<a id="tor-spec.txt-6.4"></a>

# Remote hostname lookup

To find the address associated with a hostname, the OP sends a
RELAY_RESOLVE message containing the hostname to be resolved with a NUL
terminating byte. (For a reverse lookup, the OP sends a RELAY_RESOLVE
message containing an in-addr.arpa address.) The OR replies with a
RELAY_RESOLVED message containing any number of answers. Each answer is
of the form:

```text
       Type   (1 octet)
       Length (1 octet)
       Value  (variable-width)
       TTL    (4 octets)
   "Length" is the length of the Value field.
   "Type" is one of:

      0x00 -- Hostname
      0x04 -- IPv4 address
      0x06 -- IPv6 address
      0xF0 -- Error, transient
      0xF1 -- Error, nontransient

    If any answer has a type of 'Error', then no other answer may be
    given.

    The 'Value' field encodes the answer:
        IP addresses are given in network order.
        Hostnames are given in standard DNS order ("www.example.com")
          and not NUL-terminated.
        The content of Errors is currently ignored. Relays currently
          set it to the string "Error resolving hostname" with no
          terminating NUL. Implementations MUST ignore this value.

    For backward compatibility, if there are any IPv4 answers, one of those
    must be given as the first answer.

    The RELAY_RESOLVE messge must use a nonzero, distinct streamID; the
    corresponding RELAY_RESOLVED message must use the same streamID.  No stream
    is actually created by the OR when resolving the name.
```