Cannot open FTP connection when server address is FQDN - c

Question

My FTP program written in C only works when the server address is an IP address. But the connection fails when the server address is a Fully Qualified Domain Name (FQDN).
When ftp_host is FQDN, the ftp connection fails to open. Please help.

Answer 1

I'd strongly recommend using the getaddrinfo() function, as gethostbyname() is obsolete. This also has the advantage that it's easy to convert to IPv6, either now or later.
I'm assuming you only care about IPv4 addresses, so here's an example of a function which will take a hostname as its parameter and fill in a struct sockaddr_in for you:
int get_ftp_addr(const char *hostname, struct sockaddr_in *addr)
{
char host_buffer[256];
struct addrinfo hints;
struct addrinfo *result;
struct sockaddr_in *res_addr;
int error = -1;
char *colon;
snprintf(host_buffer, sizeof(host_buffer), "%s", hostname);
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
colon = strchr(host_buffer, ':');
if (colon) {
*colon = '\0';
error = getaddrinfo(host_buffer, colon + 1, &hints, &result);
} else {
error = getaddrinfo(host_buffer, "ftp", &hints, &result);
}
if (error != 0 || !result) {
return error;
}
res_addr = (struct sockaddr_in*)(result->ai_addr);
memcpy(addr, res_addr, sizeof(struct sockaddr_in));
freeaddrinfo(result);
return 0;
}
Note that I only take a copy of the string to avoid modifying the version in the caller - don't feel obliged to keep that part if you don't want to, but personally I think it makes a cleaner interface.
This function will accept both IP addresses in dotted quad notation and fully-qualified hostnames, as getaddrinfo() accepts both of those. If a port is specified with a colon it will be used, otherwise the default FTP port will be used.
A zero return value indicates success. A positive return can be passed into gai_strerror() to get a string error code, or you can check the getaddrinfo() man page for the possible error codes. A return of -1 indicates a successful result from getaddrinfo() but no result structure - I don't think this can happen, but I don't like leaving any possible return code unhandled.
There are several caveats here, the two most important are:
This code only support IPv4 as it stands, although getaddrinfo() makes it really easy to support IPv6 as well. If you want to support both then change the AF_INET in the hints structure to AF_UNSPEC and you'll get all address families. You'll need to iterate over addresses, though, and filter out only the IPv4 and IPv6 ones (see my next point).
A DNS lookup can potentially look up to multiple IP addresses - this is quite common with large sites such as Google as they use this feature for load-balancing across hosts, and also redundancy. Ideally you code should iterate across all the returned addresses and try connecting to each one until one works. For an FTP client this is perhaps overkill, but I think it's important to be aware of it.
If you want to support IPv6, or support multi-A-records (i.e. multiple addresses back from the DNS query), then you need to follow the ai_next pointer in the struct addrinfo structure - something like this:
struct addrinfo *res;
/* Assume result is initialised as above via getaddrinfo() */
for (res = result; res != NULL; res = res->ai_next) {
...
}