This is the Winsock Programmer's FAQ. See the Introduction section for history, a list of mirrors, and information about the maintainer.

This FAQ answers the most commonly-asked questions on the alt.winsock.programming and comp.os.ms-windows.programmer.tools.winsock newsgroups. The FAQ also contains a growing repository of Winsock programming information and links useful for all levels of programmers. Please email me if you have any corrections or additions for the list.

If you would like to view this FAQ off-line, you can download a ZIPped version (321 KB, last packaged 2000.08.13) of these pages.

What's New?

2000.08.13

Added info about WinPCap to the packet capturing FAQ item. (WinPCap is a free library used for writing packet sniffers.)

2000.08.12

Merged three items in the General section that talked about Winsock versions and where to get Winsock. They really were just several parts of the same question.

Updated the "When is the next revision of the Winsock spec due out?" item to note that Winsock is slowly evolving, though no new specs are coming out.

2000.08.10

Fixed a potential bug in the error message lookup function in the basic example programs' common ws_util.cpp module. (STL's lower_bound() doesn't always return "one past the end" when it fails to find what you asked for.)

Added a pair of items on peeking and out-of-band (OOB) data. These are related because they're both enabled by optional flags to recv() and send().

Alun Jones sent me a whole spate of small additions, and clarifications pointing out the places where what I meant to say and what the FAQ actually says were at odds. Several real technical problems were corrected, too.

2000.07.04

There is an implementation of the Unix poll() function for Winsock. Added info about it to the BSD compatibility article.

There were two places mentioning the RPC method of getting the machine's MAC address. Microsoft has broken this feature in at least one patch for Windows 95 and in Windows 2000. Since the feature wasn't all that reliable anyway, these references have been replaced with warnings not to use the feature. I could have just removed the references, but the feature is well-known and often recommended because it's so simple. It's the FAQ's place to stand up and say, "This is the wrong answer for this question."

2000.07.03

Added an asynchronous I/O client program to the Basic Winsock Examples section.

Added an asynchronous I/O client based on CAsyncSocket instead of calling the Winsock async API directly. These two examples are useful in and of themselves, but they're also interesting to compare to each other to see how the MFC wrapper helps (and hinders!) over the plain API.

As a result of writing those two programs, I updated the CSocket Considered Harmful article.

Some kind readers pointed out that there is in fact an SSL mechanism built into Windows NT 4.0 SP4+ and Windows 2000. The SSL FAQ item has been updated to reflect this.

2000.06.29

Updated the item on getting the machine's MAC address. It was becoming quite inaccurate.

Added an item discussing how to get SSL encryption for your programs.

2000.06.28

Added an item discussing the various reasons why NT Server is the only legitimate server platform for high loads.

Updated the connection backlog item a bit: added info on the backlog limitations in various MS OSes, and discussed why and when this matters.

Added two new items on UDP: "What is UDP? What are its limitations?", and "What is UDP good for?".

Split the links to related FAQs out of the Web Pages resource section into their own section.

2000.06.27

Added an item regarding the proper value for a server's connection backlog.

2000.06.20

Fixed the description of the Nagle algorithm. I blame the store that sold me all that Jolt. They should never let me buy such things. :)

Previous "What's New" Entries...

Contents

Section 0 - Introduction

0.1 The Original Introduction
0.2 A Bit About the Maintainer
0.3 Legal Stuff
0.4 Mirrors
0.5 What if your question is not in the FAQ?
0.6 Is there a mailing list for FAQ updates?
0.7 Language Bias
0.8 Protocol Bias
0.9 Acknowledgements

Section 1 - General Winsock Information

1.1 What is Winsock?
1.2 What's the difference between Winsock and TCP/IP?
1.3 What newsgroups and mailing lists exist for Winsock and TCP/IP?
1.4 What versions of Winsock are available?
1.5 Is there a version of Winsock for { OS/2, DOS, Unix, etc. }?
1.6 What does Winsock 2 have that Winsock 1.1 doesn't?
1.7 When is the next rev of the specification due out?
1.8 Can Winsock speak { DECNet, IPX/SPX, etc. }?

