Ubuntu’s /etc/hosts file is a local table that maps IP addresses to hostnames. To configure it safely, back it up, edit it with root privileges, preserve Ubuntu’s localhost entries, add each mapping as IP address canonical hostname aliases, then verify the result with getent or resolvectl. Changes normally take effect immediately, although an individual application may retain an older cached result.
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What the Ubuntu hosts file does
The Ubuntu hosts(5) manual describes /etc/hosts as “a simple text file that associates IP addresses with hostnames.” It is a local, static lookup table, not a DNS-zone editor. An entry affects name resolution on the Ubuntu machine where the file is stored; it does not publish a name for other computers on your network or on the internet.
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Back up the file before changing it
A misplaced character can make local services unreachable or cause commands that depend on the machine name to fail. Create a copy first:
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sudo cp /etc/hosts /etc/hosts.bakThe backup is in the same directory, so it is available even if the edited file contains an error. To restore it later, use:
sudo cp /etc/hosts.bak /etc/hostsCheck the permissions after editing. The file should remain owned by root and should not be made world-writable merely to avoid using sudo.
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Understand the required line format
Each non-comment line contains an address first, the canonical hostname second, and optional aliases after that:
IP_address canonical_hostname [aliases...]Separate fields with spaces or tabs. A # begins a comment; anything after it on that line is ignored. Hostnames can contain letters, numbers, hyphens, and periods, subject to the hostname rules documented in hosts(5). Do not put the hostname before the address.
Ubuntu’s standard entries
A typical desktop or server file resembles this:
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 or address | Purpose |
|---|---|
127.0.0.1 localhost |
IPv4 loopback name for the local computer. |
127.0.1.1 myhost.example.test myhost |
Common Ubuntu convention for the local machine’s hostname and optional short alias. |
192.168.1.10 fileserver.example.test fileserver |
An example private-network server mapping; use the server’s actual address and names. |
::1 localhost ip6-localhost ip6-loopback |
IPv6 loopback names. |
ff02::1 and ff02::2 |
Common IPv6 multicast names for all nodes and all routers. |
Keep the existing localhost and IPv6 lines intact unless you have a documented reason to change them. Removing or repurposing loopback names can break software that assumes they resolve locally.
Edit /etc/hosts safely
1. Open the file with a root-aware editor
The preferred approach is sudoedit, which opens a temporary copy in your configured editor and writes it back with the correct privileges:
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sudoedit /etc/hostsIf you specifically want Nano, run:
sudo nano /etc/hostsDo not edit a copy in your home directory and assume it changes the system file. The saved file must be /etc/hosts itself.
2. Preserve the existing local entries
Before adding anything, identify the current 127.0.0.1, 127.0.1.1, and IPv6 lines. Ubuntu’s official example uses 127.0.1.1 for the machine’s fully qualified name in addition to the short name. Leave unrelated aliases on those lines unless you are deliberately changing the system hostname.
3. Add the mapping
For a server at 192.168.1.10, add one line such as:
192.168.1.10 fileserver.example.test fileserverThe first name is the canonical name for the address. The second is an alias, so both fileserver.example.test and fileserver resolve to 192.168.1.10 on this Ubuntu installation. Use one line for multiple names that intentionally share the same address; do not create contradictory lines for the same name.
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For a local development site, a loopback mapping is common:
127.0.0.1 app.example.test appThis sends both names to the local machine. It does not make the site reachable from another computer. To point a name at another host, use that host’s reachable IPv4 or IPv6 address instead.
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4. Save and exit
In Nano, press Ctrl+O, confirm the filename, then press Ctrl+X. With sudoedit, save and close the editor; the temporary file is copied back only if the editor exits successfully.
5. Verify the file and mapping
First inspect the resulting lines:
grep -v '^[[:space:]]*#' /etc/hostsThen test through the system name-service path:
getent hosts fileserver
resolvectl query fileserver.example.test
ping -c 1 fileservergetent hosts is a useful first test because it asks the configured name-service stack rather than reading the file in isolation. On systems using systemd-resolved, resolvectl query shows the resolver’s answer. A successful ping confirms that the name resolved and that an ICMP response was received; a failed ping does not by itself prove that the hosts entry is wrong because firewalls may block ICMP.
Examples for common configurations
One hostname with a short alias
10.0.0.25 printer.office.test printerBoth names resolve to the same address locally. Ensure that the address is stable; a DHCP lease change can make a static entry point to the wrong device.
Several names on one local service
192.168.1.20 git.office.test git source-controlAliases are space-separated and remain on the same line as the canonical name.
An IPv6-only target
2001:db8:1234::20 service.example.test serviceUse the actual IPv6 address assigned in your network. Do not substitute the documentation prefix above for a real production address.
Comments for maintenance
192.168.1.10 fileserver.example.test fileserver # office NASComments help future administrators understand why an entry exists. They are ignored by the resolver.
