Chrome Networking: DNS Prefetch & TCP Preconnect(igvita.com)
igvita.com
Chrome Networking: DNS Prefetch & TCP Preconnect
http://www.igvita.com/2012/06/04/chrome-networking-dns-prefetch-and-tcp-preconnect/
28 comments
This is actually quite fascinating. The lengths the Chrome team is going to, in the name of page-rendering speed, is really astonishing.
This got me thinking...
When TLS, cookies or same origin policy are not a concern, why not resolve resource domains on the server side and output for example <img src="hxxp://10.0.0.1/picture.jpg"> to avoid that extra client side DNS request completely?
When TLS, cookies or same origin policy are not a concern, why not resolve resource domains on the server side and output for example <img src="hxxp://10.0.0.1/picture.jpg"> to avoid that extra client side DNS request completely?
Yup, we can do that too! I didn't mention it in the writeup, but you can also insert <link rel="dns-prefetch" href="http://www.domain.com/>; hints into the head of the document to tell the browser to pre-resolve certain domains.
An often overlooked reason for supporting this is to pre-resolve hostnames behind a redirect. If you have a "http://a.com/resource, which redirects to "http://b.com/resource, and you know this relationship while you're generating the page, then injecting a dns-prefetch link can help hide the cost of the extra DNS lookup when the browser is following the 3xx.
An often overlooked reason for supporting this is to pre-resolve hostnames behind a redirect. If you have a "http://a.com/resource, which redirects to "http://b.com/resource, and you know this relationship while you're generating the page, then injecting a dns-prefetch link can help hide the cost of the extra DNS lookup when the browser is following the 3xx.
DNS pre-fetching is a great way for spammers to determine whether or not an email has been read when the recipient is using webmail with a browser which supports it. Although, I believe DNS pre-fetching is disabled over https by default on all current browsers which support it.
EDIT: This is one of the things that a website I developed tests for: https://emailprivacytester.com/
EDIT: This is one of the things that a website I developed tests for: https://emailprivacytester.com/
This only works when there's only one site hosted at 10.0.0.1. When an HTTP request is made, it looks like this (not trying to be pedantic here, I'm sure you know how HTTP requests are made):
That could of course be solved with something like
GET /picture.jpg HTTP/1.1
Host: host.sample.com
[Other header: value]...
That could of course be solved with something like
<img src="hxxp://host.sample.com/picture.jpg" ip="10.0.0.1">
or even
<img src="hxxp://10.0.0.1/picture.jpg" host="host.sample.com">Or you could just "virtual host" by initial URL path, like "hxxp://10.0.0.1/host.sample.com/picture.jpg" or "hxxp://10.0.0.1/p2ezQ/picture.jpg".
Of course... This is actually a much better solution than introducing a img tag attribute. It would be more visible at the template level and (at least the first example) requires minimal configuration.
Out of curiosity: do you do (or know someone who does) this?
Do you think the functionality is worth the code to implement it?
Out of curiosity: do you do (or know someone who does) this?
Do you think the functionality is worth the code to implement it?
No, I don't know anyone who does this. This is just a curiosity for me.
No idea if it's worth it. Measure. Build a test case that implements a limited version or emulates this somehow. Test it against some group for a few weeks or months. Did it benefit you, made no difference or harm your goals? Did it give you something unexpected that you can turn into your benefit?
No idea if it's worth it. Measure. Build a test case that implements a limited version or emulates this somehow. Test it against some group for a few weeks or months. Did it benefit you, made no difference or harm your goals? Did it give you something unexpected that you can turn into your benefit?
Generally, nothing forces one DNS to resolve to one ip and one ip to correspond to one DNS. So you're losing both a simple way to do load-balancing and virtual hosts.
That being said, both can be worked around by other ways. You definitely lose a way to do geographical load-balancing, though (like in a CDN).
That being said, both can be worked around by other ways. You definitely lose a way to do geographical load-balancing, though (like in a CDN).
This only moves the logic you need to do in CDN DNS server to somewhere in your web serving stack.
You could still implement crude geographical load-balancing by simply looking at client IP address for example at your load balancer/reverse proxy or the HTML serving server itself.
Nothing forces you to serve same IP address to each client. For media streaming, you can do some IP round robining to balance load. For serving CSS, images, scripts, etc. you'd want to hash some relatively static client derived attribute to pick a server to avoid defeating client side caching.
At its simplest, map first octet of client IP to optimal geographical load-balancing IP(s). I'm sure there'd be some sub-optimal choices, but it'd still be a reasonably good approximation. This works, because first octet determines at least whether a given IP is assigned to RIPE, ARIN, APNIC, specific organization, etc. You get about country level accuracy, at worst about continent.
Or use a full fledged geo IP database to find an optimal server IP for each client IP.
Of course this isn't very practical, because requires more maintenance and is not effortless like using traditional CDN DNS resolution based methods. Maybe some CDN vendor could offer this in a more managed, easily integrable fashion.
I'm sure it could be made work, if the potential latency win is worth the effort for you...
You could still implement crude geographical load-balancing by simply looking at client IP address for example at your load balancer/reverse proxy or the HTML serving server itself.
Nothing forces you to serve same IP address to each client. For media streaming, you can do some IP round robining to balance load. For serving CSS, images, scripts, etc. you'd want to hash some relatively static client derived attribute to pick a server to avoid defeating client side caching.
At its simplest, map first octet of client IP to optimal geographical load-balancing IP(s). I'm sure there'd be some sub-optimal choices, but it'd still be a reasonably good approximation. This works, because first octet determines at least whether a given IP is assigned to RIPE, ARIN, APNIC, specific organization, etc. You get about country level accuracy, at worst about continent.
Or use a full fledged geo IP database to find an optimal server IP for each client IP.
Of course this isn't very practical, because requires more maintenance and is not effortless like using traditional CDN DNS resolution based methods. Maybe some CDN vendor could offer this in a more managed, easily integrable fashion.
I'm sure it could be made work, if the potential latency win is worth the effort for you...
That breaks NAT64+DNS64, for one. (T-Mobile says IPv6-native with NAT64+DNS64 for legacy [IPv4] connections is the future on their network.)
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