Forum: CFEngine Help
Subject: /etc/resolv.conf: if updating, backup first
Author: matt_garman
Link to topic: https://cfengine.com/forum/read.php?3,23954,23954#msg-23954
I'm looking to create a promise that sets up name resolution on my systems; in
particular, managing the /etc/resolv.conf file.
My first pass uses the cfengine_stdlib.cf and looks like this:
bundle agent resolv
{
vars:
"search" string => "mydomain.com";
"nameservers" slist => { "192.168.184.7", "192.168.187.103" };
files:
"${sys.resolv}" -> "Some Stakeholder"
comment => "Make sure file ${sys.resolv} is correct",
edit_line => resolvconf( "${search}", "${nameservers}" );
}
The problem is, this always edits the file. CFEngine nicely creates the
"cf-before-edit" backup file... but since it always edits the file, it always
overwrites the backup.
Consider the scenario where someone didn't know CFE was managing this file, and
created a local edit. Then CFE runs twice, now that local edit is gone forever.
So then I thought, I'll just make a backup of the file before I edit it, so I
did this:
# ... bundlesequence => { "resolv_backup", "resolv" }; ...
bundle agent resolv_backup
{
vars:
"backup_file" string => "${sys.resolv}.backup.${sys.cdate}";
files:
"${backup_file}" -> "Some Stakeholder"
copy_from => perms_cp("${sys.resolv}");
}
That solves my "local edit lost forever" problem, but results in creating way
more backups than I need.
What I decided would be best for my situation is as follows:
- keep a static copy of the target file somewhere within CFE's directory
- compare the current /etc/resolv.conf to this static copy
- if the files are the same, do nothing
- if the files are different, create a backup of /etc/resolv.conf, then
overwrite with the static file
Expressed as a shell script:
DIFF_RESULT=$(diff -q /etc/resolv.conf.cf-before-edit /etc/resolv.conf)
if [ "x${DIFF_RESULT}" != "x" ]; then
cp /etc/resolv.conf /etc/resolv.conf.backup.$(date +%Y%m%d.%k%M%S)
cp -f /var/cfengine/masterfiles/resolv.conf /etc/resolv.conf
fi
...Which I could just have CFE run for me. But I'm assuming there's a more
elegant "CFE-native" way to do such a thing. What's the right approach here?
Thanks!
Matt
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