Elementary Name and Address Conversions Chapter9 Elementary Name and Address Conversions

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

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

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

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

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

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

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 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);

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);