FAQ
I've been getting a few avahi-daemon errors in /var/log/messages, eg
-------------------------------
Jan 11 00:40:24 helen avahi-daemon[12732]: Invalid query packet.
Jan 11 00:40:29 helen last message repeated 17 times
-------------------------------

(This is on a CentOS-5.7 server.)

So I looked up avahi on the web, but as far as I could see
it is not doing anything essential;
so I was wondering if stopping avahi-daemon would have any bad effect?


--
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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  • Rilindo Foster at Jan 10, 2012 at 7:57 pm

    On Jan 10, 2012, at 7:51 PM, Timothy Murphy wrote:

    I've been getting a few avahi-daemon errors in /var/log/messages, eg
    -------------------------------
    Jan 11 00:40:24 helen avahi-daemon[12732]: Invalid query packet.
    Jan 11 00:40:29 helen last message repeated 17 times
    -------------------------------

    (This is on a CentOS-5.7 server.)

    So I looked up avahi on the web, but as far as I could see
    it is not doing anything essential;
    so I was wondering if stopping avahi-daemon would have any bad effect?


    --
    Timothy Murphy
    e-mail: gayleard /at/ eircom.net
    tel: +353-86-2336090, +353-1-2842366
    s-mail: School of Mathematics, Trinity College Dublin


    _______________________________________________
    CentOS mailing list
    CentOS at centos.org
    http://lists.centos.org/mailman/listinfo/centos
    Avahi is a mdns daemon. You can safely disable it in most cases.
  • Timothy Murphy at Jan 11, 2012 at 6:42 am

    Rilindo Foster wrote:

    So I looked up avahi on the web, but as far as I could see
    it is not doing anything essential;
    so I was wondering if stopping avahi-daemon would have any bad effect?
    Avahi is a mdns daemon. You can safely disable it in most cases.
    But what applications use mdns?

    As far as I can see, it is some sort of rival to dhcpd.
    Is it only used within local LANs?
    Is it used, for example, by CUPS to identify printers?
    When, if ever, would it be used in a home network?

    --
    Timothy Murphy
    e-mail: gayleard /at/ eircom.net
    tel: +353-86-2336090, +353-1-2842366
    s-mail: School of Mathematics, Trinity College Dublin
  • William Warren at Jan 11, 2012 at 6:47 am

    On 1/11/2012 6:42 AM, Timothy Murphy wrote:
    Rilindo Foster wrote:
    So I looked up avahi on the web, but as far as I could see
    it is not doing anything essential;
    so I was wondering if stopping avahi-daemon would have any bad effect?
    Avahi is a mdns daemon. You can safely disable it in most cases.
    But what applications use mdns?

    As far as I can see, it is some sort of rival to dhcpd.
    Is it only used within local LANs?
    Is it used, for example, by CUPS to identify printers?
    When, if ever, would it be used in a home network?
    http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=mdns
    <http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=mdns>

    multicast dns. How it applies to cent though i don't know at this instant.
  • John R Pierce at Jan 11, 2012 at 7:16 am

    On 01/11/12 3:47 AM, William Warren wrote:
    multicast dns. How it applies to cent though i don't know at this instant.
    its part of multimedia home network plug and play, I believe... lets
    media boxes find media servers, and such. if you were to serve up
    streaming media on a home network, it would be a useful thing to have.
    otherwise? meh.



    --
    john r pierce N 37, W 122
    santa cruz ca mid-left coast
  • Timothy Murphy at Jan 11, 2012 at 8:40 am

    John R Pierce wrote:

    its part of multimedia home network plug and play, I believe... lets
    media boxes find media servers, and such. if you were to serve up
    streaming media on a home network, it would be a useful thing to have.
    otherwise? meh.
    Could you give a concrete example of such a setup, please?

    I must admit I'm rather confused by UPnP.
    Is it intended for devices that don't have an IP address?
    Or how does it fit in with dhcpd?

