TLS certificates for internal services done right
Comments
EvanAnderson
stock_toaster
Once dns-persist-01 becomes available/usable[1], it should make dns validation even easier.
mrl5
OP here. Actually I tried to use it but apparently prematurely -> https://github.com/acmesh-official/acme.sh/issues/7085#issue...
Once it's supported I think my next iteration will be DNS persist + internal ip addresses on the public zone.
Thank you all for comments and feedback! It's cool to see real interest in this blog post
isomorphic
Thank you, I was unaware of that. It looks like it's already support in the acme.sh client, but there is a Let's Encrypt discussion saying it's still pending at LE:
https://community.letsencrypt.org/t/dns-persist-01-deploymen...
I wonder if the interim version has been rolled out to some CAs.
xg15
At this point, can't we make a standard that skips the middlemen and just embeds the certificate directly inside the DNS entry? It seems the challenge system is converging towards that anyway.
ivlad
Or, just use IPv6 and host Internet services in a routable address. Then, use ACLs at web server / proxy / L7 lb level to allow acme-challenge unauthenticated but everything else authenticated.
Lets encrypt supports IPv6 for validation.
theK
I never understood the issue with DNS-01. if you have a process that you trust to maintain a zone's TLS identity what is the big deal about letting it control a record in that zone?
thayne
Another big problem with split-horizon is you can get clients that cache the dns result before connecting to the vpn, then after connecting, can't actually use the service, because it is using the public ip instead of the private one.
waynesonfire
> because it is using the public ip instead of the private one.
For this edge case, the issue is not with DNS; it's that the router is not configured to allow internal clients to access the network through the public IP, vis-à-vis hairpin routing.
throw0101d
Just use IPv6: no "internal" (10/8) or "external" address, just an address. ;)
selfmodruntime
Also be careful when using split DNS and Tailscale, which increasingly won't work without MagicDNS enabled.
JoeBOFH
And you still have issues sometimes around dns not able to figure out which interface to use. We had issues around that before.
techcode
Yes - literally just pick the right Caddy (or similar) image with Let's Encrypt client included - and you can simply add a tag/label to your docker compose files for each of self hosted services to get a real SSL, that auto renewed ...etc.
With one of self hosted services being Adguard-Home can do both ADs blocking and internal DNS... The public DNS records for your "internal use only" domain remain empty.
s8kur
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bruce511
Came here to say the same. I use DNS-Challenge rather than HTTP-challenge, and that makes internal servers trivial.
You need a DNS provider which supports API calls (I use DNSimple) but the core is all very straightforward.
To prevent having to include DNSimple authentication on the client's internal server I have a small API server on the web which does the Acme work.
isomorphic
I do this exactly, using ACME DNS:
https://github.com/acme-dns/acme-dns
CAUTION, though, the last time I downloaded a binary release, ClamAV triggered on it, so I kept my old version which worked. I was using the 1.0 series (without any problems!), and now it seems the project has picked up development again with a 2.0 series.
throw0101d
> You need a DNS provider which supports API calls (I use DNSimple) but the core is all very straightforward.
Library/CLI that speaks the API of several dozen DNS providers so you don't have to re-invent the wheel:
bombcar
Just LE a wild card cert and slap it everywhere.
EvanAnderson
I get antsy about a private key being in lots of places. You've also got to worry about renewal so you might as well just have "consumer" of the wildcard just provision its own non-wildcard certificate.
jve
... Could you please provide a solution? What should I do in my homelab?
Saying something isn't bad without pointing to right direction makes my insides hurt a little bit.
EvanAnderson
I feel like I did. >smile<
> Use DNS validation to allow these internal services to pull ACME certs...
The major ACME clients support DNS validation. If your DNS host doesn't have an API that your ACME client supports either get a new ACME client or a new DNS host.
raffraffraff
Hmm. I don't really care enough about leaking home network host names because they are all super generic names like 'router', 'laptop', 'tv', 'nas'. So I use my public zone on cloudflare. I just use internal ip addresses (eg: nas.example.com = 10.1.2.3) on the public zone and DNS01 challenge for let's encrypt. Anyone can resolve the ip for any of my hosts, but obviously you'd need to be on the wireguard vpn to hit them.
This means that I can always use public DNS servers like 1.1.1.1, 8.8.8.8, nextDNS etc
This is not "done right" by any stretch but it's extremely low effort to set up and has never once failed me, unlike countless complex meshy things.
graton
I do the same thing. I'm not worried about them seeing my FQDNs.
