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/etc is the standard Unix/Linux directory for host-specific system configuration: local, system-wide files that tell installed programs and services how one machine, virtual machine, container, or image should operate. It is normally part of the root filesystem and is needed early in boot. The Filesystem Hierarchy Standard (FHS) defines configuration files as local, non-executable control files; application binaries and runtime state belong elsewhere.
The important operational qualification is that /etc is a configuration interface, not always the component doing the work. A file may be read directly, generated by NetworkManager or cloud-init, represented by a symlink into /run, or superseded by a systemd drop-in. Before editing anything, identify which program owns and consumes it.
What “host-specific” means
“Host-specific” means configuration for one installed system, rather than generic program data shipped under /usr or personal preferences kept in a user’s home directory. The host can be bare metal, a VM, cloud instance, container, or immutable image; it does not have to be unique hardware.
| Scope | Typical location | Example |
|---|---|---|
| Persistent host configuration | /etc |
/etc/hostname |
| Runtime/generated state | /run |
Generated resolver data |
| Kernel and device state | /proc, /sys |
Kernel settings and devices |
| Installed software | /usr, /opt |
Program files |
| Logs, queues, caches | /var |
Service logs |
| User preferences | /home, often ~/.config |
Per-user shell settings |
The name is often explained historically as “et cetera,” but the FHS definition is functional: /etc contains host-level configuration. Configuration normally goes in subdirectories such as /etc/ssh, /etc/systemd, or /etc/NetworkManager. Software installed under /opt conventionally uses /etc/opt/<application>.
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The files you encounter most often
| Path | Purpose | Editing caution |
|---|---|---|
/etc/hostname |
Persistent static hostname in the systemd model | May be changed by cloud or provisioning tools |
/etc/hosts |
Local IP-to-name mappings | Does not publish DNS records; NSS controls whether it is consulted |
/etc/resolv.conf |
Resolver nameservers, search domains, and options | Often a symlink or generated file |
/etc/nsswitch.conf |
Sources and order for accounts, hosts, services, and more | Affects many applications; syntax and modules vary |
/etc/fstab |
Persistent filesystem mounts | Errors can cause emergency-mode boot |
/etc/passwd, /etc/shadow, /etc/group |
Local account and group databases | Use account-management commands instead of hand editing |
/etc/systemd/system/ |
Administrator units and overrides | Prefer drop-ins over copying vendor units |
/etc/ssh/sshd_config |
SSH daemon policy | Run sshd -t before reloading |
/etc/profile.d/ |
System-wide login-shell snippets | Not every shell, service, or GUI session reads it |
Hostname, hosts, DNS, and NSS are different layers
/etc/hostname: the local identity
On systemd-based Linux, /etc/hostname normally contains one newline-terminated static hostname. Comments beginning with # are ignored. The systemd documentation recommends a DNS-compatible single label for the static hostname and documents a maximum of 64 characters. A human-readable “pretty” hostname is a separate field.
hostname
hostnamectl status
cat /etc/hostnameTo set the persistent hostname on a system with systemd hostnamed:
sudo hostnamectl set-hostname server01
hostnamectl status
hostnameFor a display name that is not suitable as a DNS label:
sudo hostnamectl set-hostname "Application Server" --prettyhostnamectl is not universal Unix functionality. It requires systemd integration. Changing the local hostname also does not create DNS records, update certificates, alter monitoring inventories, or guarantee that cloud-init will not change it again.
/etc/hosts: a local static table
The hosts(5) manual defines records as an address followed by a canonical name and optional aliases:
127.0.0.1 localhost
127.0.1.1 server01.example.test server01
::1 localhost ip6-localhost ip6-loopbackThe 127.0.1.1 convention is distribution-dependent; do not assume every system uses it. Entries are useful for loopback identity, isolated networks, temporary overrides, testing, and bootstrapping. They affect local lookups only and do not publish anything to the wider network.
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Whether an application consults this file, and in what order, depends on /etc/nsswitch.conf. Check the actual path with:
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getent hosts server01
getent hosts server01.example.testgetent exercises the system’s Name Service Switch path. Tools such as dig query DNS directly and therefore may not reproduce what ordinary applications do.
/etc/resolv.conf: resolver-client settings
The resolv.conf(5) manual documents directives such as nameserver, search, and options. This file configures a resolver library; it is not a DNS server and cannot create DNS records.
ls -l /etc/resolv.conf
readlink -f /etc/resolv.conf
cat /etc/resolv.conf
resolvectl status
getent hosts example.comNever replace it blindly. NetworkManager, DHCP clients, VPN software, cloud-init, or systemd-resolved may regenerate it. On systemd-resolved systems, durable settings may belong in /etc/systemd/resolved.conf or /etc/systemd/resolved.conf.d/, as described in the resolved.conf documentation.
/etc/nsswitch.conf: who gets asked
NSS controls sources for databases such as passwd, group, shadow, hosts, services, and networks. A line such as hosts: files dns usually checks local files before DNS, but systems may include resolve, myhostname, LDAP, mDNS, or other modules. The installed modules and distribution configuration determine exact behavior.
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Filesystems: /etc/fstab
fstab describes filesystems, mount points, types, options, dump settings, and filesystem-check order. Review edits carefully, especially for network or removable filesystems:
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findmnt --verify
sudo mount -amount -a attempts to mount eligible entries and can have side effects. A wrong UUID, unavailable device, bad option, or missing nofail can delay boot or trigger emergency mode.
