99 lines
4.1 KiB
Plaintext
99 lines
4.1 KiB
Plaintext
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/**
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\page tutorial1_mx Tutorial 1: Querying for MX records
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\dontinclude ldns-mx.c
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The full source code can be found in \link examples/ldns-mx.c \endlink
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ldns-mx is a simple tool that queries your default caching forwarder for
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the MX (Mail exchange) record of the given domain.
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<pre>
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% ldns-mx nlnetlabs.nl
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nlnetlabs.nl. 86400 IN MX 100 omval.tednet.nl.
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nlnetlabs.nl. 86400 IN MX 50 open.nlnetlabs.nl.
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</pre>
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First of all, we need to include the correct header files, so
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that all functions are available to us:
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\skip include
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\until dns.h
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In this case we have used a configure script to generate a config.h file
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that does all our inclusions for us, so that it can be compiled on
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multiple platforms. If your platform supports the include files \c
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stdint.h and \c stdlib.h, you can include these instead of using a
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configure script.
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The first included files are prerequisites that ldns needs to function.
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The last one, of course, includes the functions of ldns itself.
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In our main function, we declare some variables that we are going to use:
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\skipline ldns_resolver
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\until ldns_status
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- The \c ldns_resolver structure keeps a list of nameservers, and can perform queries for us
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- An \c ldns_rdf is a basic data type of dns, the RDATA. See \ref design for a description about the building blocks of DNS.
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In this case, \c domain will be used to store the name the user specifies when calling the program
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- An \c ldns_pkt is a DNS packet, for instance a complete query, or an answer
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- The \c ldns_rr_list structure contains a list of DNS Resource Records (RRs). In this case, we will store the MX records we find in the list.
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- \c ldns_status is the basic type for status messages in ldns. Most functions will return a value of this type.
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First, we parse the command line argument (checks omitted on this page, see full source code), and store it in our \c domain variable:
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\skipline ldns_dname_new_frm_str
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This function takes a string containing a domain name (like
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"nlnetlabs.nl") and returns an \c ldns_rdf representing that name. If
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somehow the given string can not be parsed it returns NULL.
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Then, we create the resolver structure:
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\skipline ldns_resolver_new
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Most of the functions work like this, the first argument is a pointer to
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the structure where ldns should store its results (which is also a
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pointer). The function returns a status code indicating success
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(\ref LDNS_STATUS_OK) or an error number. Remember that these types of
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functions allocate memory that you should free later (using the
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ldns_free_<type> functions).
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The second argument is the filename that contains information about the
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resolver structure that is to be created. If this argument is NULL,
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/etc/resolv.conf is used. The syntax of the file is like that of
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/etc/resolv.conf.
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We tell the resolver to query for our domain, type MX, of class IN:
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\skipline ldns_resolver_query
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\until )
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The last argument contains flags to influence the type of query the
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resolver structure sends. In this case, we want the nameserver to use
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recursion, so that we'll get the final answer. Therefore, we specify the
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\ref LDNS_RD (Recursion Desired) flag.
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This should return a packet if everything goes well.
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We get all RRs of type MX from the answer packet and store them in our list:
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\skipline ldns_pkt_rr_list_by_type
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\until )
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If this list is not empty, we sort and print it:
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\skipline ldns_rr_list_sort
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\skipline ldns_rr_list_print
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And finally, just to be proper, we free our allocated data:
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\skipline free(
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\until resolver_deep_free
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For structures that can contain other ldns structures, there are two types of free() function available
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- \c ldns_free_<type> frees only the allocated data for the structure itself.
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- \c ldns_deep_free_<type> frees the structure, and ALL structures that
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are nested in it. For example, of you \c deep_free an ldns_rr_list,
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all \c ldns_rr structures that were present in the list are also
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freed.
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*/
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