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Support for UNC Naming and MUP

The multiple UNC provider (MUP) is a kernel-mode component responsible for channeling all remote file system accesses using a Universal Naming Convention (UNC) name to a network redirector (the UNC provider) that is capable of handling the remote file system requests. MUP is involved when a UNC path is used by an application as illustrated by the following example that could be executed from a command line:

notepad \\server\public\readme.txt

MUP is not involved during an operation that creates a mapped drive letter (the "NET USE" command, for example). This operation is handled by the multiple provider router (MPR) and a user-mode WNet provider DLL for the network redirector. However, a user-mode WNet provider DLL can communicate directly with a kernel-mode network redirector driver during this operation.

On Microsoft Windows Server 2003, Windows XP, and Windows 2000, remote file operations performed on a mapped drive that does not represent a Distributed File System (DFS) drive don't go through MUP. These operations go directly to the network provider that handled the drive letter mapping.

For network redirectors that conform to the Windows Vista redirector model, MUP is involved even when a mapped network drive is used. File operations performed on the mapped drive go through MUP to the network redirector. Note that in this case MUP simply passes the operation to the network redirector involved.

MUP is part of the mup.sys binary, which also includes the Microsoft DFS client in Windows Server 2003, Windows XP, and Windows 2000.

A kernel network redirector will normally also have a user-mode WNet provider DLL to support establishing connections to remote resources (mapping drive letters to remote resources, for example). The MPR is a user-mode DLL that establishes network connections based on queries to WNet providers. Calls to the MPR would result from any of the following:

A "net use x: \\server\share" command issued from a command prompt.

A network drive letter connection established from Windows Explorer

Direct calls to WNet functions.

A network redirector must register with MUP to handle UNC names. There can be multiple UNC providers registered with MUP. These UNC providers can be one or more of the following:

  • Network mini-redirectors based on RDBSS

  • Legacy redirectors not based on RDBSS

MUP determines which provider can handle a UNC path in a name-based operation, typically an IRP_MJ_CREATE request. This is referred to as "prefix resolution." Prior to Windows Vista, the prefix resolution operation serves two purposes:

  • The name-based operation which resulted in the prefix resolution is routed to the provider claiming the prefix. If successful, MUP ensures that subsequent handle-based operations (IRP_MJ_READ and IRP_MJ_WRITE, for example) go to the same provider completely bypassing MUP.

  • The provider and the prefix it claimed are entered in a prefix cache maintained by MUP. For subsequent name-based operations, MUP uses this prefix cache to determine whether a provider has already claimed a prefix before attempting to perform a prefix resolution. Each entry in this prefix cache is subject to a timeout (referred to as TTL) once it is added to the cache. An entry is thrown away after this timeout expires, at which point of time MUP will perform prefix resolution again for this prefix on a subsequent name-based operation.

MUP performs prefix resolution by issuing an IOCTL_REDIR_QUERY_PATH request to network redirectors registered with MUP. The input and output buffers for IOCTL_REDIR_QUERY_PATH are as follows:

Parameter available atData structure format

Input buffer

IrpSp-> Parameters.DeviceIoControl.Type3InputBuffer


Output buffer




The IOCTL and the data structures are defined in ntifs.h. The buffers are allocated from non-paged pool.

Network redirectors should only allow kernel-mode senders of this IOCTL, by verifying that the RequesterMode member of the IRP structure is KernelMode.

MUP uses the QUERY_PATH_REQUEST data structure for the request information.

typedef struct _QUERY_PATH_REQUEST {
    ULONG                PathNameLength;
    PIO_SECURITY_CONTEXT SecurityContext;
    WCHAR                FilePathName[1];

Structure MemberDescription


The length, in bytes, of the Unicode string contained in the FilePathName member.


A pointer to the security context.


A non-NULL terminated Unicode string of the form \<server>\<share>\<path>. The length of the string, in bytes, is specified by the PathNameLength member.


UNC providers should use the QUERY_PATH_RESPONSE data structure for the response information.

typedef struct _QUERY_PATH_RESPONSE {
    ULONG  LengthAccepted;

Structure MemberDescription


The length, in bytes, of the prefix claimed by the provider from the Unicode string path specified in the FilePathName member of the QUERY_PATH_REQUEST structure.


Note that IOCTL_REDIR_QUERY_PATH is a METHOD_NEITHER IOCTL. This means that the input and output buffers might not be at the same address. A common mistake by UNC providers is to assume that the input buffer and the output buffer are the same and use the input buffer pointer to provide the response.

