/usr/share/opensips/pi_http/registrar-mod is in opensips-http-modules 2.2.2-3build4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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	<mod><mod_name>aliases</mod_name>
		<cmd><cmd_name>show</cmd_name>
			<db_table_id>aliases</db_table_id>
			<cmd_type>DB_QUERY</cmd_type>
			<query_cols>
				<col><field>id</field><link_cmd>update</link_cmd></col>
				<col><field>username</field></col>
				<col><field>domain</field></col>
				<col><field>contact</field></col>
				<col><field>received</field></col>
				<col><field>path</field></col>
				<col><field>expires</field></col>
				<col><field>q</field></col>
				<col><field>callid</field></col>
				<col><field>cseq</field></col>
				<col><field>last_modified</field></col>
				<col><field>flags</field></col>
				<col><field>cflags</field></col>
				<col><field>user_agent</field></col>
				<col><field>socket</field></col>
				<col><field>methods</field></col>
				<col><field>sip_instance</field></col>
				<col><field>attr</field></col>
			</query_cols>
		</cmd>
		<cmd><cmd_name>add</cmd_name>
			<db_table_id>aliases</db_table_id>
			<cmd_type>DB_INSERT</cmd_type>
			<query_cols>
				<col><field>username</field></col>
				<col><field>domain</field></col>
				<col><field>contact</field></col>
				<col><field>received</field></col>
				<col><field>path</field></col>
				<col><field>expires</field></col>
				<col><field>q</field></col>
				<col><field>callid</field></col>
				<col><field>cseq</field></col>
				<col><field>last_modified</field></col>
				<col><field>flags</field></col>
				<col><field>cflags</field></col>
				<col><field>user_agent</field></col>
				<col><field>socket</field></col>
				<col><field>methods</field></col>
				<col><field>sip_instance</field></col>
				<col><field>attr</field></col>
			</query_cols>
		</cmd>
		<cmd><cmd_name>update</cmd_name>
			<db_table_id>aliases</db_table_id>
			<cmd_type>DB_UPDATE</cmd_type>
			<clause_cols>
				<col><field>id</field><operator>=</operator></col>
			</clause_cols>
			<query_cols>
				<col><field>username</field></col>
				<col><field>domain</field></col>
				<col><field>contact</field></col>
				<col><field>received</field></col>
				<col><field>path</field></col>
				<col><field>expires</field></col>
				<col><field>q</field></col>
				<col><field>callid</field></col>
				<col><field>cseq</field></col>
				<col><field>last_modified</field></col>
				<col><field>flags</field></col>
				<col><field>cflags</field></col>
				<col><field>user_agent</field></col>
				<col><field>socket</field></col>
				<col><field>methods</field></col>
				<col><field>sip_instance</field></col>
				<col><field>attr</field></col>
			</query_cols>
		</cmd>
		<cmd><cmd_name>delete</cmd_name>
			<db_table_id>aliases</db_table_id>
			<cmd_type>DB_DELETE</cmd_type>
			<clause_cols>
				<col><field>id</field><operator>=</operator></col>
			</clause_cols>
		</cmd>
	</mod>
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