We have a code to find the DNS entry of a host that has a trailing '-' in its url (format example: mysite-.watch.com):

if(gethostbyname_r(host,host_ent,host_buffer,host_buffer_size,&host_error)==NULL)
{
//failed
}

But when remove the '-' from the host name the code does not return failure. IE and nslookup works fine.

What we need to do in such a case?

Below are my personal findings:


Example 1:

> host nyustern-.collegemailer.com
nyustern-.collegemailer.com has address 209.200.118.155

> host nyustern.collegemailer.com
nyustern.collegemailer.com has address 209.200.118.155

Example 2:

> nslookup nyustern-.collegemailer.com
Note: nslookup is deprecated and may be removed from future releases.
Consider using the `dig' or `host' programs instead. Run nslookup with
the `-sil[ent]' option to prevent this message from appearing.
Server: xx.xxx.xx.xxx
Address: xx.xxx.xx.xxx#xx

Non-authoritative answer:
Name: nyustern-.collegemailer.com
Address: 209.200.118.155

> nslookup nyustern.collegemailer.com
Note: nslookup is deprecated and may be removed from future releases.
Consider using the `dig' or `host' programs instead. Run nslookup with
the `-sil[ent]' option to prevent this message from appearing.
Server: xx.xxx.xx.xxx
Address: xx.xxx.xx.xxx#xx

Non-authoritative answer:
Name: nyustern.collegemailer.com
Address: 209.200.118.155


Eample 3:

This code uses gethostbyname_r method to determine the address:-

> a.out nyuster-n.collegemailer.com
addresses: 209.200.118.155
> a.out ny-uster-n.collegemailer.com
addresses: 209.200.118.155
> a.out ny-uster-n-.collegemailer.com
unknown host `ny-uster-n-.collegemailer.com'
> a.out ny---uster-n.collegemailer.com
addresses: 209.200.118.155
> a.out nyustern.collegemailer.com
addresses: 209.200.118.155
>

I am unsure whether '-' in hostname ignored or does not has a functionality. In such a case having a trailing '-' is just a corner off case.

Looks like that particular domain is running some funky resolver which resolves almost anything. I'd guess their resolver is configured to resolve everything, but has a bug.

Punycode - Wikipedia, the free encyclopedia has some special cases relating to hyphens in host names.

I am quiet unsure now of what extra we need to code here. Is there any library we need to include? How can we resolve this issue?

What does h_errno say when you get the failure? The man page seems to suggest to use getnameinfo() instead, have you tried that? My version of gethostbyname_r returns an int, do you have a different implementation?

I did a sample code:

Code:

#include <netdb.h>
#include <stdio.h>
#include <alloca.h>

int
main (void)
{
struct hostent hostbuf, *hp;
size_t hstbuflen;
char *tmphstbuf;
int res;
int herr;

hstbuflen = 1024;
tmphstbuf = (char *)alloca (hstbuflen);

res = gethostbyname_r ("nyustern-.collegemailer.com", &hostbuf, tmphstbuf, hstbuflen, &hp, &herr);
herror("Error");
printf("Hsterror [%s]\n" , hstrerror (h_errno ));

printf ("gethostbyname_r returned: %d\n", res);
printf ("herr: %d\n", herr);
printf ("hp is %s\n", hp == NULL ? "null" : "set");
}

Output:

cc test_host.cpp && a.out

Error: Unknown host
Hsterror [Unknown host]
gethostbyname_r returned: 0
herr: 3
hp is null


The getnameinfo() function takes a socket address as input and returns the corresponding node name and service location.
Can you be more specific regards to my requirement?

This appears to be illegal according to rfc 952.
RFC 952 (rfc952) - DoD Internet host table specification