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SSL-PROBLEMS: describes common curl+SSL problems

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Daniel Stenberg 2014-03-08 22:21:15 +00:00
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docs/Makefile.am
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# | (__| |_| | _ <| |___
# \___|\___/|_| \_\_____|
#
# Copyright (C) 1998 - 2013, Daniel Stenberg, <daniel@haxx.se>, et al.
# Copyright (C) 1998 - 2014, Daniel Stenberg, <daniel@haxx.se>, et al.
#
# This software is licensed as described in the file COPYING, which
# you should have received as part of this distribution. The terms
@ -37,7 +37,8 @@ EXTRA_DIST = MANUAL BUGS CONTRIBUTE FAQ FEATURES INTERNALS SSLCERTS \
README.win32 RESOURCES TODO TheArtOfHttpScripting THANKS VERSIONS \
KNOWN_BUGS BINDINGS $(man_MANS) $(HTMLPAGES) HISTORY INSTALL \
$(PDFPAGES) LICENSE-MIXING README.netware DISTRO-DILEMMA INSTALL.devcpp \
MAIL-ETIQUETTE HTTP-COOKIES LIBCURL-STRUCTS SECURITY RELEASE-PROCEDURE
MAIL-ETIQUETTE HTTP-COOKIES LIBCURL-STRUCTS SECURITY RELEASE-PROCEDURE \
SSL-PROBLEMS
MAN2HTML= roffit < $< >$@

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SSL problems
First, let's establish that we often refer to TLS and SSL interchangably as
SSL here. The current protocol is called TLS, it was called SSL a long time
ago.
There are several known reasons why a connection that involves SSL might
fail. This is a document that attempts to details the most common ones and
how to mitigate them.
CA certs
CA certs are used to digitally verify the server's certificate. You need a
"ca bundle" for this. See lots of more details on this in the SSLCERTS
document.
SSL version
Some broken servers fail to support the protocol negotiation properly that
SSL servers are supposed to handle. This may cause the connection to fail
completely. Sometimes you may need to explicity select a SSL version to use
when connecting to make the connection succeed.
An additional complication can be that modern SSL libraries sometimes are
built with support for older SSL and TLS versions disabled!
SSL ciphers
Clients give servers a list of ciphers to select from. If the list doens't
include any ciphers the server wants/can use, the connection handshake
fails.
curl has recently disabled the user of a whole bunch of seriously insecure
ciphers from its default set (slightly depending on SSL backend in use).
You may have to explicitly provide an alternative list of ciphers for curl
to use to allow the server to use a WEAK cipher for you.
Note that these weak ciphers are identified as flawed. For example, this
includes symmetric ciphers with less than 128 bit keys and RC4.
References:
http://tools.ietf.org/html/draft-popov-tls-prohibiting-rc4-01
Allow BEAST
BEAST is the name of a TLS 1.0 attack that surfaced 2011. When adding means
to mitigate this attack, it turned out that some broken servers out there in
the wild didn't work properly with the BEAST mitigation in place.
To make such broken servers work, the --ssl-allow-beast option was
introduced. Exactly as it sounds, it re-introduces the BEAST vulnerability
but on the other hand it allows curl to connect to that kind of strange
servers.