7

I needed to automatically get my own WAN-IP-adress from my router. I found this question and, among others, a solution with dig was proposed:

dig +short myip.opendns.com @resolver1.opendns.com

It works perfectly, but now i want to understand what it is doing. Here is what I (hope to) understand so far (please correct me, if I am wrong):

  • +short just gives me a short output
  • @resolver1.opendns.com is the dns server, which is asked which ip adress belongs to the given domain

What's not clear to me is myip.opendns.com. If I would write www.spiegel.de instead, I would get the ip adress of the domain www.spiegel.de, right? With myip.opendns.com I get the WAN-IP of my router. So is myip.opendns.com just emulating a domain, which is resolved to my router? How does it do it? Where does it get my ip from? And how is it different to what webpages, like e.g. www.wieistmeineip.de, are doing? They also try to get my ip.

In the answer of Krinkle on the question I mentioned, it is stated, that this "dns-approach" would be better than the "http-approach"? Why is it better and what is the difference?

There has to be a difference, because the WAN-IP, I get from dig +short myip.opendns.com @resolver1.opendns.com (ip1) is the one I can also see in the web interface of my router, whereas www.wieistmeineip.de (and other similar sites too) is giving me another ip adress (ip2). I could imagine that my ISP is using some kind of sub-LAN, so that my requests to webservers are going through another (ISP-) router which has ip2, so that www.wieistmeineip.de is just seeing this adress (ip2). But, again, what is myip.opendns.com doing then?

Additonally: Opening ip1 from within my LAN is giving me the test website from my raspi, opening it from the outside of my LAN (mobile internet) does not work. Does it mean, that ip1 is no proper "internet ip" but more like a LAN ip?

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First to summarize the general usage of dig: it requests the IP assigned to the given domain from the default DNS server. So e.g. dig google.de would request the IP assigned to the domain google.de. That would be 172.217.19.99.

The command you mentioned is:

dig +short myip.opendns.com @resolver1.opendns.com

So what this command does is: it sends an request for the IP of the domain myip.opendns.com to the DNS server resolver1.opendns.com. This server is programmed that (if this special domain is requested) the IP the request comes from is send back.

The reasons why the method of querying the WAN IP using DNS is better were mentioned by krinkle: standardised, more stable and faster.

The reason I could imagine for those two IPs is that your router caches DNS requests and returns an old IP. Another problem colud be DualStack Lite. That is often used by new internet contracts. Do you know wheter your ISP is using DS Lite?

  • Took me a few minutes of head scratching to work this one out. I started with “it can't”, then tested it, and changed my answer to “HOW?”. I then realised that the server can get you IP address as it is in every packet that you send. Therefore this DNS server has a special case for myip.opendns.com (that is a bit cool) – ctrl-alt-delor Jan 6 '17 at 17:32
  • -So, if I understand your answer correctly, both methods are using the same mechanism (just looking at the ip, the request came from; but in case of "dns approach" in a more standardised way)? -Whether my ISP uses DS Lite I don't know. How can i find out? -Regarding your idea of my router caching old ... stuff (ips, dns requests?): Could you elaborate on that? – Beate Bier Jan 8 '17 at 22:12
  • Actually I don't think, that's the case. I think, the correct ip is displayed by my router to me. This is also the one that the dig command gives me. But the guy of my ISP on the phone told me, that this ip should not be a "public" one, therefor my raspi is not reachable and these websites are giving me other ips. But then it remains: How are dig and opendns.com getting my "internal" (intermediate-internal) ip? – Beate Bier Jan 8 '17 at 22:15
3

This service has been shut down by Cisco.

Google provides the same service.

ipv4

dig -4 TXT +short o-o.myaddr.l.google.com @ns1.google.com

or ipv6

dig -6 TXT +short o-o.myaddr.l.google.com @ns1.google.com
  • welcome to U&L, note that accepted answer may or may not work, in my case it didn't work from home (at least from one of my ISP), but ir work from some host at work (2 tested out of many). – Archemar Jan 22 at 13:50

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