ssl version and cipher suites of the client - ssl

Question

I'm working on a soap server, that will serve some old embedded computers with an legacy soap protocol.
I write it in go and so far used just plain http, but in production it must use ssl encryption. So I've just created a key and a cert (from this site) and used the http.ListenAndServeTLS function.
But now the computers cannot connect and the server is just printing a handshake error:
server.go:2848: http: TLS handshake error from [::1]:38790: tls: no cipher suite supported by both client and server
In the docs, for the computers, isn't the supported ssl version or the ciphers. So I wanted to know, how to find out the client's ssl version, and also the available cipher suites that the client supports.
And then how can I configure the golang http server so it will support the selected ciphers.

Answer 1

There seems to be two questions here, so let's do this in two parts:
Finding the client's TLS version and supported cipher suites:
To do this, you need to set the GetConfigForClient field of the tls.Config object.
This field takes a method with signature:
func(*ClientHelloInfo) (*Config, error)
It is called on receipt of a Client Hello message with a ClientHelloInfo struct. This struct contains the following fields of interest to you:
// CipherSuites lists the CipherSuites supported by the client (e.g.
// TLS_RSA_WITH_RC4_128_SHA).
CipherSuites []uint16
// SupportedVersions lists the TLS versions supported by the client.
// For TLS versions less than 1.3, this is extrapolated from the max
// version advertised by the client, so values other than the greatest
// might be rejected if used.
SupportedVersions []uint16
Please read the comments around GetConfigForClient and ClientHelloInfo for exactly how GetConfigForClient should behave, and for field details.
Specifying server-supported version and cipher suites:
This is also done through the tls.Config object using the following fields:
// CipherSuites is a list of supported cipher suites. If CipherSuites
// is nil, TLS uses a list of suites supported by the implementation.
CipherSuites []uint16
// MinVersion contains the minimum SSL/TLS version that is acceptable.
// If zero, then TLS 1.0 is taken as the minimum.
MinVersion uint16
// MaxVersion contains the maximum SSL/TLS version that is acceptable.
// If zero, then the maximum version supported by this package is used,
// which is currently TLS 1.2.
MaxVersion uint16
For example, you could set your tls.Config with the following fields:
CipherSuites: []uint16{
tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
etc...
tls.TLS_RSA_WITH_AES_256_CBC_SHA,
},
MinVersion: tls.VersionTLS12,
The full list of supported cipher suites is in the tls docs.