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6
7SSL problems
8
9  First, let's establish that we often refer to TLS and SSL interchangeably as
10  SSL here. The current protocol is called TLS, it was called SSL a long time
11  ago.
12
13  There are several known reasons why a connection that involves SSL might
14  fail. This is a document that attempts to details the most common ones and
15  how to mitigate them.
16
17CA certs
18
19  CA certs are used to digitally verify the server's certificate. You need a
20  "ca bundle" for this. See lots of more details on this in the SSLCERTS
21  document.
22
23CA bundle missing intermediate certificates
24
25  When using said CA bundle to verify a server cert, you will experience
26  problems if your CA cert does not have the certificates for the
27  intermediates in the whole trust chain.
28
29SSL version
30
31  Some broken servers fail to support the protocol negotiation properly that
32  SSL servers are supposed to handle. This may cause the connection to fail
33  completely. Sometimes you may need to explicitly select a SSL version to use
34  when connecting to make the connection succeed.
35
36  An additional complication can be that modern SSL libraries sometimes are
37  built with support for older SSL and TLS versions disabled!
38
39SSL ciphers
40
41  Clients give servers a list of ciphers to select from. If the list doesn't
42  include any ciphers the server wants/can use, the connection handshake
43  fails.
44
45  curl has recently disabled the user of a whole bunch of seriously insecure
46  ciphers from its default set (slightly depending on SSL backend in use).
47
48  You may have to explicitly provide an alternative list of ciphers for curl
49  to use to allow the server to use a WEAK cipher for you.
50
51  Note that these weak ciphers are identified as flawed. For example, this
52  includes symmetric ciphers with less than 128 bit keys and RC4.
53
54  References:
55
56  https://tools.ietf.org/html/draft-popov-tls-prohibiting-rc4-01
57
58Allow BEAST
59
60  BEAST is the name of a TLS 1.0 attack that surfaced 2011. When adding means
61  to mitigate this attack, it turned out that some broken servers out there in
62  the wild didn't work properly with the BEAST mitigation in place.
63
64  To make such broken servers work, the --ssl-allow-beast option was
65  introduced. Exactly as it sounds, it re-introduces the BEAST vulnerability
66  but on the other hand it allows curl to connect to that kind of strange
67  servers.
68