Discussion

SSH has no Host header

dlenski: SSH is an incredibly versatile and useful tool, but many things about the protocol are poorly designed, including its essentially made-up-as-you-go-along wire formats for authentication negotiation, key exchange, etc.In 2024-2025, I did a survey of millions of public keys on the Internet, gathered from SSH servers and users in addition to TLS hosts, and discovered—among other problems—that it's incredibly easy to misuse SSH keys in large part because they're stored "bare" rather than encapsulated into a certificate format that can provide some guidance as to how they should be used and for what purposes they should be trusted:https://cryptographycaffe.sandboxaq.com/posts/survey-public-....

Eikon: Well, a port per user would work as well.

otterley: Not needing a different port. Middleboxes often block ssh on nonstandard ports.

Charon77: They don't want each vm to have different public IP

CGamesPlay: Using nonstandard ports would break the `ssh foo.exe.dev` pattern.This could also have been solved by requiring users to customize their SSH config (coder does this once per machine, and it applies to all workspaces), but I guess the exe.dev guys are going for a "zero-config, works anywhere" experience.

gsich: Middleboxes are not relevant in this scenario.

3r7j6qzi9jvnve: I wonder if it's something like https://github.com/cea-hpc/sshproxy that sits in the middle (with decryption and everything) or if they could do this without setting up a session directly with the client.Well, we're implicitly trusting the host when running a VM anyway (most of the time), but it's something I'd want to check before buying into the service.EDIT: Ah, it's probably https://github.com/boldsoftware/sshpiperwill try to remember to look later.

kro: [delayed]

morpheuskafka: They are saying they want to directly SSH into a VM/container based on the web hostname it serves. But that's not how the HTTP traffic flows either. With only one routable IP for the host, all traffic on a port shared by VMs has to go to a server on the host first (unless you route based on port or source IP with iptnbles, but that is not hostname based).The HTTP traffic goes to a server (a reverse proxy, say nginx) on the host, which then reads it and proxies it to the correct VM. The client can't ever send TCP packets directly to the VM, HTTP or otherwise. That doesn't just magically happen because HTTP has a Host header, only because nginx is on the host.What they want is a reverse proxy for SSH, and doesn't SSH already have that via jump/bastion hosts? I feel like this could be implement with a shell alias, so that:ssh user@vm1.box1.tld becomes: ssh -j jumpusr@box1.tld user@vm1And just make jumpusr have no host permissions and shell set to only allow ssh.

krautsauer: SSH waits for the server key before it presents the client keys, right? Does this mean that different VMs from different users have the same key? (Or rather, all VMs have the same key? A quick look shows s00{1,2,3}.exe.xyz all having the same key.) So this is full MitM?

otterley: This is admittedly a clever trick, but I can’t help but wonder where it breaks. There seems to be an invariant that the number of backends a public key is mapped to cannot exceed the number of proxy IPs available. The scheme probably works fine if most people are only using a small number of instances, though.

w-ll: -p ?

Charon77: What good does certificate format do? Certainly won't make people not reuse it the same way.> where the affected users might be surprised or alarmed to learn that it is possible to link these real-world identities.I feel like it's obvious that ssh public keys publically identifies me, and if I don't want that, I can make different keys for different sites.

cortesoft: They just need to set the limit on the number of VMs per user to be less than or equal to the number of public IPs they have available. As long as two users don't try to share a key, you are good... which should be easy, just don't let them upload a key that another user has already uploaded.

Dylan16807: Why would anyone configure it to do that?Like, I understand the really restrictive ones that only allow web browsing. But why allow outgoing ssh to port 22 but not other ports? Especially when port 22 is arguably the least secure option. At that point let people connect to any port except for a small blacklist.

josephcsible: Middlebox operators aren't known for making reasonable or logical decisions.

spanjer: this is a test comment

cortesoft: I mean, anytime you use the cloud for anything, you are giving MITM capabilities to the hosting provider. It is their hardware, their hypervisors... they can access anything inside the VMs

Dylan16807: If jump host shell aliases were a valid option, then setting a port would be a much easier valid option.

otterley: I’m not a network security expert, so I don’t know the threat model. I just know that this is a thing companies do sometimes.

otterley: Uh, why not? Unless your SSH client is on the same network as theirs, there are going to be middleboxes somewhere in the path.

bauruine: ssh by default sends all your public keys to a server. Yes you can limit some keys to specific hosts but it's very easy to dox yourself.

miyuru: > We cannot issue an IPv4 address to each machine without blowing out the cost of the subscription. We cannot use IPv6-only as that means some of the internet cannot reach the VM over the web. That means we have to share IPv4 addresses between VMs.Give a user a option for use IPv6 only, and if the user need legacy IP add it as a additional cost and move on.Trying to keep v4 at the same cost level as v6 is not a thing we can solve. If it was we wouldn't need v6.

YesThatTom2: This is the way.

c45y: I would love it if more systems just understood SRV records, hostname.xyz = 10.1.1.1:2222So far it feels like only LDAP really makes use of it, at least with the tech I interact with

iwontberude: I love that kubernetes does this for cluster service domain names

XorNot: The solution to this is TLS SNI redirecting.You can front a TLS server on port 443 and then redirect without decrypting the connection based on the SNI name to your final destination host.

dotwaffle: That's the point, though. An SSH key gives authentication, not authorization. Generally a certificate is a key signed by some other mutually trusted authority, which SSH explicitly tried to avoid.

lmm: The point is that they want the simple UX of "ssh vm1.box1.tld" takes you to the same machine that browsing to vm1.box1.tld takes you to, without requiring their users to set any additional configuration.

snvzz: The solution is ipv6.