    --
    Timothy Murphy
    e-mail: gayleard /at/ eircom.net
    tel: +353-86-2336090, +353-1-2842366
    s-mail: School of Mathematics, Trinity College Dublin
  • Les Mikesell at Jan 11, 2012 at 9:03 am

    On Wed, Jan 11, 2012 at 7:40 AM, Timothy Murphy wrote:
    John R Pierce wrote:
    its part of multimedia home network plug and play, I believe... ?lets
    media boxes find media servers, and such. ? ?if you were to serve up
    streaming media on a home network, it would be a useful thing to have.
    otherwise? ? meh.
    Could you give a concrete example of such a setup, please?

    I must admit I'm rather confused by UPnP.
    Is it intended for devices that don't have an IP address?
    Or how does it fit in with dhcpd?
    It is for devices with IP, but to find names that aren't officially
    registered in a DNS server. For example if you have a Playstation 3,
    or a newer blu-ray player that supports network streaming it will use
    DHCP to get an address. But then suppose you install your own DLNA
    media server like ps3mediaserver (or have windows 7 home premium which
    includes one). Without registering your new server name in DNS, the
    device will be able to find the service if it is on the same lan. I
    think Macs use it to find printers too.

    --
    Les Mikesell
    lesmikesell at gmail.com
  • John R Pierce at Jan 11, 2012 at 4:22 pm

    On 01/11/12 6:03 AM, Les Mikesell wrote:
    It is for devices with IP, but to find names that aren't officially
    registered in a DNS server. For example if you have a Playstation 3,
    or a newer blu-ray player that supports network streaming it will use
    DHCP to get an address. But then suppose you install your own DLNA
    media server like ps3mediaserver (or have windows 7 home premium which
    includes one). Without registering your new server name in DNS, the
    device will be able to find the service if it is on the same lan. I
    think Macs use it to find printers too.
    its to find SERVICES that aren't registered in regular DNS, not hostnames.

    for instance, yes, said playstation will ask "are there any media
    servers out here?" and get the IPs of them so it can query them, ge
    their capabilities, and display them to the user as sources for
    music/movies/etc.



    --
    john r pierce N 37, W 122
    santa cruz ca mid-left coast
  • Florin Andrei at Jan 11, 2012 at 8:10 pm

    On 01/11/2012 06:03 AM, Les Mikesell wrote:
    It is for devices with IP, but to find names that aren't officially
    registered in a DNS server. For example if you have a Playstation 3,
    or a newer blu-ray player that supports network streaming it will use
    DHCP to get an address. But then suppose you install your own DLNA
    media server like ps3mediaserver (or have windows 7 home premium which
    includes one). Without registering your new server name in DNS, the
    device will be able to find the service if it is on the same lan. I
    think Macs use it to find printers too.
    Wait a sec, I have that setup (just mediatomb instead of ps3mediaserver)
    and there's no avahi on my network. Yet the PS3 is perfectly capable of
    discovering and using the DLNA server.

    It might be useful for *something* but it doesn't appear to be required
    in this case.
  • Warren Young at Jan 11, 2012 at 9:12 pm

    On 1/11/2012 6:10 PM, Florin Andrei wrote:
    On 01/11/2012 06:03 AM, Les Mikesell wrote:

    It is for devices with IP, but to find names that aren't officially
    registered in a DNS server. For example if you have a Playstation 3,
    or a newer blu-ray player that supports network streaming it will use
    DHCP to get an address. But then suppose you install your own DLNA
    media server like ps3mediaserver (or have windows 7 home premium which
    includes one). Without registering your new server name in DNS, the
    device will be able to find the service if it is on the same lan. I
    think Macs use it to find printers too.
    Wait a sec, I have that setup (just mediatomb instead of ps3mediaserver)
    and there's no avahi on my network. Yet the PS3 is perfectly capable of
    discovering and using the DLNA server.
    You're talking about the inverse case of Les. An MDNS server on your
    Linux box lets it find services on the network via MDNS. So, you could
    store movies on the PS3 and maybe play them on the Linux desktop without
    knowing the PS3's IP address, if you used an mdns/avahi-aware player
    program.