I use the form of hostname.int.example.com for everything inside my home network. None of which is accessible to the outside world. I use LetsEncrypt with DNS validation to get the certificates.
tzs
If you are going to have all the home stuff on a subdomain (int.example.com) would it work to delegate int.example.com to a DNS server running at home what has internet access, and could handle the ACME DNS challenges for machines on int.example.com?
If it does then you don't have to mess with your public DNS whenever you want to add or renew certificates for home machines.
I'm using the free DNS my registrar provides, which doesn't provide API access unless you upgrade to their paid DNS service and so if I could use a local DNS server for the ACME challenges for the home network I could pick one that is friendly to automation.
throw0101a
> I use the form of hostname.int.example.com […]
Note that int is a valid TLD:
luckman212
Fair, but what about names that are specific enough to give an attacker a clue to a potential attack surface, like "authelia.example.com" - now they know you've likely got an Authelia setup, and can start digging for exploitable CVEs etc. I'm in the process of removing all my individual certs and replacing with a wildcard cert served by Traefik. Is that a bad idea?
bbkane
Can they dig for exploitable CVEs if they're not on the Wireguard network? It is a clue to your infrastructure, but I personally think the simplicity is worth it.
icedchai
Do the names resolve to publicly routeable IPs? If not, I wouldn't worry about it.
nijave
My IaC is on public GitHub. They could do a network scan to find software then fingerprint to find version anyway.
Removing attack surface is better than trying to hide it.
akerl_
How many people out there have attackers doing individualized research to identify services on their home LAN so they can chain a network attack with CVEs in their self-hosted service?
dizhn
Subdomains with wildcard dns along with wildcard certificates solves that issue. You only expose a wildcard domain with no subdomains.
thayne
I don't think this is any less right than using split horizon. IMHO, there is no "right" way to do it. Every approach has downsides and tradeoffs.
icedchai
This is similar to what I do, except I have my own authoritative DNS servers instead of Cloudflare.
I'd prefer this over split DNS, any day.
rao-v
At that point why not just use the .ts.net addresses Tailscale provides for free?
raffraffraff
I get to use the same host names and IP addresses at home or on the VPN. I run a simple wireguard server on a Raspberry pi, and the whole setup has been rock solid.
I did set up tailscale, way back. After using it a few times to test, it failed me when I really needed it (I was out of the country and it failed - can't remember exactly what went wrong but it wwas 100% 'in my tailscale account'). I immediately dropped it and went back to OpenVPN (shit but reliable) before building my current setup.
kennywinker
Because hostname.tail62bc83.ts.net is a mouthful.
boscillator
The real answer here is that configuring HTTPS clients to trust a self-signed cert (or signed by an internal CA) shouldn't be as difficult as it is. I find it extremely annoying that every programming language has it's own idea of where certificates should live instead of just checking the os trust store.
greengreengrass
Possibly a separate concern, but I have some degree of confidence that the requirements and oversight of the CA/B forum (or whomever else determines which root certs go into bundles) are sufficiently strict, and issuers kept under sufficient scrutiny, that I tend to trust those more than I trust myself to secure my own root CA keys adequately. The ideal would be for people setting up their own PKI to ensure their root uses the Name Constraints extension, but the default “can sign anything for any host” I fear makes it easy for people to install their own self-pwn device, and probably left the private key lying around on a box exposed to the Internet.
* with some notable root certs that I have… questionable… trust and confidence are not simply controlled by certain state actors.
WhyNotHugo
Personally, I use a custom local CA with name constraints so that it can only sign domains for The .internal TLD. This is the most important bit: because if the cert is ever leaked, it cannot be used to MITM connections to other domains.
I have to secure the CA's key, but I also have to secure all the keys for the certificate it signs, both being a similar level of challenge.
For personal use, or for very small organisations, using a passphrase-protected Yubikey as a "cheap HSM" should suffice.
thayne
I wish that in addition to the CA setting Name Constraints (and client software validating that), that end users could add additional constraints when adding a new trusted CA, so that even if the CA cert doesn't have Name Constraints, you can restrict it to a specific domain.
afarah1
Even if all applications look at the OS trust store, in my experience there's always a gap distributing the CA to every consumer, leading to time spent on debugging from time to time... Maybe that's not the case in perfectly homogeneous or sufficiently small environments where every team uses the same infra / stack.
