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gethostbyname(), gethostbyaddr()

Get an IP address for a hostname, or vice-versa

Prototypes

#include <sys/socket.h>
#include <netdb.h>

struct hostent *gethostbyname(const char *name);
struct hostent *gethostbyaddr(const char *addr, int len, int type);

Description

These functions map back and forth between host names and IP addresses. After all, you want an IP address to pass to connect(), right? But no one wants to remember an IP address. So you let your users type in things like "www.yahoo.com" instead of "66.94.230.35".

gethostbyname() takes a string like "www.yahoo.com", and returns a struct hostent which contains tons of information, including the IP address. (Other information is the official host name, a list of aliases, the address type, the length of the addresses, and the list of addresses--it's a general-purpose structure that's pretty easy to use for our specific purposes once you see how.)

gethostbyaddr() takes a struct in_addr and brings you up a corresponding host name (if there is one), so it's sort of the reverse of gethostbyname(). As for parameters, even though addr is a char*, you actually want to pass in a pointer to a struct in_addr. len should be sizeof(struct in_addr), and type should be AF_INET.

So what is this struct hostent that gets returned? It has a number of fields that contain information about the host in question.

char *h_name

The real canonical host name.

char **h_aliases

A list of aliases that can be accessed with arrays--the last element is NULL

int h_addrtype

The result's address type, which really should be AF_INET for our purposes..

int length

The length of the addresses in bytes, which is 4 for IP (version 4) addresses.

char **h_addr_list

A list of IP addresses for this host. Although this is a char**, it's really an array of struct in_addr*s in disguise. The last array element is NULL.

h_addr

A commonly defined alias for h_addr_list[0]. If you just want any old IP address for this host (yeah, they can have more than one) just use this field.

Return Value

Returns a pointer to a resultant struct hostent or success, or NULL on error.

Instead of the normal perror() and all that stuff you'd normally use for error reporting, these functions have parallel results in the variable h_errno, which can be printed using the functions herror() or hstrerror(). These work just like the classic errno, perror(), and strerror() functions you're used to.

Example

int i;
struct hostent *he;
struct in_addr **addr_list;
struct in_addr addr;

// get the addresses of www.yahoo.com:

he = gethostbyname("www.yahoo.com");
if (he == NULL) { // do some error checking
    herror("gethostbyname"); // herror(), NOT perror()
    exit(1);
}

// print information about this host:
printf("Official name is: %s\n", he->h_name);
printf("IP address: %s\n", inet_ntoa(*(struct in_addr*)he->h_addr));
printf("All addresses: ");
addr_list = (struct in_addr **)he->h_addr_list;
for(i = 0; addr_list[i] != NULL; i++) {
    printf("%s ", inet_ntoa(*addr_list[i]));
}
printf("\n");

// get the host name of 66.94.230.32:

inet_aton("66.94.230.32", &addr);
he = gethostbyaddr(&addr, sizeof(addr), AF_INET);

printf("Host name: %s\n", he->h_name);

See Also

gethostname(), errno, perror(), strerror(), struct in_addr

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