Layered Service Provider

Layered Service Provider (LSP) is a deprecated feature of the

TCP/IP traffic taking place between the Internet and the applications that are accessing the Internet (such as a web browser, the email client, etc.). For example, it could be used by malware to redirect web browers to rogue websites, or to block access to sites like Windows Update

. Alternatively, a computer security program could scan network traffic for viruses or other threats. The Winsock Service Provider Interface (SPI) API provides a mechanism for layering providers on top of each other. Winsock LSPs are available for a range of useful purposes, including parental controls and Web content filtering. The parental controls web filter in Windows Vista is an LSP. The layering order of all providers is kept in the Winsock Catalog.

Details

Unlike the well-known

TCP/IP

protocol stack. Another use for LSPs would be to develop specialized URL filtering software to prevent Web browsers from accessing certain sites, regardless of the browser installed on a desktop. The Winsock 2 SPI allows software developers to create two different types of service providers—transport and namespace. Transport providers (commonly referred to as protocol stacks) are services, which supply functions that set up connections, transfer data, exercise flow control, error control, and so on. Namespace providers are services that associate the addressing attributes of a network protocol with one or more human-friendly names and enable protocol-independent name resolution. The SPI also allows you to develop two types of transport service providers—base and layered service providers.

Base service providers implement the actual details of a transport protocol: setting up connections, transferring data, and exercising flow control and error control. Layered service providers implement only higher-level custom communication functions and rely on an existing underlying base provider for the actual data exchange with a remote endpoint.

Winsock 2 LSPs are implemented as Windows

function, WSPStartup. All other transport SPI functions are made accessible to ws2_32.dll or an upper chain layered provider via the LSP's dispatch table. LSPs and base providers are strung together to form a protocol chain. The LSP DLL has to be registered using a special LSP registrant which instructs Winsock

2, the loading order of the LSPs (there can be more than one LSP installed) and which protocols to intercept.

LSPs work by intercepting Winsock 2 commands before they are processed by ws2_32.dll; they can therefore modify the commands, drop a command, or just log the data which makes them a useful tool for malware, network filters, network intercepters, and stream based sniffers. Sniffing network traffic through LSP can sometimes be troublesome since anti-virus vendors typically flag such activity as malicious — a network packet analyzer is therefore a better alternative for capturing network traffic.

A feature of LSP and Winsock proxy sniffing is that they allow traffic to be captured from a single application and also enable traffic going to localhost (127.0.0.1) to be sniffed on Windows.^[1]

There are two kinds of LSP: IFS and non IFS LSP. Currently most LSPs on the market are non IFS. The difference between the two LSPs is that non IFS LSPs modify the

socket handle to a non valid Windows IFS handle and therefore the LSP must implement all Winsock

2 methods. IFS LSPs, on the other hand, preserve the socket handle, which allows the LSP to implement only the functions it wants to intercept. IFS LSPs have much less performance impact than non IFS LPS, but they are limited by the fact that they cannot inspect or modify data on the receive path.

Deprecation and LSP bypass

LSPs have been deprecated since Windows Server 2012.^[2] Systems that include LSPs will not pass the Windows logo checks.^[3] Windows 8 style "metro" apps that use networking will automatically bypass all LSPs. The Windows Filtering Platform provides similar functionality and is compatible with both Windows 8 style "metro" apps and conventional desktop applications.

Corruption issues

A major issue with LSPs is that any bugs in the LSP can cause applications to break. For example, an LSP that returns the wrong number of bytes sent through an interface can cause applications to go into an infinite loop while waiting for the network stack to indicate that data has been sent.

Another major common issue with LSPs was that if they were to be removed or unregistered improperly or if the LSP was buggy, it would result in corruption of the Winsock catalog in the registry, and the entire TCP/IP stack would break and the computer could no longer access the network.

LSP technology is often exploited by

spam

e-mail. If a malware LSP is not removed correctly, older versions of Windows may be left without a working network connection.

Such potential loss of all network connectivity is prevented in Windows XP Service Pack 2, Windows Server 2003 Service Pack 1 and all later Windows operating systems, in which Winsock has the ability to self-heal after a user uninstalls such an LSP.^[4]

Installed LSPs can be viewed using the XP/Vista

Windows Defender

's Software Explorer or using third-party utilities.

References

External links

[1] "Proxocket - A Winsock Proxy Sniffer", NETRESEC Network Security Blog, 2011

[2] "Features Removed or Deprecated in Windows Server 2012".

[3] "Device.DevFund Requirements".

[4] How to determine and recover from Winsock2 corruption

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