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What /etc/hosts does
/etc/hosts is a local text table that associates IP addresses with hostnames. It is useful when one Ubuntu computer needs a fixed name for another machine, when a local development service should be reachable by a memorable name, or when the computer’s own hostname must resolve consistently. It is not a DNS-zone editor and it does not configure the DNS servers used for names that are not listed locally.
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Each active entry puts the address first, the canonical hostname second, and any aliases after that. Spaces or tabs separate the fields. A # starts a comment, so text after it on the same line is ignored. Hostnames can contain letters, numbers, hyphens and periods, subject to the start-and-end rules documented by the Ubuntu hosts(5) manual.
Before editing: understand the files involved
/etc/hosts versus /etc/hostname
/etc/hosts maps names to addresses. /etc/hostname stores the local system hostname. They serve different purposes, but the local name should be represented consistently in both files. If you are changing the machine’s hostname, update the appropriate 127.0.1.1 entry before applying the new hostname, then remove the obsolete alias after the change.
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/etc/hosts versus DNS and /etc/resolv.conf
A hosts-file entry is a static answer on this computer. It does not publish a name to your LAN or to the internet, and it cannot provide DNS record types such as MX. Ubuntu Server guidance treats static hostnames as local /etc/hosts entries and recommends using resolvectl and systemd-resolved for resolver configuration instead of manually replacing /etc/resolv.conf.
What systemd-resolved does with the file
When systemd-resolved is active, it reads the hosts mappings locally and answers them without sending those names to the network. Its documentation notes that localhost and localhost.localdomain are synthesized as 127.0.0.1 and ::1. Hosts-file processing can be disabled with ReadEtcHosts=no; if that setting is in use, entries in the file will not be consulted by systemd-resolved.
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Back up the current file
Create a copy before making any change. The backup command preserves the original file in the same directory:
sudo cp /etc/hosts /etc/hosts.bakThe command should return to the shell without an error. If you later need to undo the edit, compare the files first and restore the backup only when you are certain it is the correct version for this machine:
sudo cp /etc/hosts.bak /etc/hostsRestoring a backup replaces the current file, so do not use the restore command as a first diagnostic step. Inspect the current contents and keep a copy of any deliberate entries you still need.
Edit /etc/hosts safely
Use sudoedit
The preferred interactive method is:
sudoedit /etc/hostssudoedit opens a temporary working copy in the configured editor and writes the result back with root privileges. Save the file and exit the editor normally. If no editor is configured, use the direct Nano command below.
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Use Nano directly
sudo nano /etc/hostsIn Nano, save with Ctrl+O, confirm the filename, and exit with Ctrl+X. The command prompt may ask for your password because writing under /etc requires administrative privileges.
Do not replace the file with a partial template
Start from the existing contents. Ubuntu installations commonly include IPv4 localhost, a local-name line, IPv6 localhost, and IPv6 multicast entries. Deleting those lines can break programs that expect standard localhost names or prevent the local hostname from resolving as intended.
What an Ubuntu hosts file should look like
A typical file has entries similar to these:
127.0.0.1 localhost
127.0.1.1 myhost.example.test myhost
192.168.1.10 fileserver.example.test fileserver
::1 localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters| Line | Purpose |
|---|---|
127.0.0.1 localhost |
IPv4 loopback name. Keep this standard mapping unless you have a documented reason to change it. |
127.0.1.1 myhost.example.test myhost |
Common Ubuntu convention for the computer’s fully qualified name and its short alias. |
192.168.1.10 fileserver.example.test fileserver |
Example static mapping for another host on the local network; replace the address and names with your actual values. |
::1 localhost ip6-localhost ip6-loopback |
IPv6 loopback names. |
ff02::1 ip6-allnodes and ff02::2 ip6-allrouters |
IPv6 multicast names commonly present in Ubuntu’s example. |
The first two fields are not interchangeable: placing a hostname before an address makes the line invalid for hosts-file lookup. Keep aliases on the same line as the canonical name, separated by spaces or tabs.
Add a mapping for another machine
- Determine the address that should be used. For the example below, the server address is
192.168.1.10. - Choose one canonical hostname and, if useful, one or more short aliases. The canonical name appears immediately after the address.
- Add a line to
/etc/hosts:
192.168.1.10 fileserver.example.test fileserver- Save the file, then query the short alias and the fully qualified name:
getent hosts fileserver
resolvectl query fileserver.example.testgetent hosts exercises the system name-service stack. resolvectl query is especially useful when systemd-resolved is active. Both commands should show the address associated with the name. A successful lookup confirms that the name resolves; it does not prove that the remote computer is powered on or accepting connections.
Test actual reachability separately
To send one ICMP request using the name, run:
ping -c 1 fileserverA successful name lookup followed by a failed ping usually points to reachability, firewall, routing or service availability rather than a hosts-file syntax problem. Conversely, an unknown-host error from getent or resolvectl means the name, address, spelling or resolver path needs attention first.
