getservbyname函数和getservbyport函数究竟有什么用呢？

在学习Winsock网络编程的时候，看到函数getservbyname，文档上说该函数的功能是根据服务的名称获得服务的信息，于是我写了如下的测试程序，结果发现这里所说的获得服务的信息，不能获得我机器上已经开放的服务信息，而只能获得标准的服务，如www，ftp等服务信息。程序如下：

/*根据服务名获取服务的信息，顺便学习Winsock函数getservbyname的用法*/
#include
#include
#include
using namespace std;

int main(int argc, char* argv[]){
if(argc != 2){
  cout << "Please input the Server Name." << endl;
  exit(0);
}

WORD wVersionRequested;
WSADATA wsaData;
int err;

wVersionRequested = MAKEWORD(2,2);
err = WSAStartup(wVersionRequested, &wsaData);
if(err != 0){
  cout << "Can not find a usable Winsock dll." << endl;
  return 0;
}

//输出WSADATA中的信息
cout << "输出WSADATA中的信息" << endl;
cout << "wsaData.wVersion:" << (int)LOBYTE(wsaData.wVersion) << "." << (int)HIBYTE(wsaData.wVersion) << endl;
cout << "wsaData.wHighVersion:" << (int)LOBYTE(wsaData.wHighVersion) << "." << (int)HIBYTE(wsaData.wHighVersion) << endl;
cout << "wsaData.szDescription:" << wsaData.szDescription << endl;
cout << "wsaData.szSystemStatus:" << wsaData.szSystemStatus << endl;
if(wsaData.iMaxSockets == 0){
  cout << "wsaData.iMaxSockets:" << "Ignore" << endl;
}else{
  cout << "wsaData.iMaxSockets:" << wsaData.iMaxSockets << endl;
}
if(wsaData.iMaxUdpDg == 0){
  cout << "wsaData.iMaxUdpDg:" << "Ignore" << endl;
}else{
  cout << "wsaData.iMaxUdpDg:" << wsaData.iMaxUdpDg << endl;
}
if(wsaData.lpVendorInfo == 0){
  cout << "wsaData.lpVendorInf" << "Ignore" << endl;
}else{
  cout << "wsaData.lpVendorInf" << wsaData.lpVendorInfo << endl;
}

//调用getservbyname
servent* server_info;
server_info = getservbyname(argv[1], NULL);
//输出server_info
cout << "输出servent中的信息" << endl;
if(server_info != NULL){
  cout << "server_info->s_name:" << server_info->s_name << endl;
  for(int i=0; server_info->s_aliases[i] != NULL; i++){
   cout << "server_info->s_aliases:" << server_info->s_aliases[i] << endl;
  }
  cout << "server_info->s_port:" <s_port) << endl;
  cout << "server_info->s_prot" << server_info->s_proto << endl;
}

WSACleanup( );
return 0;

}

测试结果如下：
GetServByName  www
输出结果如下：
输出WSADATA中的信息
wsaData.wVersion:1.1
wsaData.wHighVersion:2.2
wsaData.szDescription:WinSock 2.0
wsaData.szSystemStatus:Running
wsaData.iMaxSockets:32767
wsaData.iMaxUdpDg:65467
wsaData.lpVendorInf?aA
输出servent中的信息
server_info->s_name:http
server_info->s_aliases:www
server_info->s_aliases:www-http
server_info->s_port:80
server_info->s_prottcp

但是，我的机器上开放了Server服务，而执行
GetServByName Server
却得不到任何信息，输出如下：
输出WSADATA中的信息
wsaData.wVersion:1.1
wsaData.wHighVersion:2.2
wsaData.szDescription:WinSock 2.0
wsaData.szSystemStatus:Running
wsaData.iMaxSockets:32767
wsaData.iMaxUdpDg:65467
wsaData.lpVendorInf?aA
输出servent中的信息

有此可见，调用getservbyname只能得到标准的服务，如www,ftp等的信息，但是对于这些标准的信息，我们完全没有必要调用这个函数，不是吗？

为了进一步观察是否能够得到其它一些服务的信息，我又写了以下的程序，使用getservbyport函数遍历从0到62235的端口，看看能得到哪些服务的信息

/*根据服务端口获取服务的信息，顺便学习Winsock函数getservbyport的用法*/
#include
#include
#include
using namespace std;

int main(int argc, char* argv[]){

WORD wVersionRequested;
WSADATA wsaData;
int err;

wVersionRequested = MAKEWORD(2,2);
err = WSAStartup(wVersionRequested, &wsaData);
if(err != 0){
  cout << "Can not find a usable Winsock dll." << endl;
  return 0;
}

