ssh/knownhosts: check more than one key #254

caarlos0 · 2023-03-22T16:53:10Z

I believe this fixes golang/go#36126 .

The problem was that it was keeping only the first known key of each
type found.
If you have a server advertising multiple keys of the same type,
you might get a missmatch key error.

Per sshd(8) man page, it should allow reapeatable hosts with
different host keys, although it don't specify anything about
hosts being from different types:

It is permissible (but not recommended) to have several lines or
different host keys for the same names.  This will inevitably happen when
short forms of host names from different domains are put in the file.  It
is possible that the files contain conflicting information;
authentication is accepted if valid information can be found from either
file.

So, this changes knownhosts behavior to accept any of the keys for a
given host, regardless of type.

Fixes #36126

gopherbot · 2023-03-22T17:00:10Z

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gopherbot · 2023-03-22T17:06:30Z

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lonnywong · 2023-07-04T01:39:09Z

Any progress? Love to see this merged. Thanks!

evanelias · 2023-07-04T17:43:08Z

One potential downside to this PR is it changes the documented behavior of knownhosts.KeyError.Want, re: "For each key algorithm there can be one hostkey"

type KeyError struct {
	// Want holds the accepted host keys. For each key algorithm,
	// there can be one hostkey.  If Want is empty, the host is
	// unknown. If Want is non-empty, there was a mismatch, which
	// can signify a MITM attack.
	Want []KnownKey
}

@lonnywong out of curiosity, do you really have 2 keys of the same type for the same hostname (same situation as golang/go#36126, which would be solved by this PR)? Or do you have a server presenting multiple hostkeys of different types, some of which aren't in the client knownhosts file (see golang/go#29286)?

Both issues result in the same user-facing error on the client side, but they have different root causes. In my experience, the latter situation is much more common.

I have a widely-used package, github.com/skeema/knownhosts, which uses some trickery to provide a workaround for golang/go#29286, although it doesn't help with golang/go#36126.

Just mentioning this here because github.com/skeema/knownhosts currently assumes the "For each key algorithm there can be one hostkey" logic remains the case, although I don't think it would break anything if that changes -- pretty sure it would just lead to potential duplicate entries in ssh.ClientConfig.HostKeyAlgorithms which hopefully is harmless? Anyway that should be easy to solve if it is a problem. Not sure if other packages out there depend on that logic more strictly though.

lonnywong · 2023-07-04T23:36:51Z

@evanelias Thanks very much for your help. My case is multiple hostkeys of different types. I didn’t find golang/go#29286 before. I’ll try your awesome package https://github.com/skeema/knownhosts this weekend. Thanks again.

Currently the behavior of HostKeyAlgorithms never contains duplicates, only by virtue of golang.org/x/crypto/ssh/knownhosts exposing a maximum of one key per algorithm in its KeyError.Want slice. However, that upstream behavior could theoretically change in the future, especially since golang.org/x/crypto is versioned as a pre-v1 module, and the one-key-per-type behavior is only documented as a comment (e.g. not part of any type or function signature). This commit makes our HostKeyAlgorithms function more robust / future-proof by ensuring that its result does not contain duplicates, regardless of upstream behavior. This means if golang/go#28870 is solved (for example by golang/crypto#254), there should not be any harm to our behavior here in github.com/skeema/knownhosts.

gopherbot · 2023-09-25T15:25:59Z

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caarlos0 · 2024-02-29T15:21:46Z

anything I can do to help move this forward?

ianlancetaylor · 2025-02-18T02:29:37Z

@caarlos0 We do code review on Gerrit, and as noted above this pull request is https://go.dev/cl/478535. If you look there you will see some unresolved comments. The first step to moving this forward is resolving them one way or another. Thanks.

caarlos0 · 2025-02-18T12:34:34Z

@ianlancetaylor yep, I'm aware, the comments were posted after I asked here, though... anyway, will fix them

gopherbot · 2025-02-18T12:43:58Z

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gopherbot · 2025-02-18T12:43:58Z

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gopherbot · 2025-02-18T12:43:59Z

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gopherbot · 2025-02-18T12:44:00Z

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gopherbot · 2025-02-18T12:44:01Z

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gopherbot · 2025-02-18T12:44:04Z

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gopherbot · 2025-02-18T12:44:09Z

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The problem was that it was keeping only the first known key of each type found. If you have a server advertising multiple keys of the same type, you might get a missmatch key error. Per `sshd(8)` man page, it should allow reapeatable hosts with different host keys, although it don't specify anything about hosts being from different types: > It is permissible (but not recommended) to have several lines or > different host keys for the same names. This will inevitably happen when > short forms of host names from different domains are put in the file. It > is possible that the files contain conflicting information; > authentication is accepted if valid information can be found from either > file. So, this changes `knownhosts` behavior to accept any of the keys for a given host, regardless of type. Fixes #36126 Signed-off-by: Carlos Alexandro Becker <caarlos0@users.noreply.github.com>

gopherbot · 2025-02-18T12:54:24Z

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caarlos0 mentioned this pull request Mar 22, 2023

ssh: handshake failed: possible man-in-the-middle attack: knownhosts: key mismatch charmbracelet/wishlist#152

Open

lonnywong mentioned this pull request Jun 19, 2023

tssh无法连接远程机器 WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! trzsz/trzsz-ssh#20

Closed

Kemystra mentioned this pull request Dec 10, 2023

Cant create SFTP repository. Kopia does not support ed25519 keys kopia/kopia#2443

Open

Reallyfe approved these changes Feb 18, 2025

View reviewed changes

caarlos0 changed the title ~~crypto/ssh/knownhosts: check more than one key~~ ssh/knownhosts: check more than one key Feb 18, 2025

caarlos0 force-pushed the known_hosts-keys branch from adcf924 to 58ab267 Compare February 18, 2025 12:43

caarlos0 force-pushed the known_hosts-keys branch from 58ab267 to 2ccd649 Compare February 18, 2025 12:45

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ssh/knownhosts: check more than one key #254

ssh/knownhosts: check more than one key #254

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