James' Tech Blog

This is the second post in the series “Remote admin your mama’s gentoo boxen” This post I will be going over how I am going to handle the “dynamic” IP address on the WAN of my router. I say “dynamic” because I’m not really sure how dynamic it is. It’s likely only dynamic in the way that Comcast could give it to someone else (like if they wanted to pay for it) and there’s nothing I could do about it (unless I told them I would pay more for it!).

You probably just said to yourself, “DyDNS! Don’t they have services for that already?” The answer is yeah, they do, if you want your router to have a URL and probably be poked at more. Oh and dydns has sent me emails before about my IP not being “dynamic enough” and that I should “use static registration!” Besides, this way I can encrypt my IP not only when I update it, but when the client retrieves it. Not to mention client authentication.

Here is the scheme:

Router runs ash script reporting IP to remote server

Server does it’s thing and records IP

Client runs bash script getting IP from remote server

From there the client knows the router IP and can connect with reverse SSH.

Here comes some code. I would have prefered to write this in PERL but my router doesn’t have PERL! Also, less dependencies on client. Everyone has a shell. I decided to send the IP as hexadecimal without separators. As I’m not really proficient at shell scripts I looked up how to convert IP’s to hex: http://blog.mecworks.com/articles/2007/07/05/bash-scripting-tip-convert-ip-address-to-hex/ I did some fun stuff for the client. It probably looks horrible, but it should work.

First up is the router script:

#!/bin/sh
SUCCESS=1
while [ $SUCCESS -eq "1" ]
do
RTR_IP=`ifconfig eth0.1 | awk -F "[: ]+" 'NR==2 {print $4}'`
UPDATE_IP=`printf '%02X' ${RTR_IP//./ }; echo`
SUCCESS=`wget -q -O - "http://www.ultravstudios.com/myphpscript.php?update=$UPDATE_IP"`
sleep 5
done

I’ll break it down for y’all. It loops until it works. We use ifconfig to get the IP of WAN (eth0.1), use printf to turn it into 8 character hexadecimal, send it to the special secret PHP page. The PHP page will return a value 0 on success 1 on error. Next is client side

#!/bin/bash
GET_IP=`wget -q -O - "http://www.ultravstudios.com/myphpscript.php?get=0"`
K=`printf "%d". "0x"${GET_IP:0:2} "0x"${GET_IP:2:2} "0x"${GET_IP:4:2} "0x"${GET_IP:6:2}`
RTR_IP=$(echo ${K%\.})

So this one’s really easy.  Get the value from the special secret PHP page. Turn it back to decimal with period separators. Chop the last period off. Okay, so that’s just the part to retrieve the IP. Next is my PHP code for the remote server. It seems like WordPress doesn’t like me adding php tags?!

$filename = "supersecretfilename";
//open file and get old ip
$file = fopen($filename, 'r');
$old_ip = fgets($file);
if ($_REQUEST['get'] == "0") {//the client does want the ip
print $old_ip;
fclose($file);
return;
}
//so the rest means that the client does not want ip
if (is_null($_REQUEST['update']) || $_REQUEST['update'] == "") {//there is not data
fclose($file);
print "1"; //tell client there was error
}
elseif ($_REQUEST['update'] == $old_ip) {
//there was no update close file and return
fclose($file);
print "0";
return;
}
else {//ip is new
fclose($file);
$file2 = fopen($filename, 'w');
fwrite($file2, $_REQUEST['update']);
fclose($file2);
print "0";
}

Apparently I comment my php code more than my bash! So it’s pretty dumb right now. There is no authentication, no encryption, very few fail-safes. If my router is somehow having problems updating the IP, like the script doesn’t work, it will try forever until it works. I assure you, this is just the beginning, a quick hack to get things rolling.