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ssh: send ext-info-c only once

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In accordance to RFC8308, send ext-info-c only during the first key
exchange. Some server implementations such as OpenSSH 7 will send an
extInfoMsg message each time when ext-info-c is received. This results
in a closed connection, as our client does not expect this message while
handling the mux.

See https://bugzilla.mindrot.org/show_bug.cgi?id=2929 regarding the
behaviour of OpenSSH if it sees ext-info-c in later key exchanges.

Fixes golang/go#51808

Change-Id: Id94f1ef73cec6147136246b0b6048b57db92660d
GitHub-Last-Rev: fcfe5ed37306136219854031abc809e0dc9b3124
GitHub-Pull-Request: golang/crypto#208
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/394134
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
This commit is contained in:
Peter Verraedt
2022-03-21 15:39:16 +00:00
committed by Filippo Valsorda
parent 3147a52a75
commit 2c7772ba30
1 changed files with 6 additions and 4 deletions
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ssh/handshake.go
+6 -4
View File
@@ -479,10 +479,12 @@ func (t *handshakeTransport) sendKexInit() error {
// As a client we opt in to receiving SSH_MSG_EXT_INFO so we know what
// algorithms the server supports for public key authentication. See RFC
// 8303, Section 2.1.
msg.KexAlgos = make([]string, 0, len(t.config.KeyExchanges)+1)
msg.KexAlgos = append(msg.KexAlgos, t.config.KeyExchanges...)
msg.KexAlgos = append(msg.KexAlgos, "ext-info-c")
// 8308, Section 2.1.
if firstKeyExchange := t.sessionID == nil; firstKeyExchange {
msg.KexAlgos = make([]string, 0, len(t.config.KeyExchanges)+1)
msg.KexAlgos = append(msg.KexAlgos, t.config.KeyExchanges...)
msg.KexAlgos = append(msg.KexAlgos, "ext-info-c")
}
}
packet := Marshal(msg)