Add SNI parser

.gitignore

@@ -102,3 +102,6 @@ venv.bak/
 
 # mypy
 .mypy_cache/
+
+# Editors
+.vscode/

setup.cfg

@@ -0,0 +1,17 @@
+[isort]
+multi_line_output = 4
+indent = "    "
+not_skip = __init__.py
+force_sort_within_sections = true
+sections = FUTURE,STDLIB,INBETWEENS,THIRDPARTY,FIRSTPARTY,LOCALFOLDER
+default_section = THIRDPARTY
+forced_separate = tests
+combine_as_imports = true
+use_parentheses = true
+
+[yapf]
+based_on_style = chromium
+indent_width = 4
+
+[flake8]
+max-line-length = 80

snitun/exceptions.py

@@ -0,0 +1,8 @@
+"""SniTun Exceptions."""
+
+class SniTunError(Exception):
+    """Base Exception for SniTun exceptions."""
+
+
+class ParseSNIError(SniTunError):
+    """Invalid ClientHello data."""

snitun/server/sni.py

@@ -0,0 +1,121 @@
+"""TLS ClientHello parser."""
+import logging
+
+from ..exceptions import ParseSNIError
+
+_LOGGER = logging.getLogger(__name__)
+
+TLS_HEADER_LEN = 5
+TLS_HANDSHAKE_CONTENT_TYPE = bytes(0x16)
+TLS_HANDSHAKE_TYPE_CLIENT_HELLO = bytes(0x01)
+
+
+def parse_tls_sni(data: bytes) -> str:
+    """Parse TLS SNI extention."""
+
+    if len(data) < TLS_HEADER_LEN:
+        _LOGGER.debug("Invalid TLS header")
+        raise ParseSNIError()
+
+    # If TLS handshake
+    if data[0] != TLS_HANDSHAKE_CONTENT_TYPE:
+        _LOGGER.debug("Not TLS handshake received")
+        raise ParseSNIError()
+
+    # Check compatible ClientHello
+    if int(data[1]) < 3:
+        _LOGGER.debug("Received ClientHello without SNI support")
+        raise ParseSNIError()
+
+    # Calculate TLS record size
+    tls_size = int(data[3] << 8) + int(data[4]) + TLS_HEADER_LEN
+    if len(data) < tls_size:
+        _LOGGER.debug("Can't calculate the TLS record size")
+        raise ParseSNIError()
+
+    # Check if handshake is a ClientHello
+    pos = TLS_HEADER_LEN + 1
+    if data[pos] != TLS_HANDSHAKE_TYPE_CLIENT_HELLO:
+        _LOGGER.debug("Invalid ClientHello type")
+        raise ParseSNIError()
+
+    # Seek fixed length header part
+    pos += 38
+
+    # Seek SessionID
+    try:
+        pos += 1 + int(data[pos])
+    except IndexError:
+        _LOGGER.debug("Invalid SessionID")
+        raise ParseSNIError() from None
+
+    # Seek Cipher Suites
+    try:
+        pos += 2 + int(data[pos] << 8) + int(data[pos + 1])
+    except IndexError:
+        _LOGGER.debug("Invalid CipherSuites")
+        raise ParseSNIError() from None
+
+    # Seek Compression Methods
+    try:
+        pos += 1 + int(data[pos])
+    except IndexError:
+        _LOGGER.debug("Invalid CompressionMethods")
+        raise ParseSNIError() from None
+
+    # Check data buffer + extension size
+    if pos + 2 > len(data):
+        _LOGGER.debug("Mismatch Extension TLS header")
+        raise ParseSNIError()
+
+    # Process extension
+    return _parse_extension(data, pos)
+
+
+def _parse_extension(data: bytes, pos: int) -> str:
+    """Parse TLS ClientHello Extension."""
+
+    # Seek Extension start
+    try:
+        tls_extension_size = int(data[pos] << 8) + int(data[pos + 1])
+        pos += 2
+    except IndexError:
+        raise ParseSNIError() from None
+
+    # Check data buffer + extension size
+    if pos + tls_extension_size > len(data):
+        _LOGGER.debug("Mismatch Extension TLS header")
+        raise ParseSNIError()
+
+    # Loop over extension until we have our SNI
+    while pos + 4 <= len(data):
+
+        # SNI?
+        if data[pos] == 0x00 and data[pos + 1] == 0x00:
+            return _parse_host_name(data, pos + 4)
+
+        pos = 4 + int(data[pos + 2] << 8) + int(data[pos + 3])
+
+    _LOGGER.debug("Can't find any ServerName Extension")
+    raise ParseSNIError()
+
+
+def _parse_host_name(data: bytes, pos: int) -> str:
+    """Parse TLS ServerName Extension."""
+
+    # Skeep list size
+    pos += 2
+
+    while pos + 3 < len(data):
+        size = int(data[pos + 1] << 8) + data[pos + 2]
+
+        # Unknown server name type
+        if data[pos] != 0x00:
+            _LOGGER.debug("Unknown ServerName type")
+            pos += 3 + size
+            continue
+
+        return str(data[pos + 3:size])
+
+    _LOGGER.debug("Not found any valid ServerName")
+    raise ParseSNIError()