Is this ok [y/N]: y
Downloading Packages
Getting kernel-2.4.21-63.EL.i686.rpm
kernel-2.4.21-63.EL.i686. 100% |=========================| 7.9 MB    09:56     
warning: rpmts_HdrFromFdno: V3 DSA signature: NOKEY, key ID 025e513b
Error: Could not find the GPG Key necessary to validate pkg /var/cache/yum/update/packages/kernel-2.4.21-63.EL.i686.rpm
Error: You may want to run yum clean or remove the file:
 /var/cache/yum/update/packages/kernel-2.4.21-63.EL.i686.rpm
Error: You may also check that you have the correct GPG keys installed

centos 기준으로 설명합니다.

우선 배포판 확인하기
cat /etc/*release


해결하기
배포판이 3.x 일 경우 : rpm --import http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-3
배포판이 4.x 일 경우 : rpm --import http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-4
배포판이 5.x 일 경우 : rpm --import http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5