WhyNotHugo
Yeah, adding CAs to the store sucks. On Linux it needs to be in /etc/ssl/certs (this varies slighly per distribution), which is only writable by root. A single user can't trivially trust a CA, and a great deal of applications/libraries don't support overriding the store's path.
skywhopper
Is there a commonly used language other than Java that doesn’t just defer to the OS trusted CAs by default?
michaelt
The behaviour of curl depends on how your version was built.
Python? The widely used 'requests' relies on 'certifi' and skips the OS store - but 'pip' on the other hand does use the OS store.
A tool that might use java, like a database or IDE, means it might have its own store.
Node? Make sure you set NODE_USE_SYSTEM_CA=1
Firefox and Chrome AFAIK both have their own stores.
Building a Docker container? That's intentionally isolated from the host, of course your container won't inherit the OS trusted CAs. Running a VM locally? Same.
Installed something using snap? The container-like isolation means it won't pick up the OS trusted CAs.
And of course you need the certs set up right on your cloud servers, your CI servers, the dozen different smartphones the mobile team uses for testing.
brewmarche
Python for example, although nowadays you can simply install pip_system_certs to change that behaviour.
A lot of non-language tools bring their own certificate bundle as well, like uv, git, curl and Firefox. (I think they might all be the same Mozilla bundle even).
However, it seems like the situation here has improved slightly: git can read Windows certificates now once a flag has been set, Firefox has a flag for Windows and macOS, and it supports p11-kit. curl can be built with Windows/macOS support and respects OpenSSL environment variables. uv can be configured to use system-certs.
And obviously any VM or container has to be set up separately. That often includes stuff like pipelines in forges. As mentioned in the sibling comment, at least here it's definitely by design.
jeroenhd
Node and (probably by extension) Electron infuriate me with their custom requirements.
Several Rust libraries also tend to default to a predefined set of certificates (which makes sense for libraries supposed to run on bare metal as there are no system certificates there, but that's not really a problem on most Linux installs). I make it a point to always use the native OS roots in the code I write, but unfortunately that's not universal.
Having to bind-mount certificates inside of docker containers is also always an annoyance I forget about until I see the first TLS errors in the logs, but that's by design and probably a good thing.
spacebanana7
It's hard to get right when OSs, programming languages, browsers and sometimes other applications have their own opinions about trust stores. I understand why our IT department want corporate devices to use internal CA certs on paper but it just breaks stuff in the real world.
patrakov
My preferred procedure is to use DNS-01 validation and have no publicly accessible "A" or "AAAA" record for internal services.
Or even a more extreme example: https://crt.sh/?id=27555237869 (sorry for any possible crt.sh downtime) - the domain name in question never existed in public or private DNS by itself. It is used only for a WPA3-Enterprise network, as the CN that WiFi clients expect to be present in the RADIUS server certificate, but never resolve. In the public DNS, only the "_acme-challenge" TXT record exists.
otabdeveloper4
Sounds bonkers. Why not make an overlay LAN and host your own DNS server in 10.0.0.0/8?
patrakov
I do have a DNS server in my LAN, with some records served to internal clients only. But the _acme-challenge record needs to be public for the DNS-01 validation to succeed.
The point was that you can obtain a certificate for a domain name without creating any records other than the _acme-challenge TXT record. I.e., that the domain might be completely empty all the time except for this record.
wrxd
I use the acme dns-1 challenge on my public domain. That gives you certificates you can use as you see fit, without needing to expose anything else to the public internet.
I also use Tailscale so I configure my DNS to use my Tailscale IP addresses. If you don’t want to expose them on a public DNS server you can add them only to an internal DNS server.
throw0101d
> I use the acme dns-1 challenge on my public domain.
See also perhaps DNS aliasing in case you are not able to dynamically update your 'primary' domain, but can update a secondary or sub-domain:
* https://github.com/acmesh-official/acme.sh/wiki/DNS-alias-mo...
So if "example.com" is control by Corporate IT, and they don't want 'random' folks fiddling with it, then you can create a "dnsauth.example.com" and point the dns-1 challenge record from "…foo.example.com" to "foo.dnsauth.example.com" (or a completely different domain, like "…example.net").
