Network Discovery and Name Resolution under Windows 10 in a Home Network (zero-configuration networking)

Network discovery (for example, the “discovery” performed by the “Network” item in the left (navigation) pane of “File Explorer” under Windows 10) in a home network should “just work” in the sense of discovering and displaying the network devices that are known to be in the home network. However, one often reads in technical support forums that “network discovery” is not working to some extent; sometimes this results from outdated expectations (for example, that the “net view” command is the full extent of “network discovery”) but sometimes also from old network equipment that does not support newer discovery mechanisms or from network equipment that has been configured not to respond to network discovery requests (perhaps for security reasons).

Let’s first consider how “network discovery” works and what can be done to influence its behaviour.

The Microsoft interface IFunctionDiscovery is the entry point into performing network discovery in the same style as File Explorer. The method CreateInstanceCollectionQuery of this interface is called first with either a “layered category” (e.g. "Layered\Microsoft.Networking.Devices") which will use a collection of providers appropriate to the layer or a “provider category” (e.g. “Provider\Microsoft.Networking.WSD”) which will use a specific provider/technology/protocol.

Some of the providers that are relevant to discovering networking devices are:

Provider\Microsoft.Networking.WSD
Provider\Microsoft.Networking.SSDP
Provider\Microsoft.Networking.Netbios

Network discovery can take some time, so the method that executes the discovery normally returns a “pending” status (E_PENDING) and delivers discovery results to its caller asynchronously (as they happen). The main work of discovery is performed in the “Function Discovery Provider Host” (fdPHost) service.

One piece of advice that one often sees on the Internet is to ensure that Windows services used in the discovery process are running and/or configured to run. This is not something that I would recommend. The relevant services (e.g. fdPHost, FDResPub, SSDPSRV) are normally configured as “demand” start; some may also include “trigger” configuration (e.g. FDResPub triggers on specific event values of the Microsoft-Windows-NetworkProfileTriggerProvider ETW provider); some are defined as “dependencies” for other services; some services explicitly start other services. The ability of a service to operate is also often dependent on Windows Firewall rules (that are also actively maintained and changed as system events occur). Manual interference should be a last step, guided by evidence that there is actually a misconfiguration, rather than a first/early troubleshooting step.

The progress of network discovery can be followed using ETW. A combination of the providers Microsoft-Windows-FunctionDiscovery, Microsoft-Windows-WFP (to check for firewall packet drops) and Microsoft-Windows-PktMon (or equivalent, to observe the actual network protocol interactions) is often a good combination.

Web Services Dynamic Discovery (WS-Discovery or WSD)

The Microsoft.Networking.WSD provider is the provider most likely to detect computers and file servers on the home network. During the discovery operation, the fdPHost service sends WSD Probe messages to the WSD IPv4 and IPv6 multicast addresses defined by the WSD protocol. If and when the fdPHost receives a ProbeMatch message, it sends a Get request to the responder (via TCP) to obtain a Get response. In the case of Windows computers, the responder is the FDResPub (Function Discovery Resource Publication) service

The key information in the Get response is contained within the wsdp:Relationship/wsdp:Host/pub:Computer element. As the [MS-PBSD] document says, if the computer is domain joined then the value will be of the form “<NetBIOS_Computer_Name>/Domain:<NetBIOS_Domain_Name>”, if the computer is in a workgroup then the value will have the form “<NetBIOS_Computer_Name>\Workgroup:<Workgroup_Name>”, otherwise it will have the form “<NetBIOS_Computer_Name>\NotJoined”.

Network Discovery via IFunctionDiscovery finds all of these variants and File Explorer displays all of the results that represent domain joined or workgroup computers, but it does not display computers that report “not joined”. FDResPub uses the NetGetJoinInformation API to obtain workgroup/domain information; it normally obtains the information when the service starts, so if the LanmanWorkstation service (which serves the NetGetJoinInformation request) has not (completely) started when FDResPub calls NetGetJoinInformation, then the published information will state that the computer is “not joined”. 

