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| '''Domain Name Resolution''' ('''DNS''') on Linux is done through a modular system that supports historical operation as well as modern software solutions. This system can lead to frustration, as when settings seem to magically reset on startup. | '''Domain Name Resolution''' ('''DNS''') on Linux is done in a very similar way to [[BSD/DNS|DNS on BSD]]. It is worth reiterating that [[Bind|DNS was invented for BSD]]. |
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| == Domain name resolution == | == Name resolution == |
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| The '''Name Service Switch''' ('''NSS''') file (`/etc/nsswitch.conf`) defines the order of operations for various services, among them being name resolution. | The [[Linux/NsSwitchConf|Name Service Switch (NSS) configuration file]] (`/etc/nsswitch.conf`) defines the order of operations for various services, among them being name resolution. |
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| See [[Linux/nsswitch.conf|here]] for more details on configuring `/etc/nsswitch`. |
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| The '''hosts''' file (`/etc/hosts`) is a list of addresses and names, especially for local hosts and machines. A basic hosts file looks like: | The [[Linux/Hosts|hosts file]] (`/etc/hosts`) is a list of addresses and names, especially for local hosts and machines. A basic hosts file looks like: |
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| See [[Linux/hosts|here]] for more details on configuring `/etc/hosts`. |
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| The '''resolver''' configuration file (`/etc/resolv.conf`) is a list of nameservers to query for name resolution. The file is read sequentially for up to 3 nameservers for each lookup. As such, changes are effective immediately. | The [[Linux/ResolvConf|resolver configuration file]] (`/etc/resolv.conf`) is a list of nameservers to query for name resolution. Changes are immediately effective. |
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| If the resolver file is being configured directly (which is rare-see below), then it should look like: | If the resolver file is being configured directly, then it should look like: |
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| See [[Linux/resolv.conf|here]] for more details on configuring `/etc/resolv.conf`. | Services that manipulate the resolver file include: * [[Linux/SystemdResolved|systemd-resolved(8)]] and it's userland utility `resolvectl(1)` * [[Linux/SystemdResolved#ResolveConf|systemd-resolveconf(8)]] * [[OpenResolv|openresolve]] with [[Linux/ResolvConfConf|resolveconf.conf(5)]] * [[Dhcpcd|dhcpcd(8)]] * [[NetworkManager|NetworkManager(8)]] === Utilities === * `drill(1)`, used as `drill NAME @SERVER TYPE` * [[Bind/Dig|dig(1)]] |
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| == Multicast domain name resolution == | == Multicast name resolution == |
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| '''Multicast domain name resolution''' ('''mDNS''') is an expansion of the DNS protocol making use of the reserved address space. By convention, the `.local` domain is reserved for mDNS. | '''Multicast domain name resolution''' ('''mDNS''') is an expansion of the DNS protocol. An mDNS broadcaster makes use of port 5353 and resolves names in the `.local` domain. |
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| ---- == Link-local multicast name resolution == '''Link-local multicast name resolution''' ('''LLMNR''') allows hosts to resolve names for other hosts on the same local link. Services listen on `224.0.0.252:5355` and `ff02::1:3:5355`. ---- == Debugging DNS == === Utilities === A number of tools exist for debugging DNS on Linux: * `drill` * `dig` (from the `bind` project, sometimes bundled with `dnsutils`) * `resolvectl` (from `systemd-resolved`), as in `resolvectl status` ---- == Programs that overwrite resolver files == === dhcpcd === '''`dhcpcd`''' is primarily a DHCP client. It will try to send DHCP information to `resolvconf`, but if that service is unavailable, it will itself generate `/etc/resolv.conf`. This latter behavior can be disabled by editing `/etc/dhcpcd.conf`: {{{ nohook resolv.conf }}} For most use cases, it is sufficient to provide a header file (`/etc/resolv.conf.head`) that `dhcpcd` will insert at the top of the new resolver file. ---- === openresolv === '''`openresolv`''' is an implementation of the `resolvconf` protocol. It is (optionally or otherwise) used by many programs: `dhcpcd`, `iwd`, `NetworkManager`, `netctl`, `openvpn`, and `wireguard`. To disable `openresolv`, set `resolveconf=NO` in the configuration file. ---- === systemd-resolvconf === '''`systemd-resolvconf`''' is a compatibility layer between `systemd-resolved` and the `resolvconf` protocol. It should be used if a program necessarily calls `resolvconf` but control needs to be returned to `systemd`. ---- === systemd-resolved === '''`systemd-resolved`''' is a multi-layered application, supporting modern DNS (and mDNS, and LLMNR) resolution: 1. a daemon handling name resolution through a dbus API, with all modern features (i.e. DNSSEC) 2. an NSS plugin (`resolve`) which re-implements most of the stack (''including'' reading the hosts file) 3. a DNS stub listener on 127.0.0.53:53 The recommended configuration of `/etc/nsswitch.conf` is as follows: {{{ hosts: mymachines resolve [!UNAVAIL=return] myhostname files dns }}} Then, the recommendation is to link the DNS stub file (which contains just the stub listen address, `127.0.0.53`) to `/etc/resolv.conf`. {{{ ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf }}} On the other hand, to disable the stub listener (so as to run a different DNS server), edit `/etc/systemd/resolved.conf` as follows: {{{ DNSStubListener=no }}} |
Note that there is also partial/coincidental support for '''link-local multicast name resolution''' ('''LLMNR''') in many of the Linux implementations of mDNS. LLMNR is a similar but deliberately incompatible protocol (using port 5355) and also with a limited scope for features. |
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DNS on Linux
Domain Name Resolution (DNS) on Linux is done in a very similar way to DNS on BSD. It is worth reiterating that DNS was invented for BSD.
Contents
Name resolution
Name Service Switch
The Name Service Switch (NSS) configuration file (/etc/nsswitch.conf) defines the order of operations for various services, among them being name resolution.
A minimal configuration looks like...
hosts: files dns
This configuration will require a fully configured hosts file, as seen below.
Consider instead this configuration, which makes use of libraries and services from the systemd project. This will enable some omissions from the hosts file.
hosts: files mymachines myhostname dns
Hosts
The hosts file (/etc/hosts) is a list of addresses and names, especially for local hosts and machines. A basic hosts file looks like:
127.0.0.1 localhost
Resolver
The resolver configuration file (/etc/resolv.conf) is a list of nameservers to query for name resolution. Changes are immediately effective.
If the resolver file is being configured directly, then it should look like:
nameserver 8.8.8.8
Services that manipulate the resolver file include:
systemd-resolved(8) and it's userland utility resolvectl(1)
Utilities
drill(1), used as drill NAME @SERVER TYPE
Multicast name resolution
Multicast domain name resolution (mDNS) is an expansion of the DNS protocol. An mDNS broadcaster makes use of port 5353 and resolves names in the .local domain.
Note that there is also partial/coincidental support for link-local multicast name resolution (LLMNR) in many of the Linux implementations of mDNS. LLMNR is a similar but deliberately incompatible protocol (using port 5355) and also with a limited scope for features.