Get all IP numbers of your machine and how to pass through an authenticating proxy

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Introduction

I needed some code that would give me the IP number of my machine. Not only the number assigned to it by DHCP, either on a LAN or by my ISP when dialing up, but also my IP number as seen from the web, if my machine is behind a router/firewall.

It took a while before I figured it all out, and in the process I have seen a lot of code from others. I borrowed some code here and there, but I don't remember exactly what part, so it is hard to give credits to the authors of that code. Sorry about that. Hope I can make it up by sharing this code with you.

Relevant functions in the sample source code

The sample program does essentially two things: it reports the IP numbers as 'known' by the TCP/IP software on your own machine ('internal' IP numbers) and it reports the IP numbers of your machine as seen from the web ('external' IP numbers). Of course one of the internal numbers will be the same as the external number if there is no router/firewall between your machine and the Internet.

Internal IP numbers

Internal IP numbers are reported by function FindLocalAddress(). It calls GetIpAddrTable(), one of the IP Helper functions, which returns a block of data with this structure:

typedef struct _MIB_IPADDRTABLE
{
    DWORD dwNumEntries;
    MIB_IPADDRROW table[ANY_SIZE];
} MIB_IPADDRTABLE, *PMIB_IPADDRTABLE;

dwNumEntries contains the number of entries in 'table'.

The structure of 'table' looks like this:

typedef struct _MIB_IPADDRROW
{
    DWORD        dwAddr;
    DWORD        dwIndex;
    DWORD        dwMask;
    DWORD        dwBCastAddr;
    DWORD        dwReasmSize;
    unsigned short    unused1;
    unsigned short    wType;
} MIB_IPADDRROW, *PMIB_IPADDRROW;

dwAddr is the address we are after. FindLocalAddresses() fetches that number and converts it to a xxx.xxx.xxx.xxx string by calling function inet_ntoa().

External IP numbers

External IP numbers are reported by function FindWebAddress(). It does this by making a connection to URL 'checkip.dyndns.org'. To see why, open this URL in your browser. It returns the IP number of your machine as seen from this site. If your machine is behind a firewall/router this number will differ from the internal IP numbers found earlier. Of course in this case the IP number found will in fact be the IP number of your router/firewall.

FindWebAddress() uses the WinINet API to make a connection to 'checkip.dyndns.org' and to read and parse its HTML source to find the IP number hidden in this HTML source.

How to pass through an authenticating proxy server

FindWebAddress() also shows you how to pass through a proxy server that requires authentication. The sample program creates its CInternetSession object with option INTERNET_OPEN_TYPE_PRECONFIG. This means that if your Internet Explorer configuration contains a proxy server definition ("Tools - Internet Options - Connection tab - LAN Settings..."), the Internet connection will be made through this proxy server. In that case you will know that you have to pass through an authenticating proxy when your first attempt to connect to 'checkip.dyndns.org' fails with return code 407. The sample program will then ask you to specify a username and a password, and tries to connect to 'checkip.dyndns.org' through the proxy server.

About the author

My name is Nico Cuppen. I live in Haarlem, The Netherlands and I like programming. That's how I became a shareware author. You can find more info about me and my products on www.nicocuppen.com.