A workaround for the above problem is to add a service dependency to the FDResPub service on the LanmanWorkstation service. The problem could be called a “bug” and it has a simple source code fix. FDResPub calls NetGetJoinInformation specifying the name of the local computer as the system for which the information should be retrieved; if NetGetJoinInformation fails with RPC_S_SERVER_UNAVAILABLE and a system name was specified then a failure code is returned to the caller (NERR_WkstaNotStarted), but if no system name was specified (a null was passed as parameter, implying the local system) then NetGetJoinInformation uses other local mechanisms to obtain join information and returns a success code to the caller.

This discovery mechanism should discover all devices (Windows, Apple, Linux, Network Attached Storage (NAS), etc.) that support WS-Discovery, have a WS-Discovery publisher service running and are not blocking WS-Discovery messages via firewall mechanisms.

For Windows systems, the “Network and Sharing Centre, Advanced sharing settings” dialog (on each Windows system in the home network) should be the only thing that needs to be checked to ensure that network discovery is correctly configured.

Simple Service Discovery Protocol (SSDP)

The Microsoft.Networking.SSDP provider “discovers” most of the printers, scanners, displays, etc. in the home network. The SSDPSRV service periodically multicasts SSDP M_SEARCH requests and observes SSDP NOTIFY announcements. When network discovery is started, fdPHost retrieves a list of responses from SSDPSRV via RPC. The fdPHost then retrieves detailed information about the service by querying the Location URL in the SSDP response. For services hosted on Windows systems (perhaps directly attached printers, music and video libraries, etc.), the upnphost (UPnP Device Host) service is normally the process that is listening at the Location URL.

NetBIOS

The Microsoft.Networking.Netbios provider essentially performs a classic “net view” command, using the WNetOpenEnum/WNetEnumResource/WNetCloseEnum API.

A prerequisite for this resolution mechanism is that NetBIOS over TCP/IP is enabled. By default, the relevant setting is set to “Use NetBIOS from the DHCP server. If static IP address is used or the DHCP server does not provide NetBIOS setting, enable NetBIOS over TCP/IP”.

If SMBv1 is installed, then this method should produce the classically expected results. If SMBv1 is not installed/enabled then this discovery method will only work in the computer has been elected as the “Master Browser” of a workgroup.

If the local computer is not the Master Browser, then the local computer will try to negotiate a connection with the Master Browser. Normally, the newest SMB protocol version available to both parties will be negotiated – typically SMBv3. From a network trace perspective, it seems as though the negotiation has been concluded successfully, but post processing by the client causes the connection to be disconnected.

The stack on the client (local computer) when a disconnection is initiated looks like this:

mrxsmb!SmbCeDisconnectServerConnections+0x2d6:
mrxsmb20!MRxSmb2HandOverSrvCall+0x2054:
mrxsmb!SubRdrClaimSrvCall+0x90:
mrxsmb!SmbCeCompleteSrvCallConstructionPhase2+0x146:
mrxsmb!SmbCeCompleteServerEntryInitialization+0x176:
mrxsmb!SmbCeCompleteNegotiatedConnectionEstablishment+0x155:
mrxsmb!SmbNegotiate_Finalize+0x5b:

Some code in mrxsmb20!MRxSmb2HandOverSrvCall decides that a disconnect is necessary and a quick look at that routine shows that the condition is ConnectionType == Tdi. Possible values for ConnectionType are Tdi (TDI - Transport Driver Interface), Wsk (Windows Kernel Sockets), Rdma (Remote Direct Access Memory) and VMBUS.

TDI is a deprecated technology and is used by "NetBIOS over TCP/IP" (netbt.sys). It seems as though the client will refuse to use SMBv2/3 in conjunction with "NetBIOS over TCP/IP".

