www.facebook.com") to IPv4 address (e.g. "31.13.75.17") and vice versa. Functions called gethostbyname and gethostbyaddr, while technically deprecated, are still so prevalent that you should know how to use them.struct hostent *gethostbyname(const char *name);
struct hostent *gethostbyaddr(const char *addr, int len, int type);struct hostent with network information about some machine on the Internet.
gethostbyname assumes its argument is a host name (e.g. "www.google.com").gethostbyaddr assumes the first argument is a binary representation of an IP address (e.g. not the string "171.64.64.137", but the base address of a character array with ASCII values of 171, 64, 64, and 137 laid down side by side in network byte order, which is the same as big endian order.sizeof(struct in_addr)) and the third is typically the AF_INET constant.struct hostent packages all of the information about a particular host:struct in_addr {
unsigned int s_addr // four bytes, stored in network byte order (big endian)
};
struct hostent {
char *h_name; // official name of host
char **h_aliases; // NULL-terminated list of aliases
int h_addrtype; // host address type (typically AF_INET for IPv4)
int h_length; // address length (typically 4)
char **h_addr_list; // NULL-terminated list of IP addresses
}; // h_addr_list is really a struct in_addr ** when hostent contains IPv4 addressesstruct in_addr is a one-field record modeling an IPv4 address.
s_addr field packs each figure of a dotted quad (e.g. 171.64.64.136) into one of its four bytes. Each of these four numbers numbers can range from 0 up through 255.struct hostent is used for all IP addresses, not just IPv4 addresses. For non-IPv4 addresses, h_addrtype, h_length, and h_addr_list carry different types of data than they do for IPv4.www.facebook.com", but network communication ultimately works with IP addresses like "31.13.75.17".
gethostbyname and gethostbyaddr are used to manage translations between the two.static void publishIPAddressInfo(const string& host) {
struct hostent *he = gethostbyname(host.c_str());
if (he == NULL) { // NULL return value means resolution attempt failed
cout << host << " could not be resolved to an address. Did you mistype it?" << endl;
return;
}
cout << "Official name is \"" << he->h_name << "\"" << endl;
cout << "IP Addresses: " << endl;
struct in_addr **addressList = (struct in_addr **) he->h_addr_list;
while (*addressList != NULL) {
char str[INET_ADDRSTRLEN];
cout << "+ " << inet_ntop(AF_INET, *addressList, str, INET_ADDRSTRLEN) << endl;
addressList++;
}
}h_addr_list is typed to be a char * array, implying it's an array of C strings, perhaps dotted quad IP addresses. However, that's not correct. For IPv4 records, h_addr_list is an array of struct in_addr *s.inet_ntop function places a traditional C string presentation of an IP address into the provided character buffer, and returns the the base address of that buffer.h_addr_list array until it lands on a NULL.static void publishIPAddressInfo(const string& host) {
struct hostent *he = gethostbyname(host.c_str());
if (he == NULL) { // NULL return value means resolution attempt failed
cout << host << " could not be resolved to an address. Did you mistype it?" << endl;
return;
}
cout << "Official name is \"" << he->h_name << "\"" << endl;
cout << "IP Addresses: " << endl;
struct in_addr **addressList = (struct in_addr **) he->h_addr_list;
while (*addressList != NULL) {
char str[INET_ADDRSTRLEN];
cout << "+ " << inet_ntop(AF_INET, *addressList, str, INET_ADDRSTRLEN) << endl;
addressList++;
}
}Hostname Resolution: IPv4
www.yale.edu is the exception. It looks like Yale relies on a content delivery network called Cloudflare, and www.yale.edu is catalogued as an alias.myth61$ ./resolve-hostname
Welcome to the IP address resolver!
Enter a host name: www.google.com
Official name is "www.google.com"
IP Addresses:
+ 216.58.192.4
Enter a host name: www.coinbase.com
Official name is "www.coinbase.com"
IP Addresses:
+ 104.16.9.251
+ 104.16.8.251
Enter a host name: www.yale.edu
Official name is "www.yale.edu.cdn.cloudflare.net"
IP Addresses:
+ 104.16.140.133
+ 104.16.141.133
Enter a host name: www.okcupid.com
Official name is "www.okcupid.com"
IP Addresses:
+ 198.41.209.132
+ 198.41.209.133
+ 198.41.208.132
+ 198.41.209.131
+ 198.41.208.133
Enter a host name: www.wikipedia.org
Official name is "www.wikipedia.org"
IP Addresses:
+ 198.35.26.96
Enter a host name:
All done!
myth61$Hostname Resolution: IPv6
A more generic version of gethostbyname—inventively named gethostbyname2—can be used to extract IPv6 address information about a hostname.
