On Sat, Jan 14, 2012 at 02:17:56PM +0000, Timothy Murphy wrote:
> OK, I should have said "a rival to ARP + dhcp".
But it's not; ARP+dhcp is all about mapping MAC<->IP. mDNS is dealing
with name<->IP.
mDNS competes with _DNS_; it's a way of doing local DNS without needing
a DNS server.
> 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.
ARP gets the MAC address for an IP. It has nothing to do with names.
> 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 ?
If you only have 1 or 2 machines that are statically configured (always
the same IP), then that's fine. But in the modern home there's a lot
devices that get different addresses (printers, cellphones, BluRay
players, DVRs, games consoles, TVs, even remote controls) and very few
people bother to configure them for static IP address. Many people
don't even know _how_ to configure them. They just turn them on and hope.
> 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.
It's "zero configuration"; turn a device on, the device gets an address
from a DHCP server (typically your home router) and then tells the local
subnet "Hi! I'm tivo! I can do X,Y,Z".
Now for most of my devices I don't use it, myself; I have a home grown
superior config that builds dhcpd, ip4 DNS, ip6 DNS in an programmatic
way from a single config file. Nice and simple.
But... it's possible that, unknown to me, I am using it! When I connect
my android phone to my wifi network it can automatically find my printer
and I can print directly from the phone to the printer. That's probably
using mDNS without my knowing it (the printer has mDNS capability), making
use of the DNS-SD capabilities of mDNS.
Do you need avahi on a CentOS machine? Depends on if you use client
software that does auto-discovery using the API. Does it hurt? Probably
not.
--
rgds
Stephen
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