    The plug-and-play nature of MDNS would evaporate if you had to set up a
    Linux box on the LAN just to act as MDNS server.
  • Les Mikesell at Jan 11, 2012 at 9:27 pm

    On Wed, Jan 11, 2012 at 8:12 PM, Warren Young wrote:
    It is for devices with IP, but to find names that aren't officially
    registered in a DNS server. ?For example if you have a Playstation 3,
    or a newer blu-ray player that supports network streaming it will use
    DHCP to get an address. ?But then suppose you install your own DLNA
    media server like ps3mediaserver (or have windows 7 home premium which
    includes one). ? Without registering your new server name in DNS, the
    device will be able to find the service if it is on the same lan. ?I
    think Macs use it to find printers too.
    Wait a sec, I have that setup (just mediatomb instead of ps3mediaserver)
    and there's no avahi on my network. Yet the PS3 is perfectly capable of
    discovering and using the DLNA server.
    You're talking about the inverse case of Les. ?An MDNS server on your
    Linux box lets it find services on the network via MDNS. ?So, you could
    store movies on the PS3 and maybe play them on the Linux desktop without
    knowing the PS3's IP address, if you used an mdns/avahi-aware player
    program.
    No, mediatomb and ps3mediaserver are both servers (slightly different
    capabilities) and the ps3 is still a client/player.
    The plug-and-play nature of MDNS would evaporate if you had to set up a
    Linux box on the LAN just to act as MDNS server.
    It's multicast - the client can make a query and anything on the lan
    can answer so the applications providing the service can respond on
    their own. There is probably a way to set up a server that collates
    things across lan segments or configure routers to forward, but I'm
    not that familiar with it and it isn't necessary in the usual case of
    a single LAN subnet.

    --
    Les Mikesell
    lesmikesell at gmail.com
  • Gordon Messmer at Jan 13, 2012 at 1:27 pm

    On 01/11/2012 05:10 PM, Florin Andrei wrote:
    Wait a sec, I have that setup (just mediatomb instead of ps3mediaserver)
    and there's no avahi on my network. Yet the PS3 is perfectly capable of
    discovering and using the DLNA server.
    Avahi allows the workstation running it to advertise and solicit mDNS
    information. Your mediatomb and PS3 are running software that does the
    same thing that Avahi does.

    mDNS is peer-to-peer. By design, it does not need a centralized server.
    Avahi does not enable mDNS on the network (as a server), it enables
    your workstation to participate (as a peer).
  • Timothy Murphy at Jan 14, 2012 at 7:19 am

    Gordon Messmer wrote:

    Wait a sec, I have that setup (just mediatomb instead of ps3mediaserver)
    and there's no avahi on my network. Yet the PS3 is perfectly capable of
    discovering and using the DLNA server.
    Avahi allows the workstation running it to advertise and solicit mDNS
    information. Your mediatomb and PS3 are running software that does the
    same thing that Avahi does.
    If running mediatomb avoids the necessity for Avahi,
    can you give a concrete example of a situation where Avahi _is_ needed?

    --
    Timothy Murphy
    e-mail: gayleard /at/ eircom.net
    tel: +353-86-2336090, +353-1-2842366
    s-mail: School of Mathematics, Trinity College Dublin
  • Gordon Messmer at Jan 15, 2012 at 12:43 am

    On 01/14/2012 04:19 AM, Timothy Murphy wrote:
    If running mediatomb avoids the necessity for Avahi,
    can you give a concrete example of a situation where Avahi_is_ needed?
    I did. "If two PCs were running a collaborative editor, like gobby,
    they'll use mDNS to find each other. Chat clients such as GNOME's and
    iChat will locate other chat clients on the LAN if configured to do so.
    Rhythmbox will use mDNS to locate DAAP servers for media."

    CUPS will also use Avahi to locate networked printers.
  • Marko Vojinovic at Jan 15, 2012 at 8:39 am

    On Saturday 14 January 2012 21:43:00 Gordon Messmer wrote:
    On 01/14/2012 04:19 AM, Timothy Murphy wrote:
    If running mediatomb avoids the necessity for Avahi,
    can you give a concrete example of a situation where Avahi_is_ needed?
    I did. "If two PCs were running a collaborative editor, like gobby,
    they'll use mDNS to find each other. Chat clients such as GNOME's and
    iChat will locate other chat clients on the LAN if configured to do so.
    Rhythmbox will use mDNS to locate DAAP servers for media."

    CUPS will also use Avahi to locate networked printers.
    Pulseaudio will use Avahi for audio streaming over a LAN.