struct hostent *gethostbyname2(const char *name, int af);gethostbyname2: AF_INET and AF_INET6.gethostbyname2(host, AF_INET) is equivalent to a call to gethostbyname(host)
gethostbyname2(host, AF_INET6) still returns a struct hostent *, but the struct hostent is populated with different values and types:h_addrtype field is set to AF_INET6,h_length field houses a 16 (or rather, sizeof(struct in6_addr)), andh_addr_list field is really an array of struct in6_addr pointers, where each struct in6_addr looks like this:
struct in6_addr {
u_int8_t s6_addr[16]; // 16 bytes (128 bits), stored in network byte order
};IPv6 version of the publishIPAddressInfo we wrote earlier (we call it publishIPv6AddressInfo).static void publishIPv6AddressInfo(const string& host) {
struct hostent *he = gethostbyname2(host.c_str(), AF_INET6);
if (he == NULL) { // NULL return value means resolution attempt failed
cout << host << " could not be resolved to an address. Did you mistype it?" << endl;
return;
}
cout << "Official name is \"" << he->h_name << "\"" << endl;
cout << "IPv6 Addresses: " << endl;
struct in6_addr **addressList = (struct in6_addr **) he->h_addr_list;
while (*addressList != NULL) {
char str[INET6_ADDRSTRLEN];
cout << "+ " << inet_ntop(AF_INET6, *addressList, str, INET6_ADDRSTRLEN) << endl;
addressList++;
}
}gethostbyname2, and notice the explicit use of AF_INET6, struct in6_addr, and INET6_ADDRSTRLEN.myth61$ ./resolve-hostname6
Welcome to the IPv6 address resolver!
Enter a host name: www.facebook.com
Official name is "star-mini.c10r.facebook.com"
IPv6 Addresses:
+ 2a03:2880:f131:83:face:b00c:0:25de
Enter a host name: www.microsoft.com
Official name is "e13678.dspb.akamaiedge.net"
IPv6 Addresses:
+ 2600:1406:1a:386::356e
+ 2600:1406:1a:397::356e
Enter a host name: www.google.com
Official name is "www.google.com"
IPv6 Addresses:
+ 2607:f8b0:4005:801::2004
Enter a host name: www.berkeley.edu
Official name is "www-production-1113102805.us-west-2.elb.amazonaws.com"
IPv6 Addresses:
+ 2600:1f14:436:7800:4598:b474:29c4:6bc0
+ 2600:1f14:436:7801:15f8:d879:9a03:eec0
Enter a host name: www.harvard.edu
www.harvard.edu could not be resolved to an address. Did you mistype it?
Enter a host name:
All done!
myth61$struct sockaddr { // generic socket
unsigned short sa_family; // protocol family for socket
char sa_data[14];
// address data (and defines full size to be 16 bytes)
};The sockaddr_in is used to model IPv4 address/port pairs.
sin_family field should always be initialized to be AF_INET, which is a constant used to be clear that IPv4 addresses are being used. If it feels redundant that a record dedicated to IPv4 needs to store a constant saying everything is IPv4, then stay tuned.sin_port field stores a port number in network byte (i.e. big endian) order.sin_addr field stores an IPv4 address as a packed, big endian int, as you saw with gethostbyname and the struct hostent.sin_zero field is generally ignored (though it's often set to store all zeroes). It exists to pad the record up to 16 bytes.struct sockaddr_in { // IPv4 socket address record
unsigned short sin_family;
unsigned short sin_port;
struct in_addr sin_addr;
unsigned char sin_zero[8];
};struct sockaddr_in6 { // IPv6 socket address record
unsigned short sin6_family;
unsigned short sin6_port;
unsigned int sin6_flowinfo;;
struct in6_addr sin6_addr;
unsigned int sin6_scope_id;
};sockaddr_in6 is used to model IPv6 address/port pairs.
sin6_family field should always be set to AF_INET6. As with the sin_family field, sin6_family field occupies the first two bytes of surrounding record.sin6_port field holds a two-byte, network-byte-ordered port number, just like sin_port does.struct in6_addr is also wedged in there to manage a 128-bit IPv6 address.sin6_flowinfo and sin6_scope_id are beyond the scope of what we need, so we'll ignore them.struct sockaddr { // generic socket
unsigned short sa_family; // protocol family for socket
char sa_data[14];
// address data (and defines full size to be 16 bytes)
};struct sockaddr_in { // IPv4 socket address record
unsigned short sin_family;
unsigned short sin_port;
struct in_addr sin_addr;
unsigned char sin_zero[8];
};struct sockaddr_in6 { // IPv6 socket address record
unsigned short sin6_family;
unsigned short sin6_port;
unsigned int sin6_flowinfo;;
struct in6_addr sin6_addr;
unsigned int sin6_scope_id;
};struct sockaddr is the best C can do to emulate an abstract base class.
