How to get my own IP address without DNS

Hi.

I am trying to get my own IP address. I know that there is a way to use
gethostname and gethostbyname, but this has a problem.
1. I cannot use this if no DNS is specified.
2. If several IP addresses are given for my domain name and
the ordering is round robin, I cannot tell which one is my real IP address.

Is there any way to get my own IP address in C or C++ without using DNS?
(such as ifconfig)

Kazuh

Kazu wrote:
> Hi.
>
> I am trying to get my own IP address. I know that there is a way to use
> gethostname and gethostbyname, but this has a problem.
> 1. I cannot use this if no DNS is specified.
> 2. If several IP addresses are given for my domain name and
> the ordering is round robin, I cannot tell which one is my real IP address.
>
> Is there any way to get my own IP address in C or C++ without using DNS?
> (such as ifconfig)

This question is off topic for comp.lang.c++.

But as a hint: Yes, it's possible. Get a list of the interfaces and
their addresses. Java does this with
NetworkInterface.getNetworkInterfaces(). The C if_nameindex() function
gets you your interface name, but it's not obvious to me how you get
from there to an if_addr structure. See net/if.h.

Jacques.

> I am trying to get my own IP address.

What platform? I'll assume Unix. comp.unix.programmer might be a
better place to ask.

Try "strace /sbin/ifconfig" (or read the source) to see how it does it.
I think it will read from /proc on Linux.
It might be hard to do this in a portable way. It could be better to
popen("ifconfig") and parse its output.

--Phil.

Kazu wrote:
> Hi.
>
> I am trying to get my own IP address. I know that there is a way to use
> gethostname and gethostbyname, but this has a problem.
> 1. I cannot use this if no DNS is specified.
> 2. If several IP addresses are given for my domain name and
> the ordering is round robin, I cannot tell which one is my real IP address.
>
> Is there any way to get my own IP address in C or C++ without using DNS?
> (such as ifconfig)

The easiest way is to parse the /etc/hosts file, that your system
should use if there's no DNS server.

> Kazuh

Regards.

--

Jose Maria Lopez Hernandez
Director Tecnico de bgSEC
(E-Mail Removed)
bgSEC Seguridad y Consultoria de Sistemas
http://www.bgsec.com
ESPAÑA

The only people for me are the mad ones -- the ones who are mad to live,
mad to talk, mad to be saved, desirous of everything at the same time,
the ones who never yawn or say a commonplace thing, but burn, burn, burn
like fabulous yellow Roman candles.
-- Jack Kerouac, "On the Road"

you can use system call ioctl with SIOCGIFADDR as shown below

1) struct ifreq ifr; struct sockaddr_in saddr;
2) fd=socket(PF_INET,SOCK_STREAM,0)
3) strcpy(ifr.ifr_name,"name of interface");
4) ioctl(fd,SIOCGIFADDR,&ifr);
5) saddr=*((struct sockaddr_in *)(&(ifr.ifr_addr))); /* is the address
*/
6) saddr.sin_addr.s_addr is the address of the interface in integer
format

> 1) struct ifreq ifr; struct sockaddr_in saddr;
> 2) fd=socket(PF_INET,SOCK_STREAM,*0)
> 3) strcpy(ifr.ifr_name,"name of interface");
> 4) ioctl(fd,SIOCGIFADDR,&ifr);
> 5) saddr=*((struct sockaddr_in *)(&(ifr.ifr_addr))); /* is the address

<snip>

Pretty clever!


--
Raqueeb Hassan
Bangladesh

Raqueeb Hassan wrote:
>>1) struct ifreq ifr; struct sockaddr_in saddr;
>>2) fd=socket(PF_INET,SOCK_STREAM,*0)
>>3) strcpy(ifr.ifr_name,"name of interface");
>>4) ioctl(fd,SIOCGIFADDR,&ifr);
>>5) saddr=*((struct sockaddr_in *)(&(ifr.ifr_addr))); /* is the address
>
>
> <snip>
>
> Pretty clever!
>

A deeper treatment is in:

W. Richard Stevens, UNIX Network Programming

--

Tauno Voipio
tauno voipio (at) iki fi