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  • Trailing Dots in Domain Names
    using your PATH variable it will directly run the file you named When you type emacs you re not necessarily sure what exact program you re running when you type usr bin emacs that ambiguity is removed In DNS your search domains play a similar role If you have apple com microsoft com linux com and malicioushackers net in your DNS searchlist then when you type a hostname like www you ll actually get one of www apple com www microsoft com www linux com or www malicioushackers net It may not always be clear to you which one you re going to get especially if DHCP options are inserting extra entries into your DNS searchlist like myisp com as they usually do Most people have an intuitive understanding of this when they type a single label name with no dots because they don t expect a single label name to resolve as is What surprises most people is that if you type a multi label name containing dots then it s still relative to your DNS searchlist unless you remember the trailing dot on the end to make it fully qualified So if you type www someco com and you have comcast net in your DNS searchlist then you could end up actually getting www someco com comcast net That web page probably doesn t exist but it could At the very least all these extra DNS lookups are inefficient and a waste of time At worst you could end up accessing a malicious web site set up to deceive you To avoid the inefficiency of these extra lookups and perhaps more importantly to avoid the pointless ambiguity and potential unreliability that results most Bonjour DNS SD software uses fully qualified DNS domain names with dot at the end When you resolve a service to its hostname using DNS SD you know that hostname is complete it s not a partial name relative to whatever happens to be in the user s DNS searchlist at that moment This advice holds any time you re using a hostname that s generated programatically as opposed to input entered by the user For example if you write software that has a DNS name of one of your severs embedded in it for some reason e g for update checking then that name should have a dot at the end If it doesn t have a dot at the end you risk it being looked up relative to the user s current DNS searchlist Since you probably don t know or care what s going to be in your user s DNS searchlist that s almost never what you actually want Normally it will find the right host but in the event of DNS failures it will then go off down the user s searchlist looking for other matches which results in time delay doing pointless lookups and at worst could even result in it connecting to the wrong host How this affects

    Original URL path: http://www.dns-sd.org/TrailingDotsInDomainNames.html (2016-02-14)
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  • Why GUIDs are not ideal as service identifiers
    Martha would be confusing It forces Joe to be aware that what really matters is the GUIDs and the friendly names are just unreliable decoration that can t really be trusted to mean very much If the user connects to Joe and there s an entity on the network currently called Joe then the most logical and understandable outcome is probably to connect to the thing currently called Joe not to something else that used be called Joe some time in the past GUIDs Globally Unique IDentifiers are not Globally Unique It s easy to delude yourself into thinking that using GUIDs makes your software simpler and therefore more reliable If you can assume that your identifiers are permanent unchanging and unique then you don t have to make your code gracefully handle duplicates or support renaming entities since GUIDs never change Sure they re ugly as hell and a burden on end users but if they make the product more reliable then that s worth it right Unfortunately it s too easy for GUIDs not to remain unique To take one familiar example every iTunes music library has a GUID When you access a shared iTunes music library over the network iTunes identifies it by GUID That way even if you change the user visible name iTunes still recognizes it as the same library The problem is what does same mean If you buy a new Mac and copy all your files from your old Mac is it still the same music library Well yes because the GUID is in the library file and you copied that file along with all the others so it s the same GUID and therefore the same library But what happens now if you give your old Mac to your little sister and

    Original URL path: http://www.dns-sd.org/DNS-SD-GUIDs.html (2016-02-14)
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  • DNS-SD TXT record format
    everything that comes after the optional port number in an HTTP URL as defined in section 3 2 2 of RFC 2616 HTTP 1 1 3 2 2 http URL http URL http host port abs path query If you are writing a web browser then the path key gives you the text that should appear directly after GET in an HTTP GET request If you are writing a tool to generate a URL to send to a web browser for display then you should generate a URL of the form shown below where host and port are obtained from the information in the service s SRV record and path is obtained from the path key in the TXT record http host port path The path key in the TXT record MUST include the leading slash of the abs path e g the following is a legal DNS SD TXT record for an http tcp service path thepage html If the path key is missing or present with no value or empty value or otherwise invalid then the path is assumed to be Although strictly speaking usernames and passwords are not supposed to appear in HTTP URLs people frequently use

    Original URL path: http://www.dns-sd.org/txtrecords.html (2016-02-14)
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  • Using Apple iChat with Wide-Area DNS Service Discovery (DNS-SD)
    peers on the local link which can be discovered using link local Multicast DNS However Wide Area DNS SD makes it is possible also to discover services beyond the local link using the same DNS SD APIs To have iChat take advantage of this enter the following commands in a Terminal window defaults write com apple iChat SubNet UseGlobalDomain bool YES killall iChatAgent Provided you have Wide Area DNS SD

    Original URL path: http://www.dns-sd.org/iChatWideArea.html (2016-02-14)
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