TZubiri: Op solved a problem and your comment is "I wouldn't have solved the problem".>legacy IPlol

thaumaturgy: Yeah, I ran into this problem too. I tried a few different hacky solutions and then settled on using port knocking to sort inbound ssh connections into their intended destinations. Works great.I have an architecture with a single IP hosting multiple LXC containers. I wanted users to be able to ssh into their containers as you would for any other environment. There's an option in sshd that allows you to run a script during a connection request so you can almost juggle connections according to the username -- if I remember right, it's been several years since I tried that -- but it's terribly fragile and tends to not pass TTYs properly and basically everything hates it.But, set up knockd, and then generate a random knock sequence for each individual user and automatically update your knockd config with that, and each knock sequence then (temporarily) adds a nat rule that connects the user to their destination container.When adding ssh users, I also provide them with a client config file that includes the ProxyCommand incantation that makes it work on their end.Been using this for a few years and no problems so far.

charcircuit: You don't need SSH. Installing an SSH server to such a VM is a hold over from how UNIX servers worked. It puts you in the mindset of treating your server as a pet and doing things for a single vm instead of having proper server management in place. I would reconsider if offering ssh is an actual requirement here or if it could be better served by offering users a proper control panel to manage and monitor the vms.

jiehong: It’s also similar with mDNS on local networks. It’s actually nice!Overall, DNS features are not always well implemented on most software stack.A basic example is the fact that DNS resolution actually returns a list of IPs, and the client should be trying them sequentially or in parallel, so that one can be down without impact and annoying TTL propagation issues. Yet, many languages have a std lib giving you back a single IP, or a http client assuming only one, the first.

binarin: In kinda the same situation, I was using username for host routing. And real user was determined by the principal in SSH certificate - so the proxy didn't even need to know the concrete certificates for users; it was even easier than keeping track of user SSH keys.Certificate signing was done by a separate SSH service, which you connected too with enabled SSH agent forwarding, pass 2FA challenge, and get a signed cert injected into your agent.

J-Kuhn: But... this doesn't work for SSH, which is the problem here?

9dev: Asking back, when I limit the outgoing connections from a network, why would I account for any nonstandard port and make the ruleset unwieldy, just in case someone wanted to do something clever?

ulrikrasmussen: Wouldn't a much simpler approach be to have everyone log in to a common server which sits on a VPN with all the VMs? It introduces an extra hop, but this is a pretty minor inconvenience and can be scripted away.

TZubiri: I think the vulnerability would be that not only the host can now MITM, but other co-tenants would have the capability to bypass that MiTM protection.

ulrikrasmussen: I also wonder what happens if you want to grant access to your VM to additional public keys and one of those public keys happen to already be routed to a different VM on the same IP.

9dev: It's a nice solution for sure, but a problem by choice. You could just have an AAAA record for the domain in addition to the A record, and as GP pointed out, resolve SSH sessions via the IPv6. If the user wants SSH to work with IPv4 for whatever reason—I see the point that there may be some web visitors without IPv6 still, but devs?—they could pay a small extra for a dedicated IPv4 address.

TZubiri: It's hard to think of a clearer example for the concept of Developer Experience.One similar example of SSH related UX design is Github. We mostly take the git clone git@github.com/author/repo for granted, as if it were a standard git thing that existed before. But if you ever go broke and have to implement GitHub from scratch, you'll notice the beauty in its design.

ulrikrasmussen: I had never thought about that. Seems like an easy problem to fix by sending salted hashes instead.

XorNot: SSH has ProxyCommand which accepts the %h template.Provided your users will configure something a little - or you provide a wrapping command - you can setup the tunneling for them.

grey-area: Doesn’t it try one key at a time rather than send all?

asmor: This is great if you have IPv6 support from your ISP. Not so great if you don't.Before someone mentions tunnels: Last time I tried to set up a tunnel Happy Eyeballs didn't work for me at all; almost everything went through the tunnel anyway and I had to deal with non-residential IP space issues and way too much traffic.

mystifyingpoi: Could you suggest an alternative then? Something that is feature complete with SSH server, and also free.

charcircuit: I have not worked in the server management in many years, but with how cheap code is with AI rolling your own dashboard may not be such a bad idea.>with SSH serverMy comment was about how you do not need an ssh server. The idea of a server exposing a command line that allows potentially anything to be done is not necessary in order to manage and monitor a server.

geocar: > The HTTP traffic goes to a server (a reverse proxy, say nginx) on the host, which then reads it and proxies it to the correct VM.That's one implementation. Another implementation is the proxy looks at the SNI information in the ClientHello and can choose the correct backend using that information _without_ decrypting anything.Encrypted SNI and ECH requires some coordination, but still doesn't require decryption/trust by the proxy/jumpbox which might be really important if you have a large number of otherwise independent services behind the single address.

unsnap_biceps: The server matches your purposed public key with one in the authorized keys file. If you don't want to expose your raw public key to the server, you'll need to generate and send the hashed key format into the authorized keys file, which at that point is the same as just generating a new purpose built key, no? Am I missing something?

sirl1on: Treating your server as pet may perfectly fine. Not everything has to be fully automated cloud cluster cattle.

miyuru: I complained as a yearly tradition for couple of years to get v6 enabled in my ISP. They had the core network enabled on World IPv6 Launch in 2012, but not deployed to end customers.One simple way to check if your ISP have some kind of IPv6 netowork is to see if CDN domains given by YouTube and Facebook have AAAA records.We shouldn't have to ask for ISPs to add IPv6 support but here we are.

unsnap_biceps: It does, and there's typically a maximum number of attempts (MaxAuthTries defaults to 6 IIRC) before the server just rejects the connection attempt.

lifthrasiir: They could buy a dedicated IPv4 address, but that address still has to be tunneled through IPv4 networks if that dev has no access to IPv6 networks. Thus DX still suffers.