Send the correct SNI from tsh to auth server #3872
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awly
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russjones and
webvictim
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russjones
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Jun 25, 2020
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| // Per https://tools.ietf.org/html/rfc5246#section-7.4.4 the total size of | ||
| // the known CA subjects sent to the client can't exceed 2^16-1 (due to | ||
| // 2-byte length encoding). The crypto/tls stack will panic if this | ||
| // happens. To make the error less cryptic, catch this condition and return | ||
| // a better error. | ||
| // | ||
| // This may happen with a very large (>500) number of trusted clusters, if | ||
| // the client doesn't send the correct ServerName in its ClientHelloInfo | ||
| // (see the switch at the top of this func). | ||
| var totalSubjectsLen int64 | ||
| for _, s := range pool.Subjects() { | ||
| // Each subject in the list gets a separate 2-byte length prefix. | ||
| totalSubjectsLen += 2 | ||
| totalSubjectsLen += int64(len(s)) | ||
| } |
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
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retest this please |
This was referenced Jun 29, 2020
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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