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  1. #1

    Anyway to know how often do ISPs update DNS?

    Anyway to know how often do each ISP update DNS?

    I want to know that in order to know if DNS fail over is a good solution for my visitors.

  2. #2
    BTW I've just learned that "anyway" is not the same as "any way". What I wanted to say in the las post was "any way" .

    "Anyway" I think you understood the message

  3. #3
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    1-48 hours for propogation

  4. #4
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    you can help it along a bit by changing your TTL values
    Graham Craig

    "IT'S NOT HOW GOOD YOU ARE, IT'S HOW BAD YOU WANT IT."

  5. #5
    I think you didn't understand my question. I want to check each DNS from the main ISP used for my visitors. I have a list of the main DNS in Spain (most of my visitors are from there). Is there any way to check each DNS? I think that TTL showed from making a ping means another value. This script maybe is useful but for example in that case it shows 48 hours. But I think that is not what I want to know. My question is the limit of the DNS for the TTL of every domain. I mean 80.58.0.97 it seems like it refresh the database of TTL every 48 hours, so if you change your TTL value you have to wait a maximum of 48 hours for that DNS. But if you change the nameservers in the registrar you don't have to wait 48 hours for that DNS, you have to wait the TTL that the DNS saved for your domain. My question is that many ISP doesn't allow any TTL and how to know which is the limit of each TTL. I assume that the unique way to know is testing each DNS changing the TTL of a domain, is that correct?

  6. #6
    Join Date
    Nov 2005
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    Quote Originally Posted by EnAlgures
    Anyway to know how often do each ISP update DNS?

    I want to know that in order to know if DNS fail over is a good solution for my visitors.
    Whenever they feel like it.

    Many ISP's (mainly large ISPs) ignore the TTL's that your DNS server issues with DNS lookups. The problem is that you will never be able to figure out exactly what DNS information each of your users is getting unless you specifically ask each and every of them, and hope that they know how to find it and give it to you.

    Also, you would proably need to ask your users what their DNS servers are currently configured as, since in most cases the only way to find out what DNS servers a particular user on a particular ISP would be provided with (they are often assigned by DHCP anyway) is to actually use that ISP (most ISP's don't list the DNS servers that their customers use, and in some cases those DNS servers aren't even available to anyone not on their network).

    If an ISP is providing invalid information to its users, then there is little that you can do other than ask the user to complain to their ISP. There is no guaranteed way to "force" a DNS server to take updated DNS records.

  7. #7
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    Sterling, VA
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    ISPs and DNS...fishy

    ISPs will not disclose this information therefore it makes for a murky answer - but from my experience in the world of DNS and your question begs two answers.

    1. DNS changes are usually propogated in 24-72 hours from the leading ISPs. Average TTLs are set to 24 hours, so if you were to make a change to your DNS or want to failover immediately, you can expect to have cache poisoning and ISPs that will not adhere to your lowering of the TTLs anyway.

    "My question is that many ISP doesn't allow any TTL and how to know which is the limit of each TTL. I assume that the unique way to know is testing each DNS changing the TTL of a domain, is that correct?"

    2. You are right, and there is a way around this problem. We actually have nameservers in the same cage space as the leading ISPs worldwide hooked up by a piece of ethernet so you can actualy set your TTLs to 0 (zero) seconds on our network and be able to have realtime propogation and realtime failover for complete network availability. These ISPs have to adhere to our TTLs because we're sitting in the same rack and have this peering relationship however most public nameservers will not get this treatment.

    So, you may want to consider your options.
    James Dobbs
    Biz Dev Director
    Neustar UltraDNS
    (703) 547-6001

  8. #8
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    Quote Originally Posted by UltraDNS
    We actually have nameservers in the same cage space as the leading ISPs worldwide hooked up by a piece of ethernet so you can actualy set your TTLs to 0 (zero) seconds on our network and be able to have realtime propogation and realtime failover for complete network availability. These ISPs have to adhere to our TTLs because we're sitting in the same rack and have this peering relationship
    How do you define "leading ISPs"? Is there a list we can see?
    Chris

    "Some problems are so complex that you have to be highly intelligent and well informed just to be undecided about them." - Laurence J. Peter

  9. #9
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    sure AOL, Verio, Earthlink, Telstra, Cablevision, Cox, Verio, and about 20 more.
    James Dobbs
    Biz Dev Director
    Neustar UltraDNS
    (703) 547-6001

  10. #10
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    Interesting, thanks. Any in Spain? (for the benefit of the OP). Do you have an estimate of the percentage of global / US / Spanish users who are connected through your linked ISPs?

    Quote Originally Posted by EnAlgures
    I have a list of the main DNS in Spain (most of my visitors are from there). Is there any way to check each DNS? I think that TTL showed from making a ping means another value.
    You won't get the true TTL values they're using. The best you can do is check the records they return at regular intervals and try to estimate for yourself what they're doing (at the command line use nslookup or dig @nameserver domain, for scripting Net::DNS is good).
    Chris

    "Some problems are so complex that you have to be highly intelligent and well informed just to be undecided about them." - Laurence J. Peter

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