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plugin/bind: Fix listener collision notes in readme (#5349)

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* clarify listener collision notes in readme

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

* formatting

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>
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Chris O'Haver 2022-04-28 10:06:15 -04:00 • committed by GitHub
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plugin/bind/README.md
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@ -84,10 +84,17 @@ You can exclude some addresses by their IP or interface name (The following will
## Bugs
When defining more than one server block, take care not to bind more than one server to the same
address and port. Doing so will result in unpredictable behavior (requests may be randomly
served by either server). Keep in mind that *without* the *bind* plugin, a server will bind to all
interfaces, and this will collide with another server if it's using *bind* to listen to an interface
### Avoiding Listener Contention
TL;DR, When adding the _bind_ plugin to a server block, it must also be added to all other server blocks that listen on the same port.
When more than one server block is configured to listen to a common port, those server blocks must either
all use the _bind_ plugin, or all use default binding (no _bind_ plugin). Note that "port" here refers the TCP/UDP port that
a server block is configured to serve (default 53) - not a network interface. For two server blocks listening on the same port,
if one uses the bind plugin and the other does not, two separate listeners will be created that will contend for serving
packets destined to the same address. Doing so will result in unpredictable behavior (requests may be randomly
served by either server). This happens because *without* the *bind* plugin, a server will bind to all
interfaces, and this will collide with another server if it's using *bind* to listen to an address
on the same port. For example, the following creates two servers that both listen on 127.0.0.1:53,
which would result in unpredictable behavior for queries in `a.bad.example.com`: