Re: SSL support in Ncat - client version issue: what do other apps do?

[David Fifield <david () bamsoftware com>]

On Sun, Feb 08, 2009 at 02:38:18PM +0100, Kristof Boeynaems wrote:
> On Sun, Feb 8, 2009 at 11:37 AM, Brandon Enright <bmenrigh () ucsd edu> wrote:
> Additionally, we might use the SSL_set_ssl_method() method
> (http://www.openssl.org/docs/ssl/SSL_CTX_set_ssl_version.html) to
> change the connection method of the current SSL object (and thus
> reusing the object). This seems to be the best idea.
> Alternatively, we might use the SSL_CTX_set_ssl_version() method (also
> listed on that page) to change the sslnfo->ctx and then create a new
> SSL object from this new context. This will affect all new SSL objects
> created based on this context though, which is more likely to have
> repercussions on other functionality.

I looked at the source code of some other applications to see how they
handle SSL client connections. None that I found use the fallback/retry
technique that has been discussed (and implemented at
http://seclists.org/nmap-dev/2009/q1/0430.html).

Wget only supports one client method at a time, but it provides
command-line options to choose which method to use. If you say
--secure-protocol=auto, it uses SSLv23_client_method, if you say
--secure-protocol=TLSv1, it uses TLSv1_client_method, etc. See
src/openssl.c.
http://www.gnu.org/software/wget/manual/html_node/HTTPS-_0028SSL_002fTLS_0029-Options.html

cURL is the same, only supporting one method at a time but providing
options --tlsv1, --sslv2, --sslv3. If none of those are given the
SSLv23_client_method method is used. See lib/ssluse.c.

socat again only supports one method per connection. It has a method
option to choose which one is used: method=SSLv2, method=SSLv23, etc.
See xio-openssl.c.
http://www.dest-unreach.org/socat/doc/socat.html#GROUP_OPENSSL

That seems to be a common solution: provide SSL version selection for
use against those servers that require it, and use SSLv23_client_method
by default.

Doing this would be adequate for Ncat's needs. Nmap's version detection
is another story. You may hit servers requiring any version of SSL
during a single run, so it's not enough to have one global setting.
Falling back to different client methods is a good solution if the
implementation can be worked out.

David Fifield

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