DNS: Separate out CIFS DNS Resolver code

Separate out the DNS resolver key type from the CIFS filesystem into its own
module so that it can be made available for general use, including the AFS
filesystem module.

This facility makes it possible for the kernel to upcall to userspace to have
it issue DNS requests, package up the replies and present them to the kernel
in a useful form.  The kernel is then able to cache the DNS replies as keys
can be retained in keyrings.

Resolver keys are of type "dns_resolver" and have a case-insensitive
description that is of the form "[<type>:]<domain_name>".  The optional <type>
indicates the particular DNS lookup and packaging that's required.  The
<domain_name> is the query to be made.

If <type> isn't given, a basic hostname to IP address lookup is made, and the
result is stored in the key in the form of a printable string consisting of a
comma-separated list of IPv4 and IPv6 addresses.

This key type is supported by userspace helpers driven from /sbin/request-key
and configured through /etc/request-key.conf.  The cifs.upcall utility is
invoked for UNC path server name to IP address resolution.

The CIFS functionality is encapsulated by the dns_resolve_unc_to_ip() function,
which is used to resolve a UNC path to an IP address for CIFS filesystem.  This
part remains in the CIFS module for now.

See the added Documentation/networking/dns_resolver.txt for more information.

Signed-off-by: Wang Lei <wang840925@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/net/dns_resolver/Kconfig b/net/dns_resolver/Kconfig
new file mode 100644
index 000000000000..2ec47cb5d0d6
--- /dev/null
+++ b/net/dns_resolver/Kconfig
@@ -0,0 +1,27 @@
+#
+# Configuration for DNS Resolver
+#
+config DNS_RESOLVER
+	tristate "DNS Resolver support"
+	depends on NET && KEYS
+	help
+	  Saying Y here will include support for the DNS Resolver key type
+	  which can be used to make upcalls to perform DNS lookups in
+	  userspace.
+
+	  DNS Resolver is used to query DNS server for information.  Examples
+	  being resolving a UNC hostname element to an IP address for CIFS or
+	  performing a DNS query for AFSDB records so that AFS can locate a
+	  cell's volume location database servers.
+
+	  DNS Resolver is used by the CIFS and AFS modules, and would support
+	  samba4 later.  DNS Resolver is supported by the userspace upcall
+	  helper "/sbin/dns.resolver" via /etc/request-key.conf.
+
+	  See <file:Documentation/networking/dns_resolver.txt> for further
+	  information.
+
+	  To compile this as a module, choose M here: the module will be called
+	  dnsresolver.
+
+	  If unsure, say N.