yum is not working

[yufan]

could someone tell me what wrong is with this:

[root at localhost yufan]# yum update
Setting up Update Process
Setting up repositories
updates-released 100% |=========================| 951 B 00:00
extras 100% |=========================| 1.1 kB 00:00
http://linux.admin.uillinois.edu/pub/linux/fedora/linux/core/4/i386/os/repodata/repomd.xml:
[Errno 4] IOError: HTTP Error 403: Proxy-Connection: Keep-Alive
Connection: Keep-Alive
Date: Sat, 12 Nov 2005 02:48:22 GMT
Server: Apache/2.0.46 (Red Hat)
Content-Length: 353
Content-Type: text/html; charset=iso-8859-1
Trying other mirror.
base 100% |=========================| 1.1 kB 00:00
Reading repository metadata in from local files
primary.xml.gz 100% |=========================| 357 kB 00:50
http://ftp.lug.ro/fedora/linux/core/updates/4/i386/repodata/primary.xml.gz:
[Errno -1] Metadata file does not match checksum
Trying other mirror.
primary.xml.gz 100% |=========================| 354 kB 00:14
http://fedora.ngi.it/updates/4/i386/repodata/primary.xml.gz: [Errno -1]
Metadata file does not match checksum
Trying other mirror.
http://ftp.chg.ru/pub/Linux/fedora/core/updates/4/i386/repodata/primary.xml.gz:
[Errno 4] IOError: HTTP Error 404: Proxy-Connection: Keep-Alive
Connection: Keep-Alive
Date: Sat, 12 Nov 2005 02:49:09 GMT
Server: Apache/1.3.33 (Unix)
Transfer-Encoding: chunked
Content-Type: text/html; charset=iso-8859-1
Trying other mirror.
primary.xml.gz 100% |=========================| 355 kB 00:15
http://fedora.secsup.org/core/updates/4/i386/repodata/primary.xml.gz: [Errno
-1] Metadata file does not match checksum
Trying other mirror.
primary.xml.gz 100% |=========================| 357 kB 00:21
http://ftp.udl.es/pub/fedora/linux/core/updates/4/i386/repodata/primary.xml.gz:
[Errno -1] Metadata file does not match checksum
Trying other mirror.
primary.xml.gz 100% |=========================| 357 kB 00:27
http://ftp.iasi.roedu.net/mirrors/fedora.redhat.com/core/updates/4/i386/repodata/primary.xml.gz:
[Errno -1] Metadata file does not match checksum
Trying other mirror.
primary.xml.gz 100% |=========================| 355 kB 00:20
http://mirrors.kernel.org/fedora/core/updates/4/i386/repodata/primary.xml.gz:
[Errno -1] Metadata file does not match checksum
Trying other mirror.
primary.xml.gz 100% |=========================| 357 kB 00:16
http://www.muug.mb.ca/pub/fedora/linux/core/updates/4/i386/repodata/primary.xml.gz:
[Errno -1] Metadata file does not match checksum
Trying other mirror.
primary.xml.gz 100% |=========================| 248 kB 00:08

it keeps printing all kinds of error messages and never start to update.what's
wrong?
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