christoph ender's


monday the 20th of may, 2024

influx: get minium time value from all series

In order to free some diskspace from my influx databases, which are storing all the performance metrics from my icinga installations, I've been trying to remove the older, no longer relevant records, from the database. In order to get an overview over the current data, I've been trying to find out for how long I've been storing data at all. However, it appears there's no easy way to query the mininum time value for all series/measurements.

→ read more …
saturday the 4th of may, 2024

macOS: delv says “no crypto support”

delv is a tool for DNS lookup and validation. I've been trying to check the DNSSEC status for some domains, on macOS however, there's just an error message.

→ read more …
sunday the 21st of april, 2024

dane and tlsa basics

DANE – DNS-based Authentication of Named Entities – stores hash digests of certificates in TLSA DNS resource records. In combination with DNSSEC it is possible to verify certificates without any CA using DNS alone, thereby eliminating man-in-the-middle and downgrade attacks.

→ read more …
friday the 5th of april, 2024

kobold letters

Turns out specific e-mails, called Kobold letters, may change their contents when they're forwarded, simply by putting properly coded CSS into the mail.

→ read more …
wednesday the 3rd of april, 2024

minimal inwx certbot handler

I've been trying to implement the ACME DNS-01 challenge with certbot and INWX. I'm doing this for a MFA / MobileTAN protected account, which limits the choice of API clients to the INWX PHP client.

→ read more …
wednesday the 27th of march, 2024

removal from private docker registry

It seems there's no easy way to remove tags / images from a private docker registry. It appears the most straightforward way is to get the manifest digest and to put it into a HTTP delete request.

→ read more …
sunday the 24th of march, 2024

accessing a private docker registry

While pushing and pulling to a private docker registry can be done via built-in methods, examining the contents apparently can only be done using separate HTTP queries.

→ read more …
saturday the 23rd of march, 2024

apt: packages kept back

Some day, when applying upgrades with the apt command line interface, the tool might state that some packages were kept back.

→ read more …
monday the 18th of march, 2024

smtp: dsn versus mdn

There are two ways to get information about successful e-mail deliveries: DSN and MDN.

→ read more …
monday the 11th of march, 2024

spf records for helo/ehlo

While running various tests for mail servers, I stumbled upon SpamAssassin's SPF_HELO_NONE warning. It incurs a negative score of 0.001, and the short description complains that “HELO does not publish an SPF Record”.

→ read more …
saturday the 27th of january, 2024

smtp smuggling

At the end of 2023, news about a novel kind of attack on SMTP servers was published: SEC Consult found a way to concatenate multiple e-mails in a way that made many systems process the second mail as if it had been submitted on it's own – along with all the privileges that were granted for the submission of the first one. While the MTA itself isn't attacked, this can be used to make any affected mail server act like an open relay.

→ read more …
thursday the 9th of november, 2023

migrating swap partitions

After replacing a VM's swap partition with another one, I noticed that the system took considerably longer to boot.

→ read more …
saturday the 14th of october, 2023

no identification using MAC address with dhcpv6

I've been trying quite unsuccessful to exclude certain clients from getting an IPv6 via DHCP in a network. Since I just wanted to exclude specific interfaces I've used the MAC address instead of the DUID – the “DHCP unique identifier”, see DHCPv6. As it turns out, although the option to exclude/identify clients via MAC addresses may be present in dhcpv6 servers, it can't be used reliably at all. As the official kea documentation states: “Unfortunately, the DHCPv6 protocol does not provide any completely reliable way to retrieve that information.”

friday the 13th of october, 2023

microsoft-activision takeover

Today, on friday the 13th, microsoft acquired activision, and with it the trademark and all that is left of Infocom, Inc. If the name “Infocom” is known to you at all, you might want to consider sharing “Microsoft consumes Activision; and a plea” or share the associated post on mastodon.

saturday the 7th of october, 2023

get mac address for ipv6

For IPv6, hosts use Neighbor Discovery instead of ARP for IPv4. Accordingly, one can use the ndisc6 tool to look for the MAC address in question.

→ read more …
friday the 6th of october, 2023

kea dhcpv6 fails to bind link-local

Lately, after setting up an instance of an isc kea dhcp6 server , I noticed that after a reboot it would be inactive, although it had been started properly. Turned out that it simply couldn't bind the link-local address.

→ read more …
tuesday the 19th of september, 2023

simplest hd keep-awake

I've been trying to get some long-type smartctl tests to run through uninterrupted. Since they're taking about 11 hours for a 4TB hd – yes the old, spinning ones – these were so far always interrupted by the hd going to sleep. After looking at some measures to deactive the various sleep mechanisms I found the best and simplest one.

→ read more …
tuesday the 12th of september, 2023

prompt failover with isc-kea-dhcp

After migrating to the new isc kea dhcp server - the successor to the older isc dhcp server – I've struggled a bit to get a server pair to do a proper failover when one of the servers fails. Turned out that there's a max-unacked-clients parameter, which tells the system how many dhcp clients need to have sent out dhcp requests before the failover occurs. By default, this is set to 5, so until you dont have five different clients waiting for an IP address, nothing's going to happen. I ended up simply setting this to 0, so once the timeout set in max-response-delay is met, there's always a guaranteed failover to the surviving server.

thursday the 7th of september, 2023

no local nagios dhcp check

One of my server pairs is running icinga and a dhcp server on each of them in HA mode for redundancy reasons. I've been trying to monitor the dhcp service using the nagios check_dhcp plugin. With the servers checking themselves, however, I mostly got many CRITICAL: No DHCPOFFERs were received replies.