If the local computer is the Master Browser, it has access to the list of servers via local mechanisms and the results are made available to the user of IFunctionDiscovery. Users of IFunctionDiscovery, such as Windows File Explorer, typically recognize that some systems have been discovered by more than one mechanism (perhaps WSD and NetBIOS) and display just a single entry for such systems in their user interface.

Name Resolution

If network discovery fails to discover some resource (for example, a file server), it may still be possible to reference the resource by name (rather than by IP address; IP addresses are typically not permanently assigned but rather leased, so it is difficult to be certain of the IP address in the long term in a home network). Name resolution uses different protocols to network discovery and these may well work, even if discovery has failed.

Windows uses 3 mechanisms to resolve names: multicast DNS (mDNS), Link-Local Multicast Name Resolution (LLMNR) and NetBIOS Name Service (NBNS). Name resolution via all applicable mechanisms is normally started in parallel (i.e. the mechanisms are not tried sequentially, waiting for one method to fail before the next is tried). If NetBIOS over TCP/IP is disabled or the name being queried is not NetBIOS compatible (e.g. it is longer than 15 characters) then the NetBIOS Name Service resolution method is not used.

PktMon

Judging by Web search results, Windows 10 has included a new network traffic capturing mechanism since October 2018; however, two and a half years later, it still seems to be largely unknown (I only discovered it a few days ago).

Microsoft provides and supports a “classic” NDIS Filter traffic capturing mechanism (NdisCap, its associated Microsoft-Windows-NDIS-PacketCapture ETW provider and a PowerShell cmdlet Add-NetEventPacketCaptureProvider) and previously also supported a Windows Filtering Platform capture mechanism (WFPCapture, its associated Microsoft-Pef-WFP-MessageProvider ETW provider and a PowerShell cmdlet Add-NetEventWFPCaptureProvider). The new mechanism (PktMon and its associated Microsoft-Windows-PktMon ETW provider) does not yet have a PowerShell cmdlet to add it to ETW tracing sessions.

According to the PktMon home page, “[PktMon] is especially helpful in virtualization scenarios, like container networking and SDN, because it provides visibility within the networking stack”. The mechanism that allows PktMon to intercept a packet at various points in its transition through the network stack are additional “hooks” introduced into NDIS.sys. Some typical stack traces of the points at which PktMon is invoked are:

PktMon!PktMonPacketLogCallback+0x19
ndis!PktMonClientNblLog+0xbd
ndis!PktMonClientNblLogNdis+0x2b
ndis!ndisCallSendHandler+0x3ca4b
ndis!ndisInvokeNextSendHandler+0x10e
ndis!NdisSendNetBufferLists+0x17d

PktMon!PktMonPacketLogCallback+0x19
ndis!PktMonClientNblLog+0xbd
ndis!PktMonClientNblLogNdis+0x2b
ndis!ndisMIndicateNetBufferListsToOpen+0x3e95c
ndis!ndisMTopReceiveNetBufferLists+0x1bd
ndis!ndisCallReceiveHandler+0x61
ndis!ndisInvokeNextReceiveHandler+0x1df
ndis!ndisFilterIndicateReceiveNetBufferLists+0x3be91
ndis!NdisFIndicateReceiveNetBufferLists+0x6e

PktMon can be seen as an improvement on NdisCap. The main advantage (in my opinion) is that PktMon can be loaded and started without requiring rebinding of the network stack. As I have mentioned in other articles, rebinding the network stack can, under unfortunate circumstances, be a risky undertaking. The new ability to intercept packets at various points in the network stack is something that I have never personally had a need to use but is probably welcomed by those who have had difficulty in diagnosing network problems in “virtualization scenarios”.

One thing that PktMon cannot do is to trace loopback traffic, since the Windows loopback implementation does not use NDIS (WFP mechanisms and raw sockets can capture such traffic).

