在青云上创建centos 6.4后，习惯性的安装emacs.结果遇到报错信息：
# yum install emacs
Loaded plugins: fastestmirror
Determining fastest mirrors
Error: Cannot retrieve metalink for repository: epel. Please verify its path and try again
解决方法是更新ca-certificates：
# yum upgrade ca-certificates --disablerepo=epel
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: mirrors.skyshe.cn
* extras: ftp.sjtu.edu.cn
* rpmfusion-free-updates: ftp.sjtu.edu.cn
* rpmfusion-nonfree-updates: ftp.sjtu.edu.cn
* updates: mirrors.skyshe.cn
base | 3.7 kB 00:00
base/primary_db| 4.6 MB 00:04
emacs | 2.9 kB 00:02
emacs/primary_db | 37 kB 00:01
extras | 3.4 kB 00:00
extras/primary_db | 37 kB 00:00
rpmfusion-free-updates | 2.7 kB 00:00
rpmfusion-free-updates/primary_db | 272 kB 00:00
rpmfusion-nonfree-updates | 2.2 kB 00:00
rpmfusion-nonfree-updates/primary_db | 65 kB 00:00
updates| 3.4 kB 00:00
updates/primary_db | 5.2 MB 00:05
Setting up Upgrade Process
Resolving Dependencies
--> Running transaction check
---> Package ca-certificates.noarch 0:2010.63-3.el6_1.5 will be updated
---> Package ca-certificates.noarch 0:2015.2.6-65.0.1.el6_7 will be an update
--> Processing Dependency: p11-kit-trust >= 0.18.4-2 for package: ca-certificates-2015.2.6-65.0.1.el6_7.noarch
--> Processing Dependency: p11-kit >= 0.18.4-2 for package: ca-certificates-2015.2.6-65.0.1.el6_7.noarch
--> Running transaction check
---> Package p11-kit.x86_64 0:0.18.5-2.el6_5.2 will be installed
--> Processing Dependency: libtasn1.so.3(LIBTASN1_0_3)(64bit) for package: p11-kit-0.18.5-2.el6_5.2.x86_64
--> Processing Dependency: libtasn1.so.3()(64bit) for package: p11-kit-0.18.5-2.el6_5.2.x86_64
---> Package p11-kit-trust.x86_64 0:0.18.5-2.el6_5.2 will be installed
--> Running transaction check
---> Package libtasn1.x86_64 0:2.3-6.el6_5 will be installed
--> Processing Conflict: p11-kit-trust-0.18.5-2.el6_5.2.x86_64 conflicts nss < 3.14.3-33
--> Restarting Dependency Resolution with new changes.
--> Running transaction check
---> Package nss.x86_64 0:3.14.0.0-12.el6 will be updated
--> Processing Dependency: nss = 3.14.0.0-12.el6 for package: nss-tools-3.14.0.0-12.el6.x86_64
--> Processing Dependency: nss = 3.14.0.0-12.el6 for package: nss-sysinit-3.14.0.0-12.el6.x86_64
---> Package nss.x86_64 0:3.21.0-0.3.el6_7 will be an update
--> Processing Dependency: nss-util >= 3.21.0 for package: nss-3.21.0-0.3.el6_7.x86_64
--> Processing Dependency: nss-softokn(x86-64) >= 3.14.3-22 for package: nss-3.21.0-0.3.el6_7.x86_64
--> Processing Dependency: nspr >= 4.11.0 for package: nss-3.21.0-0.3.el6_7.x86_64
--> Processing Dependency: libnssutil3.so(NSSUTIL_3.21)(64bit) for package: nss-3.21.0-0.3.el6_7.x86_64
