Delays between "connection received" and "connection authenticated" because of localhost entries in hba

Delays between "connection received" and "connection authenticated" because of localhost entries in hba

Hi,

we're facing a strange issue with delays between "connection received" and "connection authenticated".

# select version();
                                                              version

-----------------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 15.6 (Ubuntu 15.6-1.pgdg22.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 11.4.0-1ubuntu1~22.04)
11.4.0,64-bit 
(1 ligne)

I know, this is not the latest minor version.

# \! cat /etc/os-release | head -1
PRETTY_NAME="Ubuntu 22.04.4 LTS"

What we see in the log is this (around 4 seconds delay):

2024-05-07 15:29:50.244 CEST [369909] LOG:  connection received: host=xx.xx.xx.100 port=48434
2024-05-07 15:29:54.518 CEST [369909] LOG:  connection authenticated: identity="xxxxxx" method=md5
(/etc/postgresql/15/main/pg_hba.conf:121)

The matching line is this ( I know md5 ):
host    all             xxxxx     xx.xx.xx.0/24            md5

What we've found out so far is, that this only happens if we have a localhost(or any other hostname) line before the
linematching our connection, something like this: 
host    replication     xxxxx     localhost                md5
host    all             xxxxx     xx.xx.xx.0/24            md5

We don't see it if we do it like this:
host    all             xxxxx     xx.xx.xx.0/24            md5
host    replication     xxxxx     localhost                md5

Has anyone experienced such a behavior? It seems clear that this is somehow related to name resolution but we couldn't
reproducesomething like this on the OS using dig (in a loop several hundred of times). It is also only happening from
timeto time, and not constantly. 

What we're basically looking for is a way to prove the assumption without involving PostgreSQL at all, if that does
makesense? 

Many thanks in advance
Daniel