.pQd

detecting duplicate network packets from linux command line

by .pQd
while going down the rabbit hole of bed network performance at work we’ve narrowed the problem cause to duplicate packets showing up on a specific segment of leased metro Ethernet service. tshark filter helped us to see when duplicates occurred on monitored link we’ve got pair of computers – A and B connected at both ... Read More

resizing btrfs on top of luks

by .pQd
recently we needed to expand storage space available on one of our servers. originally it was using RAID10 on 4 4TBSSD drives handled by Dell’s PERC h730p controller, we wanted to add 2 more 4TB drives and go from 8 to 12TB array. we’ve decided to be brave and use RAID10 -> RAID10 array expansion. ... Read More

backup via ssh-tunneled connection

by .pQd
in one context i need to pull backup from server C to server A. normally i’d use rsync with direct ssh connection as a transport method. in this particular case C is not directly reachable from A, so i need to use ssh tunneling to connect from A to C via jump-host B: public ssh ... Read More

fighting a false-positive flagging by multiple antivirus vendors

by .pQd
recently i woke up to this: “Hi, some of our employees are using your application. This morning they have received an upgrade notification (in yellow banner) to get the latest version of your app. Our anti-virus/malware has triggered on your module called “somefile.exe” detected at risk being a “Trojan.Gen.MBT “. below few resources that i’ve ... Read More

apache2 – reverse proxy with sticky sessions and fail-over

by .pQd
i needed to set up apache2 as a reverse proxy that will forward requests to few backends. yes – i know that there are better tools to do it – like haproxy or nginx – but in this case apache2 was preferred for the simplicity of the setup. requirements: sticky sessions – in normal conditions ... Read More

guacamole under Debian

by .pQd
Apache Guacamole is a clientless remote desktop gateway – with it you can access RDP-enabled Windows PC using ordinary web browser and HTTP[S]. below – notes taken while setting it up under Debian 10.

useful network-related tools

by .pQd
internet-wide scanners: https://censys.io/ https://www.shodan.io/ ssl: https://www.ssllabs.com/ smtp: https://www.mail-tester.com/ dns: http://dns.squish.net/

upgrading firmware on Intel’s SATA SSD drives behind Dell’s h730p RAID

by .pQd
fetch Intel_SSD_Data_Center_Tool from here. the zip archive contained, among other, .deb for 64 bit OS – that worked for me under debian stretch. to see drives run: to upgrade the firmware where 0 corresponds to the Index of drive returned by the first command the same likely works for other LSI/Avago cards – no matter ... Read More

iostat -x 1 reporting 100% utilization of nearly-idle NVMe drives

by .pQd
after an upgrade to debian buster i’ve noticed that both iostat -x 1 and munin’s diskstats_utilization report that NVMe drives are busy most of the time. some empirical tests showed that disks are actually idle, performance did not drop. upgrade to 5.2 kernel resolved the miss-reporting issue.

openvpn – “OpenSSL: error:1408518A:SSL routines:ssl3_ctx_ctrl:dh key too small” after upgrade to Debina Buster

by .pQd
another thing to adjust after upgrade to Buster. on openvpn endpoint that in it’s config – /etc/openvpn/whatever.conf – had: dh dh1024.pem the vpn service did not start. tail -f /var/log/syslog showed: apparently the new version of openssl no longer accepts 1024 Diffie Hellman group. solution: and change in /etc/openvpn/whatever.conf – from dh dh1024.pem to dh ... Read More

bind9 fails to start after an upgrade to Debian Buster

by .pQd
i’ve done a routine upgrade to Buster on some server and all went fine… except bind9 no longer started. every attempt to restart it ended up with long waiting and an error message: logs did not contain anything interesting – actually they showed that bind would start and then after ~ a minute was shut ... Read More

allowing custom file extensions to be uploaded to wordpress

by .pQd
WordPress restricts types of files you can upload. it not only checks the extension of what you’re adding to the media library but also its mime type. so you cannot fool it by uploading .exe renamed to .jpeg – if you try it, you’ll get an error saying: “Sorry, this file type is not permitted ... Read More

“error: internal error: unable to execute QEMU command ‘transaction’: Could not create file: Permission denied” when creating a snapshot under KVM

by .pQd
debian buster brings apparmor. apparmor brings problems – eg it’s too restrictive for libvirt KVM guests and does not allow KVM to create snapshot-related files in VM’s folders. so far i did not find a clean and generic way to address it so i had to disable apparmor for libvirt by adding security_driver = “none” ... Read More

let’s encrypt via proxy server under debian

by .pQd
i prefer to have strict DROP policy for the outgoing traffic from production servers. let’s encrypt API endpoint is behind Akamai’s CDN and IP address to which acme-v02.api.letsencrypt.org resolves changes frequently. i don’t like playing whack-a-mole every 3 months so i’ve: set up a squid-based proxy server that allows for filtering based on domain names: ... Read More

kvm: disabled by bios

by .pQd
does your kvm VM feel sluggish? mine did feel pretty slow.. as it turned out virt-install quietly overrode ‘hvm’ preferences and created a fully emulated qemu vm when it failed to make use of hardware-assisted virtualization.

bios upgrade on Dell PowerEdge T20 via AMT

by .pQd
prepare the floppy image file that later can be mounted via AMT, include in it just the bios update file – in my case it was PET20A18.exe from here. once the file is ready use the Manageability Commander Tool > Remote Control > Take control mount such img file as a virtual, oversize floppy. Using ... Read More

bridging lan segments across untrusted links

by .pQd
we’ve run out of the office space in one of the locations. in short term it was not possible to find a suitable and large enough place to rent so we had to split and relocate some of the staff to another building few kilometers away. it’s possible that we’ll shuffle people and servers between ... Read More

multi-master mysql replication with servers on 3 different continents

by .pQd
at work i’m using mysql replication quite extensively. first it was a straightforward one-way replication that has been rock-solid for us since 2009. in 2012, for another type of data, we’ve started using master-master setup. initially the servers were in different European countries, eventually the secondary site was moved to North America while primary one ... Read More