→ read more …
tuesday the 22nd of august, 2023

btrfs send and receive

brtfs snapshots are great for incremental backups – just create a snap from a working directory and keep on happily working on the original folder as you please: btrfs makes sure that only incremental changes from the snapshot to the current state will occupy space.

→ read more …
saturday the 19th of august, 2023

working around microsoft blacklisting

Catching up on yesterday's post: It's hard to deny that self-hosting mail for individuals or smaller companies has become a much greater challenge nowadays.

→ read more …
friday the 18th of august, 2023

office 365 “junks” microsoft mail

With all the fuzz these days about getting mail from stand-alone running smtp servers to be recognized as non-junk by the big platforms, it's quite funny to see that even Microsoft can't keep up: On a company's exchange account, which I've been assigned to use, microsoft now sorts its very own e-mails advertising the new teams app and other things into the “junk” folder all by itself.

wednesday the 16th of august, 2023

btrfs snapshot's exclusive space

How much space does a btrfs snapshot actually exclusively allocate? One simply has to run a btrfs fi du -s backup-* in order to see which space is shared between the snapshots and which is exclusively used by the snapshot listed.

→ read more …
saturday the 12th of august, 2023

strict versus real-strict imapsync

imapsync is an extremely useful tool for the migration of imap accounts. While trying to migrate accounts with a very large number of messages, I encountered a few warnings about duplicates. The imapsync FAQ says the it's a problem with message identification – imapsync by default uses the Message-ID: and Received: headers to identify messages on both sides, which may fail when, for example, imap servers change one or more of these headers.

→ read more …
friday the 11th of august, 2023

forcing windows to use openvpn-dns

While providing windows dial-in vpn clients with the dns servers addresses of the internal network using the dhcp-option DNS parameter, I found out that the name resolution didn't work reliably. After some research it turned out that this was due to windows just adding the provided dns addresses to the ones already present on the system, and using all of them for the actual name resolution.

→ read more …
saturday the 29th of july, 2023

defer domain-specific postfix delivery

Some time ago I had to migrate a mail server running multiple domains, whereby these domains were to be moved one after another instead of moving everyhing at once. That meant that the reception of mail had to be paused for specific domains only during the migration of the messages, update of the MX record and so on.

→ read more …
tuesday the 25th of july, 2023

proxying via ssh

One of the recurring jobs coming up when running mail servers is to get the IPs of your mail servers off various blacklists where they happen to turn up for in part completely unknown reasons. In order to get an IP from a blacklist the list owners have invented various ways to achieve this, and one I recently came across required some confirmation on their website while having my user agent coming from the blocked mail IP in question.

→ read more …
tuesday the 2nd of may, 2023

debian on nipogi-jk06

I've been looking for two simple budget machines to run debian with icinga nodes in HA-mode on. Usually the raspberrys I've been so far using would've been enough, but since the supply chain shortage it's been practically impossible to get new ones, except for creatively overprized ones.

→ read more …
friday the 28th of april, 2023

fixed ipv6 assignment

While SLAAC is very conveninent to get multiple hosts configured with minimum effort for ipv6, it's often nice to have a set of shorter addresses for some hosts – it's much easier to remember fd00:0:0:10::1 than fd00:0:0:10:3047:8f88:6801:87b0.

→ read more …
tuesday the 25th of april, 2023

multi-gateway openvpn server

Lately, I had to provide access to a private network over the internet using openvpn. For redundancy reasons, it had to be accessible via two separate gateways, so that whenever one failed, the private network would still be accessible using the alternative gateway. I'm skipping a lot of headache requirements / givens and just describe the solution core.

→ read more …
wednesday the 15th of march, 2023

handling multiple ssh identities

Once you're using multiple identities for services like github or gitlab, along with multiple SSH keys for authentication with these systems, there's the need to tell SSH which of your keys should be used for a new connection. This can, for example, be achived using a combination of the IdentityFile and IdentitiesOnly statements.

→ read more …
monday the 20th of february, 2023

persistent dummy NICs

For monitoring purposes of a raspi device, which only has dynamic IP addresses assigned, I needed a virtual dummy NIC which can be assigned a static IP.

→ read more …
friday the 10th of february, 2023

icinga cluster check

In case all satellites from a non-master zone are going offline at once – if, for example, the only connection to the zone has gone down – there are initially no notifications since there's no entitiy left which could relay messages to the parent/master zone.

→ read more …
sunday the 1st of january, 2023

sherlock holmes in the public domain

Although the copyright for the Sherlock Holmes canon had already expired almost everywhere in the world, some stories remained copyrighted in the U.S. until the end of 2022. Starting January 1st, 2023, the last stories from the casebook of Sherlock holmes entered the public domain and can now also be downloaded legally from the U.S.

→ read more …
tuesday the 20th of december, 2022

write down everything

Reading through Brendan O'Leary's post “What I learned at GitLab that I don't want to forget” I was struck immediately by the very first point he brought up: “Write down everything”, simply because over the last few years I've come to realize that this turned out to be the one of the most important aspects of my work.

→ read more …
friday the 2nd of december, 2022

ipv4 address blocks for documentation

Turns out the IETF has assigned three subnets for the sole purpose of documentation. RFC 5737 says: The blocks (TEST-NET-1), (TEST-NET-2), and (TEST-NET-3) are provided for use in documentation.

saturday the 5th of november, 2022


Having to keep a large number of systems operational requires some kind of monitoring, which in turn needs to be able to connect to the monitored systems. So far I've set up connections using OpenVPN or SSH, but using wireguard turned out to provide the best of both worlds.

→ read more …