[Yum] Yum fails to work properly

[Benjamin Sher]

Dear friends:

[Using Scientific Linux 4, a derivative of RHEL4]

I've been using Scientific Linux (SL) for several weeks. Have had no 
problem with yum until two days ago.

At first, I got the following error message:

[sher at localhost ~]$ yum list available |more
Cannot open/read repomd.xml file for repository: dag
Cannot find repomd.xml file for dag
Setting up Repos
[sher at localhost ~]$

Then, a day later I got a different error message. Please see below:

Here is the output of my /etc/yum.repos.d subdirectory:

["sl" stands for Scientific Linux, a derivative of Fedora4/RH4]

Benjamin Sher wrote:
[sher at localhost ~]$ cd /etc/yum.repo
bash: cd: /etc/yum.repo: No such file or directory
[sher at localhost ~]$ cd /etc/yum.repos.d
[sher at localhost yum.repos.d]$ ls
dag.repo   dag.repo.bk  sl-contrib.repo  sl.repo     sl-rhaps.repo
dag.repo~  flash.repo   sl-errata.repo   sl.repo.bk
[sher at localhost yum.repos.d]$ ls -l
total 72
-rw-r--r--  1 root root 113 Jun 23 10:56 dag.repo
-rw-r--r--  1 root root 113 Apr 20 09:27 dag.repo~
-rw-r--r--  1 root root 113 Apr 20 09:27 dag.repo.bk
-rw-r--r--  1 root root 212 Apr 20 09:27 flash.repo
-rw-r--r--  1 root root 127 Apr 20 09:27 sl-contrib.repo
-rw-r--r--  1 root root 128 Apr 20 09:27 sl-errata.repo
-rw-r--r--  1 root root 119 Apr 20 09:27 sl.repo
-rw-r--r--  1 root root 119 Apr 20 09:27 sl.repo.bk
-rw-r--r--  1 root root 131 Apr 20 09:27 sl-rhaps.repo
[sher at localhost yum.repos.d]$ cat dag.repo
[dag]
name=DAG rpms
baseurl=ftp://ftp.scientificlinux.org/linux/extra/dag/redhat/el4/en/$basearch/dag/ 

enabled=1
[sher at localhost yum.repos.d]$               
[sher at localhost yum.repos.d]$ yum list available
Setting up Repos
Reading repository metadata in from local files
dag       : ################################################## 2480/2480
MD Read   : ################################################## 1447/1447
sl-base   : ################################################## 1447/1447
Error: Caching enabled but no local cache of 
//var/cache/yum/sl-errata/primary.xml.gz from sl-errata
[sher at localhost yum.repos.d]$


I have one other question, please: does it matter which yum repository 
one uses? Is this something that each distro decides for itself. In 
other words, should I stick to my distro's repository? What are your 
suggestions?

Please remember that I am an ordinary user.

Thank you so much.

Benjamin