dn_comp()--Compress Domain Name

 #include <sys/types.h>
 #include <netinet/in.h>
 #include <arpa/nameser.h>
 #include <resolv.h>

 int dn_comp(unsigned char *expanded_domain_name,
             unsigned char *compressed_domain_name,  
             int  answer_buffer_length,
             unsigned char **domain_name_pointers,
             unsigned char **last_domain_name)

The dn_comp() function is used to compress an expanded domain name.

Authorities and Locks

None.

Parameters

expanded_domain_name

(Input) The pointer to the expanded domain name.

compressed_domain_name

(Output) The pointer to where the compressed domain name will be stored.

answer_buffer_length

(Input) The size of the compressed_domain_name buffer.

domain_name_pointers

(Input) The pointer to an array of pointers to previously compressed domain names in the current message.

last_domain_name

(Input) The pointer to the end of the array specified by domain_name_pointers.

Return Value

dn_comp() returns an integer. Possible values are:

• -1 (unsuccessful)
  
  
• n (successful), where n is the size of the compressed domain name.

  dn_comp() compresses the domain name pointed to by expanded_domain_name. The result is placed in compressed_domain_name.

Error Conditions

When the dn_comp() function fails, it does not set specific errno or h_errno values. An error occurs under the following conditions:

• NULL pointer(s) passed to the function.
  
  
• Invalid pointer(s) passed to the function.
  
  
• Compressed_domain_name too small for the compressed domain name.

Usage Notes

1. domain_name_pointers[0] points to the beginning of the DNS packet. The list of pointers ends with a NULL pointer. After domain_name_pointers[0] is initialized to the beginning of the packet and domain_name_pointers[1] is initialized to NULL, dn_comp() updates the list each time it is called.
   
   
2. dn_comp() calls dn_find() to attempt to locate the different parts of the domain name being compressed.
   
   
3. dn_comp() expects EBCDIC data as input. The output from dn_comp() is also EBCDIC.

Related Information

• dn_expand()--Expand Domain Name
  
  
• dn_find()--Search for Compressed Domain Name
  
  
• dn_skipname()--Skip over Compressed Domain Name

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