Thread: getaddrinfo() can't resolve hostname

Hi,

I have recently started network Programming for Linux (though I use windows, I believe it's still compatible via cygwin).

Anyways, I have a small code where I pass in the hostname and the program returns the IP addresses for this host. It works fine with common websites like google, yahoo etc.

I can also get my own PC's IP addresses by passing in MYPC-PC as the argument. However, if I try to resolve any other computer's name that is connected to the SAME network as my PC, I get the following error:
"Non-recoverable failure in name resolution."

I never had this problem in C#, so I don't know how to work my way around this problem. I hope someone can help me out.

Thanks in advance

How are you formatting the name?

Do you have DNS on your router/switch?
What are you using as your Primary DNS...

For it to resove LAN addresses you have to be using the DNS in your router...
I use it in my software and it's never missed resolving a lan address yet.

Originally Posted by chaos5687

I name it as "Other-pc", that worked on C# so I assumed it wouldn't cause any problems in C either.

It'd be great if you could give me an example on formatting the name

Do you mean formatting like "other-pc.some.text.net" ?

Just use the computer's name... My computers are named Volts, Amps, Watts, Joules and Ohms... So the Name parameter below would be simply "Amps" to get it's IP.

BUT...

You have to be using DNS from a local router or switch for this to work. Your ISP's DNS (for example) cannot look inside your network.

Code:

// resolve server name and port to useable LAN addr
BOOL GetHostAddr(PTCHAR Name, WORD Port, PSOCKADDR Addr)
  { ADDRINFOT   hints = {0};    // hints for search
    PADDRINFOT  res   = NULL;   // pointer to search result
    // fill in struct
    hints.ai_flags    = AI_PASSIVE;
    hints.ai_family   = AF_INET;
    hints.ai_socktype = SOCK_DGRAM;
    hints.ai_protocol = IPPROTO_UDP;

    // get ip address
    if (GetAddrInfo(Name,NULL,&hints,&res))
      return 0;     \\ !0 == failure
    // transfer to local IP struct
    memcpy(Addr,res->ai_addr,sizeof(SOCKADDR));
    ((PSOCKADDR_IN) Addr)->sin_family = AF_INET;
    ((PSOCKADDR_IN) Addr)->sin_port   = htons(Port);
    // release memory 
    FreeAddrInfo(res);
    return 1; }

Take a look at the docs... GetAddressInfo(), far as I know is an IPv4 function. IPV6 has it's own separate function... Plus precious few LANS are running ipv6 right now, routers and switches more than a couple of months old can't handle it...

Originally Posted by ssharish2005

As far as i know the ai_family should be either set to AF_INET or AF_INET6. Anything else would undefined. Check the spec. AF_INET would return IPv4 and AF_INET6 would return IPV6 address respectively.
getaddrinfo(3) - Linux man page

ssharish

In that case it would have to be called twice... once for each.

I have just checked the man pages and apparently getaddrinfo() doesn't mind hints.ai_family being set to AF_UNSPEC.In that case it will return both available IPv4 and IPv6 addresses...
Also when I enter my own computer's name, it returns 4 IP addresses, 3 being IPv6 and 1 being IPv4...

Well, that's weird, so I will do a bit more research on that and write here if I find anything out

Thanks for your help