ssh: golang/crypto/ssh prioritizes server's kex algo instead of parsing known_hosts first #1526

kayrus · 2016-04-01T08:06:46Z

Original issue was reported here: coreos/bugs#1186

The problem is that golang ssh implementation compares client's ServerHostKeyAlgos and server's ServerHostKeyAlgos and chooses first match depending on the order in supportedHostKeyAlgos variable:

// supportedKexAlgos specifies the supported host-key algorithms (i.e. methods
// of authenticating servers) in preference order.
var supportedHostKeyAlgos = []string{
        CertAlgoRSAv01, CertAlgoDSAv01, CertAlgoECDSA256v01,
        CertAlgoECDSA384v01, CertAlgoECDSA521v01,

        KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521,
        KeyAlgoRSA, KeyAlgoDSA,
}

If remote host had KeyAlgoRSA key before and it was written into client's .fleetctl/known_hosts, then SSH on the remote host was upgraded (i.e. ecdsa-sha2-nistp256 support was added), then crypto/ssh automatically chooses first match: KeyAlgoECDSA256 and doesn't take into consideration already existing fingerprints. This causes Failed building SSH client: ssh: handshake failed: host key mismatch error message on the fleetctl client's side.

The text was updated successfully, but these errors were encountered:

kayrus · 2016-04-01T08:40:10Z

OpenSSH logic:

load known_hosts keys
parse them, determine whether remote hostname/IP corresponds to the known_hosts
set hostkeyalgs order (order_hostkeyalgs)
send SSH2_MSG_KEXINIT
receive host's fingerprint and compare it to the entry inside known_hosts

fleetctl+golang/crypto/ssh logic:

send SSH2_MSG_KEXINIT
receive host's fingerprint and compare it to the entry inside known_hosts

kayrus · 2016-04-01T09:15:51Z

ClientConfig struct contains HostKeyAlgorithms. We can use ParseKnownHosts crypto/ssh function to parse known_hosts and set HostKeyAlgorithms inside sshClientConfig.

Here is an example on how to parse known_hosts files https://github.com/golang/crypto/blob/master/ssh/keys_test.go#L391

Retrieve remote host Key Algorithms from known_host if they are there and use them to perform ssh handshake. Otherwise fallback to default values suggested by remote. This patch is based from a previous patch written by: kayrus <kay.diam@gmail.com> Resolves coreos#1526 and coreos/bugs#1186

kayrus mentioned this issue Apr 1, 2016

[WIP] ssh: define known_hosts key algo before handshake #1527

Closed

tixxdz mentioned this issue Apr 1, 2016

ssh: define the list of Key Algorithms of remote hosts before handshake #1529

Merged

tixxdz closed this as completed in #1529 Apr 4, 2016

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ssh: golang/crypto/ssh prioritizes server's kex algo instead of parsing known_hosts first #1526

ssh: golang/crypto/ssh prioritizes server's kex algo instead of parsing known_hosts first #1526

kayrus commented Apr 1, 2016

kayrus commented Apr 1, 2016

kayrus commented Apr 1, 2016

pFad - (p)hone/(F)rame/(a)nonymizer/(d)eclutterfier! Saves Data!

ssh: golang/crypto/ssh prioritizes server's kex algo instead of parsing known_hosts first #1526

ssh: golang/crypto/ssh prioritizes server's kex algo instead of parsing known_hosts first #1526

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kayrus commented Apr 1, 2016

kayrus commented Apr 1, 2016

kayrus commented Apr 1, 2016

pFad - (p)hone/(F)rame/(a)nonymizer/(d)eclutterfier! Saves Data!