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ellie288
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Message 1 of 7
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Virgin media hostnames don't resolve to IP addresses.

Why don't virgin media hostnames resolve to IP addresses?

Reverse DNS resolution works just fine, but not forward resolution. Let me give an example.

My IP is  [removed] and I enter:

 

 

I get the following response:

 

198.184.192.80.in-addr.arpa domain name pointer cpc128308-linl10-2-0-cust197.know.cable.virginm.net.

 

However, if I then attempt forward DNS resolution of this hostname, it fails!

 

$ host cpc128308-linl10-2-0-cust197.know.cable.virginm.net

 

I receive the following response:

 

Host cpc128308-linl10-2-0-cust197.know.cable.virginm.net not found: 2(SERVFAIL)

 

(The proper response would be to return the IP address).

Why do virgin media not return an IP address for the hostname? RFC 1912 states that IP addresses and hostnames should resolve in both directions. This seems to be the case for most other ISPs.

This causes problems with connecting to servers that require Forward-confirmed reverse DNS: https://en.wikipedia.org/wiki/Forward-confirmed_reverse_DNS

Could someone with tech knowledge explain what is going on here?

 

 [MOD EDIT: Personal and private information has been removed from this post. Please do not post personal or private information in your public posts. Please review the Forum Guidelines]

 

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DJ_Shadow1966
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Re: Virgin media hostnames don't resolve to IP addresses.

Hello

What system are you doing this on, as when I do the same with nslookup which is the correct tool for doing dns lookup it works with no issues on my connection. Although it doesnt seemn to working on yours I get the same results.

Right the issue you having is the reverse will only work when it has a service like no-ip.com setup on it, this will set the dns up fully for the ip/hostname.

If this is too do with email servers there is the problem as we all have dynamic IP addresses although it may change rarely, you cannot run email servers properly on a dynamic IP due to this problem.

Mike

 

 

 

 

I dont work for VirginMedia all opinions are my own.
39 years in IT.
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ellie288
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Re: Virgin media hostnames don't resolve to IP addresses.

 

What system are you doing this on, as when I do the same with nslookup which is the correct tool for doing dns lookup it works with no issues on my connection. Although it doesnt seemn to working on yours I get the same results.

 Debian GNU/Linux. I have also tried nslookup on my hostname too, but I get the same SERVFAIL response.

I have now tried both host/nslookup against other users IPs/hostnames (found on this forum) and resolution from hostname to IP does indeed work for theirs. Just not with mine!

I am actually using a separate custom DynDNS domain for my IP so that I can log into my computer remotely for troubleshooting. Surely that wouldn't cause the problem though as forward resolution would still be on the virginm.net domain?

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ellie288
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Re: Virgin media hostnames don't resolve to IP addresses.

Upon further inspection, it seems that virgin media hostnames have 2 separate formats, and it varies between customers. Some have this format:

cpc[num]-knowx-x-x-cust[num].y-y.cable.virginm.net

Others have a different format:

cpc[num]-[place]x-x-x-cust[num].know.cable.virginm.net

 Resolution seems to work on the first format, but not on the 2nd format!

 

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Very Insightful Person
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Re: Virgin media hostnames don't resolve to IP addresses.

First off, you do need to realise that RFC1912 is an opinion piece.  It does not specify an internet standard. 

Second - Even if it were a standard, The RFC’s are very careful to distinguish between SHOULD and MUST

  • MUST - An absolute requirement.
  • SHOULD - recommended practice, but there may be times when this requirement may be ignored.

I fell into the same trap when going through the DHCP RFC’s

With regard to Forward-reverse confirmed DNS, there are very few services on the internet that use it.  Best practice calls for outbound mail SERVERS to use this in order to guarantee inbound servers will talk to them, but you should not be running a mail server off a DHCP allocated IP address anyway.

Tim

 

As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.

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ellie288
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Re: Virgin media hostnames don't resolve to IP addresses.

Thanks for your response. I am not running a mail server. I am trying to log in to remote servers using SSH, but I am unable to do so with certain ones that use TCP wrappers. It also effects IRC, though not gravely.

I understand that RFC's are technically only 'recommendations'. I seem to have identified a bug here though: Certain Virgin Media customer's hostnames (ones that follow a particular format) do not resolve properly, whereas it works perfectly fine for customers who have the other format. Why is this? Shouldn't it work for all customers?

Regardless of the status of RFC's or mail servers, or even ssh servers, the point still stands: hostname-to-IP resolution works fine for certain VM customers, but not for others. I would like to know why that is and if VM intend to fix it.

 

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chenks
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Message 7 of 7
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Re: Virgin media hostnames don't resolve to IP addresses.

i would tend to lean on the side that the version that does resolve is most likely a bug, rather than the other way round.

you should assume something is not intended to work unless explicitly stated that it does.
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