Re: A mechanism to encode HTTP version information in DNS

"Adrien W. de Croy" <> Sun, 17 February 2013 22:33 UTC

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From: "Adrien W. de Croy" <>
To: "Phillip Hallam-Baker" <>
Cc: "Eliot Lear" <>, " Group" <>
Date: Sun, 17 Feb 2013 22:31:23 +0000
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Subject: Re: A mechanism to encode HTTP version information in DNS
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basically anything in the name is not discoverable.

So if the name contains _tcp, we're not doing discovery on transport.  
We're presuming there is a _tcp deployment etc.

We should take a step back and ask what are we trying to discover.  It 
makes not much sense to me to leave transport out of that.

------ Original Message ------
From: "Phillip Hallam-Baker" <>
To: "Adrien W. de Croy" <>
Cc: "Eliot Lear" <>om>; " Group" 
Sent: 18/02/2013 11:05:44 a.m.
Subject: Re: A mechanism to encode HTTP version information in DNS
>On Sun, Feb 17, 2013 at 4:31 PM, Adrien W. de Croy <> 
>>I've always thought one of the issues with SRV is the transport is in 
>>the wrong part - the name, so you can't specify that with the result.
>>For something new, we should look at putting the transport into the RR 
>>data instead of the name
>>then it makes it possible to deploy over multiple transports later on, 
>>e.g. SCTP, UDP whatever if desired.
>That is exactly what the URI RR does, the target is a URI which can of 
>course specify a new scheme.
>I think it best to treat _http._tcp as simply a code point 
>corresponding to the http URI scheme and the _tcp part as just a 
>If you wanted to specify a different transport then you would either 
>specify a different scheme or you would specify it as a discovery 
>parameter for that scheme.
>>------ Original Message ------
>>From: "Eliot Lear" <>
>>To: "Phillip Hallam-Baker" <>
>>Cc: " Group" <>
>>Sent: 17/02/2013 11:59:24 p.m.
>>Subject: Re: A mechanism to encode HTTP version information in DNS
>>>You're hitting at the heart of a key issue, and rather than focus on 
>>>mechanisms, I urge folk to take a look at the very beginning of 
>>>draft-lear-httpbis-svcinfo-rr where I state design goals.  One of 
>>>those goals is to not harm latency.  What you describe below 
>>>introduces a dependency of  QNAME on target, forcing an additional 
>>>query, which is one of the key issues with SRV.
>>>Now, I am not saying that's the wrong thing to do.  But I am saying 
>>>that it violates that stated design goal.  Maybe the goal is the 
>>>wrong one to have?
>>>On 2/14/13 9:16 PM, Phillip Hallam-Baker wrote:
>>>>Encoding HTTP version information in DNS is easy if you don't 
>>>>particularly care about using DNS properly or want to do anything 
>>>>more than encode HTTP version information.
>>>>Doing it well gets rather more complex. A DNS query costs a round 
>>>>trip so you would ideally like to make it pay. Also the process of 
>>>>deploying DNS records takes some time and it is better to reuse an 
>>>>existing record but only if that will not create ambiguity.
>>>>Looking again at the URI record, I think that we could use it to 
>>>>provide a HTTP version flag and other useful features in the DNS. In 
>>>>particular we can use the URI record to effect a HTTP redirect in 
>>>>DNS (a UDP round trip) rather than require a TCP round trip. It also 
>>>>provides for fault tolerance and load balancing and works well with 
>>>>Web Services.
>>>>The format of the URI record is currently: <priority> <weight> 
>>>>Priority - uint16
>>>>Weight - uint16
>>>>Target - string
>>>>While Target is technically a list of string entries it is not a 
>>>>good idea to depend on the string boundaries for technical reasons 
>>>>and so multiple strings should probably be considered equivalent to 
>>>>the result of concatenating them together.
>>>>For example: two servers offering HTTP service for ''
>>>>  URI 10 10 ""
>>>>  URI 10 10 ""
>>>>OK so that is not very interesting but the existing but the URI 
>>>>scheme also permits services to be advertised under a different 
>>>>scheme such as https or coap (or ftp if you must!)
>>>>So to force an upgrade to TLS we might specify the following:
>>>>  URI 10 10 ""
>>>>  URI 10 10 ""
>>>>Or to advertise multiple protocols:
>>>>  URI 10 10 ""
>>>>  URI 10 10 "coap://"
>>>>Or to map a domain to a path on another server:
>>>>  URI 10 10 
>>>>All those capabilities are useful in the context of HTTP discovery. 
>>>>They allow a redirect to be effected through the DNS rather than 
>>>>require a server deployment. But it would be much nicer if we could 
>>>>encode both a target URI and some description of that target to 
>>>>allow client selection. For example:
>>>>* IP version
>>>>* HTTP protocol version
>>>>* HSTS data
>>>>We don't always need this data but when we do it is very useful. But 
>>>>it turns out that the existing URI record might already meet this 
>>>>need since a URI cannot have a space inside it and so we can use 
>>>>that as a delimeter between the target URI and any parameters. We 
>>>>are currently discussing the details of this on DNSEXT but it looks 
>>>>like it works fine.
>>>>  URI 10 10 " ipv4 
>>>>  URI 10 10 " http2"
>>>>  URI 10 10 " sts"
>>>>The same mechanism can be used to effect pinning or to alert the 
>>>>client to the existence of a DANE record.
>>>>Knowing whether the site supports IPv4 or IPv6 or both allows us to 
>>>>optimize any A record lookup.
>>>>We could even specify the ASN number of the server IP address in the 
>>>>URI record. Why might you want to know that? Well it allows a client 
>>>>to select a server likely to be closer
>>>>The same mechanism can be used for a Web Service only there we would 
>>>>use the protocol prefix of the Web Service rather than HTTP.