Extract Server Name Indication (SNI) from TLS client hello

Question

How would you extract the Server Name Indication from a TLS Client Hello message. I'm curently struggling to understand this very cryptic RFC 3546 on TLS Extensions, in which the SNI is defined.

Things I've understood so far:

• The host is utf8 encoded and readable when you utf8 enocde the buffer.
• Theres one byte before the host, that determines it's length.

If I could find out the exact position of that length byte, extracting the SNI would be pretty simple. But how do I get to that byte in the first place?

Answer 1

I did this in sniproxy, examining a TLS client hello packet in Wireshark while reading that RFC is a pretty good way to go. It's not too hard, just lots of variable length fields you have to skip past and check checking if you have the correct element type.

I'm working on my tests right now, and have this annotated sample packet that might help:

const unsigned char good_data_2[] = {
    // TLS record
    0x16, // Content Type: Handshake
    0x03, 0x01, // Version: TLS 1.0
    0x00, 0x6c, // Length (use for bounds checking)
        // Handshake
        0x01, // Handshake Type: Client Hello
        0x00, 0x00, 0x68, // Length (use for bounds checking)
        0x03, 0x03, // Version: TLS 1.2
        // Random (32 bytes fixed length)
        0xb6, 0xb2, 0x6a, 0xfb, 0x55, 0x5e, 0x03, 0xd5,
        0x65, 0xa3, 0x6a, 0xf0, 0x5e, 0xa5, 0x43, 0x02,
        0x93, 0xb9, 0x59, 0xa7, 0x54, 0xc3, 0xdd, 0x78,
        0x57, 0x58, 0x34, 0xc5, 0x82, 0xfd, 0x53, 0xd1,
        0x00, // Session ID Length (skip past this much)
        0x00, 0x04, // Cipher Suites Length (skip past this much)
            0x00, 0x01, // NULL-MD5
            0x00, 0xff, // RENEGOTIATION INFO SCSV
        0x01, // Compression Methods Length (skip past this much)
            0x00, // NULL
        0x00, 0x3b, // Extensions Length (use for bounds checking)
            // Extension
            0x00, 0x00, // Extension Type: Server Name (check extension type)
            0x00, 0x0e, // Length (use for bounds checking)
            0x00, 0x0c, // Server Name Indication Length
                0x00, // Server Name Type: host_name (check server name type)
                0x00, 0x09, // Length (length of your data)
                // "localhost" (data your after)
                0x6c, 0x6f, 0x63, 0x61, 0x6c, 0x68, 0x6f, 0x73, 0x74,
            // Extension
            0x00, 0x0d, // Extension Type: Signature Algorithms (check extension type)
            0x00, 0x20, // Length (skip past since this is the wrong extension)
            // Data
            0x00, 0x1e, 0x06, 0x01, 0x06, 0x02, 0x06, 0x03,
            0x05, 0x01, 0x05, 0x02, 0x05, 0x03, 0x04, 0x01,
            0x04, 0x02, 0x04, 0x03, 0x03, 0x01, 0x03, 0x02,
            0x03, 0x03, 0x02, 0x01, 0x02, 0x02, 0x02, 0x03,
            // Extension
            0x00, 0x0f, // Extension Type: Heart Beat (check extension type)
            0x00, 0x01, // Length (skip past since this is the wrong extension)
            0x01 // Mode: Peer allows to send requests
};