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, 9 insertions, 8 deletions

diff --git a/fs/cifs/Kconfig b/fs/cifs/Kconfig
index 5739fd7f88b4..57f0aa9f141f 100644
--- a/fs/cifs/Kconfig
+++ b/fs/cifs/Kconfig
@@ -71,14 +71,14 @@ config CIFS_WEAK_PW_HASH
           If unsure, say N.
 
 config CIFS_UPCALL
-          bool "Kerberos/SPNEGO advanced session setup"
-          depends on CIFS && KEYS
-          help
-            Enables an upcall mechanism for CIFS which accesses
-            userspace helper utilities to provide SPNEGO packaged (RFC 4178)
-            Kerberos tickets which are needed to mount to certain secure servers
-            (for which more secure Kerberos authentication is required). If
-            unsure, say N.
+        bool "Kerberos/SPNEGO advanced session setup"
+        depends on CIFS && KEYS
+        select DNS_RESOLVER
+        help
+          Enables an upcall mechanism for CIFS which accesses userspace helper
+          utilities to provide SPNEGO packaged (RFC 4178) Kerberos tickets
+          which are needed to mount to certain secure servers (for which more
+          secure Kerberos authentication is required). If unsure, say N.
 
 config CIFS_XATTR
         bool "CIFS extended attributes"
@@ -122,6 +122,7 @@ config CIFS_DEBUG2
 config CIFS_DFS_UPCALL
           bool "DFS feature support"
           depends on CIFS && KEYS
+          select DNS_RESOLVER
           help
             Distributed File System (DFS) support is used to access shares
             transparently in an enterprise name space, even if the share