--> Processing Dependency: libnssutil3.so(NSSUTIL_3.17.1)(64bit) for package: nss-3.21.0-0.3.el6_7.x86_64
--> Processing Dependency: libnssutil3.so(NSSUTIL_3.15)(64bit) for package: nss-3.21.0-0.3.el6_7.x86_64
--> Running transaction check
---> Package nspr.x86_64 0:4.9.2-1.el6 will be updated
---> Package nspr.x86_64 0:4.11.0-0.1.el6_7 will be an update
---> Package nss-softokn.x86_64 0:3.12.9-11.el6 will be updated
---> Package nss-softokn.x86_64 0:3.14.3-23.el6_7 will be an update
--> Processing Dependency: nss-softokn-freebl(x86-64) >= 3.14.3-23.el6_7 for package: nss-softokn-3.14.3-23.el6_7.x86_64
---> Package nss-sysinit.x86_64 0:3.14.0.0-12.el6 will be updated
---> Package nss-sysinit.x86_64 0:3.21.0-0.3.el6_7 will be an update
---> Package nss-tools.x86_64 0:3.14.0.0-12.el6 will be updated
---> Package nss-tools.x86_64 0:3.21.0-0.3.el6_7 will be an update
---> Package nss-util.x86_64 0:3.14.0.0-2.el6 will be updated
---> Package nss-util.x86_64 0:3.21.0-0.3.el6_7 will be an update
--> Running transaction check
---> Package nss-softokn-freebl.x86_64 0:3.12.9-11.el6 will be updated
---> Package nss-softokn-freebl.x86_64 0:3.14.3-23.el6_7 will be an update
--> Finished Dependency Resolution
Dependencies Resolved
=============================
Package Arch Version Repository Size
=============================
Updating:
ca-certificates noarch 2015.2.6-65.0.1.el6_7 updates1.2 M
nss x86_643.21.0-0.3.el6_7 updates858 k
Installing for dependencies:
libtasn1 x86_642.3-6.el6_5 base 238 k
p11-kit x86_640.18.5-2.el6_5.2base94 k
p11-kit-trust x86_640.18.5-2.el6_5.2base71 k
Updating for dependencies:
nspr x86_644.11.0-0.1.el6_7updates114 k
nss-softokn x86_643.14.3-23.el6_7 updates262 k
nss-softokn-freeblx86_643.14.3-23.el6_7 updates167 k
nss-sysinit x86_643.21.0-0.3.el6_7updates 46 k
nss-tools x86_643.21.0-0.3.el6_7updates435 k
nss-util x86_643.21.0-0.3.el6_7updates 67 k
Transaction Summary
=============================
Install 3 Package(s)
Upgrade 8 Package(s)
Total download size: 3.5 M
Is this ok [y/N]: y
Downloading Packages:
(1/11): ca-certificates-2015.2.6-65.0.1.el6_7.noarch.rpm | 1.2 MB 00:01
(2/11): libtasn1-2.3-6.el6_5.x86_64.rpm| 238 kB 00:00
(3/11): nspr-4.11.0-0.1.el6_7.x86_64.rpm | 114 kB 00:00
(4/11): nss-3.21.0-0.3.el6_7.x86_64.rpm| 858 kB 00:00
(5/11): nss-softokn-3.14.3-23.el6_7.x86_64.rpm | 262 kB 00:00
(6/11): nss-softokn-freebl-3.14.3-23.el6_7.x86_64.rpm | 167 kB 00:00
(7/11): nss-sysinit-3.21.0-0.3.el6_7.x86_64.rpm| 46 kB 00:00
(8/11): nss-tools-3.21.0-0.3.el6_7.x86_64.rpm | 435 kB 00:00
(9/11): nss-util-3.21.0-0.3.el6_7.x86_64.rpm | 67 kB 00:00
