装了个CentOS4.3Server，只有一张CD，装完后安装pptpd（http://www.poptop.org/），按照文档http://www.members.optushome.com.au/~wskwok/poptop_ads_howto_1.htm 安装。安装dkms-2.0.10-1.noarch.rpm成功，安装kernel_ppp_mppe-1.0.2-3dkms.noarch.rpm时出现：
DKMS: add Completed.
Module build for the currently running kernel was skipped
since the kernel source for this kernel does not seem to be
installed.
上网狂搜一轮，找到一个如何安装kernel源码和建立build环境的帖子（http://www.centos.org/modules/newbb/viewtopic.php?topic_id=4072），照着做了，出错依旧。又找到一个帖子说yum install kernel-devel就行了，结果还是不行。哦，我的uname -r是2.6.9-34.ELsmp，yum list|grep kernel看一下，还有个kernel-smp-devel，那就yum install kernel-smp-devel吧，怎么还不行？到底什么情况下才会出这个错误信息呢，再找，在kernel_ppp_mppe的安装源码目录/var/lib/dkms/kernel_ppp_mppe/1.0.2/source下（安装过程中有提示）找到一个kernel_ppp_mppe.spec文件中有这样一段：
if [ `uname -r | grep -c "BOOT"` -eq 0 ] && [ -e /lib/modules/`uname -r`/build/include ] ; then
dkms build -m kernel_ppp_mppe -v %{version}
dkms install -m kernel_ppp_mppe -v %{version}
elif [ `uname -r | grep -c "BOOT"` -gt 0 ]; then
echo -e "Module build for the currently running kernel was skipped"
echo -e "since you are running a BOOT variant of the kernel."
else
echo -e "Module build for the currently running kernel was skipped"
echo -e "since the kernel source for this kernel does not seem to be"
echo -e "installed."
原来不存在/lib/module/`uname -r`/build/include时就会报这个错。我的uname -r是2.6.9-34.ELsmp，在/lib/modules/2.6.9-34.ELsmp下根本没有build目录，可/lib/modules/2.6.9-34.0.2.ELsmp下有一个叫build的symbolic link指向/usr/src/kernels/2.6.9-34.0.2.EL-smp-i686，再看看我刚才装的kernel-smp-kernel版本是2.6.9-34.0.2，难道是版本升级了的问题？在/lib/modules/2.6.9-34.ELsmp下ln -s /usr/src/kernels/2.6.9.34.0.2.ELsmp build，再rpm -ivh kernel_ppp_mppe-1.0.2-3dkms.noarch.rpm，一切正常，modprobe ppp-compress-18 && echo ok 显示ok，终于成功了。
升级ppp到2.4.3（pptpd1.3.0要求ppp2.4.3以上）：rpm -Uvh ppp-2.4.3-5.rhel4.i386.rpm
安装pptpd：rpm -ivh pptpd-1.3.0-0.i386.rpm
终于搞定了。