Sendmail takes a long time to start

Posted: 2012-07-26 13:39:39 by Alasdair Keyes

Direct Link | RSS feed

I had someone come to me today with a CentOS 5.x machine that was taking upwards of 10 minutes to get to the login prompt.

When I rebooted the box, It hung for a long time on Sendmail and the Sendmail sm-client services

Starting Sendmail:

Between the two of them they took over 8 minutes to start. Helpfully /var/log/messages and /var/log/maillog didn't have any entries about why it took so long. After a bit of poking it turns out that he had changed the hostname from kvmhost01.local to kvmhost01.testlocal but hadn't updated /etc/hosts so sendmail couldn't find the IP associated with the hostname.

# Do not remove the following line, or various programs
# that require network functionality will fail.       kvmhost01.testlocal localhost.localdomain localhost
::1             localhost6.localdomain6 localhost6

It's only a small thing but obviously makes a big difference to Sendmail.

If you found this useful, please feel free to donate via bitcoin to 1NT2ErDzLDBPB8CDLk6j1qUdT6FmxkMmNz

Network Ranges By Country/Continent

Posted: 2012-04-05 12:46:45 by Alasdair Keyes

Direct Link | RSS feed

Whilst investigating the source of some network attacks this morning, I came across the following website

It provides lists of Network blocks by Country/Continent, if you notice a large number of attacks from a specific geographical area, you can find other IP blocks from the same area to add firewall rules.

According to the website, the data may become a paid-for service soon, so you may want to get the information while it's free!

If you found this useful, please feel free to donate via bitcoin to 1NT2ErDzLDBPB8CDLk6j1qUdT6FmxkMmNz

KVM Console Not working

Posted: 2011-11-25 13:09:11 by Alasdair Keyes

Direct Link | RSS feed

I've noticed that with KVM virtual machines, the kvm console <domainid> doesn't appear to work, when running you get

# virsh console MyVM
Connected to domain MyVM
Escape character is ^]

Hitting enter gives you nothing It appears that this isn't bug with KVM, rather the Guest OS isn't aware that it should start up a ttyS for KVM to connect to. When starting the machine, enter into grub and add the following to the kernel options...


Then when the virtual machine starts you can connect again using virsh and hit enter a couple of times

[root@inth1-vdc-lvh01 ~]# virsh console MyVM
Connected to domain MyVM
Escape character is ^]

CentOS Linux release 6.0 (Final)
Kernel 2.6.32-71.el6.x86_64 on an x86_64

centos-6-0 login:

Voila... Obviously this isn't going to work for Windows :)

If you found this useful, please feel free to donate via bitcoin to 1NT2ErDzLDBPB8CDLk6j1qUdT6FmxkMmNz

BASH v4 Regular Expressions

Posted: 2011-03-05 14:09:48 by Alasdair Keyes

Direct Link | RSS feed

I've recently built a new backup server using NILFS for it's handy snapshotting capability. I'll get onto that in another post, but because NILFS support was only added into the Linux 2.6.30 kernel, our current CentOS 5 build's don't have support for it as they only run 2.6.18.

To overcome this, I built the new box up with Ubuntu 10.10 Server. However, it appears that I'd never written any BASH scripts using regular expressions in Ubuntu. I copied my custom backup scripts from our old box to the new backup server and ran them and it errored on some path name checks I performed. Upon investigation it appears that BASH v4's regular expression handling isn't backwards compatible with version 3. Specifically the use of the $ character as the End of Line character

As a test...


if [[ $TESTDIR =~ '/$' ]]; then
        echo "$TESTDIR has a trailing slash";
        echo "$TESTDIR has no trailing slash";

The output is

/tmp/a/ has no trailing slash

Which doesn't seem too clever to me. After some looking through the bash documentation I found out about the 'shopt' builtin which allows you to change additional shell behavior.

Long story short, so I don't have to go round changing all the regex in my code, if I add something like this to the top of all my scripts, they can port between Bash v3 and v4 seamlessly...


