Chapter9

Elementary Name and Address Conversions

IntroductionIntroduction Domain Name Server gethostbyname Function. gethostbyaddr Function. uname Function. gethostname Function.

getsvrbyname & getsvrbyport Function.

Domain Name SystemDomain Name System

FQDN(Fully qualified domain name) ex) comedu.chungbuk.ac.kr.

RRs(Resource Records)Types Description Exam

A Maps hostname into 32-bit IPv4 203.255.74.123AAAA Maps hostname into 128-bit IPv6 pp.238PTR Map IP addresses into hostnames

For An IPv4 address the 4bytes of32bit address are reversed (Dec.) IPv6 reversed sequence. (Hex)

123.74.255.203.in-addr.arpaIPv6 pp.238

MX Priority of mail serviceCNAME Service name map into server

namewww and ftp

Resolvers and Name ServersResolvers and Name Servers

Applicationcode

Resolvercode

Localname server

Othername server

Resolverconfiguration

files

Function call Function return

UDP request

UDP reply

Application

/etc/resolv.conf

Typical arrangement of clients, resolvers, and name serversTypical arrangement of clients, resolvers, and name servers

gethostbyname Function#include <netdb.h>

struct hostent *gethostbyname (const char *hostname);

struct hostent {char *h_name; /* official (canonical) name of host */char **h_aliases; /* pointer to arrayof opinters to alises names */int h_addrtype; /* host address type : AF_INET or AF_INET6 */int h_length; /* length of address : 4 or 6 */char **h_addr_list; /* ptr to array of ptrs with IPv4 or IPv6 addrs */

};

hostent{}

h_name

h_aliases

h_addrtype

h_length

h_addr_list

official hostname \0

NULL

NULL

Alises #1 \0

Alises #2 \0

IP addr #1

IP addr #2

IP addr #3h_length =4

In_addr{}

In_addr{}

In_addr{}

AF_INET

4

RES_USE_INET6 Resolver Option

1. #include <resolv.h> res_init(); _res.option |= RES_USE_INET6;

2. export RES_OPTION=inet6; (shell once) (in .profile One User Only)

3. options inet6 (in /etc/resolv.conf) if all program support IPv6

#include <netdb.h>

struct hostent *gethostbyname2 (const char *hostname, int family);

gethostbyname2 Function

RES_USE_INET6 optionoff on

gethostbyname(host)

Search for A records. Iffound, return IPv4addresses( h_length=4 ).Else error.

Search for AAAA records. Iffound, return IPv6addresses(h_length=16).Else Search for A records. Iffound, return IPv4-mapped IPv6addresses(h_length=16).Else error.

Gethostbyname2(host,AF_INET)

Search for A records. Iffound, return IPv4addresses( h_length=4 ).Else error.

Search for A records. If found,return IPv4-mapped IPv6addresses(h_length=16).Else error.

Gethostbyname2(host,AF_INET6)

Search for AAAA records.If found, return IPv4addresses(h_length=16).Else error.

Search for AAAA records. Iffound, return IPv6addresses(h_length=16).Else error.

gethostbyaddr Function

#include <netdb.h>struct hostent *gethostbyaddr(const chr *addr, size_t len,int family);

The field of interest in this structure is normally h_name, the canonical hostname

uname Function#include <netdb.h>

int uname(struct utsname *name);#define _UTS_NAMESIZE 16#define _UTS_NODESIZE 256

struct ntsname {char sysname[_UTS_NAMESIZE]; /* name of this operationg system */char sysname[_UTS_NAMESIZE]; /* name of this node */char sysname[_UTS_NAMESIZE]; /* O. S. release level */char sysname[_UTS_NAMESIZE]; /* O. S. version level */char sysname[_UTS_NAMESIZE]; /* hardware type */

};

gethostname Function#include <netdb.h>

int gethostname(char *name, size_t namelen);

getservbyname Function#include <netdb.h>

struct servent *getservbyname(const char *servname, const char *protoname);

struct servent {char *s_name; /* official service name */char **s_ aliases; /* aliases list*/int s_port; /* port number, network by order */char *s_proto; /* protocol to use */

};

getservbyport Function#include <netdb.h>

struct servent *getservbyport(int port, const char *protoname);