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by Colin Cogle
Published September 24, 2022.
SSHFP records are a vital part of securing SSH. However, Microsoft's DNS Server doesn't give us an easy way to add them. Here's how.
SSH servers all have fingerprints to identify themselves. These little hashes of the host's public key are exchanged with clients, and remembered in a Trust On First Use (TOFU) security model.
The authenticity of host 'www.someserver.net (2001:db8::49)' can't be established. ED25519 key fingerprint is SHA256:1z0nm0KwN5cnOSA9HQ0VzjUR9VaPUd+/tB5FtTB1NdQ=. No matching host key fingerprint found in DNS. This key is not known by any other names Are you sure you want to continue connecting (yes/no/[fingerprint])?
Now, let's be honest. SSH is an incredibly high-value target, but how many of us have actually taken the time to manually verify an SSH key? Raise your hands. Yeah, I'm not raising mine, either.
TOFU is a decent security model (it's used by Gemini, too!), because it protects all connections to the server — except for the very first one.
You might have noticed that there is a line up there that reads, "No matching host key fingerprint found in DNS". Longtime Linux system administrators might know about the SSHFP DNS record, defined way back in RFC 4255. Adding one to a standard DNS server such as BIND9 is as simple as making the records and copying them into your zone file.
$ ssh-keygen -r myservername | tee ssh-keys.txt myservername IN SSHFP 1 2 d73d279b42b037972739203d1d0d15ce3511f5568f51dfbfb41e45b5307535d4 myservername IN SSHFP 3 2 95941914e0c29ca8a104bf4b6c8890c7c172f7ce610bc331ea6aab1537852ead myservername IN SSHFP 4 2 6c0461c9809836eb61b0e3a07b31a99522452a7e28499a39aa8423a17b8e2f2a $ ssh-keys.txt >> /etc/bind9/myzonefile.dns
If you're lucky enough to be using the BIND DNS server, congratulations! Your work is done. However, if this was all I was writing about, it wouldn't make for a useful article.
However, some of us administer Windows networks, which sometimes means that Active Directory Domain Services (AD DS) is in the mix; and, if that be true, then one would be foolish not to use the Microsoft DNS Server alongside that. Though it lags *far* behind BIND in terms of feature and resource record support, it does provide a GUI (albeit dated) and a robust PowerShell module.
While it does not support SSHFP records natively, the DNS Server included with all versions of Windows Server will at least accept, store, and serve SSHFP records. The problem, though, is how to get them saved, as neither the GUI nor the PowerShell cmdlets will let you input one. (Neither will dnscmd, but that thing is deprecated, anyway.)
You might be wondering, though. What is the point of storing SSHFP records when you have a Windows-centric network? Simple: you gain the same benefits.
You will need to be running Windows Server 2012 or later. While this should work with any version of the Microsoft DNS Server, both the RFC — and common sense — strongly recommend you use DNSSEC to secure these records.
If you are running Windows Server 2008 R2 or older, stop right here and evaluate your personal and professional life decisions. Not only has been out of extended support for quite a long time, but its DNSSEC implementation is rudimentary at best.
You will need to gather the SSH fingerprints for all of your hosts. You can log on and run ssh-keygen -r to generate them interactively, or for things like printers and other network devices, a tool like ssh-keyscan can read them remotely.
If you have a macOS or Linux machine with the nsupdate tool on there, you can skip this section. Otherwise, download the BIND9 Tools for Windows to obtain a copy of nsupdate.exe, which will allow us to spit arbitrary DNS records into our zone:
Set-Location $env:Temp Invoke-WebRequest -OutFile 'BIND.zip' -Uri 'https://downloads.isc.org/isc/bind9/9.16.32/BIND9.16.32.x64.zip' Expand-Archive -Path 'BIND.zip'
The nsupdate tool has its own syntax. You can copy and paste all of the hostnames and fingerprints that you've gathered from across your network, and combine them into one big Notepad document.
You can also use this PowerShell to turn your `ssh-keygen -r` output into a file that can be used by nsupdate. Here is an example:
Example: Sample nsupdate input
# Edit this next line with the name of your DNS zone!
$MyDomainName = 'example.com'
$sshfp = ''
Get-Content 'Output of ssh-keygen -r.txt' | ForEach-Object -ScriptBlock `
{
# Each line of the input file will need to be modified to add some more
# items. It must be prefixed with "update add" and have a TTL inserted
# after the hostname (even though the DNS zone will likely override it).
$x = $_ -Split "[\t\s]"
$sshfp += "update add $($x[0]).$MyDomainName 3600 $($x[1..5]).`r`n"
}
# Prepare the NSUpdate command.
$NSUpdateCommand = "server $DNSServer
zone $MyDomainName
$sshfp
send"
Security-conscious people might be thinking that using freeware to inject random DNS records is a very bad thing, and they are correct. Fortunately, by default, Windows-hosted zones default to only allowing secure dynamic updates (authenticated by Kerberos). To accomplish our goal, we will need to temporarily relax security rules.
Set your zone update settings to allow secure and non-secure updates, but don't forget to change it back later. You can point and click, or use PowerShell.
1. In the DNS snap-in, right-click your zone and choose Properties.
2. Under Dynamic Updates, choose Secure and Nonsecure.
3. Click OK.
Image: Screenshot of zone properties
Or:
Set-DnsServerPrimaryZone -Name $MyDomainName -DynamicUpdate 'NonsecureAndSecure'
Now, don't linger, since we've left our zone open to exploit.
Now for the coup de grâce. You can copy and paste your file directly into nsupdate.exe. Alternatively, if you've been following along with my PowerShell commands, you can simply do this:
$NSUpdateCommand | .\nsupdate.exe
Before you celebrate, we need to restore safety and sanity; either skip back a section and undo your changes, or run this PowerShell command:
Set-DnsServerPrimaryZone -Name $MyDomainName -DynamicUpdate 'Secure'
Once this is done, you will see some unknown records listed in your list of DNS entries. They show up as an unknown type because the DNS Server service is an afterthought to Microsoft.
Image: Zone entries, showing the SSHFP records
Unfortunately, because Microsoft's DNS Server service lags so far behind the curve, you will not be able to edit or properly view the record. Just be happy we got this far. You can, though, double-click on it and view the raw binary data, for what that's worth. They show up uneditable because the DNS Server service is an afterthought to Microsoft.
Image: Properties of an "unknown" (SSHFP) record
Now that your SSHFP DNS records are live (and hopefully secured), let's try connecting again. If your records are present and correct, you will see output like this.
The authenticity of host 'www.someserver.net (2001:db8::49)' can't be established. ED25519 key fingerprint is SHA256:1z0nm0KwN5cnOSA9HQ0VzjUR9VaPUd+/tB5FtTB1NdQ=. Matching host key fingerprint found in DNS. This key is not known by any other names Are you sure you want to continue connecting (yes/no/[fingerprint])?
It's a bit of a roundabout method, unfortunately. It would be great if Microsoft's DNS Server gave us an easier way to add and modify SSHFP records (and DANE/TLSA records, for that matter). However, this will work, and this will help you secure your internal environment.
Finally, bookmark this page. This isn't a shameless plug, but a warning that, as you replace devices or regenerate SSH host keys, you will need to regenerate your SSHFP records.
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Previous Blog: "A File Format to Aid in Security Vulnerability Disclosures"
Next Blog: "Rescuing a UniFi Cloud Key Gen2 Plus"
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"SSHFP Records on Windows Server" by Colin Cogle is licensed under
Creative Commons Attribution-ShareAlike 4.0 International (CC-BY-SA).
