers that provide authoritative data for the
zone
] names of the servers for the sub-domains
] zone management parameters
Zone partitioning of the DNS name space:
|(C.ISI.EDU,SRI-NIC.ARPA
| A.ISI.EDU)
MIL EDU ARPA
|(SRI-NIC.ARPA, |(SRI-NIC.ARPA, |
| A.ISI.EDU | C.ISI.EDU) |
BRL NOSC DARPA | IN-ADDR SRI-NIC ACC
UCI MIT | UDEL YALE
|(XX.LCS.MIT.EDU, ISI
|ACHILLES.MIT.EDU) |(VAXA.ISI.EDU,VENERA.ISI.EDU,
from RCF 1034: Domain Concepts and Facilities by Mockpetris
Authoritative name servers:
] a server may be an authoritative source for zero or more zones
] data for a zone is entered into a local master file
] the master (primary) server reads the zone data directly from the
master file
] secondary authoritative servers download zone data from
primary server
] secondary servers periodically check their version number
against the master server
Sample query of the root server :
. IN SOA SRI-NIC.ARPA. HOSTMASTER.SRI-NIC.ARPA. (
870611 ;serial 1800 ;refresh every 30 min 300 ;retry every 5 min 604800 ;expire after a week
- ;minimum of a day NS A.ISI.EDU NS C.ISI.EDU. NS SRI-NIC.ARPA.
MIL. 86400 NS SRI-NIC.ARPA. 86400 NS A.ISI.EDU.
EDU. 86400 NS SRI-NIC.ARPA. 86400 NS C.ISI.EDU.
SRI-NIC.ARPA. A 26.0.0. A 10.0.0. MX 0 SRI-NIC.ARPA. HINFO DEC-2060 TOPS
ACC.ARPA. A 26.6.0.
HINFO PDP-11/70 UNIX
MX 10 ACC.ARPA.
... Continued in next column
...Continued from previous column USC-ISIC.ARPA. CNAME C.ISI.EDU. 73.0.0.26.IN-ADDR.ARPA. PTR SRI-NIC.ARPA. 65.0.6.26.IN-ADDR.ARPA. PTR ACC.ARPA. 51.0.0.10.IN-ADDR.ARPA. PTR SRI-NIC.ARPA. 52.0.0.10.IN-ADDR.ARPA. PTR C.ISI.EDU.
103.0.3.26.IN-ADDR.ARPA. PTR A.ISI.EDU.
A.ISI.EDU. 86400 A 26.3.0.
C.ISI.EDU. 86400 A 10.0.0.
Using nstest to query name servers (example 1):
visual8% nstest 129.115.102.
amedusa.cs.utsa.edu
;; res_mkquery(0, medusa.cs.utsa.edu, 1, 1) ;; res_send()
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46777 ;; flags: rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; medusa.cs.utsa.edu, type = A, class = IN ;; Querying server (# 1) address = 129.115.102.
;; got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46777 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0;; medusa.cs.utsa.edu, type = A, class = IN medusa.cs.utsa.edu. 1D IN A 129.115.11.
Using nstest to query name servers (example 2):
nmedusa.cs.utsa.edu ;; res_mkquery(0, medusa.cs.utsa.edu, 1, 2)
;; res_send() ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46778
;; flags: rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; medusa.cs.utsa.edu, type = NS, class = IN ;; Querying server (# 1) address = 129.115.102.
;; got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46778
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 ;; medusa.cs.utsa.edu, type = NS, class = IN
cs.utsa.edu. 1D IN SOA jazz.cs.utsa.edu. root.jazz.cs.utsa.edu. ( 424 ; serial 3H ; refresh 1H ; retry 1W ; expiry 1D ) ; minimum
The resolver library:
cc [ flag ... ] file ... -lresolv -lsocket -lnsl [ library ... ]
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
int res_mkquery(int op, /* QUERY */
const char dname, / domain */
int class, /* IN */
int type, /* A, NS, etc. */
const char data, / RR */
int datalen, /* length of RR */
struct rrec newrr, / use null*/
uchar_t buf, / answer */
int buflen /* answer length */);
The resolver library:
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
int res_send(uchar_t msg, / preformated msg */
int msglen,
uchar_t *answer,
int anslen);
The resolver library:
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
Int res_query(const char *dname,
int class,
int type,
uchar_t *answer,
int anslen);
For next time:
] Read CDK Chapter 9.3-9.
] Try some examples of nstest before the next class