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Changing Ubuntu’s system hostname
If you are renaming the Ubuntu machine itself, keep /etc/hostname and /etc/hosts consistent. Ubuntu guidance places the new name on the 127.0.1.1 line before applying the hostname change. For example, a machine currently using ubuntu can be prepared with:
127.0.1.1 ubuntu newnameApply the hostname change with the normal hostname tool:
sudo hostnamectl set-hostname newnameAfter the change, edit the line so the new canonical name remains and the old alias is removed when it is no longer needed:
127.0.1.1 newnameCheck both files:
cat /etc/hostname
getent hosts "$(hostname)"The exact fully qualified name used in your environment may include a domain suffix. Keep the spelling and capitalization consistent in the hostname file, the 127.0.1.1 entry, certificates, service configuration, and automation that refers to the machine.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsHow /etc/hosts interacts with systemd-resolved and DNS
The hosts file supplies static local answers. It does not configure DNS servers, search domains, DHCP behavior, or network interfaces. Ubuntu Server documentation recommends using resolvectl and the systemd-resolved configuration for resolver settings rather than manually replacing /etc/resolv.conf.
systemd-resolved reads hosts-file mappings locally and immediately under its normal configuration. Those names are not sent to upstream DNS. Its documentation also states that localhost and localhost.localdomain are synthesized as 127.0.0.1 and ::1. Hosts-file support can be disabled with ReadEtcHosts=no; if that setting is present, entries in /etc/hosts will not be used by that resolver.
Do not use /etc/hosts to create mail routing, service-discovery, or other DNS record types. The mapping is an address-to-name association only. A name can resolve correctly with getent while an application that needs an MX, SRV, TXT, or other record still requires DNS.
Troubleshoot a mapping that does not work
The name returns no address
- Confirm that the line is not commented out and that the address appears before the hostname.
- Check for spelling differences between the command,
/etc/hostname, and/etc/hosts. Hostname comparisons can be affected by exact spelling, so test the fully qualified and short forms separately. - Make sure there are no hidden characters or accidental punctuation. Display the file with line numbers if necessary:
sudo nl -ba /etc/hosts. - Run
getent hosts name. If it fails, the system name-service stack did not obtain an answer from the configured sources.
The wrong address is returned
- Search for duplicate entries for the same hostname and remove or correct stale ones.
- Verify that the address belongs to the intended machine and that a DHCP lease or network redesign has not changed it.
- Test the exact name used by the application. A short alias and a fully qualified hostname can be separate entries.
One application still uses the old address
The hosts-file change normally takes effect without a reboot, but applications may cache name lookups independently. Close and restart only the affected program or service first. Browsers, language runtimes, containerized applications, and long-running daemons can each have their own cache or resolver behavior. If getent and resolvectl query show the new address, the remaining problem is likely inside that application or its network namespace rather than in the file.
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This warning commonly means the machine’s current hostname does not resolve locally. Compare:
hostname
cat /etc/hostname
getent hosts "$(hostname)"
Ensure the hostname shown by hostname appears on the appropriate 127.0.1.1 line (or on another deliberate local address) and that /etc/hostname contains the same name. Correct the spelling, save the file, and run the getent test again. Do not “fix” the warning by deleting the localhost entries.
resolvectl does not reflect the entry
- Check whether
systemd-resolvedis active and whether its configuration hasReadEtcHosts=no. - Use
getent hosts nameto test the complete name-service configuration even whenresolvectlis unavailable or a different resolver is in use. - Inspect the exact file being used. A container, chroot, virtual machine, or separate network namespace can have its own
/etc/hosts.
Ping fails after the name resolves
A resolved name proves only that an address was selected. Check reachability, routing, service ports, and firewall policy separately. Use a protocol-specific test such as the service’s client or a TCP connection test when ICMP is filtered.
Quick Recap
Operational and security considerations
- Limit edits to the mappings you actually need. A hosts entry overrides normal DNS behavior for that name on the local machine.
- Review entries after moving a service, changing a DHCP reservation, or restoring a system image; stale addresses can silently redirect administrative commands and applications.
- Do not copy an untrusted hosts file onto a server. A malicious entry can redirect software downloads, update endpoints, login portals, or internal services without changing the network’s DNS.
- Use version control or a configuration-management system for fleets, while preserving file ownership, permissions, localhost lines, and machine-specific differences.
- Keep a dated or clearly named backup before bulk changes. Restore the backup if a change causes name-resolution failures, then reapply entries one at a time.
Quick verification checklist
- Create a backup with
sudo cp /etc/hosts /etc/hosts.bak. - Edit with
sudoedit /etc/hostsorsudo nano /etc/hosts. - Retain
127.0.0.1 localhost, the IPv6 localhost line, and the existing multicast entries unless a documented configuration requires otherwise. - Write new lines as
address canonical-name alias, with spaces or tabs between fields. - Confirm the address and spelling, then save the file.
- Run
getent hosts name; on systemd-resolved systems also runresolvectl query name. - Restart only an application that continues to use a cached result.
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