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POHow to enumerate all IP addresses attached to a machine, in POSIX C?
 

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  1. POHow to enumerate all IP addresses attached to a machine, in POSIX C?
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    <p><strong>Background:</strong></p> <p>I'm writing a daemon that makes outgoing TCP/IP connections. It will be running on machines with multiple (non-loopback) IP addresses. I'd like the users to be able to specify, in the daemon's config file, which IP address(es) to use for outgoing connections, or <code>*</code> to use all.</p> <p>The addresses will be used in a rotation, each connection going out from the IP address used least recently. This behavior is important, as is <code>*</code> being a replacement for "all" so daemons running on multiple machines can point to the same config file on a fileshare, and have each use its own set of IP addresses.</p> <p><strong>Problem:</strong></p> <p>How do I get a list of all the IP addresses a machine can make outgoing (i.e. to any other computer) connections on? Given a list of all IP addresses, how would I filter out loopback addresses?</p> <p>I'm in C, and if possible I'd like to use POSIX only, but the daemon will probably only ever run on Linux boxes, so I'd accept a Linux-centric answer.</p> <p>Each IP address will be available on exactly one (possibly virtual) network device and vice versa, so a way to enumerate network devices and get associated IP addresses would also suffice, though I wouldn't really be happy about it. (Side questions: Is it even possible to associate multiple IP addresses with a single device? How 'bout the same IP under multiple devices? Not important.)</p> <p><strong>Insufficient Solutions:</strong></p> <ul> <li><code>gethostname()</code>/<code>gethostbyname()</code> (as <a href="https://stackoverflow.com/questions/122208/get-the-ip-address-of-local-computer">this question</a>). Using that method, I only ever get 127.0.0.1 back (or .1.1 in Debian). I suspect this is because the hostname of the machine is in the <code>hosts</code> file, and that's as far as <code>gethostbyname()</code> checks. (I believe that's why in Debian I always get 127.0.1.1: Debian defaults to adding localhost as 127.0.0.1 and the machine's hostname as 127.0.1.1 to the <code>hosts</code> file, right?) I'd like a solution that ignores <code>hosts</code> and gives me everything actually there.</li> <li>I've had no more luck with <code>getaddrinfo()</code> than <code>gethostname()</code>/<code>gethostbyname()</code>. It seems to be bound by the same problem. I tested this passing the machine's hostname and a <code>NULL</code> service (port) into it; the docs say passing a <code>NULL</code> hostname AND a <code>NULL</code> service is illegal, and this is backed up by testing. Not sure how else to ask it for everything on the machine, but I'm open to suggestions in this vein.</li> <li>EDIT: <a href="https://stackoverflow.com/questions/259389/finding-an-ip-from-an-interface-name/259424#259424">this answer</a> shows how to get the IP address from a device name, but doesn't show how to enumerate the device names. Any ideas?</li> </ul> <p>FINAL EDIT: I've accepted <a href="https://stackoverflow.com/questions/1160963/how-to-enumerate-all-ip-addresses-attached-to-a-machine-in-posix-c/1161018#1161018">caskey's answer</a> to give him the credit for pointing me in the direction of how this needs to be done. I've posted my <a href="https://stackoverflow.com/questions/1160963/how-to-enumerate-all-ip-addresses-attached-to-a-machine-in-posix-c/1162010#1162010">own answer</a> listing the source code of how exactly to do it in case anyone else needs it.</p>
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    1. COI meant `man netdevice` but then I realized it wasn't POSIX, so it's moot.
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