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Our GSSAPI code only allows packet sizes up to 16kB. However it emerges that during authentication, larger packets might be needed; various authorities suggest 48kB or 64kB as the maximum packet size. This limitation caused login failure for AD users who belong to many AD groups. To add insult to injury, we gave an unintelligible error message, typically "GSSAPI context establishment error: The routine must be called again to complete its function: Unknown error". As noted in code comments, the 16kB packet limit is effectively a protocol constant once we are doing normal data transmission: the GSSAPI code splits the data stream at those points, and if we change the limit then we will have cross-version compatibility problems due to the receiver's buffer being too small in some combinations. However, during the authentication exchange the packet sizes are not determined by us, but by the underlying GSSAPI library. So we might as well just try to send what the library tells us to. An unpatched recipient will fail on a packet larger than 16kB, but that's not worse than the sender failing without even trying. So this doesn't introduce any meaningful compatibility problem. We still need a buffer size limit, but we can easily make it be 64kB rather than 16kB until transport negotiation is complete. (Larger values were discussed, but don't seem likely to add anything.) Reported-by: Chris Gooch <cgooch@bamfunds.com> Fix-suggested-by: Jacob Champion <jacob.champion@enterprisedb.com> Author: Tom Lane <tgl@sss.pgh.pa.us> Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com> Discussion: https://postgr.es/m/DS0PR22MB5971A9C8A3F44BCC6293C4DABE99A@DS0PR22MB5971.namprd22.prod.outlook.com Backpatch-through: 13
src/backend/libpq/README.SSL
SSL
===
>From the servers perspective:
Receives StartupPacket
|
|
(Is SSL_NEGOTIATE_CODE?) ----------- Normal startup
| No
|
| Yes
|
|
(Server compiled with USE_SSL?) ------- Send 'N'
| No |
| |
| Yes Normal startup
|
|
Send 'S'
|
|
Establish SSL
|
|
Normal startup
>From the clients perspective (v6.6 client _with_ SSL):
Connect
|
|
Send packet with SSL_NEGOTIATE_CODE
|
|
Receive single char ------- 'S' -------- Establish SSL
| |
| '<else>' |
| Normal startup
|
|
Is it 'E' for error ------------------- Retry connection
| Yes without SSL
| No
|
Is it 'N' for normal ------------------- Normal startup
| Yes
|
Fail with unknown
---------------------------------------------------------------------------
Ephemeral DH
============
Since the server static private key ($DataDir/server.key) will
normally be stored unencrypted so that the database backend can
restart automatically, it is important that we select an algorithm
that continues to provide confidentiality even if the attacker has the
server's private key. Ephemeral DH (EDH) keys provide this and more
(Perfect Forward Secrecy aka PFS).
N.B., the static private key should still be protected to the largest
extent possible, to minimize the risk of impersonations.
Another benefit of EDH is that it allows the backend and clients to
use DSA keys. DSA keys can only provide digital signatures, not
encryption, and are often acceptable in jurisdictions where RSA keys
are unacceptable.
The downside to EDH is that it makes it impossible to use ssldump(1)
if there's a problem establishing an SSL session. In this case you'll
need to temporarily disable EDH (see initialize_dh()).