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https://issues.apache.org/jira/browse/HADOOP-8758?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13461308#comment-13461308
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Kan Zhang commented on HADOOP-8758:
-----------------------------------

Daryn, as I explained before, this JIRA is to remove the requirement on 
Kerberos as the only external auth method and allow for other auth methods like 
SIMPLE and, in particular, DIGEST-MD5 itself to be used as external auth 
methods. If you want to do the removing in HADOOP-8779 and enable SIMPLE auth 
to be used as an alternative to Kerberos, that's fine. Once HADOOP-8779 is 
done, this JIRA will be trivial.

You might have some misunderstanding of DIGEST-MD5 and how it's used in Hadoop 
RPC. You can't delegate SASL DIGEST-MD5 callbacks to a LoginContext which 
validates client credentials on LDAP, since in DIGEST-MD5, clients will not 
send their credentials to server. DIGEST-MD5 is a challenge-response protocol. 
Server needs to know client credentials (token passwords) to perform 
authentication and hence the need for callbacks. Secret managers are there to 
serve SASL DIGEST-MD5 callbacks.
                
> Support for pluggable token implementations
> -------------------------------------------
>
>                 Key: HADOOP-8758
>                 URL: https://issues.apache.org/jira/browse/HADOOP-8758
>             Project: Hadoop Common
>          Issue Type: Improvement
>          Components: ipc, security
>            Reporter: Kan Zhang
>            Assignee: Kan Zhang
>
> Variants of the delegation token mechanism have been employed by different 
> Hadoop services (NN, JT, RM, etc) to re-authenticate a previously 
> Kerberos-authenticated client. While existing delegation token mechanism 
> compliments Kerberos well, it doesn't necessarily have to be coupled with 
> Kerberos. In principle, delegation tokens can be coupled with any 
> authentication mechanism that bootstraps security. In particular, it can be 
> coupled with other token implementations that use the same DIGEST-MD5 auth 
> method. For example, a token can be pre-generated in an out-of-band manner 
> and configured as a shared secret key between NN and JT to allow JT to make 
> initial authentication to NN. This simple example doesn't deal with token 
> renewal etc, but it helps to illustrate the point that if we can support 
> multiple pluggable token implementations, it opens up the possibility for 
> different users to plug in the token implementation of their choice to 
> bootstrap security. Such token based mechanism has advantages over Kerberos 
> in that 1) it doesn't require Kerberos infrastructure, 2) it leverages 
> existing SASL DIGEST-MD5 auth method and doesn't require adding a new RPC 
> auth method.

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