Section 2 - Issues for New Winsock Programmers

2.1 Are there any sample apps on the Net?
2.2 Do I need to initialize the WSAData structure before calling WSAStartup?
2.3 I'm getting link errors when compiling Winsock programs. What's wrong?
2.4 If I write a Winsock program, will I be able to communicate with a Unix Sockets program?
2.5 Can I use Winsock with { My Favorite Language }?
2.6 Are there any tools available for debugging Winsock programs?
2.7 How do I get a readable error message from a Winsock error number?
2.8 Winsock keeps returning the error WSAEWOULDBLOCK. What's wrong with my program?
2.9 How can I test my Winsock application without setting up a network?
2.10 What's the proper way to close a TCP socket?
2.11 Is it possible to close the connection "abnormally"?
2.12 How do I detect when my TCP connection is closed?
2.13 How do I detect an abnormal network disconnect?
2.14 How can I change the timeout for a Winsock function?
2.15 What is peeking (MSG_PEEK), and why is it bad?
2.16 What is out-of-band data (MSG_OOB), and why is it bad?
2.17 If MSG_PEEK and MSG_OOB are bad, what do I pass for send() and recv()'s flags parameter?

Section 3 - Intermediate Winsock Issues

3.1 How do I speak { HTTP, POP3, SMTP, FTP, Telnet, NNTP, etc. } with Winsock?
3.2 How can I encrypt my TCP stream with SSL/TLS?
3.3 How do I get my IP address from within a Winsock program?
3.4 What's the proper way to impose a packet scheme on a stream protocol like TCP?
3.5 How do I write my program to work through a firewall?
3.6 What if the firewall does not support SOCKS?
3.7 I'm writing a server. What's a good network port to use?
3.8 What is UDP? What are its limitations?
3.9 What is UDP good for?
3.10 How do I send a broadcast packet?
3.11 Is Winsock thread-safe?
3.12 If two threads in an application call recv() on a socket, will they each get the same data?
3.13 Is there any way for two threads to be notified when something happens on a socket?
3.14 How do I detect if the modem is connected?
3.15 How can I get the local username?
3.16 Windows 9x's Dial Up Networking keeps popping up an automatic dial window, even when it isn't necessary. Can I make it stop?
3.17 I've heard that asynchronous sockets are unreliable. Is this true?
3.18 What is the Nagle algorithm?
3.19 When should I turn off the Nagle algorithm?
3.20 What is TCP's sliding window?
3.21 What is the silly window syndrome?
3.22 What is the delayed ACK algorithm?
3.23 What platform should I deploy my server on?

Section 4 - Advanced Winsock Issues

4.1 How can I open a raw data socket?
4.2 How can I capture packets on a LAN with Winsock?
4.3 How can I change the IP or TCP header of a packet?
4.4 How can I "ping" another machine with Winsock?
4.5 How do I pass a socket from one process to another?
4.6 Is it possible to create sockets that map to a DLL rather than an application?
4.7 How can I get access to the {route, ARP, interface, etc.} table?
4.8 How do I get the MAC (a.k.a. hardware) address of the local Ethernet adapter?
4.9 How many simultaneous sockets can I have open with Winsock?
4.10 What are the "64 sockets" limitations?
4.11 How do I make Winsock use a specific network interface?
4.12 What is the { SYN, ACK, FIN, RST } bit?
4.13 Is it a bad idea to bind() to a particular port in a client program?
4.14 What is the TCP window size?
4.15 What is the connection backlog?

Section 5 - Resources

5.1 Winsock Libraries and Controls
5.2 Sample Winsock Applications and Tutorials
5.3 Debugging Tools (packet sniffers and shims)
5.4 Useful Web Pages
5.5 Other FAQs
5.6 MS Knowledge Base Articles
5.7 Winsock SDKs and Specifications
5.8 Miscellaneous

Section 6 - Examples

6.1 Several Basic Winsock Programs
6.2 Pinging a Host Using Microsoft's ICMP.DLL
6.3 Pinging a Host Using Raw Sockets
6.4 How to Pass a Socket Handle in Win32
6.5 Getting the Local IP Address
6.6 Getting the Network Interface List
6.7 How to Get the MAC (a.k.a. "hardware") Address, NetBIOS method
6.8 How to Get the MAC (a.k.a. "hardware") Address, SNMP method
6.9 How to Get the MAC (a.k.a. "hardware") Address, RPC method
6.10 Finding the Maximum Number of Open Sockets
6.11 How to Get the Local User Name

Section 7 - Articles

Winsock for the Impatient by Warren Young
Which I/O Strategy Should I Use? by Warren Young
How to Use TCP Effectively by Warren Young
The Lame List by the Winsock Vendor Community
Debugging TCP by Warren Young
Winsock's Compatibility With BSD Sockets by Warren Young
WsControl() Revealed by Tom Sanfilippo
CSocket Considered Harmful by Warren Young

Section 8 - Book and Software Reviews

Section 9 - Glossary