There are DNS servers written strictly focused on this use case:
* https://github.com/acme-dns/acme-dns
Also code that handles a bunch of DNS provider APIs so you don't have to roll your own for ACME client hooks:
adontz
Moreover you can delegate domain to improve security.
https://www.eff.org/deeplinks/2018/02/technical-deep-dive-se...
samgranieri
Split horizon DNS is not something I'm willing to do. I'll just rock out with .internal or .home.arpa, have step-ca and bind communicate to each other, either in step-issuer in kube or maybe even rfc2136 if i feel like a bit of the dns-01 strategy is in order. I slap the internal ca root certs everywhere, and keep my home infra out of the crt.sh logs.
I get it, I could just do *.mydomain.com and slap that wildcard cert everywhere, but it's still in the public logs..
xurukefi
Or...
- Don't use split DNS. Don't use any special internal or dev domain. Leave it to your infrastructure to route/NAT those public IPs to your internal network.
- Don't use the HTTP-01 challenge. Use DNS-01.
- Don't run your own internal CA. Use Let's Encrypt. If you care about name leakage (CT Logs), use wildcard certs. Use a central reverse proxy/load balancer for termination.
Walf
>Don't use split DNS
So what's your solution when you have a wholly private service that will never have a public v4 address, nor a publicly routable v6? How do clients get the address for a nice domain name without the addresses in public DNS?
I use acme.sh with DNS validation, and use common domains that have both public and private services on subdomains. I use split horizon so private.domain.example resolves only on LAN and VPN, and public.domain.example resolves everywhere, but the address changes depending on the network one is connected to.
xurukefi
> How do clients get the address for a nice domain name without the addresses in public DNS?
They don’t. You put the addresses in public DNS.
master_crab
Wait…what? You have a fully internal service and you dont have an internal DNS server? I guess those exist in theory, but not in practice. :)
If you have a heavy enough tech stack to run fully internal services, than you can also run an internal DNS service (even pihole is enough) and load internal only entries there.
Or add everything to your hosts file if you have a central config service.
jeroenhd
Using wildcard certs and/or a central reverse proxy defeats the purpose of internal TLS.
Split-horizon DNS for a publicly usable domain is almost always a bad idea, but running your own ACME server is pretty easy (maybe 10 lines of Caddy config) and using an internal domain (an actual one, not a randomly picked TLD you don't think exists yet) solves the problem pretty easily. You'll want a safe backup for your root certificate private key, of course, but that's pretty much all you need to really worry about.
preisschild
> Use a central reverse proxy/load balancer for termination.
If you do that anyways, you could also use something like oidc authN/authZ on the reverse proxy level and just expose it to the internet.
You dont even need to self host the oidc idp, you can use Google/Github or even something like ATProto
Plasmoid
Let's encrypt is great but if you operate any kind of scale you can quickly hit their rate limit. AWS only recently started allowing you to pull the certificate out of ACM.
Hamuko
I use a registered domain with DNS validation and then CNAMEs that I resolve locally. Basically:
1. Register a domain ("server.com") and put it on some public DNS that can do DNS validation with acme.sh.
2. Use DNS validation to get a certificate on your domain from Let's Encrypt. You can just grab a wildcard one ("*.server.com").
3. CNAME all of your services on a public DNS to an internal address ("email.server.com" → "server.internal", "plex.server.com" → "server.internal").
4. Resolve your internal address on a local DNS server with an A record ("server.internal" → 192.168.0.123). This can often just be done on your router.
iambvk
I like this approach. Thank you.
Would it work if a user's device that is already connected to the VPN, but has custom DNS override to say 8.8.8.8 ? How can I allow my users to be able to use 8.8.8.8 DNS override and still work seamlessly?
Hamuko
I think you'd at least have to have the fallback DNS set to your local DNS resolver in order to get the server.internal resolved to a A record.
thomashabets2
Personally, I hate split horizon DNS. I prefer the "BeyondCorp" model. I MUCH prefer putting an mTLS cert in my trusted devices over relying on VPNs in same devices. I've yet to see a "clever" DNS setup not cause annoyances.
Specifically grafana is nice to be able to see on the phone, and split horizon DNS and corp VPN is a hassle, to say the least, on phones.
I bet you can do it with HA-Proxy, but I use https://github.com/ThomasHabets/sni-router
cactacea
Zero trust or bust. I actually cringed when I read "done right" applied to split-horizon DNS.
pizzalife
I don't agree that tunneling everything through some external facing proxy is "TLS certificates for internal services done right".
nijave
Arguably it's 1/2. You can put public certs on proxy then give proxy private CA to backend services. Then you don't need public certs for all the private stuff nor need to trust the private CA on all your devices.
jmbwell
I don’t know much in this space, but I find myself wishing there was a dead simple self hosted CA solution and also that trust on first use (à la ssh) was A Thing for self-managed root certs in client implementations. TOFU is such an elegant, good-enough solution for these use cases. Fixed deployment is always still an option, but in this day and age it feels so much like we are unnecessarily still dealing with solved problems
nijave
On k8s, there's cert-manager but also you need k8s...