BASH_VER=`/bin/bash --version | grep 'version 4'`;
if [ -n "$BASH_VER" ]; then
        echo "Detected BASH v4 - Switching on Version 3 compatibility"
        shopt -s compat31
        echo "Not Detected BASH v4"


if [[ $TESTDIR =~ '/$' ]]; then
        echo "$TESTDIR has a trailing slash";
        echo "$TESTDIR has no trailing slash";

Produces more expected output

Detected BASH v4 - Switching on Version 3 compatibility
/tmp/a/ has a trailing slash

Or if you're not bothered about nice output, shorten the check to..

if [ -n "`/bin/bash --version | grep 'version 4'`" ]; then
        shopt -s compat31

If you found this useful, please feel free to donate via bitcoin to 1NT2ErDzLDBPB8CDLk6j1qUdT6FmxkMmNz

FLOSS and CentOS

Posted: 2010-11-18 16:44:24 by Alasdair Keyes

Direct Link | RSS feed

Earlier today I watched this podcast from FLOSS (Free Libre Open Source Software) on the TWiT Network whose special guest was Karanbir Singh from the CentOS project. Karanbir talked about CentOS, it's start in life, it's relationship with the Redhat Enterprise Linux system and his experiences with the project which were very interesting to listen to.

Being a lead developer on CentOS, Karanbir commits most of the package updates for the CentOS 5 systems I use at both home and work and I get many emails from him via the CentOS Announce list advising of new package commits. In a strange way it was interesting to see the person who I've never met and probably never will but has huge influence over my work life.

If you're interested in hearing about Open Source projects or use CentOS on any machines, I'd highly recommend spending an hour and watching (or listening) to this.

If you found this useful, please feel free to donate via bitcoin to 1NT2ErDzLDBPB8CDLk6j1qUdT6FmxkMmNz

Centos/Redhat 6

Posted: 2010-11-11 13:00:01 by Alasdair Keyes

Direct Link | RSS feed

Redhat have just released their latest Enterprise Linux offering Redhat EL 6. According to the CentOS twitter feed it will be 4-6 weeks before CentOS 6 is built and released publicly.

The two biggest changes for me are that exim is now no longer in the Redhat Repository and Xen DomU supported kernels have been dropped in favour of KVM technology.

Exim is the only MTA I really want to use, so I won't be switching to Postfix or sendmail. Thankfully, rebuilding the Latest Exim Source RPM from Redhat 5 on Redhat 6 seems very easy, so it shouldn't cause too much of an issue that I can see. Once CentOS 6 is released, I'll post an article about getting Exim running on 6, so keep posted.

I'm very intrigued to use KVM, I've had very little experience of it, but I've used Xen a lot, building our entire Linux VPS platform around it. The "bonusses" of KVM are looking interesting, especially the memory overcommit facility that's available. If I can get some hardware to test with I'll look at the ease of building up a redundant KVM cluster.

If you found this useful, please feel free to donate via bitcoin to 1NT2ErDzLDBPB8CDLk6j1qUdT6FmxkMmNz

Jumbo Frames

Posted: 2010-10-01 18:35:44 by Alasdair Keyes

Direct Link | RSS feed

We'd been having some issues with retransmit errors on our iSCSI network, so we enabled jumbo frames to allow frames up to 9000 bytes instead of the standard 1500. The change was very easy to implement...

ifconfig ethx mtu 9000

And then updated the /etc/sysconfig/network-scripts/ifcfg-ethx file to include the line


(We use CentOS)

Using the Dell SANHQ software showed a marked increase in the speed of large iSCSI transactions, However we discovered that some of our CentOS based Xen hosts were dropping incoming packets on the iSCSI interface

$ ifconfig ethx | grep dropped
          RX packets:1883666 errors:0 dropped:12636 overruns:0 frame:0
          TX packets:1696267 errors:0 dropped:0 overruns:0 carrier:0

This was a concern as we'd not had any issues with this before, although some dropped packets can be forgiven, in this case we seemed to have problems with our Hosts kernel panicing and rebooting. We couldn't work out what was causing these dropped packets and tried a number of things including increasing the incoming TCP buffer, but to no avail.