There are 3 main components of PktMon: the driver (PktMon.sys), a DLL (PktMonApi.dll) and an executable (PktMon.exe).

PktMon.sys

PktMon.sys is the core component. It is controlled via a small set of IOCTLs (to start, stop and query a capture; add, remove and list packet filters; list traceable components; reset trace counters) and the keywords used in the ETWENABLECALLBACK (Config, Rundown, NblParsed, NblInfo and Payload).

The information in the list of traceable components will seem familiar to anyone who has used the kernel debugger extensions “!ndiskd.miniports”, “!ndiskd.protocols” and “!ndiskd.filters”. The list of components is not only available via the IOCTL but is also included (in a different form) at the end of an ETW trace if the “Rundown” keyword is enabled.

The packet filtering possibilities are of the address/protocol-type/port type rather than what Microsoft sometimes calls OLP (Offset value, bit Length, and value Pattern). The output of the command “pktmon filter add help” accurately reflects the filtering possibilities. The filtering mechanism does not allow “negative” conditions to be expressed – for example, one can’t specify “ignore RDP” (as one might wish to do if one is logged onto a system via RDP).

Similar to both NdisCap and WFPCapture, PktMon must be explicitly loaded and/or started before it can generate any events; just starting an ETW trace session containing Microsoft-Windows-PktMon is not enough to capture trace data.

PktMonApi.dll

PktMonApi.dll currently has 9 exports, which are mostly just simple wrappings around IOCTLs to PktMon.sys:

PktmonAddFilter
PktmonGetComponentList
PktmonGetFilterList
PktmonGetStatus
PktmonRemoveAllFilters
PktmonResetCounters
PktmonStart
PktmonStop
PktmonUnload

PktMonApi.dll is not used by PktMon.exe (which contains its own simple wrappings around the IOCTLs).

PktMon.exe

PktMon.exe has several facets: it can configure and control PktMon.sys via its IOCTLs, it can manage ETW trace sessions, it can extract information from the Microsoft-Windows-PktMon ETL and save it in various formats (including “pcapng”), and it can perform “tcpdump” style simple formatting of packets captured and display in real-time.

As mentioned, PktMon.sys must be explicitly managed in order for Microsoft-Windows-PktMon to capture data. It would be ideal (for me) if Microsoft-Windows-PktMon could just be included in a Windows Performance Recorder (WPR) Profile along with other providers and use of advanced ETW features (such as stack traces, SID information, etc.). Since that is not possible, one must be content with the limited ETW configuration options of PktMon.exe (provider, keywords and level) – a similar situation to that with NdisCap and “netsh trace”.

The PktMon home page currently says “Packet drops from Windows Firewall are not visible through Packet Monitor yet” (my emphasis of “yet”). I would not expect Windows Firewall drop detection to be included in PktMon.sys, since they deal with different technologies. By including both Microsoft-Windows-PktMon and Microsoft-Windows-WFP in ETW trace sessions, one can see both the drops types detected by PktMon and the drops caused by Windows Firewall. Perhaps the “yet” is just a nod to future improvements in the PktMon.exe user interface to present a unified view of the output of the two providers.

The configuration defaults of PktMon.exe cause packets to be logged at all interception points (--comp all); if the intent of a capture is just to analyse the traffic in a tool like Wireshark, this can cause each packet to be repeated twenty or more times in the network trace. One can tackle this by selecting specific components when exporting captured data to the pcapng format, but I prefer to use “PktMon start” with the “--comp nics” qualifier.

Microsoft Message Analyzer

Microsoft Message Analyzer (MMA) was discontinued more-or-less contemporaneously with the introduction of PktMon and the PktMon team provides no support for MMA. The OPN (Open Protocol Notation) below allows the ETL output of PktMon to be viewed comfortably in MMA (if one still has a copy installed).

