Re: [nft PATCH v2] List handles of added rules if requested

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Fri, May 05, 2017 at 04:33:23PM +0200, Phil Sutter wrote:
> Being able to retrieve an added rule's handle atomically is a crucial
> feature for scripts invoking nft command: Without it, there is no way to
> be sure a handle extracted from 'nft list ruleset' command actually
> refers to the rule one has added before or that of another process which
> ran in between.
> 
> Extracting an added rule's handle itself is not an easy task already,
> since there is a chance that a given rule is printed differently than
> when it was added before. A simple example is port number vs. service
> name:
> 
> | nft add rule ip t c tcp dport { ssh, 80 } accept
> 
> There is no way to make 'nft list ruleset' return the rule just like
> this as depending on whether '-nn' was given or not, it either prints
> the set as '{ ssh, http }' or '{ 22, 80 }' but never in the mixed form
> that was used when adding it.
> 
> This patch prints an identifying string for each added rule which may be
> used as single parameter to a later 'nft delete rule' command. So a
> simple scripting example looks like this:
> 
> | handle=$(nft add rule ip t c counter)
> | ...
> | nft delete rule $handle
> 
> Signed-off-by: Phil Sutter <phil@nwl.cc>
> ---
> Changes since v1:
> - Pass NLM_F_ECHO to kernel to leverage already existing reporting
>   infrastructure and therefore not require a seperate kernel patch.
> - Limit mnl_callback() action to NEWRULE messages - when replacing a
>   rule, it would otherwise print the deleted rule as well.

This does not work for nft -i.