Unix Network Programming Episode 77

‘gethostbyaddr’ Function

The function gethostbyaddr takes a binary IPv4 address and tries to find the hostname corresponding to that address. This is the reverse of gethostbyname.

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

‘getservbyname’ and ‘getservbyport’ Functions

Services, like hosts, are often known by names, too. If we refer to a service by its name in our code, instead of by its port number, and if the mapping from the name to port number is contained in a file (normally /etc/services), then if the port number changes, all we need to modify is one line in the /etc/services file instead of having to recompile the applications. The next function, getservbyname, looks up a service given its name.

#include <netdb.h>
struct servent *getservbyname (const char *servname, const char *protoname);

This function returns a pointer to the following structure:

struct servent {
	char *s_name; /* official service name */
	char **s_aliases; /* alias list */
	int s-port; /* port number, network-byte order */
	char *s_proto; /* protocol to use */
};

The service name servname must be specified. If a protocol is also specified (protoname is a non-null pointer), then the entry must also have a matching protocol.

The next function, getservbyport, looks up a service given its port number and an optional protocol.

#include <netdb.h>
struct servent *getservbyport (int port, const char *protoname);

#include "unp.h"

int main(int argc, char **argv)
{
    int sockfd, n;
    char recvline[MAXLINE+1];
    struct sockaddr_in servaddr;
    struct in_addr **ptr;
    struct in_addr *inetaddrp[2];
    struct in_addr inetaddr;
    struct hostent *hp;
    struct servent *sp;

    if(argc!=3){
        err_quit("usage: daytimetcpclientl <hostname> <service>");
    }

    if((hp=gethostbyname(argv[1]))==NULL)
    {
        if(inet_aton(argv[1],&inetaddr)==0)
        {
            err_quit("hostname error for %s: %s",argv[1], hstrerror(h_errno));
        }else{
            inetaddrp[0]=&inetaddr;
            inetaddrp[1]=NULL;
            pptr=inetaddrp;
        }
    }
    else
    {
        pptr=(struct in_addr **)hp->h_addr_list;
    }

    if((sp=getservyname(argv[2],"tcp"))==NULL)
    {
        err_quit("getservbyname error for %s", argv[2]);
    }
    for(;*pptr!=NULL;pptr++)
    {
        sockfd=Socket(AF_INET, SOCK_STREAM,0);

        bzero(&servaddr, sizeof(servaddr));
        servaddr.sin_family=AF_INET;
        servaddr.sin_port=sp->s_port;
        memcpy(&servaddr.sin_addr, *pptr, sizeof9struct in_addr));
        printf("trying %s\n", Sock_ntop((SA *)&servaddr, sizeof(servaddr)));

        if(connect(sockfd, (SA *)&servaddr, iszoef9servaddr))==0)
            break;
        err_ret("connect error");
        close(sockfd);
    }
    if(*pptr==NULL)
    {
        err_quit("unable to connect");
    }

    while((n=Read(sockfd, recvline,MAXLINE))>0)
    {
        recvlien[n]=0;
        Fputs(recvline, stdout);
    }
    return 0;
}

Our daytime client that uses gethostbyname and getservbyname