The OPN is short and it works (for me), but it includes some “design” decisions and is incomplete (no support for the MBB (Mobile BroadBand) NDIS medium type, for example, and probably in many unknown/unanticipated ways).

module PktMon;

using Microsoft_Windows_PktMon;
using Standard;
using Ethernet;
using WiFi;

autostart actor PktMonPayload(ep_Microsoft_Windows_PktMon e)
{
    process e accepts m:Event_160 where m.PacketType == 1
    {
                dispatch endpoint Ethernet.Node accepts BinaryDecoder<Ethernet.Frame[m.LoggedPayloadSize < m.OriginalPayloadSize]>(m.Payload) as Ethernet.Frame;
    }

    process e accepts m:Event_160 where m.PacketType == 2
    {
                DecodeWiFiMessageAndDispatch(m.Payload);
    }
}

Saving this OPN in a file named %LOCALAPPDATA%\Microsoft\MessageAnalyzer\OPNAndConfiguration\OPNForEtw\CoreNetworking\PktMon.opn (for example) and restarting MMA should enable the functionality (MMA may take some time to completely start while it recompiles various OPN files).

Microsoft_Windows_PktMon

The Microsoft_Windows_PktMon ETW provider defines 5 keywords:

1.       Config: PktMon.exe help says “Internal Packet Monitor errors”; I have never observed any.

2.       Rundown: this causes the list of components to be logged to the ETL when PktMon is stopped.

3.       NblParsed: this causes address, protocol type, port, etc. information for each packet to be logged. The same information could be extracted from the binary payload (if present), but it is not trivial to do this because of various options at each layer (data link, network, transport).

4.       NblInfo: this seems to be a superset of the information logged as TcpipNlbOob by the Microsoft-Windows-TCPIP provider at level 17 (uninteresting for most people).

5.       Payload: this causes the raw data of the packet to be logged. The data can be truncated, if desired (truncation length information is included in the start IOCTL to PktMon.sys).

pktmon etl2pcap

Converting pktmon packet events to PCAPNG format is, in principle, relatively straightforward. There is a GitHub Microsoft repository with the code of a utility that performs the slightly more difficult task of converting “netsh trace” packet data to PCAPNG format (https://github.com/microsoft/etl2pcapng).

Perhaps surprisingly, the “pktmon etl2pcap” command (“Convert pktmon log file to pcapng format”) only produces a useful PCAPNG file if the packets were captured on an Ethernet/802.3 link. If packets were captured on a WiFi/802.11 link, the resulting PCAPNG packet is recorded as having been captured on an Ethernet link; since the datalink headers are different in content and length, a tool like Wireshark “decodes” the packet data incorrectly.

Apart from correctly identifying 802.11 frames as such (LINKTYPE_IEEE802_11), there is one other addition step needed when saving 802.11 packets to PCAPNG format: the “protected” flag in the 802.11 frame control field needs to be cleared. The “protected” flag indicates whether the frame was encrypted; packets carrying network layer data are protected/encrypted but, by the time received packets have reached the packet capture hooks, the protected/encrypted content has been decrypted – however the captured 802.11 packet header (frame control, etc.) is not updated. If the protected bit is not cleared when saving the capture then, when the capture is loaded into Wireshark, the packet is assumed to still be protected and is displayed as such (no attempt is made to decode the “encrypted” portion of the packet).

pktmon start --trace

PktMon can control (start/stop) other ETW trace providers, as can “netsh trace”, logman and wpr (Windows Performance Recorder) amongst others. However, PktMon differs from the other controllers in the default values for “keywords” and “level” passed to the providers. By default, most providers use “maximal” values for keywords and level but PktMon uses a value of 0xFFFFFFFF for the keywords value (which is actually a 64-bit value, so the high 32 bits are set to zero) and 4 for the level (the maximum value is 255 and Microsoft-Windows-TCPIP, for example, logs some events at level 17).

Although the help/documentation (“pktmon start help”) mentions this, I have been caught out more than once puzzling over why certain expected events were missing from a trace.