(10/11): p11-kit-0.18.5-2.el6_5.2.x86_64.rpm | 94 kB 00:00
(11/11): p11-kit-trust-0.18.5-2.el6_5.2.x86_64.rpm | 71 kB 00:00
=============================
Total 601 kB/s | 3.5 MB 00:06
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
Updating : nspr-4.11.0-0.1.el6_7.x86_64 1/19
Updating : nss-util-3.21.0-0.3.el6_7.x86_64 2/19
Updating : nss-softokn-freebl-3.14.3-23.el6_7.x86_64 3/19
Updating : nss-softokn-3.14.3-23.el6_7.x86_644/19
Updating : nss-sysinit-3.21.0-0.3.el6_7.x86_64 5/19
Updating : nss-3.21.0-0.3.el6_7.x86_64 6/19
Installing : libtasn1-2.3-6.el6_5.x86_64 7/19
Installing : p11-kit-0.18.5-2.el6_5.2.x86_64 8/19
Installing : p11-kit-trust-0.18.5-2.el6_5.2.x86_64 9/19
Updating : ca-certificates-2015.2.6-65.0.1.el6_7.noarch 10/19
Updating : nss-tools-3.21.0-0.3.el6_7.x86_6411/19
Cleanup: ca-certificates-2010.63-3.el6_1.5.noarch 12/19
Cleanup: nss-tools-3.14.0.0-12.el6.x86_64 13/19
Cleanup: nss-3.14.0.0-12.el6.x86_64 14/19
Cleanup: nss-sysinit-3.14.0.0-12.el6.x86_64 15/19
Cleanup: nss-softokn-3.12.9-11.el6.x86_64 16/19
Cleanup: nss-util-3.14.0.0-2.el6.x86_64 17/19
Cleanup: nspr-4.9.2-1.el6.x86_64 18/19
Cleanup: nss-softokn-freebl-3.12.9-11.el6.x86_64 19/19
Verifying : p11-kit-0.18.5-2.el6_5.2.x86_64 1/19
Verifying : nss-util-3.21.0-0.3.el6_7.x86_64 2/19
Verifying : nss-softokn-3.14.3-23.el6_7.x86_643/19
Verifying : nss-softokn-freebl-3.14.3-23.el6_7.x86_64 4/19
Verifying : nss-sysinit-3.21.0-0.3.el6_7.x86_64 5/19
Verifying : libtasn1-2.3-6.el6_5.x86_64 6/19
Verifying : nss-3.21.0-0.3.el6_7.x86_64 7/19
Verifying : ca-certificates-2015.2.6-65.0.1.el6_7.noarch 8/19
Verifying : nspr-4.11.0-0.1.el6_7.x86_64 9/19
Verifying : nss-tools-3.21.0-0.3.el6_7.x86_6410/19
Verifying : p11-kit-trust-0.18.5-2.el6_5.2.x86_6411/19
Verifying : nss-tools-3.14.0.0-12.el6.x86_64 12/19
Verifying : nss-3.14.0.0-12.el6.x86_64 13/19
Verifying : ca-certificates-2010.63-3.el6_1.5.noarch 14/19
Verifying : nss-softokn-freebl-3.12.9-11.el6.x86_64 15/19
Verifying : nss-util-3.14.0.0-2.el6.x86_64 16/19
Verifying : nss-sysinit-3.14.0.0-12.el6.x86_64 17/19
Verifying : nspr-4.9.2-1.el6.x86_64 18/19
Verifying : nss-softokn-3.12.9-11.el6.x86_64 19/19
Dependency Installed:
libtasn1.x86_64 0:2.3-6.el6_5 p11-kit.x86_64 0:0.18.5-2.el6_5.2 p11-kit-trust.x86_64 0:0.18.5-2.el6_5.2
Updated:
ca-certificates.noarch 0:2015.2.6-65.0.1.el6_7 nss.x86_64 0:3.21.0-0.3.el6_7
Dependency Updated:
nspr.x86_64 0:4.11.0-0.1.el6_7 nss-softokn.x86_64 0:3.14.3-23.el6_7 nss-softokn-freebl.x86_64 0:3.14.3-23.el6_7 nss-sysinit.x86_64 0:3.21.0-0.3.el6_7
nss-tools.x86_64 0:3.21.0-0.3.el6_7 nss-util.x86_64 0:3.21.0-0.3.el6_7
Complete!
现在可以了。