总结一下，XEP-0030 (前JEP-0030) disco#info 3个主要用途：
1. 查询服务器
<iq from=’tim@timpc/home’ to=’timpc’>
<query xmlns=’http://jabber.org/disco#info’/>
</iq>
目标主机必须返回一个IQ结果或错误。
返回结果的 <query/> 必须标明 ‘http://jabber.org/protocol/disco#info’ namespace，含有一到多个 <identity/> 一到多个 <feature/> elements.
Note: Every entity MUST have at least one identity, and every entity MUST support at least the ‘http://jabber.org/protocol/disco#info’ feature; however, an entity is not required to return a result and MAY return an error, most likely <feature-not-implemented/> or <service-unavailable/>, although other error conditions may be appropriate.
每个 <identity/> 里面必须包含 ‘category’ and ‘type’, 并可以加上一个友好的名称 ‘name’
每个 <feature/> 必须包含 ‘var’ protocol namespace or other feature offered by the entity.category/type 最好是存在一个注册表里面
2. 查询某个JID
当一个 disco#info 发送到一个普通用户JID时候，JID的服务器必须直接返回结果，不过我看不出这个请求有多大的实际用途。JEP的例子说应答服务器在确认请求方是可以信任的前提下可以返回这个用户是否注册。
向服务器查询 [email protected] 的 disco#info
<iq type='get'
from='shakespeare.lit'
to='[email protected]'
id='info2'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
服务器应答 registered account
<iq type='result'
from='[email protected]'
to='shakespeare.lit'
id='info2'>
<query xmlns='http://jabber.org/protocol/disco#info'>
<identity category='account' type='registered'/>
</query>
</iq>
也可以应答当前用户连接的 resource 的信息
<iq type='get'
from='[email protected]/balcony'
to='[email protected]/orchard'
id='info4'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
<iq type=’result’
from=’[email protected]/orchard’
to=’[email protected]/balcony’
id=’info4′>
<query xmlns=’http://jabber.org/protocol/disco#info’>
<identity
category=’client’
type=’pc’
name=’Gabber’/>
<feature var=’jabber:iq:time’/>
<feature var=’jabber:iq:version’/>
</query>
</iq>
3. 查询 conference room (muc)
<iq type=’get’
from=’[email protected]/balcony’
to=’[email protected]’
id=’info3′>
<query xmlns=’http://jabber.org/protocol/disco#info’/>
</iq>
<iq type=’result’
from=’[email protected]’
to=’[email protected]/balcony’
id=’info3′>
<query xmlns=’http://jabber.org/protocol/disco#info’>
<identity
category=’conference’
type=’text’
name=’Romeo and Juliet, Act II, Scene II’/>
<feature var=’http://jabber.org/protocol/disco#info’/>
<feature var=’http://jabber.org/protocol/muc’/>
<feature var=’http://jabber.org/protocol/feature-neg’/>
<feature var=’muc-password’/>
<feature var=’muc-hidden’/>
<feature var=’muc-temporary’/>
<feature var=’muc-open’/>
<feature var=’muc-unmoderated’/>
<feature var=’muc-nonanonymous’/>
</query>
</iq>
