plumbing: transport/ssh, auto-populate ClientConfig.HostKeyAlgorithms. Fixes #411

This commit adjusts the transport/ssh logic in command.connect(), so that it
now auto-populates ssh.ClientConfig.HostKeyAlgorithms. The algorithms are
chosen based on the known host keys for the target host, as obtained from the
known_hosts file.

In order to look-up the algorithms from the known_hosts file, external module
github.com/skeema/knownhosts is used. This package is just a thin wrapper
around golang.org/x/crypto/ssh/knownhosts, adding an extra mechanism to query
the known_hosts keys, implemented in a way which avoids duplication of any
golang.org/x/crypto/ssh/knownhosts logic.

Because HostKeyAlgorithms vary by target host, some related logic for setting
HostKeyCallback has been moved out of the various AuthMethod implementations.
This was necessary because the old HostKeyCallbackHelper is not host-specific.
Since known_hosts handling isn't really tied to AuthMethod anyway, it seems
reasonable to separate these. Previously-exported types/methods remain in
place for backwards compat, but some of them are now unused.

For testing approach, see pull request. Issue #411 can only be reproduced
via end-to-end / integration testing, since it requires actually launching
an SSH connection, in order to see the key mismatch error triggered from
https://github.com/golang/go/issues/29286 as the root cause.

1 files changed, 23 insertions, 1 deletions

diff --git a/plumbing/transport/ssh/common.go b/plumbing/transport/ssh/common.go
index 46e7913..4b9ac07 100644
--- a/plumbing/transport/ssh/common.go
+++ b/plumbing/transport/ssh/common.go
@@ -121,10 +121,15 @@ func (c *command) connect() error {
 	if err != nil {
 		return err
 	}
+	hostWithPort := c.getHostWithPort()
+	config, err = SetConfigHostKeyFields(config, hostWithPort)
+	if err != nil {
+		return err
+	}
 
 	overrideConfig(c.config, config)
 
-	c.client, err = dial("tcp", c.getHostWithPort(), config)
+	c.client, err = dial("tcp", hostWithPort, config)
 	if err != nil {
 		return err
 	}
@@ -162,6 +167,23 @@ func dial(network, addr string, config *ssh.ClientConfig) (*ssh.Client, error) {
 	return ssh.NewClient(c, chans, reqs), nil
 }
 
+// SetConfigHostKeyFields sets cfg.HostKeyCallback and cfg.HostKeyAlgorithms
+// based on OpenSSH known_hosts. cfg is modified in-place. hostWithPort must be
+// supplied, since the algorithms will be set based on the known host keys for
+// that specific host. Otherwise, golang.org/x/crypto/ssh can return an error
+// upon connecting to a host whose *first* key is not known, even though other
+// keys (of different types) are known and match properly.
+// For background see https://github.com/go-git/go-git/issues/411 as well as
+// https://github.com/golang/go/issues/29286 for root cause.
+func SetConfigHostKeyFields(cfg *ssh.ClientConfig, hostWithPort string) (*ssh.ClientConfig, error) {
+	kh, err := newKnownHosts()
+	if err == nil {
+		cfg.HostKeyCallback = kh.HostKeyCallback()
+		cfg.HostKeyAlgorithms = kh.HostKeyAlgorithms(hostWithPort)
+	}
+	return cfg, err
+}
+
 func (c *command) getHostWithPort() string {
 	if addr, found := c.doGetHostWithPortFromSSHConfig(); found {
 		return addr