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>DNS</TITLE
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>DNS</TITLE
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>eCos Reference Manual</TH
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><H1
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><A
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><A
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NAME="NET-NS-DNS-API">Chapter 45. DNS</H1
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NAME="NET-NS-DNS-API">Chapter 45. DNS</H1
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><DIV
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><DIV
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CLASS="SECT1"
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CLASS="SECT1"
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><H1
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><H1
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CLASS="SECT1"
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CLASS="SECT1"
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><A
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><A
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NAME="NET-NS-DNS-API1">DNS API</H1
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NAME="NET-NS-DNS-API1">DNS API</H1
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><P
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><P
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>The DNS client uses the normal BSD API for performing lookups:
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>The DNS client uses the normal BSD API for performing lookups:
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>gethostbyname()</TT
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>gethostbyname()</TT
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> and
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> and
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>gethostbyaddr()</TT
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>gethostbyaddr()</TT
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>.</P
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>.</P
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><P
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><P
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>There are a few restrictions:</P
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>There are a few restrictions:</P
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><P
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><P
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></P
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></P
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><UL
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><UL
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><LI
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><LI
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><P
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><P
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>Only IPv4 is supported, ie IPv6 addresses cannot be looked
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>Only IPv4 is supported, ie IPv6 addresses cannot be looked
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up.</P
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up.</P
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></LI
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></LI
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><LI
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><LI
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><P
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><P
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>If the DNS server returns multiple authoritive records
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>If the DNS server returns multiple authoritive records
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for a host name, the hostent will only contain a record for the
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for a host name, the hostent will only contain a record for the
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first entry.</P
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first entry.</P
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></LI
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></LI
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><LI
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><LI
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><P
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><P
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>The code has been made thread safe. ie multiple threads
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>The code has been made thread safe. ie multiple threads
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may call
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may call
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>gethostbyname()</TT
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>gethostbyname()</TT
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>
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>
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without causing problems to the hostent structure returned. What
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without causing problems to the hostent structure returned. What
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is not safe is one thread using both
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is not safe is one thread using both
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>gethostbyname()</TT
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>gethostbyname()</TT
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>
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>
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and
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and
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>gethostbyaddr()</TT
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>gethostbyaddr()</TT
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>.
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>.
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A call to one will destroy the results from the previous call
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A call to one will destroy the results from the previous call
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to the other function.</P
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to the other function.</P
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></LI
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></LI
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></UL
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></UL
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><P
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><P
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>To initialise the DNS client the following function must be
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>To initialise the DNS client the following function must be
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called:</P
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called:</P
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><TD
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><TD
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><PRE
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><PRE
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CLASS="PROGRAMLISTING"
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CLASS="PROGRAMLISTING"
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>#include <network.h>
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>#include <network.h>
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int cyg_dns_res_init(struct in_addr *dns_server)</PRE
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int cyg_dns_res_init(struct in_addr *dns_server)</PRE
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></TD
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></TD
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></TR
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></TR
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></TABLE
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></TABLE
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><P
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><P
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>where dns_server is the address of the DNS server
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>where dns_server is the address of the DNS server
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the client should query. On Error this function returns -1, otherwise
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the client should query. On Error this function returns -1, otherwise
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0 for success. If lookups are attemped before this function has
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0 for success. If lookups are attemped before this function has
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been called, they will fail and return NULL.</P
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been called, they will fail and return NULL.</P
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><P
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><P
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>A default, hard coded, server may be specified in the CDL option
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>A default, hard coded, server may be specified in the CDL option
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<TT
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<TT
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CLASS="LITERAL"
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CLASS="LITERAL"
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>CYGDAT_NS_DNS_DEFAULT_SERVER</TT
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>CYGDAT_NS_DNS_DEFAULT_SERVER</TT
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>. The use of this is
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>. The use of this is
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controlled by <TT
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controlled by <TT
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CLASS="LITERAL"
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CLASS="LITERAL"
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>CYGPKG_NS_DNS_DEFAULT</TT
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>CYGPKG_NS_DNS_DEFAULT</TT
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>. If this is
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>. If this is
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enabled, <TT
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enabled, <TT
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CLASS="LITERAL"
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CLASS="LITERAL"
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>init_all_network_interfaces</TT
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>init_all_network_interfaces</TT
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> will
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> will
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initialize the resolver with the hard coded address. The DHCP client
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initialize the resolver with the hard coded address. The DHCP client
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or user code my override this address by calling
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or user code my override this address by calling
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<TT
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<TT
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CLASS="LITERAL"
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CLASS="LITERAL"
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>cyg_dns_res_init</TT
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>cyg_dns_res_init</TT
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> again. </P
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> again. </P
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><P
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><P
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>The DNS client understands the concepts of the target being
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>The DNS client understands the concepts of the target being
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in a domain. By default no domain will be used. Host name lookups
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in a domain. By default no domain will be used. Host name lookups
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should be for fully qualified names. The domain name can be set
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should be for fully qualified names. The domain name can be set
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and retrieved using the functions:
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and retrieved using the functions:
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<DIV
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<DIV
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CLASS="FUNCSYNOPSIS"
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CLASS="FUNCSYNOPSIS"
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><A
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><A
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NAME="AEN14685"><P
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NAME="AEN14685"><P
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></P
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></P
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><P
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><P
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><CODE
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><CODE
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><CODE
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><CODE
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CLASS="FUNCDEF"
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CLASS="FUNCDEF"
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>int getdomainname</CODE
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>int getdomainname</CODE
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>(char *name, size_t len);</CODE
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>(char *name, size_t len);</CODE
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></P
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></P
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><P
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><P
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><CODE
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><CODE
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><CODE
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><CODE
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CLASS="FUNCDEF"
|
CLASS="FUNCDEF"
|
>int setdomainname</CODE
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>int setdomainname</CODE
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>(const char *name, size_t len);</CODE
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>(const char *name, size_t len);</CODE
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></P
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></P
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><P
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><P
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></P
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></P
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></DIV
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></DIV
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></P
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></P
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><P
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><P
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>Alternatively, a hard coded domain name can be set using CDL.
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>Alternatively, a hard coded domain name can be set using CDL.
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The boolean <TT
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The boolean <TT
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CLASS="LITERAL"
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CLASS="LITERAL"
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>CYGPKG_NS_DNS_DOMAINNAME</TT
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>CYGPKG_NS_DNS_DOMAINNAME</TT
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> enables this
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> enables this
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and the domain name is taken from
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and the domain name is taken from
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<TT
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<TT
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CLASS="LITERAL"
|
CLASS="LITERAL"
|
>CYGPKG_NS_DNS_DOMAINNAME_NAME</TT
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>CYGPKG_NS_DNS_DOMAINNAME_NAME</TT
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>.</P
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>.</P
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><P
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><P
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>Once set, the DNS client will first perform a lookup with the domain
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>Once set, the DNS client will first perform a lookup with the domain
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name appended. If this fails it will then perform a second lookup
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name appended. If this fails it will then perform a second lookup
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without the appended domain name. </P
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without the appended domain name. </P
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></DIV
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></DIV
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