Practical UNIX and Internet Security, 3rd Edition

Tips on Writing Network Programs

If you are coding a new network service, there are also a number of pitfalls you will need to consider.

Things to Do

1. Do a reverse lookup on connections when you need a hostname for any reason. After you have obtained a hostname to go with the IP address you have, do another lookup on that hostname to ensure that its IP address matches what you have.

2. Include some form of load shedding or load limiting in your server to handle cases of excessive load. Consider what should happen if someone makes a concerted effort to direct a denial of service attack against your server. For example, you may wish to have a server stop processing incoming requests if the load goes over some predefined value.

3. Put reasonable timeouts on each network-oriented read request. A remote server that does not respond quickly may be common, but one that does not respond for days may hang up your code awaiting a reply. This rule is especially important in TCP-based servers that may continue to attempt delivery indefinitely.

4. Put reasonable timeouts on each network write request. If some remote server accepts the first few bytes and then blocks indefinitely, you do not want it to lock up your code awaiting completion.

5. Make no assumptions about the content of input data, no matter what the source is. For instance, do not assume that input is null-terminated, contains linefeeds, or is even in standard ASCII format. Your program should behave in a defined manner if it receives random ...