Change Ubuntu’s own hostname
Ubuntu commonly associates the local machine name with 127.0.1.1. Suppose the current line is:
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127.0.1.1 ubuntu ubuntu-oldTo introduce newname, first place the new name on that line:
127.0.1.1 ubuntu newnameApply the hostname change using the hostname procedure appropriate to your system, then remove the old alias when it is no longer needed:
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How lookup and caching affect the result
Hosts-file modifications normally take effect immediately. The file itself does not require a reboot. However, an application can cache a previous DNS answer or retain a failed lookup for a while. If getent and resolvectl return the new address but one program still uses the old one, close and reopen that program or restart only the affected service according to its own documentation.
systemd-resolved handles hosts entries locally and does not forward those names to configured DNS servers. This makes a hosts entry suitable for a private name that should resolve only on the Ubuntu machine. It also means that editing the file will not make the name available to other computers; each client would need its own entry or a shared DNS service.
Diagnose “sudo cannot resolve my host”
A warning such as “sudo: unable to resolve host …” usually means the current hostname does not match any local hosts-file name. Work through the checks in this order:
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- Read
/etc/hostnameand note the exact hostname, including capitalization and punctuation. - Open
/etc/hostsand check that the same name appears after an address, normally on the127.0.1.1line for the local machine. - Verify that the address comes first and that the hostname is not inside a comment beginning with
#. - Keep
127.0.0.1 localhostand the IPv6 localhost line intact unless a documented system design requires otherwise. - Run
getent hosts <name>, replacing<name>with the exact local hostname. - If systemd-resolved is active, run
resolvectl query <name>as a second check.
For example, if /etc/hostname contains newname, a matching hosts entry can be:
127.0.1.1 newnameIf you need both a fully qualified name and a short name, list the fully qualified name first and the short form as an alias:
127.0.1.1 newname.example.test newnameAfter saving, run the lookup commands again. A reboot is not normally necessary; restart a particular application only if it continues to use stale data after the system resolver returns the corrected address.
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Reversing the fields
Incorrect:
fileserver 192.168.1.10Correct:
192.168.1.10 fileserverHosts-file syntax always starts with the IP address.
Using an alias on a separate line without an address
Every active line needs an address. Put related names on one line:
192.168.1.10 fileserver.example.test fileserver filesEditing without administrative privileges
A normal editor may open the file read-only or fail to save it. Use sudoedit /etc/hosts or sudo nano /etc/hosts from a user account allowed to use sudo.
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Accidentally commenting out a mapping
Anything after # is a comment. A line such as #192.168.1.10 fileserver does not create a mapping. Remove the leading # when the entry should be active.
Changing only one of the hostname files
If /etc/hostname says one name while /etc/hosts contains another, local commands and administrative tools can report resolution errors. Make the names consistent, then test with getent.
Expecting the mapping to work elsewhere
An entry is local to the computer where it was added. It does not update a router, DNS server or another Ubuntu installation. Configure those systems separately if the name must be shared.
Expecting mail or other DNS record types
The hosts file supplies address-to-name mappings. systemd-resolved’s documentation specifically notes that these mappings do not affect lookups for non-address types such as MX records. Use DNS for those records.
Use comments without hiding important entries
Comments are useful for recording why a static address exists, especially when an address may change. Keep the comment at the end of an active line:
192.168.1.10 fileserver.example.test fileserver # office storage hostDo not put explanatory text before the address unless the entire line is intentionally a comment. Keep comments factual and avoid recording passwords, tokens or other secrets in a world-readable system file.
A complete configure-and-verify workflow
- Back up the file with
sudo cp /etc/hosts /etc/hosts.bak. - Open it using
sudoedit /etc/hostsorsudo nano /etc/hosts. - Preserve the existing localhost and IPv6 lines.
- For each static name, add
IP_address canonical_hostname [aliases...]. - If changing the local hostname, make the
127.0.1.1entry and/etc/hostnameagree. - Save the file and run
getent hosts name. - When systemd-resolved is in use, run
resolvectl query name. - Use
ping -c 1 nameonly as a separate connectivity check. - If command-line lookups succeed but one application fails, restart that application rather than rebooting the computer.
When to use DNS instead
Use /etc/hosts for a small, deliberately local set of static mappings: a few lab machines, a temporary development name, or the Ubuntu computer’s own hostname. A centrally managed DNS service is more appropriate when many clients need the same names, addresses change regularly, reverse records must be maintained, or services need record types beyond address mappings. The hosts file is simple precisely because it is local and manual; that simplicity becomes a maintenance burden as the number of machines grows.
Final verification checklist
- The backup
/etc/hosts.bakexists. - Every active line begins with an IPv4 or IPv6 address.
- The canonical hostname follows the address, with aliases after it.
- Fields are separated by spaces or tabs, and comments begin with
#. 127.0.0.1 localhostand the IPv6 localhost mappings remain present unless intentionally redesigned.- The local hostname in
/etc/hostnamematches its entry in/etc/hosts. getent hosts namereturns the expected address.resolvectl query nameagrees when systemd-resolved is active.- An application that still shows old data has been restarted if necessary.
Once these checks pass, Ubuntu is using the intended local mappings, while ordinary names not listed in the file continue through the configured resolver path.
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