Most browsers support trust on first use for leaf certs
jmbwell
I guess I mean treat it as a clear first class feature. Right now most browsers treat it as an arcane error. I’m thinking more “This is the first time you’re connecting to this site. Do you trust it?”
And later if something changes, then they can do the whole DOING SOMETHING NASTY! thing, which is effectively the experience today
notTooFarGone
Firefox now started that you can't even go on the page on some occasions.
Using a browser in an air gapped environment is so much more pain than it should be.
bigfatkitten
That support is limited. Browsers refuse to support passkeys when you TOFU untrusted certs, and for good reason.
zufallsheld
Well, if your cert-manager distributes its own CA, you'd still need the clients to trust the CA, even in k8s.
xorcist
This is crazy. If you have a home network with a few internal services, or some sort of network where you don't control the endpoints, just use DNS validation. That's why it exists.
But on hosts you control, you should absolutely provision them with an identity and join the local CA. You're going to need it for a multitude of other reasons.
maqnius
Can you elaborate why one shouldn't use DNS validation for hosts you control in general?
xorcist
That's not it. If you have an internal network, where every host is provisioned by you, you already control identity.
In that case there's no need to validate anything as names, dns records, certificates and anything else should already be in place.
raquuk
I am looking forward to finally using DNS-PERSIST-01 for validation. No more dynamic DNS updates, DNS credentials or forwarding necessary.
Thom2000
Sadly most tools still doesn't support it: https://github.com/cert-manager/cert-manager/issues/8373#iss...
And then the issue is protecting the private key of the issuer and monitoring certificates (it's a good idea to do that anyway).
sandeepkd
I was under impression that I understand networking and DNS resolution. It was really hard to follow, the OP did worked hard, just not sure what exact problem was being solved with the proposed solution that isnt already been solved.
lee_ars
OP, why mess with acme.sh's cron schedule with your own cron job that calls its cron job? You might instead consider running acme.sh again with the "--install-cert" flag, coupled with its "--reloadcmd" flag, in order to automatically have it do the cert+key copying and nginx reloading for you when it renews:
acme.sh --install-cert -d grafana.tuxnet.dev --key-file /etc/ssl/private/grafana.tuxnet.dev.key --fullchain-file /etc/ssl/certs/grafana.tuxnet.dev.crt --reloadcmd "systemctl reload nginx"
Then you can ditch your custom cron and let acme handle everything on its own, as intended.
aliasxneo
I wonder if the author realizes that getting public certificates results in them being recorded in CT logs.
Thom2000
Yep. Especially with non-wildcard certs this leaks all service names (privacy concern).
master_crab
Split brain =/= done right.
This introduces all sorts of aberrant behavior. Most notably that clients will get different responses depending on their query and route that you now have to test for. It also is a “silent” deviation for audiences that may not be aware of.
I’m not sure what problem that is trying to be solved here? That the OP wanted internal users seeing a different site than external users? Or using a different route in? In both those cases the correct user behavior is to use a different DNS record.
cobertos
Why not just map the domain to an internal IP and call it a day? Then the only way it can be accessed is through a VPN. Then use a wildcard so none of leaks into cert transparency logs
throw0101d
> Then use a wildcard so none of leaks into cert transparency logs
You now also have to build infrastructure to distribute the wildcard from (presumably) central place where you generate it to all the different places where it is desired.
And hope the wildcard's private key does not leak from one of myriad of places it now lives.
mnahkies
Personally I think wildcard certs are more secure in many instances - if you're just running one load balancer/reverse proxy then all the private keys are going to the same place anyway. You get the upside of not leaking your subdomains to transparency logs, and having fewer certs to renew.
It also forces you to use DNS challenges, which means you don't require the publicly route able address.
Personally what I do is: - Predominantly use wildcards
- Run a tiny coredns instance with my A records for a internal subdomain of my normal domain
- Configure tailscale to use the coredns resolver
- Run two haproxy instances, one for internal services, one for public facing. The public facing one can't route directly to the internal services.
