Re: Proper ACL condition to block failed rDNS lookup

On 2025-06-07 17:23, AC via Exim-users wrote:
> 
> I want to block any connection that doesn't have a successful rDNS (logs 
> report "no host name found") but searching around results in so many 
> different variants of an ACL rule to do this.
> 
> What is the proper way (likely in acl_smtp_connect if I'm not mistaken) 
> that will simply reject any conection that comes from a bare IP with no 
> rDNS? Note I'm not asking that I block something where the incoming 
> hostname doesn't match the rDNS where the two might mismatch (such as an 
> MX record vs. a more general domain), I'm only looking to block anything 
> that has absolutely no rDNS value so I don't want to use the 
> "reverse_host_lookup" option.
> 
> The closest thing I found was in the mailing list from 2008:
> 
> defer
>   !condition = ${lookup dnsdb{defer_never,ptr=$sender_host_address}{yes}}
>   message    = Reverse DNS lookup failed for host $sender_host_address
> 
> 
> 
> Is this still a correct suggestion?
> 

I've added a rule in acl_smtp_connect to block connections from failed 
rDNS but occasionally one seemingly slips through. I suspect it's going 
through a different pathway but why would acl_smtp_connect not block the 
below event? There's no indication in the logs about "no host name 
found". Did this go through a different ACL entirely?

SMTP protocol synchronization error (input sent without waiting for 
greeting): rejected connection from H=[152.32.138.230] I=[10.0.0.6]:25 
input="\026\003\001\001\027\001"



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