Hanging mount (not the way you like it)

Symptom: trying

$ mount -t nfs /whatever

is just hanging there.

Troubleshooting: rpcinfo -p servername is fast and responsive. A lot of times when rpc fails nfs will fall on a backup protocol that is fairly slow. With rpcinfo -p, you can rule this out as being your problem.

portmapper is running. According to lord google, this has been the cause of the problem many a time. Not my problem though.

The server logs don’t say crapola.

What do you do?

Solution: You may just have bad syntax. Try

$ mount -t nfs /whatever/

Yes, this took me hours to figure out. Stupid slash.

From #islug (I kept the last bit because I like to talk about fscking whenever I get a chance).

<mop> Have you ever spent all day stressed out about a problem, then you figure out that it’s all just because you left a trailing front slash?
<mop> Reminds me of my first days programming and missing semi-colons, the bastards.
<twinprism> yes. :)
<TomHung> echo $iknowyourfeeling
<bradmw> mop: what language?
<mop> Today: just mounting an nfs file share (around 2 hours debugging to find out its because I mounted /whatever instead of /whatever/)
<mop> Back in the day: C
<bradmw> does linux have a scandisk type program to check floppies for bad sectors?
<mop> you can fsck a floppy
<mop> just like an apple pie
* TomHung <= fsck up
<bradmw> let me unmount my sheep first
<mop> common mistake. umount.
<mop> ;)
<TomHung> mount /dev/usb /home/sheep

Believe it or not I’ve done basically the same thing with grep -R…

Get Mail List from LDAP

Although bash is my first scripting language, it seems I am becomming more and more of a python convert. Today I needed to send an email to everyone with an account on my ldap server. Normally, I would have used something like sed and bash. Although I did still end up using bash, I forced myself to write the regular expression part in python.

The idea with languages like python is to make it so that it’s easier to write your own bit of code to do what you want than it would be to find someone else’s code and use it. This code is probably pretty useless to most people. It is not even a little efficient. But who knows, I may help a budding sysadmin who is just starting to write his own scripts. And by blogging it, hopefully I know where to find it for next time.

Here is the script to pull out user’s email addresses, one per line (which is exactly the type of file my mail client will accept to make an email list):


This program was written to get all the email addresses from raw input
and print them onto the screen
import re
    while 1:
            line = raw_input() + 'n'
            string = re.search(r"(w*.)?w*@w*.w*",line)
                print string.group()

        #this should only happen on the last iteration

I ended up just using some trivial bash stuff to do some of the processing, although this would be pretty easy to have built into the python, but it was even easier to just put it in good old familiar and unmanagable bash.

slapcat | ./mailfind.py | sort | uniq

Awesome. Now I have my mail addresses to use for my black-market viagra selling business.

Common Permission Error

One extremely important part of using any multi-user Operating System is correctly understanding permissions. Over the past couple years, I have been administering a Linux server with a lot of users, and there is one error that has popped up over and over again.  A lot of users forget or don’t realize that permission to delete a file from a directory is determined by the write flag of the directory, not the write flag of the file.

Take this scenario: Bob is an unprivileged user on a LAMP box and he is hosting a dynamic website.  He wants to allow changes to folders within his public_html directory so he can do things like upload templates, media, etc.  However, the apache user is not Bob, it is something like www-data.  As an unprivileged user, Bob cannot chown or chgrp a directory to a group he doesn’t belong to.  He might think about contacting the sysadmin, but more likely he chmods the directories to be a+w, and is careful to make the files not have the write flag (assuming that this is what determines if a file can be deleted or not). This is an incorrect assumption, and he is leaving his files to be deleted by whoever else has an account on the server.

For example:

bob@lamp:~$ ls -l     #note the test directory has o+w
total 4
drwxrwxrwx 2 bob stupid 4096 2008-07-01 10:27 test

bob@lamp:~$ cd test/

bob@lamp:~/test$ ls -l myfile     #note that myfile does not have o+w
-rw-r–r– 1 lundeen2 stupid 5 2008-07-01 10:28 myfile

bob@lamp:~/test$ su otheruser

otheruser@lamp:~/test$ rm myfile     #other random users are able to delete this file
rm: remove write-protected regular file `myfile’? y

otheruser@lamp:~/test$ ls -l
total 0

There are several ways to handle Bob’s situation.  Bob could ask a privileged user to add him to the www-data group (though this won’t work very well if all the users are part of this group) or he could ask the admin to setattr +i the file to make it undeletable (though he himself could not delete it afterward). A better way would probably be for Bob to use acls (eg setfacl) or to set the sticky bit on the directory (chmod +t).  The sticky bit might be good enough, as it is probably what Bob wanted in the first place – for other users besides himself to be able to write to his directory, but not giving those users a chance to delete his or www-data’s files. Using acls is probably best, but can also be slightly more complicated.

While the behavior of permissions may be obvious to a system administrator, to an average user, it seems it is not.  In my weekly cron scripts I have a  “find / -type d  ( -perm -o+w  -perm 1000 )” to search for all files with this permission.