- Converting
a symbolic host name
(such as "www.facebook.com") to
a binary IP address:
- struct hostent*
gethostbyname("hostname")
- Input: a string
with
a symbolic host name
(e.g.: "www.facebook.com")
- Output:
a pointer to
a
host entry typed
data structure
- Returns NULL if error (e.g., hostname is not found).
- Input: a string
with
a symbolic host name
(e.g.: "www.facebook.com")
- struct hostent*
gethostbyname("hostname")
- The host entry struct hostent
data structure:
struct hostent { char * h_name; /* The canonical (official) name of host */ // a string variable char * h_aliases[ ]; /* The list of "aliases" for the hostname */ // h_aliases is an array of string // h_aliases[i] == NULL marks the end of the array int h_addrtype; /* host address type (should be = AF_INET (2)) */ int h_length; /* length of address (= 4 bytes IP addresses) */ char * h_addr_list[ ]; /* list of addresses */ // h_addr_list is an array of int (binary IP addresses) // in network byte order // h_addr_list[i] == 0 marks the end of the array };
- Graphical representation
of the "host entry"
data structure
(with sample content
to make things clearer):
- Example output:
Enter hostname: www.yahoo.com
official hostname h_name = `ds-any-fp3-real.wa1.b.yahoo.com' Host has this alias list: Alias 1: www.yahoo.com Alias 2: fd-fp3.wg1.b.yahoo.com Alias 3: ds-fp3.wg1.b.yahoo.com Alias 4: ds-any-fp3-lfb.wa1.b.yahoo.com Host's address type = 2 (should be AF_INET = 2) Host's address length = 4 List of Internet addresses for host: IP address 1: 98.138.253.109 IP address 2: 206.190.36.45 IP address 3: 98.138.252.30 IP address 4: 206.190.36.105
- Example Program:
(Demo above code)
- Prog file: click here
Notes:
- Compile with: cc -o gethostbyname gethostbyname.c -lnsl
Example: how to use the gethostbyname() call
/* -------------------------------
Variables
------------------------------- */
char hname[100];
struct hostent *hp; /* host pointer */
int addr;
int i;
printf("Enter hostname: ");
scanf("%s", hname);
hp = gethostbyname(hname);
if (hp == NULL)
{
printf("host information for %s not found\n", hname);
exit(1);
}
/* -----------------------------------
Print the information
----------------------------------- */
/* --------------
Canonical name
-------------- */
printf("official hostname h_name = `%s'\n", hp->h_name);
/* -----------------------------
List of aliases
----------------------------- */
printf("Host has this alias list:\n");
char *p[]; /* help variable to access h_aliases[ ] */
p = hp->h_aliases; // p[i] is a string (char *)
for ( i = 0 ; p[i] != NULL; i++ )
{
printf("\tAlias %d: %s\n", i+1, p[i] );
}
printf("Host's address type = %d (should be AF_INET = %d)\n",
hp->h_addrtype, AF_INET);
/* -------------------------------------
Length of an IP address
------------------------------------- */
printf("Host's address length = %d\n", hp->h_length);
/* -------------------------------------
List of IP addresses
------------------------------------- */
printf("List of Internet addresses for host:\n");
int *a[]; /* Help variable to access h_addr_list[ ] */
a = (int **) hp->h_addr_list; // a[i] is an IP address
for (i = 0; a[i] != 0; i++)
{
printf("\tIP address %d: ", i+1);
paddr( (void*) a[i] ); // Print IP address as dotted decimal
printf("\n");
}
|
- Fact:
- We just need a (any) IP address of a IP host !!!
- Variable in the
host entry structure
that contain the
IP addresses of
the host:
h_addr_list[ ] // It's an array (multiple IP addresses)We just need 1, so the safest choice is:
h_addr_list[0] // The other entries may be invalid !!!
- The UDP sender
using
a symbolic host name:
int main(int argc, char **argv) { struct sockaddr_in dest_addr; /* Destination socket address */ struct hostent *hp; ... /* ============================================================= argv[1] is the string containing the symbolic host name ============================================================= */ hp = gethostbyname(argv[1]); // E.g. argv[1] = "wwww.google.com" if ( h == NULL ) { printf("Error... \n"); exit(1); } /* ============================================================= Use: hp->haddr_list[0] // This one is for sure valid :) Note: hp->haddr_list[0] is in network byte ordering !!! ============================================================= */ memcpy( (char *) &(dest_addr.sin_addr.s_addr), hp->h_addr_list[0], // Copy first entry and hp->h_length); // perserve byte ordering ... }Note:
- memcpy(dest, src, len)
copies
len
bytes from
src address
to
dest address
while
preserving the byte ordering
- Do not use htonl( )
because the IP address in h_addr_list[0] is already in network byte ordering !!!)
- memcpy(dest, src, len)
copies
len
bytes from
src address
to
dest address
while
preserving the byte ordering
- Example Program:
(Demo above code)
- The new UDP sender using symbolic host names Prog file: click here
- The (unmodified) UDP receiver Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile the sender: gcc -o udp4-s3 udp4-s3.c -lnsl
- To compile the receiver:
gcc -o udp4-r2 udp4-r2.c
- To run the receiver: udp4-r2 port#
- To run the sender: udp4-s3 lab??.mathcs.emory.edu port#