struct sockaddr, but many system calls will accept parameters of type struct sockaddr *.struct sockaddr *, it really accepts the address of either a struct sockaddr_in or a struct sockaddr_in6. The system call relies on the value within the first two bytes—the sa_family field—to determine what the true record type is.struct sockaddr { // generic socket
unsigned short sa_family; // protocol family for socket
char sa_data[14];
// address data (and defines full size to be 16 bytes)
};struct sockaddr_in { // IPv4 socket address record
unsigned short sin_family;
unsigned short sin_port;
struct in_addr sin_addr;
unsigned char sin_zero[8];
};struct sockaddr_in6 { // IPv6 socket address record
unsigned short sin6_family;
unsigned short sin6_port;
unsigned int sin6_flowinfo;;
struct in6_addr sin6_addr;
unsigned int sin6_scope_id;
};At this point, we know most of the directives needed to implement and understand how to implement createClientSocket and createServerSocket.
createClientSocket is the easier of the two, so we'll implement that one first. (For simplicity, we'll confine ourselves to an IPv4 world.)gethostbyname does this for us.socket system call to do this.struct sockaddr_in that packages the host and port number we're interested in connecting to.connect to do this.createClientSocket:int createClientSocket(const string& host, unsigned short port) {
struct hostent *he = gethostbyname(host.c_str());
if (he == NULL) return -1;
int s = socket(AF_INET, SOCK_STREAM, 0);
if (s < 0) return -1;
struct sockaddr_in address;
memset(&address, 0, sizeof(address));
address.sin_family = AF_INET;
address.sin_port = htons(port);
// h_addr is #define for h_addr_list[0]
address.sin_addr = *((struct in_addr *)he->h_addr);
if (connect(s, (struct sockaddr *) &address, sizeof(address)) == 0) return s;
close(s);
return -1;
}Here are a few details about my implementation of createClientSocket worth calling out:
gethostbyname first before we call socket, because we want to confirm the host has a registered IP address—which means it's reachable—before we allocate any system resources.gethostbyname is intrinsically IPv4. If we wanted to involve IPv6 addresses instead, we would need to use gethostbyname2.socket finds, claims, and returns an unused descriptor. AF_INET configures it to be compatible with an IPv4 address, and SOCK_STREAM configures it to provide reliable data transport, which basically means the socket will reorder data packets and requests missing or garbled data packets to be resent so as to give the impression that data that is received in the order it's sent.
AF_INET6 had we decided to use IPv6 addresses instead. (Other arguments are possible, but they're less common.)SOCK_DGRAM had we preferred to collect data packets in the order they just happen to arrive and manage missing and garbled data packets ourselves. (Other arguments are possible, though they're less common.)Here are a few more details:
address is declared to be of type struct sockaddr_in, since that's the data type specifically set up to model IPv4 addresses. Had we been dealing with IPv6 addresses, we'd have declared a struct sockaddr_in6 instead.
AF_INET within sin_family, since those two bytes are examined by system calls to determine the type of socket address structure.sin_port field is, not surprisingly, designed to hold the port of interest. htons—that's an abbreviation for host-to-network-short—is there to ensure the port is stored in network byte order (which is big endian order). On big endian machines, htons is implemented to return the provided short without modification. On little endian machines (like the myths), htons returns a figure constructed by exchanging the two bytes of the incoming short. In addition to htons, Linux also provided htonl for four-byte longs, and it also provides ntohs and ntohl to restore host byte order from network byte ordered figures.connect associates the descriptor s with the host/IP address pair modeled by the supplied struct sockaddr_in *. The second argument is downcast to a struct sockaddr *, since connect must accept a pointer to any type within the entire struct sockaddr family, not just struct sockaddr_ins. connect will return -1 (with errno set to ECONNREFUSED) if the server of interest isn't running.Here is the full implementation of createServerSocket (and online right here):
int createServerSocket(unsigned short port, int backlog) {
int s = socket(AF_INET, SOCK_STREAM, 0);
if (s < 0) return -1;
struct sockaddr_in address;
memset(&address, 0, sizeof(address));
address.sin_family = AF_INET;
address.sin_addr.s_addr = htonl(INADDR_ANY);
address.sin_port = htons(port);
if (bind(s, (struct sockaddr *)&address, sizeof(address)) == 0 &&
listen(s, backlog) == 0) return s;
close(s);
return -1;
}createServerSocket worth calling out:
socket is precisely the same here as it was in createClientSocket. It allocates a descriptor and configures it to be a socket descriptor within the AF_INET namespace.struct sockaddr_in here is configured in much the same way it was in createClientSocket, except that the sin_addr.s_addr field should be set to a local IP address, not a remote one. The constant INADDR_ANY is used to state that address should represent all local addresses.bind call simply assigns the set of local IP addresses represented by address to the provided socket s. Because we embedded INADDR_ANY within address, bind associates the supplied socket with all local IP addresses. That means once createServerSocket has done its job, clients can connect to any of the machine's IP addresses via the specified port.listen call is what converts the socket to be one that's willing to accept connections via accept. The second argument is a queue size limit, which states how many pending connection requests can accumulate and wait their turn to be accepted. If the number of outstanding requests is at the limit, additional requests are simply refused.