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Hannes
Asked: 2022-03-06 17:30:22 +0800 CST

How does unbound handle multiple forwarders (forward-addr)?

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According to https://nlnetlabs.nl/documentation/unbound/unbound.conf/ multiple forwarders can be configured with forward-addr

For the forward zone this list of nameservers is used to forward the queries to. The servers listed as forward-host: and forward-addr: have to handle further recursion for the query.

However there is no statement how these servers are used. Is only the first server queried? Are all simultaniously queried and fastes answer will be sent? Or are they queried in a round robin style? Or another option?

domain-name-system forwarding unbound

2 Answers

  1. Well I couldn't find a better source, but it seems that it uses the first address and then if it is not responsive after certain time, it takes the next in the list.

    I found this page that seems to be a conversation with someone from support.

    It also says that some other load balancer algorithms are not available.

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  2. Unbound's SRTT implements RFC2988 https://tools.ietf.org/html/rfc2988.

    Basically unbound will occasionally select randomly a server whose SRTT is between the fastest and (fastest + 400ms).

    Bind similarly uses a decaying function on the srtt so that it will occasionally pick a slower server instead of just sticking with the fastest one.

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