Check port reachable in C - c

Question

I have a C function to check a host and its port, when I use FQDN host name, the function return error like: connect() failed: connect time out, but if I use IP address instead, it seems ok, how to fix this?
Thanks.
#include <unistd.h>
#include <string.h>
#include <syslog.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <errno.h>
int is_network_up(char *chkhost, unsigned short chkport) {
int sock;
struct sockaddr_in chksock;
struct hostent *host = NULL;
if ((sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) == -1) {
syslog(LOG_ERR, "socket() creation error: %s", strerror(errno));
return 0;
}
memset(&chksock, 0, sizeof(chksock));
chksock.sin_family = AF_INET;
chksock.sin_port = htons(chkport);
/* get the server address */
if (inet_pton(AF_INET, chkhost, &(chksock.sin_addr.s_addr)) <= 0) {
if ((host = gethostbyname(chkhost)) == NULL) {
syslog(LOG_ERR, "%s", hstrerror(h_errno));
return 0;
}
memcpy(&(chksock.sin_addr.s_addr), &(host->h_addr_list[0]),
sizeof(struct in_addr));
}
/* try to connect */
if (connect(sock, (struct sockaddr *) &chksock, sizeof(chksock)) < 0) {
syslog(LOG_ERR, "connect() failed: %s", strerror(errno));
return 0;
}
close(sock);
return 1;
}

Answer 1

inet_pton() is the wrong task for that. It only accepts numerical addresses.
In former times, people used to use gethostbyname() for name resolution.
But as we have 2012 meanwhile, this method is outdated for several years now, as it is still restricted to AF_INET.
With the program below, you should achieve about the same and stay future compatible.
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdlib.h>
#include <stdio.h>
int is_network_up(char *chkhost, unsigned short chkport) {
int sock = -1;
struct addrinfo * res, *rp;
int ret = 0;
char sport[10];
snprintf(sport, sizeof sport, "%d", chkport);
struct addrinfo hints = { .ai_socktype=SOCK_STREAM };
if (getaddrinfo(chkhost, sport, &hints, &res)) {
perror("gai");
return 0;
}
for (rp = res; rp && !ret; rp = rp->ai_next) {
sock = socket(rp->ai_family, rp->ai_socktype,
rp->ai_protocol);
if (sock == -1) continue;
if (connect(sock, rp->ai_addr, rp->ai_addrlen) != -1) {
char node[200], service[100];
getnameinfo(res->ai_addr, res->ai_addrlen, node, sizeof node, service, sizeof
service, NI_NUMERICHOST);
printf("Success on %s, %s\n", node, service);
ret = 1; /* Success */
}
close(sock);
}
freeaddrinfo(res);
return ret;
}
int main(int argc, char** argv) {
if (argc > 1) {
printf("%s: %d\n", argv[1], is_network_up(argv[1], 22));
}
}

Answer 2

Make sure name resolution is working. See if you can ping the machine by name from the exact same environment in which your code runs.
If ping works, try telnet <machinename> <portnumber> -- If both of those work it is likely a problem with your code (which I did not look at in depth, too sleepy:).
Make sure you're converting anything returned by the OS as an ip address from network order to host order. IIRC, gethostbyname returns binary ip addresses in network order.
ntohl can be used on chksock.sin_addr.s_addr after the memcpy to achieve this.