Question

sshd -- reverse hostname lookup problem

Asked by: raytung

Hi,

I'm currently using RedHat 7.2 & 7.3. I use ssh to connect to the linux servers. Recently I have disabled the reverse name lookup in the DNS server and now whenever I try ssh to the linux servers, it paused for a while (~30 secs) before prompting for password. I've tried changing the config in /etc/sshd/sshd_config like these:

In RH 7.2:-

   ReverseMappingCheck   no

In RH 7.3:-

   VerifyReverseMapping   no


I've restarted the sshd services after changing the config files, but no improvement. And the log file /var/log/secure gives the following:-

Aug 14 09:11:21 machine1 sshd[22240]: Could not reverse map address 10.31.44.152


Does anyone know what the problem is? Did I change the wrong config? Please help!!!

Thank you!!

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Asked On
2003-08-13 at 21:26:19ID20709154
Tags

reverse

,

lookup

,

ssh

,

sshd

Topic

Linux Network Security

Participating Experts
3
Points
75
Comments
13

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Answers

 

by: ahoffmannPosted on 2003-08-14 at 02:59:02ID: 9145210

some openssh sshd can be tweaked like this:
   sshd -u0 ...

There's also a know buggy version of OpenSSH which does DNS queries, check at RH for a version without this flaw (it has to do with the kerberos authentification)

 

by: jleviePosted on 2003-08-14 at 06:39:56ID: 9146457

As an addendum to ahoffman's comment '-u0' will disable DNS lookups except when the configuration requires it. From the man page for sshd on 7.3:

            "Authentication mechanisms that may require DNS
             include RhostsAuthentication, RhostsRSAAuthentication,
             HostbasedAuthentication and using a from="pattern-list" option in
             a key file.  Configuration options that require DNS include using
             a USER@HOST pattern in AllowUsers or DenyUsers."

 

by: MrYowlerPosted on 2003-08-14 at 09:34:11ID: 9148032

Simple solution...  add 10.31.44.152 as a host in your hosts file...  :-P

That gets you a valid rDNS, and gets rid of the delay...



"Doctor!  Doctor!  It hurts when I do this!"
"Don't do that..."



:)

 

by: ahoffmannPosted on 2003-08-14 at 14:27:45ID: 9150278

duuh, mainly MrYowler gave exhausting comments, now a very brief one saying nearly about nothing, but points to the right direction:
   10.31.44.152
simply setup your own DNS or hosts file ;-)

KISS - keep it stupid simple

 

by: raytungPosted on 2003-08-14 at 18:43:54ID: 9151413

Where do I put that '-u0' option when starting sshd with:

   service sshd start

? Is there an equivalent option that can be set in sshd_config?

 

by: raytungPosted on 2003-08-14 at 19:06:28ID: 9151482

I just tried to create a file 'sshd' in /etc/sysconfig with the line:

   OPTIONS="-u0"

and restart sshd with:

   service sshd restart

Now it works!! No hosts file modification and no more delay!! :)

Just wonder if it is the right way to do? Thanks!!

 

by: jleviePosted on 2003-08-14 at 20:36:52ID: 9152271

That would be the "RedHat way" for this.

 

by: raytungPosted on 2003-08-14 at 21:17:21ID: 9152691

Hi jlevie, are you referring to the option '-u0' of sshd or the /etc/sysconfig/sshd file?
If it's latter case, is there a more generic solution then? Thx!!


 

by: MrYowlerPosted on 2003-08-15 at 23:02:16ID: 9164645

/me sends a big fat raspberry to ahoffman...  :-P

I know how to shut up.  I know how to give simple answers, too.  I usually give long ones, for the sake of ensuring that anyone who reads it, should understand.  Well, that - and to torture you...  :-P

 

by: jleviePosted on 2003-08-16 at 07:35:47ID: 9165583

I read your question as being how best to cause sshd to start up with that option. The "more generic solution" for a local LAN is to make sure that all hosts can be resolved by IP. For just a few nodes that is manageable by hosts file entries on all machines, but a local DNS server is more appropriate if there are more than a few systems involved. Now if the machines involved aren't on a local network using one of the RFC1918 private networks the solution is to use the hostname that a reverse lookup of the IP returns as the system's hostname, rather than one you might pick. It the provider didn't set up in-addr.arpa records for those IP's I'd complain to them and try to get them to fix the problem.

 

by: ahoffmannPosted on 2004-06-25 at 11:11:38ID: 11401103

I'd split points

20120131-EE-VQP-002

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