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I am using Mac/Firefox. Use System Proxy Settings.

My question is how does my off-the-shelf installed Firefox resolve local websites names, when apparently, I am not using any proxy. I just use System proxy settings. What and where from are these settings picked up?

I am able to access these local sites without any proxy. I always thought that some global DNS servers are hard-coded in browser, but they won't be of any use here, because these local sites will not be exposed to these global DNS servers.

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closed as off topic by Sven, Bart De Vos, Iain, Tom O'Connor, Chris S Nov 22 '11 at 18:31

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2 Answers 2

up vote 3 down vote accepted

The company DHCP server provides you with an IP address, but also with DNS servers.

These DNS servers know all the local websites and resolve those names for you.

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1  
So my understanding that some global DNS servers are hard-coded in browser is wrong. The DNS servers are fetched by browser at runtime from DHCP servers? –  xyz Nov 22 '11 at 12:43
    
Almost. DNS servers are commonly fetched along with the network card's IP address from the DHCP server when the system TCP/IP stack is initialized. You can also force such requests, but I don't know the exact commands on OS X. (On Windows, it's ipconfig /renew and friends.) –  Michael Kjörling Nov 22 '11 at 12:45
    
When you connect to the network, your computer asks if there is a DHCP server around. The your company DHPC responds with "I'm the DHCP, this is now your IP, and these are your new DNS servers." From there on out your computer (including your browser) uses those DNS servers to resolve any domain names. –  Kenny Rasschaert Nov 22 '11 at 12:46
    
As a follow-up question, how do I know the list of DNS servers being used by computer(including my browser)? –  xyz Nov 22 '11 at 12:52
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Open up Terminal -> nslookup mywebsite or dig mywebsite –  Bart De Vos Nov 22 '11 at 13:08

You are likely also using an internal DNS server which is able to resolve your internal host names.

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but how and when does this internal DNS server is coming into picture is my real question. Because I am not doing anything explicitly. –  xyz Nov 22 '11 at 12:42
    
You will have picked up your companies DNS servers over DHCP. Then, any DNS request goes to them. If it's an internal website the companies DNS servers will respond with the LAN IP. If it's an Internet site, then the DNS servers will pass on your request to the ISP which will then return the websites IP down the chain. Only then will your browser attempt to connect - it's not interested in Proxies until that point. –  Dan Nov 22 '11 at 12:47

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