When a UNC provider receives an IOCTL_REDIR_QUERY_PATH request, it has to determine whether it can handle the UNC path specified in the FilePathName member of the QUERY_PATH_REQUEST structure. If so, it has to update the LengthAccepted member of the QUERY_PATH_RESPONSE structure with the length, in bytes, of the prefix that it has claimed, and complete the IRP with STATUS_SUCCESS. If the provider cannot handle the specified UNC path, it must fail the IOCTL_REDIR_QUERY_PATH request with an appropriate NTSTATUS error code and must not update the LengthAccepted member of the QUERY_PATH_RESPONSE structure. Providers must not modify any of the other members or the FilePathName string under any condition.

If the \\server\share prefix name is not recognized in response to an IRP_MJ_CREATE or other IRPs that are using UNC names, the recommended NTSTATUS code to return is one of the following:


The network path cannot be located. The machine name (\\server, for example) is not valid or the network redirector cannot resolve the machine name (using whatever name resolution mechanisms are available).


The specified share name cannot be found on the remote server. The machine name (\\server, for example) is valid, but specified share name cannot be found on the remote server.


There were insufficient resources available to allocate memory for buffers.


An IOCTL_REDIR_QUERY_PATH request should only come from MUP and the requester mode of the IRP should always be KernelMode. This error code is returned if the requester mode of the calling thread was not KernelMode.


The PathNameLength member in the QUERY_PATH_REQUEST structure exceeds the maximum allowable length, UNICODE_STRING_MAX_BYTES, for a Unicode string.


If the prefix resolution operation failed due to invalid or incorrect credentials, the provider should return the exact error code returned by the remote server; these error codes must not be translated to STATUS_BAD_NETWORK_NAME or STATUS_BAD_NETWORK_PATH. Error codes like STATUS_LOGON_FAILURE and STATUS_ACCESS_DENIED serve as a feedback mechanism to the user indicating the requirement to use appropriate credentials. These error codes are also used in certain cases to prompt the user automatically for credentials. Without these error codes, the user might assume that the machine is not accessible.

If the network redirector is unable to resolve a prefix, it must return an NTSTATUS code that closely matches the intended semantics from the above list of recommended NTSTATUS codes. A network redirector must not return the actual encountered error (STATUS_CONNECTION_REFUSED, for example) directly to MUP if the NTSTATUS code is not from the above list.

The length of the prefix claimed by the provider depends on an individual UNC provider. Most providers usually claim the \\<servername>\<sharename > part of a path of the form \\<servername>\<sharename>\<path>. For example, if a provider claimed \\server\public given a path \\server\public\dir1\dir2, all name-based operations for the prefix \\server\public (\server\public\file1, for example) will be routed to that provider automatically without any prefix resolution since the prefix is already in the prefix cache. However, a path with the prefix \server\marketing\presentation will go through prefix resolution.

If a network redirector claims a server name (\\server, for example), all requests for shares on this server will go to this network redirector. This behavior is only acceptable if there is no possibility of another share on the same server being accessed by a different network redirector. For example, a network redirector claiming \\server of a UNC path will prevent access by other network redirectors to other shares on this server (WebDAV access to \\server\web, for example).

Any legacy network redirector (not based on using RDBSS) that registers as a UNC provider with MUP by calling FsRtlRegisterUncProvider will receive the IOCTL_REDIR_QUERY_PATH request.

A network mini-redirector that indicates support as a UNC provider will receive this prefix claim as if it were an IRP_MJ_CREATE call. This create request is similar to a user-mode Createfile call with FILE_CREATE_TREE_CONNECTION flag set on. A network mini-redirector will not receive the prefix claim as a call to MRxLowIOSubmit[LOWIO_OP_IOCTL]. For a prefix claim, RDBSS will send an MRxCreateSrvCall request to the network mini-redirector followed by a call to MRxSrvCallWinnerNotify and MRxCreateVNetRoot. When a network mini-redirector registers with RDBSS, the driver dispatch table for the network mini-redirector will be copied over by RDBSS to point to internal RDBSS entry points. RDBSS then receives this IOCTL_REDIR_QUERY_PATH internally for the network mini-redirector and calls MRxCreateSrvCall, MRxSrvCallWinnerNotify, and MRxCreateVNetRoot. The original IOCTL_REDIR_QUERY_PATH IRP will be contained in the RX_CONTEXT structure passed to the MRxCreateSrvCall routine. In addition, the following members in the RX_CONTEXT passed to MRxCreateSrvCall will be modified:

The MajorFunction member is set to IRP_MJ_CREATE even though the original IRP was IRP_MJ_DEVICE_CONTROL.

