Backport: send the correct SNI from tsh to auth server #3932
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SNI is used to indicate which cluster's CA to use for client cert validation. If SNI is not sent, or set as "teleport.cluster.local" (which is default in the client config), auth server will attempt to validate against all known CAs. The list of CA subjects is sent to the client during handshake, before client sends its own client cert. If this list is too long, handshake will fail. The limit is 65535 bytes, because TLS wire encoding uses 2 bytes for a length prefix. In teleport, this fits ~520-540 trusted cluster CAs. To avoid handshake failures on such large setups, all clients must send the correct SNI. In some future version, we should enforce this to catch such issues early. For now, added a debug log to report clients using the default ServerName. Also added a check for large number of CAs, to print a helpful error. Updates #3870
awly
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klizhentas,
russjones and
webvictim
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awly
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fspmarshall
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Jun 29, 2020
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Backport of #3872 into 4.3
SNI is used to indicate which cluster's CA to use for client cert
validation. If SNI is not sent, or set as "teleport.cluster.local"
(which is default in the client config), auth server will attempt to
validate against all known CAs.
The list of CA subjects is sent to the client during handshake, before
client sends its own client cert. If this list is too long, handshake
will fail. The limit is 65535 bytes, because TLS wire encoding uses 2
bytes for a length prefix. In teleport, this fits ~520-540 trusted
cluster CAs.
To avoid handshake failures on such large setups, all clients must send
the correct SNI. In some future version, we should enforce this to catch
such issues early. For now, added a debug log to report clients using
the default ServerName. Also added a check for large number of CAs, to
print a helpful error.
Updates #3870