    HTH, :-)
    Marko
  • Gordon Messmer at Jan 13, 2012 at 1:24 pm

    On 01/11/2012 05:40 AM, Timothy Murphy wrote:
    I must admit I'm rather confused by UPnP.
    UPnP is something completely different. That protocol allows devices
    behind a NAT router to request that it open a port forward to them. It
    is commonly used by game consoles to open the ports required for online
    gaming services.
  • Les Mikesell at Jan 13, 2012 at 1:42 pm

    On Fri, Jan 13, 2012 at 12:24 PM, Gordon Messmer wrote:
    On 01/11/2012 05:40 AM, Timothy Murphy wrote:
    I must admit I'm rather confused by UPnP.
    UPnP is something completely different. ?That protocol allows devices
    behind a NAT router to request that it open a port forward to them. ?It
    is commonly used by game consoles to open the ports required for online
    gaming services.
    It is also a subset of DLNA and was used before the DLNA standard (if
    you want to call it that) for media services. So much is optional in
    the DLNA standard that you can't count on any two things to
    interoperate anyway. I think most of the network media stuff is done
    by people who really want you to keep buying DVDs and Blu-ray discs so
    it is intentionally disfunctional.

    --
    Les Mikesell
    lesmikesell at gmail.com
  • Peter Brady at Jan 11, 2012 at 5:28 pm

    On 11/01/12 11:16 PM, John R Pierce wrote:
    its part of multimedia home network plug and play, I believe... lets
    media boxes find media servers, and such. if you were to serve up
    streaming media on a home network, it would be a useful thing to have.
    otherwise? meh.
    I use AVAHI for an office full of iDevices that the boss wanted to be
    able to print from. AVAHI lets me advertise the CUPS printers in a
    format that the iDevices can see and subsequently utilise.

    Otherwise I disable and remove it on my other server instances.

    Cheers
    -pete

    --
    Peter Brady
    Email: pdbrady at ans.com.au
    Home Page: http://www.simonplace.net/
    Skype: pbrady77
    Mobile: +61 410 490 797

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  • Timothy Murphy at Jan 11, 2012 at 8:26 am

    William Warren wrote:

    Avahi is a mdns daemon. You can safely disable it in most cases.
    But what applications use mdns?

    As far as I can see, it is some sort of rival to dhcpd.
    Is it only used within local LANs?
    Is it used, for example, by CUPS to identify printers?
    When, if ever, would it be used in a home network?
    http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=mdns
    <http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=mdns>
    I had looked up mdns on google, which is what you seem to be suggesting.
    But it did not give me an answer to my query above.
    Which URL did you think answered this?

    --
    Timothy Murphy
    e-mail: gayleard /at/ eircom.net
    tel: +353-86-2336090, +353-1-2842366
    s-mail: School of Mathematics, Trinity College Dublin
  • Nicolas Thierry-Mieg at Jan 11, 2012 at 8:29 am

    Timothy Murphy wrote:
    William Warren wrote:
    Avahi is a mdns daemon. You can safely disable it in most cases.
    But what applications use mdns?

    As far as I can see, it is some sort of rival to dhcpd.
    Is it only used within local LANs?
    Is it used, for example, by CUPS to identify printers?
    When, if ever, would it be used in a home network?
    http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=mdns
    <http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=mdns>
    I had looked up mdns on google, which is what you seem to be suggesting.
    But it did not give me an answer to my query above.
    Which URL did you think answered this?
    maybe this:
    http://en.wikipedia.org/wiki/Zero_configuration_networking
  • Gordon Messmer at Jan 13, 2012 at 1:22 pm

    On 01/11/2012 03:42 AM, Timothy Murphy 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).

    mDNS is used to discover services using IP multicast.
    Is it only used within local LANs?
    Unless you've done additional multicast configuration, yes.
    Is it used, for example, by CUPS to identify printers?
    Yes, it's common for recent network printers to advertise themselves
    using mDNS.
    When, if ever, would it be used in a home network?
    If two PCs were running a collaborative editor, like gobby, they'll use
    mDNS to find each other.

    Chat clients such as GNOME's and iChat will locate other chat clients on
    the LAN if configured to do so.