//调用getservbyport遍历从0到65535的端口
servent* server_info;
for(int i=0;i< 65536;i++){
  server_info = getservbyport(i, NULL);
  //输出server_info
  if(server_info != NULL){
   cout << "server_info->s_name:" << server_info->s_name << endl;
   for(int i=0; server_info->s_aliases[i] != NULL; i++){
    cout << "server_info->s_aliases:" << server_info->s_aliases[i] << endl;
   }
   cout << "server_info->s_port:" <s_port) << endl;
   cout << "server_info->s_prot" << server_info->s_proto << endl;
   cout << endl;
  }

}

WSACleanup( );
return 0;

}

执行该程序，输出结果如下：
server_info->s_name:exec
server_info->s_port:512
server_info->s_prottcp

server_info->s_name:login
server_info->s_port:513
server_info->s_prottcp

server_info->s_name:nfsd
server_info->s_aliases:nfs
server_info->s_port:2049
server_info->s_protudp

server_info->s_name:cmd
server_info->s_aliases:shell
server_info->s_port:514
server_info->s_prottcp

server_info->s_name:printer
server_info->s_aliases:spooler
server_info->s_port:515
server_info->s_prottcp

server_info->s_name:talk
server_info->s_port:517
server_info->s_protudp

server_info->s_name:knetd
server_info->s_port:2053
server_info->s_prottcp

server_info->s_name:ntalk
server_info->s_port:518
server_info->s_protudp

server_info->s_name:echo
server_info->s_port:7
server_info->s_prottcp

server_info->s_name:efs
server_info->s_port:520
server_info->s_prottcp

server_info->s_name:discard
server_info->s_aliases:sink
server_info->s_aliases:null
server_info->s_port:9
server_info->s_prottcp

server_info->s_name:systat
server_info->s_aliases:users
server_info->s_port:11
server_info->s_prottcp

server_info->s_name:daytime
server_info->s_port:13
server_info->s_prottcp

server_info->s_name:timed
server_info->s_aliases:timeserver
server_info->s_port:525
server_info->s_protudp

server_info->s_name:tempo
server_info->s_aliases:newdate
server_info->s_port:526
server_info->s_prottcp

server_info->s_name:qotd
server_info->s_aliases:quote
server_info->s_port:17
server_info->s_prottcp

server_info->s_name:courier
server_info->s_aliases:rpc
server_info->s_port:530
server_info->s_prottcp

server_info->s_name:chargen
server_info->s_aliases:ttytst
server_info->s_aliases:source
server_info->s_port:19
server_info->s_prottcp

server_info->s_name:conference
server_info->s_aliases:chat
server_info->s_port:531
server_info->s_prottcp

server_info->s_name:ftp-data
server_info->s_port:20
server_info->s_prottcp

server_info->s_name:netnews
server_info->s_aliases:readnews
server_info->s_port:532
server_info->s_prottcp

server_info->s_name:radius
server_info->s_port:1812
server_info->s_protudp

server_info->s_name:ftp
server_info->s_port:21
server_info->s_prottcp

server_info->s_name:netwall
server_info->s_port:533
server_info->s_protudp

server_info->s_name:radacct
server_info->s_port:1813
server_info->s_protudp

server_info->s_name:telnet
server_info->s_port:23
server_info->s_prottcp

server_info->s_name:smtp
server_info->s_aliases:mail
server_info->s_port:25
server_info->s_prottcp

server_info->s_name:uucp
server_info->s_aliases:uucpd
server_info->s_port:540
server_info->s_prottcp

server_info->s_name:klogin
server_info->s_port:543
server_info->s_prottcp

server_info->s_name:kshell
server_info->s_aliases:krcmd
server_info->s_port:544
server_info->s_prottcp

server_info->s_name:time
server_info->s_aliases:timserver
server_info->s_port:37
server_info->s_prottcp

server_info->s_name:new-rwho
server_info->s_aliases:new-who
server_info->s_port:550
server_info->s_protudp

server_info->s_name:rlp
server_info->s_aliases:resource
server_info->s_port:39
server_info->s_protudp

server_info->s_name:nameserver
server_info->s_aliases:name
server_info->s_port:42
server_info->s_prottcp

server_info->s_name:nicname
server_info->s_aliases:whois
server_info->s_port:43
server_info->s_prottcp

server_info->s_name:remotefs
server_info->s_aliases:rfs
server_info->s_aliases:rfs_server
server_info->s_port:556
server_info->s_prottcp

server_info->s_name:rmonitor
server_info->s_aliases:rmonitord
server_info->s_port:560
server_info->s_protudp

server_info->s_name:monitor
server_info->s_port:561
server_info->s_protudp

server_info->s_name:domain
server_info->s_port:53
server_info->s_prottcp

server_info->s_name:man
server_info->s_port:9535
server_info->s_prottcp

server_info->s_name:bootps
server_info->s_aliases:dhcps
server_info->s_port:67
server_info->s_protudp

