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I'm running a caching DNS server currently to improve latency in a network.

The question is: can I override the TTL I get from a server using BIND9 or other software on Linux?

short "dig www.google.com" here:

; <<>> DiG 9.6.1-P2 <<>> www.google.com

;; ANSWER SECTION: www.google.com. 604441 IN CNAME www.l.google.com. www.l.google.com. 300 IN A 74.125.45.147

Can I change that '300' into 15 minutes?

Thanks you so much for your time!!.

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As mentioned several times, please don't do this. Just take a quick look around on serverfault to find out how many questions there are from sysadmins who have to deal with DNS servers that don't obey the TTL. Try: serverfault.com/questions/54758/… or serverfault.com/questions/103281/… or serverfault.com/questions/96453/… – Mark Henderson♦ Feb 17 '10 at 22:03
Do not worry it was a proof of concept, that CAN be done. Also this will not propagate more than 2 people in this network. And this was part as an Academic effort to show a internet connection with the lowest latency possible. I understand your concerns. This was the first time I use this web page, and I'm amazed by the good replies. – OmniWired Feb 18 '10 at 0:10
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3 Answers

up vote 5 down vote

CAN this be done? Sure - there are broken DNS servers (e.g. the ones AOL runs) that do this, and every admin I know hates it.

SHOULD this be done? Almost certainly no.

Generally speaking the TTL was set to a particular value for a reason (in google's case, probably fault tolerance: You'll only be unable to reach google for 5 minutes if that server blows up), and you shouldn't muck about with it.

You're already getting a performance boost by keeping the google.com record in your cache for the 5 minutes it's intended to live for since your individual workstations won't be running out to the internet for resolution -- don't over-optimize and break the expected behavior :)

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should I change the question and say: how can I run a evil, not standard compliant server, so evil it cache for 15 minutes? So I can learn "what shouldn't be done". Thanks you everyone for the quick responses. – – OmniWired Feb 17 '10 at 20:33
You could TAKE OVER THE WORLD this way. Muaaahahahaha. Yes. Evil. – kubanczyk Feb 17 '10 at 20:52
yes, and learning in the process, amazing! I'm sure it can be done, but I can't seem to find an answer... Maybe with some crazy BIND9 configuration zone... – OmniWired Feb 17 '10 at 21:00
Well, my general intent was to convince you not to do it. If you insist the only way I know of would be to actually to patch BIND to add a min-cache-ttl configuration directive (analogous in function to the existing max-cache-ttl). AFAIK no caching DNS server in the wild allows you to do this, though I haven't looked at Microsoft's... – voretaq7♦ Feb 17 '10 at 21:09
I thought that as well. I'm downloading dnsmasq source as we speak. I believe will be easier to patch dnsmasq because the source code to analyze is shorter. – OmniWired Feb 17 '10 at 21:14
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up vote 2 down vote

the DIRTIEST most ugliest thing that can be done is...

1-Downloading the source 2-find the file called cache.c 3-find the function is_expired

4- Change it in this way

static int is_expired(time_t now, struct crec *crecp)
{
  if (crecp->flags & F_IMMORTAL)
    return 0;

  if (difftime(now, crecp->ttd) < 0)
    return 0;

  return 0; // IT WAS IN ONE
}

When the function ask did expire? we always saw no

In this way it will never expire and you will conquer the world.

OUTPUT:

; <<>> DiG 9.6.1-P2 <<>> www.google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28477
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;www.google.com.            IN  A

;; ANSWER SECTION:
www.google.com.     603937  IN  CNAME   www.l.google.com.
www.l.google.com.   4294966733 IN   A   209.85.195.99
www.l.google.com.   4294966733 IN   A   209.85.195.104
www.l.google.com.   4294966733 IN   A   209.85.195.147

;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Wed Feb 17 18:34:47 2010
;; MSG SIZE  rcvd: 110
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+1 for continuous urge to take over the world (and learn in the process). – kubanczyk Feb 20 '10 at 12:22
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up vote 0 down vote

See similar question here http://serverfault.com/questions/86968/dnsmasq-how-to-increase-ttl and another one here http://serverfault.com/questions/86970/is-there-an-alternative-to-dnsmasq

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in that question it doesn't fix my problem. I need to cache for longer periods of time (more than 300 seconds) one particular web site. – OmniWired Feb 17 '10 at 20:09
The dnsmasq article point to this: "Normally responses which come form /etc/hosts and the DHCP lease file have Time-To-Live set as zero, which conventionally means do not cache further. If you are happy to trade lower load on the server for potentially stale date, you can set a time-to-live (in seconds) here." And is not what I'n looking to accomplish. I want to use an address resolved by the dns for a longer period than the one I been given – OmniWired Feb 17 '10 at 20:14
@omniwired - The reason you can reduce the TTL is because TTL = "You may cache this record for at most S seconds". Good, Correct DNS servers shouldn't ever let you cache records longer than their TTL... – voretaq7♦ Feb 17 '10 at 20:22
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