Dedicated operating systems for embedded systems are fast being phased out due to their use of manual optimization, which provides high performance and small footprint, but also requires high maintenance and portability costs every time hardware evolves. In this paper, we describe an approach based on customization of generic operating system modules. Our approach uses a remote customization server to automatically generate highly optimized code that is then loaded and executed in the kernel of the embedded device. This process combines the advantages of generic systems software code (leveraging portability and evolution costs) with the advantages of customization (small footprint and low overhead). We have validated our customization infrastructure with a case study: the TCP/IP stack of the Linux kernel. We analyzed the performance and size of the customized code generated on three platforms: a Pentium III (600MHz), an ARM SA1100 (200Mhz) on a COMPAQ iPAQ, and a 486 (40MHz). The customized code runs about 25% faster and its size reduces by up to a factor of 20. The throughput of the protocol stack improves by up to 21%.