The PrefixClaim.SuppliedPathName.Buffer member is set to the FilePathName member of the QUERY_PATH_REQUEST structure.

The PrefixClaim.SuppliedPathName.Length member is set to the PathNameLength member of the QUERY_PATH_REQUEST structure.

The Create.NtCreateParameters.SecurityContext member is set to the SecurityContext member of the QUERY_PATH_REQUEST structure.

The Create.ThisIsATreeConnectOpen member is set to TRUE.

The Create.Flags member has the RX_CONTEXT_CREATE_FLAG_UNC_NAME bit set.

If the network mini-redirector wants to see details of the prefix claim, it can read these members in the RX_CONTEXT passed to MRxCreateSrvCall. Otherwise it can just attempt to connect to the server share and return STATUS_SUCCESS if the MRxCreateSrvCall call was successful. RDBSS will make the prefix claim on behalf of the network mini-redirector.

There is one case where a network mini-redirector could receive this IOCTL directly. A network mini-redirector could save a copy of its driver dispatch table before initializing and registering with RDBSS. After calling RxRegisterMinirdr to register with RDBSS, the network mini-redirector could save a copy of the new driver dispatch table entry points installed by RDBSS and restore its original driver dispatch table. The restored driver dispatch table would need to be modified so that after checking the received IRP for those of interest to the network mini-redirector, the call is forwarded to the RDBSS driver dispatch entry points. RDBSS will copy over the driver dispatch table of a network mini-redirector when the driver initializes RDBSS and calls RxRegisterMinrdr. A network mini-redirector that links against rdbsslib.lib must save its original driver dispatch table before calling RxDriverEntry from its DriverEntry routine to initialize the RDBSS static library and restore its driver dispatch table after calling RxRegisterMinrdr. This is because RDBSS copies over the network mini-redirector dispatch table in both the RxDriverEntry and RxRegisterMinrdr routines.

The order in which providers are queried during prefix resolution is controlled by the REG_SZ ProviderOrder registry value stored under the following key:


Individual provider names in the ProviderOrder registry value are separated by commas without any leading or trailing white space.

For example, this value could contain the string:


Given a UNC path \\<server>\<share>\<path>, MUP issues a prefix resolution request if the prefix (\\server\share or \\server, for example) is not found in the MUP prefix cache. MUP sends a prefix resolution request to each provider in the following order until a provider claims the prefix (or all providers have been queried):

  1. TS client (RDPNP)

  2. SMB redirector (LanmanWorkstation)

  3. WebDAV redirector (WebClient)

Changes to the ProviderOrder registry value require a reboot to take effect in MUP on Windows Server 2003, Windows XP, and Windows 2000.

MUP uses each provider name listed to find the provider's registry key under the following registry key:


MUP then reads the DeviceName value under the NetworkProvider subkey to find the device name with which the provider will register. When the provider actually registers, MUP matches the device name passed in with the list of device names of known providers and places the provider in an ordered list for the purposes of prefix resolution. The order of providers in this list is based on the order specified in the ProviderOrder registry value discussed above.

Note that this provider order is also honored by the Multiple Provider Router (MPR), the user-mode DLL that establishes network connections based on queries to WNet providers.

Prior to Windows Server 2003 with Service Pack 1 and Windows XP with Service Pack 2, MUP behavior was to issue prefix resolution requests to all the providers "in parallel" in the order specified in the ProviderOrder registry value, and then wait for all of the providers to complete the prefix resolution operation. Thus, even if the first provider claimed the prefix, MUP still waits for all the other providers to complete the prefix resolution operation. When multiple providers respond with a prefix claim, MUP selects the provider based on the order specified in ProviderOrder registry value.

On Windows XP Service Pack 2 and later and on Windows Server 2003 Service Pack 1 and later, this behavior changed slightly. MUP issues the prefix resolution request serially and stops as soon as the first provider claims the prefix. Thus, in the above example, if RDPNP claims a prefix, MUP will not call the SMB or WebDAV redirectors.

The primary reason this behavior was changed is that a "serial prefix resolution" scheme prevents cases of a network redirector with lower priority in the ProviderOrder value from causing performance issues for a network redirector of higher priority in the ProviderOrder value. For example, consider a remote server, with a firewall in place, configured to block certain types of TCP/IP packets (access to HTTP, for example), but to allow others (SMB access, for example). In this case, even if the SMB network redirector is configured as the first provider in the ProviderOrder value and claims the prefix quickly, the WebDAV redirector might significantly delay the completion of the prefix resolution by waiting for the TCP connection to timeout.



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