After a short time I noticed that the hosts running slightly earlier kernels were getting the packet loss, whereas the later ones were fine, specifically Redhat/CentOS kernels 2.6.18-164.x.x.el5xen were dropping packets, where as the 2.6.18-194.x.x.el5xen kernels were fine.

There's no special fix in this instance, if you're getting s similar issue and running the 2.6.18-164 kernel make sure you do an update. It's also just a reminder to keep your servers and especially kernels, up to date, although software is often only updated for security patches, there are many small fixes that are rolled out that fix little issues like this.

If you found this useful, please feel free to donate via bitcoin to 1NT2ErDzLDBPB8CDLk6j1qUdT6FmxkMmNz

Parallels Virtualization Engineer

Posted: 2010-08-17 18:53:00 by Alasdair Keyes

Direct Link | RSS feed

At we've recently released our Parallels Virtuozzo Based VPS systems. Parallel's container based virtualization is a very low-overhead system which allows a very high ratio of VPS guest OS's per physical hardware node.

As part of this I attended a two-day course plus various online tutorials and exams and I am now a Parallels Certified Virtualization Engineer and Parallels Certified Automation Engineer.

Which is nice.

If you are using Parallels and wish to look at getting certified with their technologies you can check them out at

If you found this useful, please feel free to donate via bitcoin to 1NT2ErDzLDBPB8CDLk6j1qUdT6FmxkMmNz

OS Deduplication with LessFS

Posted: 2010-07-18 16:41:36 by Alasdair Keyes

Direct Link | RSS feed

Working in hosting means that I often have to deal with large amounts of data. Either in customer files on a shared hosting platform or as VHDs on a Virtual platform.

Storing lots of data in any form is a pain, it's unweildy, hard to backup, hard to move and storage costs a lot, not only for money on buying disks, but if not done in a smart way, takes up cabinet space and extra money in cooling/powering servers just for their storage capacity. With the recent evolution of cheap SAN hardware with iSCSI networking data storage for small companies is getting easier, however the amount of storage being used by any company is always increasing faster than you'd like.

Many SAN providers have deduplication which can save a lot of space, however, do we really need to buy SAN technology to get the benefits of deduplication? Many companies like ours have lots of server and lots of data which we would certainly like to slim down on. I started investigating into Open Source deduplication and found a couple of contenders.

First of all is lessfs. LessFS is an Open Source deduplication filesystem written using FUSE. I thought I'd give it a go and it seems pretty good so I thought I'd run a quick tutorial on how to use it. For this demo, I used Ubuntu 10.04 LTS x86_64. I chose this over my favourite choice of CentOS as it's software and libraries are far more current which I found would help in the installing of dependencies. This was installed into a VMware VM with a 3GB / ext3 partition and a 10GB ext3 partition mounted onto /data

We could stop there but it's worth investigating the power of Deduplication and seeing how lessfs stacks up. Now lessfs creates a stats folder in your mount under .lessfs/lessfs_stats it shows how much space has been used to store files and how much has been saved using it's deduplication, lets have a look at the file

root@dedup:~# cat /mnt/less/.lessfs/lessfs_stats
      7                0                0  lessfs_stats

Lets check how much data is being used by lessfs at the moment

root@dedup:~#  du -hs /data
64M      /data

This space is just lessfs's database and other storage mechanisms. We'll write some data and see how it goes. Lets create a 200MB file full of zeros, this should be easily dedup'd as it's all identical.

root@dedup:~# dd if=/dev/zero of=/mnt/less/zero bs=1M count=200
root@dedup:~# ls -alh /mnt/less/zero
-rw-r--r-- 1 root root 200M 2010-07-16 19:40 /mnt/less/zero

You can see that the system sees it using 200MB, what does less fs think we've used

root@dedup:~# head /mnt/less/.lessfs/lessfs_stats
      7                0                0  lessfs_stats
      8        209715200             1628  zero

That's pretty good, 200MB compressed into 1K! But lessfs could be lying, as we know the actual data is stored in /data, how much has that grown.

root@dedup:~# du -hs /data
64M      /data

Absolutely nothing, it's looking good. It's worth storing some real-world data...

