Can't perform reverse DNS lookup for a (binary) IPv6 address
Tor provides the "RESOLVE_PTR" (0xF1) command as an extension to the SOCKS5 protocol, which can in theory be used to perform a reverse DNS lookup of an IPv4 or IPv6 address.
As with other SOCKS5 commands, this command takes an argument which can either be a binary IPv4 or IPv6 address, or an ASCII string.
Somewhat contrary to ticket #32314 (moved), this command works for IPv6 addresses only if the address is specified as an ASCII string (address type 3) with no brackets. If the address is specified in binary (address type 4), or as a string enclosed in brackets, Tor will reject it.