Can I disable IPv6 lookup when querying DNS?

[Ken Englander]

We have found there are significant time delays when issuing a DNS lookup by name if one of the DNS servers is down. This problem seems to be much worse when the OS attempts to look for IPv6 addresses first, then IPv4.

So, I want to know if there is a way to tell the lookup process NOT to check for IPv6 addresses or tell it to prefer IPv4 addresses?

[Anshumali]

We got it done by adding:

ipnodes: files [NOTFOUND=return]

in /etc/nsswitch.conf
You can check the syntax and modify that as per your usage.
Check it with tcpdump port 54 afterwards.

[rick jones]

Don't you mean port 53 for DNS?

[Anshumali]

Ooops..my fat fingers... For sure 53 :)

[Bill Hassell · Accepted solution]

IPv6 is embedded in very unexpected ways, haphazard is my take on it. I recently had to eliminate all IPv6 client and server responses for an (probably misguided) auditor. If you're running 11.11, you can probably remove the IPv6 patches and products with success.

nsswitch.conf MUST have:

ipnodes: files

to avoid DNS issues. Some DNS servers do not support IPv6 and some are just not configured for IPv6 support, all causing DNS resolution delays.

But for 11.23 and 11.31, I discovered that getipnodebyname, getipnodebyaddr, gethostint, getaddrinfo all have IPv6 embedded with nothing to turn that 'feature' off.

icmp will try IPv6 and if fails, IPv4 only when there is a DNS server declared
in resolv.conf.

If you want to pretty much disable IPv6 junk, you can rename the ipv6 device file:

mv /dev/ipv6 /dev/ipv6-deny

[Ken Englander]

Anshumali - You must have a different situation. I still need to query DNS for IPv4 information so I cannot setup nsswitch.conf as you suggested.

Bill - Your information is what I needed. I am a bit hesitant about renaming the driver. Have you actually done that?

Thanks all for the feedback!

[Ken Englander]

BTW - this is on 11.23 and 11.31

[rick jones]

Applications should be calling getaddrinfo() (*), and probably should be setting AI_ADDRCONFIG. If that is the case, then getaddrinfo() will only request an IPv6 address if a non-loopback IPv6 address is configured on the system. (Same for IPv4).

I would not expect gethostbyname() to ask for IPv6 addresses. If a gethostbyname() call is actually triggering queries for IPv6 addresses, that might be something worth mentioning to the Response Center.

(*) conversion from gethostbyname() to getaddrinfo() should have started years ago, so if you have apps you've not converted, get started!-)

[Ken Englander]

Hi Rick - thanks for that info. I am not sure at this point what calls are being made as this is from within Java. The doc indicates that Java 6 now checks for IPv6 addresses so I suspect it is using the getaddrinfo calls.

As for our native code, I do not know if we have updated it or not. However, if not we will be doing that in 2011.

[rick jones]

One thing you might try is making sure there are indeed no IPv6 addresses assigned on your system. If there are no IPv6 addresses assigned, and you still see attempts to get resolve to IPv6 addresses, then it suggests that asking the JVM to add that AI_ADDRCONFIG would be goodness. Further confirmation would be achieved by using a small C test program that calls getaddrinfo() with the AI_ADDRCONFIG bit set.

[Anshumali]

<>

and why did I write:
You can check the syntax and modify that as per your usage.
:) :)
However...

[Anshumali]

and i rechecked your no-DNS query thing:

what i have is:

root@xxxx:/tmp # cat /etc/nsswitch.conf |grep -e host -e ipnodes
hosts: files [NOTFOUND=continue TRYAGAIN=continue] dns [NOTFOUND=continue UNAVAIL=continue TRYAGAIN=return]
ipnodes: files [NOTFOUND=return]

and nslookup query for something which is not in /etc/hosts:

root@xxxxx:/tmp # nslookup drc-cifs-pr6
Using /etc/hosts on: sustw4

looking up FILES
Trying DNS
Name: drc-cifs-pr6.xxx.com.xx
Address: 10.101.1.154

It *goes* to DNS as per the hosts configuration in nsswitch.conf. ;)
So, bottom line is till you understand the syntax, it will work as you need.

----- There is no single answer in UNIX. ;)