What is a service wrapper ?For a service to operate it has to present a certain interface to the operating system, f.ex. so the service can be stopped and started. The implementation of this interface is very similar for many services, so to minimize the size and complexity of the actual service it is placed inside a service wrapper. The service wrapper also saves resources by only using a single proces to have several services running.
When having problems with a service wrapper, then it is usually caused by one or more of the services running within the context of the service wrapper.
Which service wrappers exist ?Some of the common service wrappers, which can be seen in the Task Manager:
- Services.exe : Services Control Manager for system services.
- Lsass.exe : Local Security Authority SubSystem for security services. More Info MS KB Q308356
- Svchost.exe : Service Host for loading dynamic-link libraries (DLL) as a services
- Dllhost.exe : DCOM DLL Host Process (Aka. COM Surrogate)supports DLL based COM objects. (Replaces Mtx.exe)
- Svrany.exe : Applications as Services Utility. Related Install an application as a Windows service.
More Info MSDN - Componentized Windows Services
Why does the service wrappers request access to the Internet ?When a service inside a service wrapper needs to access the network, then has to go through the service wrapper, so the service wrapper becomes the one trying to get access. Therefore one should be careful before using a firewall to block network access for a service wrapper. Usually one can see what service that wants access, by looking at the port number:
- DNS Client (Port 53)
- DHCP Client (Port 67)
- Windows Time (Port 123)
- SSDP Discovery Service (Port 1900/5000)
Note to see the processes along with the port numbers used:
- WinXP (Will show open ports and the process-id they belong to)
Note to convert the process-id to a process-name use TList/Tasklist (See below), and if the process is a service wrapper then one can also see the services running within the service wrapper.
- WinXP SP2 (Will show open ports and the process-name and component they belong to)
- CurrPorts by Nirsoft is a GUI version of netstat that works with WinNT/2k/XP
More Info MS Reskit: TCP and UDP Port Assignments
How to see the services currently loaded by the wrapper ?The Task Manager will only show the wrapper process itself and not the actual services. Instead use this command to see what services are hidding behind the process names:
- Open the Task Manager (Hold down the keys CTRL + ALT + ESC) and right-click the service-wrapper proces and select "Go to Service(s)"
- WinNT4/Win2k (Found on CD in X:\Support\Tools):
Why is Svchost.exe listed several times in the task list ?This is because Svchost.exe can load different groups of services, depending of the different user priviledges needed for a certain group or for isolating critical services for increased stability and ease of debugging.
To see the different groups of which Svchost uses go here with a registry editor (REG_MULTI_SZ):
Note it is possible to create a new group, and move a service started with Svchost into this group. This can be useful to diagnose which in a group of services is causing unwanted behavior like high CPU / RAM usage.
[HKEY_LOCAL_MACHINE \Software \Microsoft \Windows NT \CurrentVersion \Svchost]
NetworkService = "..." (Network User)
LocalService = "..." (Local User)
netsvcs = "..." (System User)
rpcss = "RpcSs" (System User, Critical Service)
- Find the group that contains the service to investigate (ex. netsvcs)
- Create a new group by adding another REG_MULTI_SZ value (ex. netsvcs2)
- If there exists a sub-key with the name as the group (ex. netsvcs), then make a copy of the sub-key and give it the new name (ex. netsvcs2)
- Edit the REG_MULTI_SZ for the new group (ex. netsvcs2) and add the service to investigate (ex. wuauserv)
- Edit the REG_MULTI_SZ for the original group (ex. netsvcs) and remove the service (ex. wuauserv)
- Go to the registry key for the service (ex. wuauserv) and change the ImagePath value to reflect the new group (ex. svchost.exe -k netsvcs2)
More Info MS KB Q314056
What could cause a service wrapper to crash ?If a single service running within the service wrapper performs a faulty operation, then it is possible for that single service to take down the whole service wrapper along with all other services running within the service wrapper. This can be experienced when open to these exploits:
- Blaster RPC Exploit (Services.exe / Svchost.exe)
- Sasser LSA Exploit (Lsass.exe)
- Graweg NetApi32 Exploit (Services.exe / Svchost.exe)
Why is the service wrapper using 100 percent of the CPU ?This is not normal behavior and is usually caused by a malfunctioning service running inside the service wrapper. To diagnose what service is causing the trouble:
- Look in the Event Log for error messages, and if error messages are present then use them as starting point.
- If no error messages then one can try to use the Windows Task Manager to find the service-wrapper causing the trouble, and then use the technique described above for discovering what services resides inside the service-wrapper.
- Some trojans, spyware, malware software tries to hide themselves by using executable name of a service wrapper. Make sure the system is not infected.
- If no clear evidence of what service is causing the trouble, try to close down each service in the service wrapper one by one and see when the problem goes away (be careful with critical services). Another way is to move each service out in its own service wrapper by using the method described above.
- These services are usually causing trouble for people:
Updated: 13 February 2009