[Pkg-utopia-maintainers] Bug#414569: Reverse lookup for said host is not set up

[Martin Steigerwald]

Hello!

Reverse lookup for said the host in the strace - our ldap server - is not set 
up.

ms at mango:~> host 172.21.242.9
Host 9.242.21.172.in-addr.arpa not found: 3(NXDOMAIN)

It tells so immediately. 

To my knowledge there is no strict requirement that an LDAP or any other hosts 
in a local network needs a reverse lookup set up.

I imagine there may be lots of networks where reverse lookup is not defined 
for some hosts, my network at home doesn't even have a DNS server.

At least I do not get whether avahi tries to find out about the same IP 
address again and again. Since the workstation uses LDAP I think that IP 
reverse lookup for that IP address is queried for very often. The "strace 
ssh" case was repeatable after a second. It shouldn't try to find out about 
that IP address that often IMHO. If it isn't known it should wait some time 
before it tries again. That would be an avahi-daemon issue.

Added to that I would be more reluctant to add an option to nsswitch that 
delays reverse lookups where the DNS server returns not found in a fraction 
of a second by 5 seconds or more. Its the postinst script of the package 
libnss-mdns that does it:

---------------------------------------------------------------------
    perl -i -pe '
        sub insert {
            # this also splits on tab
            my @bits=split(" ", shift);
            # do not break configuration if the "hosts" line already 
references
            # mdns
            if (grep { $_ eq "mdns4_minimal" || $_ eq "mdns4"
                || $_ eq "mdns" || $_ eq "mdns_minimal"
                || $_ eq "mdns6" || $_ eq "mdns6_minimal"} @bits) {
                return join " ", @bits;
            }
            # change "dns" into "mdns4_minimal [NOTFOUND=return] dns mdns4"
            return join " ", map {
                $_ eq "dns" ? ("mdns4_minimal","[NOTFOUND=return]",
$_,"mdns4") : $_
            } @bits;
        }
        s/^(hosts:\s+)(.*)/$1.insert($2)/e;
    ' /etc/nsswitch.conf
---------------------------------------------------------------------

I cannot remember that it asked me whether I like to do these changes. It 
maybe tries to do these changes again when the package is updated.

I recommend that "mdns4_minimal" is added by default - I doesn't create the 
timeout as I tested today -, but "mdns4" after dns lookup is not without 
asking the user first. That would be a libnss-mdns issue.

About NFS I agree with you, it likely wasn't NFS, it was the LDAP lookups and 
possibly other server IP address reverse lookups I think.

Regards,
-- 
Martin Steigerwald
Trainer / Consultant / Systems Engineer

team(ix) GmbH
Solide IT-Infrastruktur
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