When even the obscure DNS provider I'm using is supported for DNS challenges, I really don't see much upside to using HTTP challenges anymore
waynesonfire
> - Run two haproxy instances,
This made me consider my haproxy architecture. I use haproxy acl rules and the use_backend directive to enforce security policy and routing. It denies http requests based on source ip address per service (host header)
I think a good middle-ground is to use a single haproxy but two frontends; one binds to the public interface and one for private. haproxy is now no longer responsible for routing, the router is.
ramblurr
> TLS certificates for internal services* done right
* "internal services" = on a single server that is publicly routable
ivlad
Enterprise PKI is not hard and has many uses besides issuing certificates to web servers. Any company of 1000+ users or endpoints should just set one up.
I did it multiple times, most recently using YubiHSM as root key store for offline enterprise root CA.
Eduard
to help with passive reconnaissance, here are tuxnet.dev's SSL certificates and associated subdomains:
waynesonfire
¯\_(ツ)_/¯
Not quite done right.
maxgashkov
I've settled on using .internal and Knot as a authoritative NS, step CA + ACME to issue short-lived certs, and a Split DNS resolver from Tailscale as the only external dependency (mostly as a convenience for when I'm on the road).
I do have a luxury of all the homelab VMs being rebuildable via IaC, so I've just injected CA trust at that step.
The biggest PITA so far were 3rd party docker images, each with its own way to inject custom CA.
iOS devices were surprisingly easy to handle.
sigio
My setup is having a wildcard DNS record and a wildcard certificate for my 'home' domain. It has a fixed IP from my ISP, so you always end up on haproxy, which then forwards to individual ports/ip's in the internal network. I can do filtering based on source-ip from there, so traffic from myself/internal will be allowed, and outside traffic (not from some allowlisted ip's) will get blocked. ACME validation is done via DNS, so nothing needs to be accessable for that to issue certificates. Internally I will usually also use the public IP for services, so no need for a split-dns.
28304283409234
When using letsencrypt for internal services it becomes very hard to distinguish badplayer.com LE certificate from the good players. You only ensure encryption. Not identity. Do not use letsencrypt internally for things that matter.
spydum
this is all fine and good, if you are okay broadcasting your internal hostnames. I suppose it's a trade off some might make.
CartwheelLinux
There's one way around that which is requesting a wildcard cert, but then that has its own rammifications
mrl5
I've documented how to securely set up TLS certificates for internal services without creating TLS issues for http clients downstream. All thanks to split-horizon DNS, WAF and ACME protocol. All for free!
rootnod3
Uhm....source?
jabart
A Github Action running acme.sh that pushes certs to S3 solves the split dns issue for hosts, which can cause all sorts of weirdness after a while. You can then grab a cert on a schedule and even make them wildcard if you want. Then you will get NXDOMAIN if you are not on the VPN so ideally no public traffic.
llama052
DNS-01 validation is way less work than that in my experience.
gmuslera
"Right" without use case can be wrong. And by use case I include scale. For a small team, few machines, some in-place infrastructure may worth it. Smaller than that may be overkill, bigger than that may not be enough, or end being cumbersome, insecure or not work for everyone.
woranl
I use client certificate authentication (or mTLS) to access internal services. It requires the client device to present a digital certificate during the TLS handshake, so I don’t need to white list any IP addresses.
AtNightWeCode
Don't do this. Public certs are for public services.
cpach
Why does it matter?
0010010111
Feasible in local (7) btrfs.
guptadagger
im not understanding why you would need a waf at all
denkmoon
Bogus nonsense. Sign your internal certs with an internal CA, make your clients trust the CA. Use it properly.
NicolaiS
> "make your clients trust the CA"
There's no single trust store: the OS has one, Firefox, Java (cacerts), Python (certifi), Node, Go containers, all your Docker images, ...
Failure? People just toggle TLS verification off.
Easy to do in a one-man shop, but almost impossible in any big company. Try grepping for verify=False / -k / InsecureSkipVerify in any fortune 500 and you'll find plenty
nijave
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nntlol
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blueflame7
[dead]
The relative proximity of the words "done right" and "split-horizon DNS" makes my insides hurt a little bit.
Use DNS validation to allow these internal services to pull ACME certs. There's so much less headache, long-term.
Split-horizon DNS (and the tedious make-work it can create when you start needing to mirror public-accessibly records in the private DNS) has always been something to aspire to move away from in my experience.