1. Elementary Name and Address Conversions
Chapter9
Elementary Name and Address Conversions

2. Introduction Domain Name Server gethostbyname Function.
gethostbyaddr Function.
uname Function.
gethostname Function.
getsvrbyname & getsvrbyport Function.

3. Domain Name System
FQDN(Fully qualified domain name) ex) comedu.chungbuk.ac.kr.
RRs(Resource Records)

4. Resolvers and Name Servers
Application
code
Resolver
Local
name server
Other
configuration
files
Function call
Function return
UDP request
UDP reply
/etc/resolv.conf
Typical arrangement of clients, resolvers, and name servers

5. 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
Alises #1 \0
Alises #2 \0
IP addr #1
IP addr #2
IP addr #3
h_length =4
In_addr{}
AF_INET 4

6. 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

7. gethostbyname2 Function
#include <netdb.h>
struct hostent *gethostbyname2 (const char *hostname, int family);

8. 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

9. uname Function gethostname 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);

10. 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);