Accounts and authentication
/etc/passwd contains account metadata such as names, IDs, home directories, and shells. Modern systems normally keep password hashes in the protected /etc/shadow; /etc/group and /etc/gshadow hold group data. Prefer useradd, usermod, passwd, and groupadd. Protect backups because they may contain hashes or other sensitive data.
Other security-sensitive files include /etc/sudoers and files under /etc/sudoers.d/. Validate sudo policy with visudo, not a generic editor.
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Shell and login defaults
/etc/profile initializes login shells; /etc/profile.d/ provides modular snippets. Some distributions provide /etc/bash.bashrc. /etc/shells lists permitted login shells, while /etc/motd and /etc/issue provide login messages. Names and loading rules differ across shells and distributions, and these files do not automatically configure services or every SSH command.
Services and systemd
Vendor unit files commonly reside in /usr/lib/systemd/system or /lib/systemd/system. Administrator units and overrides belong in /etc/systemd/system. Do not edit vendor files: package upgrades can replace them. Prefer a drop-in created with systemctl edit example.service, then apply and inspect it:
sudo systemctl edit example.service
sudo systemctl daemon-reload
sudo systemctl restart example.service
systemctl status example.service
journalctl -u example.serviceSome systemd components use separate global drop-in directories such as /etc/systemd/*.conf.d/. Their precedence is component-specific; consult that component’s manual.
Dynamic linker paths
/etc/ld.so.conf and /etc/ld.so.conf.d/ add library search paths for systems using ldconfig:
sudo ldconfig
ldconfig -pNever add writable or untrusted directories casually. A malicious shared library in such a path can enable code execution or break system programs.
SSH and scheduled jobs
Before reloading SSH, validate its configuration:
sudo sshd -t
sudo systemctl reload sshThe service may be named ssh or sshd. Keep console or out-of-band access when changing remote login policy. System-wide cron configuration commonly uses /etc/cron.* and /etc/cron.d/; exact syntax and ownership requirements depend on the cron implementation.
How to determine who owns a file
Use inspection before editing:
stat /etc/hosts /etc/hostname /etc/resolv.conf
ls -l /etc/resolv.conf
systemctl is-active NetworkManager
systemctl is-active systemd-resolved
systemctl is-active systemd-networkd
cloud-init status“Unit not found” is normal when a distribution does not use that service. Also check for container or image-management behavior. A reboot, DHCP renewal, package update, or image replacement may regenerate a file even if your manual edit was correct.
A safe editing workflow
- Identify the owner. Check symlinks, active network managers, generators, and documentation.
- Back up the original.
sudo cp -a /etc/example.conf /etc/example.conf.bak - Edit with privilege safely.
sudoedit /etc/example.conf - Run the subsystem validator. Examples include
sudo sshd -t,findmnt --verify,visudo, or a service-specific check. - Reload or restart only what is necessary. Use
daemon-reloadafter unit changes. - Test through the real path. Use
getentfor name service,systemctl statusandjournalctlfor services, and a controlled mount test forfstab. - Keep rollback access. Remote administrators should retain console or provider recovery access for risky SSH, sudo, mount, and resolver changes.
Common failures and recovery
Hostname changed, but applications still fail
Check for a missing local mapping, absent DNS record, an application expecting an FQDN, stale certificates or Kerberos principals, monitoring inventory mismatches, or a provisioning agent that reverted the name:
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hostname --fqdn
getent hosts "$(hostname)"
cat /etc/hostsA successful hostnamectl command does not update DNS or external systems.
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Resolver edits disappear
Inspect the symlink target and active manager:
ls -l /etc/resolv.conf
readlink -f /etc/resolv.conf
resolvectl status
nmcli general statusConfigure the responsible manager instead of repeatedly overwriting its generated output.
/etc/hosts appears ignored
Check that NSS includes files, that the name matches exactly, and that you are not testing with a DNS-only tool:
grep '^hosts:' /etc/nsswitch.conf
getent hosts name.exampleIPv6 preference, caching, application-specific resolvers, and alternate NSS modules can also change the result.
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Use rescue or emergency mode, or a live environment if necessary. Remount the root filesystem read-write, restore the backup or comment out the faulty entry, then review with findmnt --verify. Test with mount -a only after checking device availability, network ordering, and options.
Containers, cloud images, and immutable systems
Containers often have a minimal or synthetic /etc. The runtime may inject /etc/hostname, generate /etc/hosts, and mount or replace /etc/resolv.conf. Many containers lack systemd, so hostnamectl, systemd-resolved, and systemd unit management may not exist.
Cloud-init and provider agents can rewrite identity and network files at first boot or on every provisioning cycle. Image-based or immutable operating systems may expose /etc as an overlay or managed writable layer; changes may need to be made through image-building or provisioning mechanisms and can disappear after replacement or rollback. These are implementation differences, not contradictions of the FHS.
When to edit directly—and when not to
Direct editing is reasonable for a documented static file that a known service reads directly, provided you have validation and rollback. Use a management command or declarative tool when NetworkManager, DHCP, cloud-init, systemd generators, a container runtime, or configuration management owns the state; when several related changes must remain consistent; or when changes must be repeatable across a fleet. Version-controlled templates and secret-management systems are safer than ad hoc public repositories.
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Quick inspection checklist
ls -la /etc
hostnamectl status
cat /etc/hostname
cat /etc/hosts
readlink -f /etc/resolv.conf
cat /etc/resolv.conf
grep '^hosts:' /etc/nsswitch.conf
getent hosts localhost
findmnt --verify
systemctl status systemd-resolvedThe central rule is simple: /etc is the persistent, host-level configuration layer, but the active behavior depends on the program or manager that consumes or generates each path. Treat ownership, validation, and rollback as part of every configuration change.
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