dhcp-server: T9267: make client-class substring matching contains-anywhere

The Mikrotik configuration referenced by T9267 uses
matching-type=substring, documented to match the value anywhere within
the option string. This PR's "substring" match instead compared a
fixed-length slice at a fixed byte offset (default 0) for exact
equality - effectively an anchored prefix match, not a substring
search - which does not reliably reproduce the ticket's own use case
of picking a device model string (e.g. "T42S") out of a longer,
variably-formatted hostname.

Rebuild "hostname substring"/"vendor-class-id substring" on Kea's
match()+lcase() instead of substring()==, so a configured value now
matches anywhere in the option content and case-insensitively (options
like vendor-class-id are not guaranteed to arrive in a fixed case).
Drop the "offset" leaf together with it, since it has no remaining
purpose once matching is no longer anchored to a fixed position, and
there is no released behavior to preserve.

Raw hex input and values containing a literal single quote are
rejected for substring matching in verify(), since neither can be
safely embedded into the generated Kea regular expression; exact
"value" matching is unaffected and still accepts both.

Reported by alexk37 in review of PR #5436.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/conf_mode/service_dhcp-server.py b/src/conf_mode/service_dhcp-server.py
index 6572b4b7f..46fd0a7bc 100755
--- a/src/conf_mode/service_dhcp-server.py
+++ b/src/conf_mode/service_dhcp-server.py
@@ -597,9 +597,15 @@ def verify(dhcp):
                             f'Client class "{class_name}": "{option_label} substring" requires a value'
                         )
                     value = match_config['substring']['value']
-                    if not pattern.match(value):
+                    if value.startswith('0x'):
+                        raise ConfigError(
+                            f'Client class "{class_name}": "{option_label} substring" only supports '
+                            f'plain text values, not raw hex'
+                        )
+                    if "'" in value:
                         raise ConfigError(
-                            f'Invalid {option_label} substring value "{value}" must be either text literal or hex string starting with 0x'
+                            f'Client class "{class_name}": "{option_label} substring" value must not '
+                            f"contain a single quote (')"
                         )
 
     return None