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engine: add note on name resolution with multiple nameservers 

Signed-off-by: David Karlsson <david.karlsson@docker.com>
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David Karlsson 2023-05-03 15:19:59 +02:00
parent 44424f2ed1
commit 0125f46e97
1 changed files with 29 additions and 6 deletions
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network/index.md
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@ -84,18 +84,17 @@ you can use the `--alias` flag to specify an additional network alias for the co
## DNS services
By default, containers inherit the DNS settings of the host, as defined in the `/etc/resolv.conf` configuration file.
By default, containers inherit the DNS settings of the host,
as defined in the `/etc/resolv.conf` configuration file.
Containers that attach to the default `bridge` network receive a copy of this file.
Containers that attach to a
[custom network](network-tutorial-standalone.md#use-user-defined-bridge-networks)
use Docker's embedded DNS server.
The embedded DNS server forwards external DNS lookups to the DNS servers configured on the host.
Custom hosts, defined in `/etc/hosts` on the host machine, aren't inherited by containers.
To pass additional hosts into container, refer to
[add entries to container hosts file](../engine/reference/commandline/run.md#add-host)
in the `docker run` reference documentation.
You can override these settings on a per-container basis.
You can configure DNS resolution on a per-container basis, using flags for the
`docker run` command when you start the container. The following table
describes the available `docker run` flags related to DNS configuration.
| Flag | Description |
| -------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
@ -104,6 +103,30 @@ You can override these settings on a per-container basis.
| `--dns-opt` | A key-value pair representing a DNS option and its value. See your operating system's documentation for `resolv.conf` for valid options. |
| `--hostname` | The hostname a container uses for itself. Defaults to the container's ID if not specified. |
### Name resolution with multiple nameservers
When you specify multiple DNS servers using `--dns` flags, name resolution may
work in a surprising or unexpected way. DNS lookup behavior depends on a number
of different factors:
- Whether the container OS runs on [musl or glibc](https://wiki.musl-libc.org/functional-differences-from-glibc.html#Name_Resolver/DNS){: target="blank" rel="noopener" }
- Whether the Docker daemon binary was [statically or dynamically linked](https://pkg.go.dev/net#hdr-Name_Resolution){: target="blank" rel="noopener" }
- If dynamically linked, which version of glibc that's used
- Whether or not [nsswitch.conf is present](https://tldp.org/LDP/nag2/x-087-2-resolv.library.html#X-087-2-RESOLV.NSSWITCH-CONF){: target="blank" rel="noopener" }
You may find that name resolution works as follows:
1. The container emits requests to **all** nameservers that you specify.
2. The container uses the first response returned by any of the nameservers.
Even if the first response is `NXDOMAIN`, or similar.
### Custom hosts
Custom hosts, defined in `/etc/hosts` on the host machine, aren't inherited by containers.
To pass additional hosts into container, refer to
[add entries to container hosts file](../engine/reference/commandline/run.md#add-host)
in the `docker run` reference documentation.
## Proxy server
If your container needs to use a proxy server, see