server_info->s_name:bootpc
server_info->s_aliases:dhcpc
server_info->s_port:68
server_info->s_protudp

server_info->s_name:tftp
server_info->s_port:69
server_info->s_protudp

server_info->s_name:gopher
server_info->s_port:70
server_info->s_prottcp

server_info->s_name:finger
server_info->s_port:79
server_info->s_prottcp

server_info->s_name:http
server_info->s_aliases:www
server_info->s_aliases:www-http
server_info->s_port:80
server_info->s_prottcp

server_info->s_name:kpop
server_info->s_port:1109
server_info->s_prottcp

server_info->s_name:kerberos
server_info->s_aliases:krb5
server_info->s_aliases:kerberos-sec
server_info->s_port:88
server_info->s_prottcp

server_info->s_name:hostname
server_info->s_aliases:hostnames
server_info->s_port:101
server_info->s_prottcp

server_info->s_name:iso-tsap
server_info->s_port:102
server_info->s_prottcp

server_info->s_name:rtelnet
server_info->s_port:107
server_info->s_prottcp

server_info->s_name:pop2
server_info->s_aliases:postoffice
server_info->s_port:109
server_info->s_prottcp

server_info->s_name:pop3
server_info->s_port:110
server_info->s_prottcp

server_info->s_name:sunrpc
server_info->s_aliases:rpcbind
server_info->s_aliases:portmap
server_info->s_port:111
server_info->s_prottcp

server_info->s_name:auth
server_info->s_aliases:ident
server_info->s_aliases:tap
server_info->s_port:113
server_info->s_prottcp

server_info->s_name:uucp-path
server_info->s_port:117
server_info->s_prottcp

server_info->s_name:nntp
server_info->s_aliases:usenet
server_info->s_port:119
server_info->s_prottcp

server_info->s_name:ntp
server_info->s_port:123
server_info->s_protudp

server_info->s_name:ldaps
server_info->s_aliases:sldap
server_info->s_port:636
server_info->s_prottcp

server_info->s_name:ldap
server_info->s_port:389
server_info->s_prottcp

server_info->s_name:epmap
server_info->s_aliases:loc-srv
server_info->s_port:135
server_info->s_prottcp

server_info->s_name:netbios-ns
server_info->s_aliases:nbname
server_info->s_port:137
server_info->s_prottcp

server_info->s_name:netbios-dgm
server_info->s_aliases:nbdatagram
server_info->s_port:138
server_info->s_protudp

server_info->s_name:netbios-ssn
server_info->s_aliases:nbsession
server_info->s_port:139
server_info->s_prottcp

server_info->s_name:imap
server_info->s_aliases:imap4
server_info->s_port:143
server_info->s_prottcp

server_info->s_name:phone
server_info->s_port:1167
server_info->s_protudp

server_info->s_name:ms-sql-s
server_info->s_port:1433
server_info->s_prottcp

server_info->s_name:doom
server_info->s_port:666
server_info->s_prottcp

server_info->s_name:ms-sql-m
server_info->s_port:1434
server_info->s_prottcp

server_info->s_name:pcmail-srv
server_info->s_port:158
server_info->s_prottcp

server_info->s_name:snmp
server_info->s_port:161
server_info->s_protudp

server_info->s_name:snmptrap
server_info->s_aliases:snmp-trap
server_info->s_port:162
server_info->s_protudp

server_info->s_name:l2tp
server_info->s_port:1701
server_info->s_protudp

server_info->s_name:print-srv
server_info->s_port:170
server_info->s_prottcp

server_info->s_name:bgp
server_info->s_port:179
server_info->s_prottcp

server_info->s_name:https
server_info->s_aliases:MCom
server_info->s_port:443
server_info->s_prottcp

server_info->s_name:pptp
server_info->s_port:1723
server_info->s_prottcp

server_info->s_name:microsoft-ds
server_info->s_port:445
server_info->s_prottcp

server_info->s_name:irc
server_info->s_port:194
server_info->s_prottcp

server_info->s_name:kpasswd
server_info->s_port:464
server_info->s_prottcp

server_info->s_name:ipx
server_info->s_port:213
server_info->s_protudp

server_info->s_name:wins
server_info->s_port:1512
server_info->s_prottcp

server_info->s_name:kerberos-adm
server_info->s_port:749
server_info->s_prottcp

server_info->s_name:kerberos-iv
server_info->s_port:750
server_info->s_protudp

server_info->s_name:isakmp
server_info->s_aliases:ike
server_info->s_port:500
server_info->s_protudp

server_info->s_name:ingreslock
server_info->s_aliases:ingres
server_info->s_port:1524
server_info->s_prottcp

看来，这个函数毕竟不是一无用处的，得到的信息还是比较多。但是我的疑问是：该函数返回的结果和某个机器上是否运行了这么多服务一点关系都没有，那么，它们究竟有什么用呢？
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