An engineer needs to collect User-ID mappings from the company's existing proxies.

To collect User-ID information from third-party proxies, Palo Alto Networks supports several methods of integrating user information. Syslog parsing allows the firewall to receive syslog messages from external services, parse them, and extract user information. X-Forwarded-For (XFF) headers, which are used in HTTP requests and proxies, can carry the original IP address of a client connecting through a proxy, and this information can be used by the firewall to map the user IDs.

Syslog is commonly used for integrating third-party devices like proxies with User-ID, and XFF headers are specifically mentioned in the context of integrating user mappings from HTTP traffic. Client probing and Server Monitoring are not the correct methods for pulling data from third-party proxies.

For further details, refer to the Palo Alto Networks documentation on User-ID integration and the "PAN-OS® Administrator’s Guide".