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如题，求大神指点，以下是安装过程。
root@ubuntu-virtual-machine:/home/ubuntu# sudo apt-get install openssh-server
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
ncurses-term ssh-import-id
Suggested packages:
rssh molly-guard openssh-blacklist openssh-blacklist-extra monkeysphere
The following NEW packages will be installed:
ncurses-term openssh-server ssh-import-id
0 upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
Need to get 775 kB of archives.
After this operation, 3,066 kB of additional disk space will be used.
Do you want to continue [Y/n]? y
WARNING: The following packages cannot be authenticated!
ncurses-term openssh-server ssh-import-id
Install these packages without verification [y/N]? y
Err http://us./ubuntu/ quantal/main ncurses-term all 5.9-10ubuntu1
404 Not Found [IP: 91.189.91.15 80]
Err http://us./ubuntu/ quantal/main openssh-server i386 1:6.0p1-3ubuntu1
404 Not Found [IP: 91.189.91.15 80]
Err http://us./ubuntu/ quantal/main ssh-import-id all 2.12-0ubuntu1
404 Not Found [IP: 91.189.91.15 80]
Failed to fetch http://us./ubuntu/pool/main/n/ncurses/ncurses-term_5.9-10ubuntu1_all.deb 404 Not Found [IP: 91.189.91.15 80]
Failed to fetch http://us./ubuntu/pool/main/o/openssh/openssh-server_6.0p1-3ubuntu1_i386.deb 404 Not Found [IP: 91.189.91.15 80]
Failed to fetch http://us./ubuntu/pool/main/s/ssh-import-id/ssh-import-id_2.12-0ubuntu1_all.deb 404 Not Found [IP: 91.189.91.15 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