root@dedup:~# scp -r /mnt/less/

This will allow some dedup, but lets see what happens if we store the same data again into a different folder

root@dedup:~# scp -r /mnt/less/Aerosmith_copy
root@dedup:~# du -hs /mnt/less/Aerosmith/
249M    /mnt/less/Aerosmith/
root@dedup:~# du -hs /mnt/less/Aerosmith_copy/
249M    /mnt/less/aerosmith_copy/

That's 500 MB we've copied to that filesystem, yet we're only using 308MB in total in the /data folder

root@dedup:~# du -hs /data/
308M    /data/

If we check the lessfs_stats file, we actually seem to see an increase in size vs compressed size. I'm not sure if this is a calculation issue or one related to block size etc. However, you wouldn't expect much with compressed files anyway.

     45          3727130          3727159  08 - Cryin'.mp3
     46          2942524          2942547  05 - Don't Stop.mp3
     47          3301450          3301476  01 - Eat The Rich.mp3

The interesting bit is when we check the section related to the duplcate set of files...

    131          3727130                0  08 - Cryin'.mp3
    132          2942524                0  05 - Don't Stop.mp3
    133          3301450                0  01 - Eat The Rich.mp3

it is deduplicating it perfectly. It has worked out that the files are copies and the copies using little to no space on disk.

There are plenty of options available with lessfs, tweaking the config file allows for encryption, dynamic defrag and transaction logging (to enable recovery after a crash) and the level of compression by using different compression systems (BZIP, GZIP, Deflate etc).

As with anything like this, I wouldn't use it on data I couldn't afford to lose until I was very sure it was stable. However, it shows plenty of promise for what lies ahead in the world of data storage.

If you found this useful, please feel free to donate via bitcoin to 1NT2ErDzLDBPB8CDLk6j1qUdT6FmxkMmNz

Apache Caching

Posted: 2010-04-18 15:51:10 by Alasdair Keyes

Direct Link | RSS feed

With the release of Apache 2.2, Apache's built in caching modules (mod_cache, mod_disk_cache and mod_mem_cache) became "production ready", although I'm not too sure as I'll discuss.

I implemented Apache's mod_disk_cache on a production server and it seemed to work very well, essentially, each request is hashed into a file path on your web server. This is very useful if you serve content from a remote share such as NFS, it will allow the web server to skip out all that unneeded network activity just to send the same 2KB image or CSS document 20,000 times. Compared to memory, disk is relatively slow but by further comparison, NFS or remote shares will crawl if you're handling lots of small documents.

However, I did find out that Wordpress sites have problems when mod_disk_cache is used. It seems to generate collisions and you will find that people will visit different pages in your Wordpress site but just keep on getting served the same page from the cache, often this seems to be the RSS feed for that site.

At first I thought that I'd just have to increase the path depth and size, however, even when increasing this to the maximum and flushing the cache, the problems seemed to stay. As a result, I skipped the disk cache and stuck with mod_mem_cache.

Obviously memory is much much faster than disk, although much less abundant, a modern server usually won't arrive with less than a 100GB disk but unless you spend serious cash, you won't have more than 8-16GB RAM. After implementing mod_mem_cache the problems with wordpress disappeared, the only thing I can think of is that either mod_mem_cache wasn't caching the requests and they all went to the data store... or Apache's disk cache implementation didn't have enough path size/depth to correctly stop collisions whereas it's memory hashing didn't have the same path limitations so was able to succesfully handle the request.

Unfortunately, there doesn't seem to be a way to view the requests stored in memory. When disk cache is being used you can look through the hashed locations on the filesystem /var/myapachecache/abc/def/123/345/.... but no such help when you're just using memory caching.

Anyway, long story short, memory caching works wonders and depending on your site. Disk caching might work wonders too, but if you seem to get collisions, just ditch it.

If you found this useful, please feel free to donate via bitcoin to 1NT2ErDzLDBPB8CDLk6j1qUdT6FmxkMmNz

IT Consultancy Services

I'm now available for IT consultancy and software development services - Cloudee LTD.

Happy user of Digital Ocean (Affiliate link)


Validate HTML 5