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Re: Re: [lwip-users] How can I get a DNS Server IP automatically

Subject: Re: Re: [lwip-users] How can I get a DNS Server IP automatically
Date: Thu, 13 Nov 2008 10:17:51 +0800

Hi, Empson and BERNON
Thank you very much for your reply!
Though DHCP server did not offer the DNS server IP adress, it offer the gateway. In most case, I think we can use gateway as our DNS server. If we need some host's IP address, just send the DNS request to the gateway. May be gateway has an DNS server address. (I am not sure if it will go on send request to its own gateway when it doesn't have a DNS server's IP.)
I have tested this case in my network. I get the host's IP by set the DNS server as default "inet_addr("") /* resolver1.opendns.com */" or my gateway.
It worked OK.
Thanks to Empson again!
Your explanation are very deep and clear. I think I have understand. May be Auto IP is a good choice for me. In fact I will build a network without a DHCP host, and they need to have IP address.
Best Regards!


发件人: David Empson
发送时间: 2008-11-13 06:19:12
收件人: Mailing list for lwIP users
主题: Re: [lwip-users] How can I get a DNS Server IP automatically
HuangZhenhua address@hidden wrote:
> Another question, What is the difference between DHCP and AutoIP?
DHCP involves a server supplying the client with an IP address, subnet mask, gateway and other configuration parameters (e.g. DNS is almost always supplied). The client may supply an identification string. The server is responsible for picking the appropriate IP address, which it might do by allocating from a pool of available addresses, or assigning a fixed address based on the client's ID or MAC address.
AutoIP (also known as APIPA for "Automatic Private IP Addressing") is a mechanism where a device assigns itself a random IP address from a particular range reserved for AutoIP ( No server is involved.The subnet mask used is always, there is no gateway, and it doesn't have any other configuration parameters such as DNS.
After picking a random AutoIP address, the device polls the network to check whether that address is already in use by something else. If it is, then the device picks another random IP address and tries again.
AutoIP is often used in ad hoc networks which are temporarily set up between two or more devices, e.g. you could use it when you plug a laptop into an embedded device via a direct Ethernet cable. If both devices use AutoIP then they will be able to talk to each other. Actually finding each other may require something like sending a broadcast ping, or using a name-based service advertising and discovery protocol like SLP, NetBIOS or Apple's Bonjour (multicast DNS, with DNS service discovery).
http://en.wikipedia.org/wiki/Zeroconf has a good overview.

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