Stephen Harris wrote:
>> >> As far as I can see, it is some sort of rival to dhcpd.
>> >
>> > No, DHCP is used to assign network addresses and routes (and other
>> > optional configuration items).
>>
>> According to the Wikipedia entry for mDNS,
>> "Using mDNS allows to determine the IP address of a host
>> without the help of a centralized DNS server".
>>
>> Isn't that more or less what I said above?
>
> It's almost the opposite. mDNS does name->IP and let's people
> find other machines; DHCP does MAC->IP and let's a machine find _itself_.
>
> Or, another way of looking at it. mDNS is a bit like ARP, but for names.
>
> ARP: In a traditional ethernet network, when you try to connect to a
> machine on your local network with the number 10.20.30.40 then your
> machine will send out an ARP broadcast packet "whois 10.20.30.40" and
> then the machine in question will respond with its MAC address and then
> the machines can talk via ethernet.
>
> mDNS does something similar, but for names mapping to IP addresses; so
> your machine will broadcast out requests for names ("whois fred") and
> get a response. mDNS-SD can also do service discovery ("who is running
> samba?", "who is running iTunes?"). This allows applications to find
> local resources.
>
> All this is done without a central server.
>
> DHCP is almost the opposite; it's for a machine to find out what _it_
> is; the machine asking "Who am I?" and the server responding "You're
> 10.20.30.40". In some cases the machine might say "Who am I? I'd like
> to be called Tom"; the dhcp server would respond "You're 10.20.30.40"
> and _might_ update a central DNS (or, more often, might not).
OK, I should have said "a rival to ARP + dhcp".
As I see it, dhcpd assigns IP addresses to the devices on a LAN,
and arp then provides a method of accessing a device
with a given IP address.
Incidentally, I don't really see why mDNS is needed on a LAN.
If a program wants to know the IP address of a device with a given name,
why can't it just look in /etc/hosts ?
I see that it might be useful in a much simpler setup,
where there is no server;
but if there is a server available, I don't really see the point of it.
--
Timothy Murphy
e-mail: gayleard /at/ eircom.net
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College Dublin
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