SOCKET NAMES AND DNS in Font

Drawer QR in Font SOCKET NAMES AND DNS

Expiration Date:28-Mar-2016 05:00:00 UTC ... Registrant Name:Python Software Foundation ... Name Server:NS2.XS4ALL.NL Name Server:NS.XS4ALL.NL And that provides our answer! Wherever you are in the world, your DNS request for any hostname within python.org must be passed on to one of the two DNS servers named in that entry. Of course, when your DNS server makes this request to a top-level domain nameserver, it does not really get back only two names like those just given; instead, it is also given their IP addresses so that it can contact them directly without incurring another round of DNS lookup. Your DNS server is now finished talking to both the root-level DNS server and the top-level .org DNS server, and can communicate directly with NS2.XS4ALL.NL or NS.XS4ALL.NL to ask about the python.org domain and, in fact, it will usually try one of them and then fall back to trying the other if the first one is unavailable. This increases the chances of you getting an answer, but, of course, it can increase the amount of time that you sit staring at your web browser before the page can actually be displayed! Depending on how python.org has its nameservers configured, the DNS server might require just one more query to get its answer, or it might take several if the organization is a large one with many departments and sub-departments that all run their own DNS servers to which requests need to be delegated. In this case, the www.python.org query can be answered directly by either of the two servers just named, and your DNS server can now return a UDP packet to your browser telling it which IP addresses belong to that hostname. Note that this process required four separate network round-trips. Your machine made a request and got a response from your own DNS server, and in order to answer that request, your DNS server had to make a recursive query that consisted of three different round-trips to other servers. No wonder your browser sits there spinning when you enter a domain name for the first time!

The foregoing explanation of a typical DNS query has, I hope, made clear that your operating system is doing a lot for you when you need a hostname looked up. For this reason, I am going to recommend that, unless you absolutely need to speak DNS for some quite particular reason, you always rely on getaddrinfo() or some other system-supported mechanism for resolving hostnames. Consider the benefits: The DNS is often not the only way that a system gets name information. If your application runs off and tries to use DNS on its own as its first choice for resolving a domain name, then users will notice that some computer names that work everywhere else on your system in their browser, in file share names, and so forth suddenly do not work when they use your application, because you are not deferring to mechanisms like WINS or /etc/hosts like the operating system itself does. The local machine probably has a cache of recently queried domain names that might already know about the host whose IP address you need. If you try speaking DNS yourself to answer your query, you will be duplicating work that has already been done.

The system on which your Python script is running already knows about the local domain nameservers, thanks either to manual intervention by your system administrator or a network configuration protocol like DHCP in your office, home, or coffee shop. To crank up DNS right inside your Python program, you will have to learn how to query your particular operating system for this information an operating-system-specific action that we will not be covering in this book. If you do not use the local DNS server, then you will not be able to benefit from its own cache that would prevent your application and other applications running on the same network from repeating requests about a hostname that is in frequent use at your location. From time to time, adjustments are made to the world DNS infrastructure, and operating system libraries and daemons are gradually updated to accommodate this. If your program makes raw DNS calls of its own, then you will have to follow these changes yourself and make sure that your code stays up-to-date with the latest changes in TLD server IP addresses, conventions involving internationalization, and tweaks to the DNS protocol itself.

Finally, note that Python does not come with any DNS facilities built into the Standard Library. If you are going to talk DNS using Python, then you must choose and learn a third-party library for doing so.