    Rhythmbox will use mDNS to locate DAAP servers for media.
  • Les Mikesell at Jan 13, 2012 at 1:37 pm

    On Fri, Jan 13, 2012 at 12:22 PM, Gordon Messmer 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).
    There is a larger 'zeroconf' context where if the request to a DHCP
    server times out, a node can assign itself something in the
    169.254.0.0/16 range. Then:
    mDNS is used to discover services using IP multicast.
    --
    Les Mikesell
    lesmikesell at gmail.com
  • Timothy Murphy at Jan 14, 2012 at 7:29 am

    Gordon Messmer wrote:
    On 01/11/2012 03:42 AM, Timothy Murphy 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?

    --
    Timothy Murphy
    e-mail: gayleard /at/ eircom.net
    tel: +353-86-2336090, +353-1-2842366
    s-mail: School of Mathematics, Trinity College Dublin
  • Stephen Harris at Jan 14, 2012 at 8:10 am

    On Sat, Jan 14, 2012 at 12:29:05PM +0000, Timothy Murphy wrote:
    Gordon Messmer wrote:
    On 01/11/2012 03:42 AM, Timothy Murphy 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).

    Interestingly, Ubuntu 11 allows you to specify "mdns4" in nsswitch.conf
    so that "ping foo.local" would find a machine on the local network called
    "foo" by mDNS. I'm not sure if there's an equiv for RH/CentOS at present.

    --

    rgds
    Stephen
  • Timothy Murphy at Jan 14, 2012 at 9:17 am

    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
  • Mikael Fridh at Jan 14, 2012 at 9:31 am

    On Jan 14, 2012 3:18 PM, "Timothy Murphy" wrote:
    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 ?
    Yeah if all servers on the lan somehow magically ended up in the hosts file
    I wouldn't install avahi either.
    but if there is a server available, I don't really see the point of it.
    I think that's been said already.
  • Stephen Harris at Jan 14, 2012 at 9:44 am

    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
  • Les Mikesell at Jan 14, 2012 at 11:56 am

    On Sat, Jan 14, 2012 at 8:17 AM, Timothy Murphy wrote:
    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.
    Somebody already said this but it isn't just host names, it is for
    services and the ports they run on.
    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 ?
    Devices aren't really the point. Start a second copy of mediatomb
    somewhere. Change the port it runs on. Start 2 copies on the same
    server on different ports. Tell the ps3 to find them. Where is the
    ps3's /etc/host file? How would you edit it - and if you could, how
    would you describe 2 of the same service on the same device? If I
    turn on a sony laptop running windows7, the ps3 sees both the windows
    media server and the sony vaio instance of the similar service.
    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.
    A visitor with a laptop uses your wifi and would like to print
    something. With apple's bonjour (which can be installed on windows
    too, and avahi probably matches) he'll see a list of available
    printers without having to configure anything. Isn't that nicer than
    having to match IP/name/protocol/port up yourself all with different
    configuration concepts?

    --
    Les Mikesell
    lesmikesell at gmail.com
  • Marc Deop at Jan 14, 2012 at 9:57 am

    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.
    Ain't it the router the one that responds?

    I mean, it usually has an ARP table to speed up things ;)

    Regards
  • Donkey Hottie at Jan 14, 2012 at 10:13 am

    14.1.2012 16:57, Marc Deop kirjoitti:
    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.
    Ain't it the router the one that responds?

    I mean, it usually has an ARP table to speed up things ;)

    Regards
    Any machine in the LAN may have and has it's own ARP table.

    --

    You will receive a legacy which will place you above want.
  • Les Mikesell at Jan 14, 2012 at 12:01 pm

    On Sat, Jan 14, 2012 at 8:57 AM, Marc Deop wrote:
    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.
    Ain't it the router the one that responds?
    No, the device with the IP responds directly.
    I mean, it usually has an ARP table to speed up things ;)
    Everything keeps an arp cache so they don't have to repeat the lookup
    for every packet, but routers expect to talk to a lot of devices and
    hold the cached pairs longer - perhaps up to 20 minutes. Most other
    devices have very short timeouts so they'll notice an IP change more
    quickly.

    --
    Les Mikesell
    lesmikesell at gmail.com

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