Let’s catpure for example only tcp-packets with syn or ack-flag set:

tcpdump "tcp[tcpflags] & (tcp-syn|tcp-ack) != 0"

Btrfs has it’s builtin tool for displaying IO stats on devices:

root@tardis:# btrfs device stats /mnt/Raid/
[/dev/sdb].write_io_errs 0
[/dev/sdb].read_io_errs 0
[/dev/sdb].flush_io_errs 0
[/dev/sdb].corruption_errs 0
[/dev/sdb].generation_errs 0
[/dev/sdc].write_io_errs 0
[/dev/sdc].read_io_errs 0
[/dev/sdc].flush_io_errs 0
[/dev/sdc].corruption_errs 0
[/dev/sdc].generation_errs 0

A simple cronjob could warn us if anything is wrong:

MAILTO=sysadmin@tardis.tdl
@hourly /sbin/btrfs device stats /mnt/Raid | grep -vE ' 0$'

It’s wisely to scrub the filesystem from time to time. Wikipedia says the following about “Data scrubbing”:

Data scrubbing is an error correction technique that uses a background task to periodically inspect main memory or storage for errors, and then correct detected errors using redundant data in form of different checksums or copies of data. Data scrubbing reduces the likelihood that single correctable errors will accumulate, leading to reduced risks of uncorrectable errors.

If we don’t scrub, it could happen that BTRFS only reads from the good drive without detecting the faulty drive

@monthly /sbin/btrfs scrub start -Bq /mnt/Raid

>

Guacamole is a clientless remote desktop gateway using HTML5. It supports the following backends: VNC, RDP, SSH and Telnet. It really looks like a promising project to me

We can use static ARP entries on hosts or routers to reduce ARP-traffic or to prevent ARP-spoofing. For such entries we need pairs of MAC-addresses and IP-addresses. The same information we also need for static DHCP-entries on a DHCP-server. Why not combine?

If someone attacks a linux-system, most administrator would block the ip-address of the attacker using iptables-rules. But there is another method to block the address of an attacker: nullroutes. Nullroutes are routes to nowhere

memcachetop is a nice perl-script for displaying live stats from memcached. The author kept portability in mind, so it has not much dependencies and might run on many systems.

For scrolling with the mouse-wheel in GNU screen, the following line is needed in ~/.screenrc:

termcapinfo xterm*|rxvt*|kterm*|Eterm* ti@:te@

It happens so often that when I start a copy-job remotely, I just realize that I forgot to start it in GNU screen. Sometimes I can just stop the job and start it again in screen, but very often stopping the job might be problematically. There is a nice tool called reptyr which can re-ptying your jobs.

Lets Encryptwas lately quite often in the media. Letsencrypt is a very easy to use tool which provides certificates for free. Those certificates are valid on most common browsers. I never understood why certificates are expensive that’s why I tried out letsencrypt(and I like it!).

In this article, I will replace all cacert-certificates on a kolab-server. Therefore I will install the letsencrypt-certificate on: apache2, cyrus-imapd and postfix.