一、调试与log
server端: /usr/sbin/sshd -d 用-d开启debug模式查看
client端: ssh -v user@host 用-v开启详细信息查看
日志: /var/log/messages 和 /var/log/secure

二、PAM认证配置
配置 /etc/pam.d/sshd

三、常见问题

主要容易引起问题的几个地方
.ssh/authorized_keys 内容设置与权限设置
selinux开启时authorized_keys无法正常调用的问题
http://forums.fedoraforum.org/showthread.php?t=267960

In case this problem is still applicable;
most (Fedora) systems set up wrong selinux permissions when root generates its ssh keys.
Check the selinux bits of root's ssh keys with "ls -lZ /root/.ssh".
You'll probably have to change the selinux bits using "chcon -R -t ssh_home_t /root/.ssh".
David

另外就是/etc/ssh/sshd_config的配置问题了
详见我另一片文章：/tech/vps-only-ssh-key-login.html