Client capabilities are specified using the "*cap" info key.

The info string is made up of single char flags (possibly with modifiers, though currently none exist).

Defined capabilities (* = not implemented):

z client can accept gzipped files.
h http transfers
f * ftp transfers
a * audio channel (voice chat)
i * irc
p pogo stick control
t team messages

For more information on z and h, see QW Download Specification.

The QuakeForge clients will not send "*cap" to the server unless "QF" or "EXT" is detected in the challenge string sent by the server.

From the QuakeForge quakeworld server:

    if (sv_extensions->int_val) {
        extended = " QF qtv EXT";
    }

    // send it to the client
    Netchan_OutOfBandPrint (net_from, "%c%i%s", S2C_CHALLENGE,
                            svs.challenges[i].challenge, extended);