75 lines
1.6 KiB
Go
75 lines
1.6 KiB
Go
package sni
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// TODO this was probably copied from somewhere that deserves attribution
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import (
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"errors"
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)
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// GetHostname uses SNI to determine the intended target of a new TLS connection.
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func GetHostname(b []byte) (string, error) {
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rest := b[5:]
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current := 0
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handshakeType := rest[0]
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current++
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if handshakeType != 0x1 {
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return "", errors.New("Not a ClientHello")
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}
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// Skip over another length
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current += 3
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// Skip over protocolversion
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current += 2
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// Skip over random number
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current += 4 + 28
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// Skip over session ID
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sessionIDLength := int(rest[current])
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current++
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current += sessionIDLength
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cipherSuiteLength := (int(rest[current]) << 8) + int(rest[current+1])
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current += 2
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current += cipherSuiteLength
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compressionMethodLength := int(rest[current])
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current++
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current += compressionMethodLength
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if current > len(rest) {
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return "", errors.New("no extensions")
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}
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current += 2
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hostname := ""
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for current < len(rest) && hostname == "" {
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extensionType := (int(rest[current]) << 8) + int(rest[current+1])
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current += 2
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extensionDataLength := (int(rest[current]) << 8) + int(rest[current+1])
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current += 2
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if extensionType == 0 {
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// Skip over number of names as we're assuming there's just one
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current += 2
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nameType := rest[current]
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current++
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if nameType != 0 {
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return "", errors.New("Not a hostname")
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}
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nameLen := (int(rest[current]) << 8) + int(rest[current+1])
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current += 2
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hostname = string(rest[current : current+nameLen])
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}
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current += extensionDataLength
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}
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if hostname == "" {
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return "", errors.New("No hostname")
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}
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return hostname, nil
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}
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