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Error Messages

Real error text seen on Linux servers, with its cause and first fix. Search this page for the words of an error; the linked topic explains the mechanism and shows the captured output. Entries are added as each module is written.

Foundations

Error Cause Fix Topic
# No SMBIOS nor DMI entry point found, sorry. The VM or container exposes no SMBIOS table Use lscpu, lsmem, lspci, or the cloud metadata service System Information
ls: cannot access '/nonexistent': No such file or directory A system call returned ENOENT Check the path; strace -e trace=file shows what was tried Architecture
not a dynamic executable Static binary, script or other architecture file <path> Architecture

Shell and CLI

Error Cause Fix Topic
bash: toool: command not found (exit 127) No alias, function, builtin or PATH entry has that name Check spelling, PATH, and the owning package Command Resolution
bash: ./tool: No such file or directory The file is not in the current directory ls -l ./tool Command Resolution
bash: /home/.../app: No such file or directory for a command that exists Bash hashed the command's old location hash -r Command Resolution
cannot execute: required file not found The interpreter in the shebang, or the ELF loader, is missing; often CRLF line endings file <path> shows CRLF terminators; strip them with sed -i 's/\r$//' Command Resolution
bash: /home/laborant/tool.sh: Permission denied (exit 126) No execute bit, or a noexec mount chmod +x, or bash <file> Command Resolution
./check.sh: 3: [[: not found Bash syntax run by dash (/bin/sh on Ubuntu) #!/bin/bash Shell Basics
not a tty A command that needs a terminal runs from cron, systemd or CI Remove prompts, or ssh -t Shell Basics
No manual entry for bash Documentation excluded at install (tsflags=nodocs, minimized image) Re-enable docs, reinstall, mandb Getting Help
sudo: mytool: command not found sudo uses secure_path, which lacks the tool's directory Full path, or extend secure_path Variables and Environment
warning: setlocale: LC_ALL: cannot change locale The requested locale is not installed, often forwarded by SSH Install the locale or unset the variable Locale and Encoding
ls: cannot access 'report': No such file or directory for a file named report 2026.txt Unquoted variable split on spaces Quote: "$file" Quoting and Expansion
/etc/demo2.conf: Permission denied after sudo echo ... > The redirection runs as the calling user Pipe into sudo tee Streams and Redirection
log.txt: cannot overwrite existing file noclobber is set Append with >>, or use the forced-overwrite operator shown in the topic Streams and Redirection
$logfile: ambiguous redirect Unquoted target variable is empty or has spaces Quote and set it Streams and Redirection
Killed (exit 137) SIGKILL, often the OOM killer dmesg -T, journalctl -k Exit Codes and Chaining
mkdir: cannot create directory ...: File exists The directory exists, so the && chain stops mkdir -p Exit Codes and Chaining
[: =: unary operator expected Unquoted empty variable inside [ ] Quote it or use [[ ]] Scripting Essentials
syntax error: unexpected end of file Missing fi, done, esac or quote bash -n script.sh Scripting Essentials

Files and Filesystem

Error Cause Fix Topic
No space left on device with free space in df -h Inodes exhausted, or a different filesystem is full df -i <path>, df -h <path> Inodes and Links
Disk full, du finds much less Deleted files still open sudo lsof +L1 Inodes and Links
dpkg-query: no path found matching pattern /etc/ssh/sshd_config File created by a maintainer script Query the directory instead Filesystem Hierarchy
cannot execute binary file: Exec format error Binary for another architecture Download the build for uname -m File Types
syntax error near unexpected token when running a downloaded binary An HTML error page was saved as the binary file <path>; download with curl -f File Types
/dev/vda: Permission denied A device node was run as a command Use lsblk or fdisk -l File Types
mkdir: cannot create directory 'a/b/c': No such file or directory Parent directory missing mkdir -p File Operations
cp: -r not specified; omitting directory 'app' Source is a directory cp -r or cp -a File Operations
rmdir: failed to remove 'app': Directory not empty rmdir removes empty directories only Check contents, then rm -r File Operations
curl: (22) The requested URL returned error: 404 HTTP error with curl -f Check the URL; the script stops as intended File Operations
ln: failed to create hard link ...: Invalid cross-device link Hard links cannot span filesystems ln -s or copy Inodes and Links
ln: ...: hard link not allowed for directory Directories cannot have extra hard links Symlink or bind mount Inodes and Links
[Errno 24] Too many open files Per-process descriptor limit reached Check for leaks; raise LimitNOFILE= File Descriptors
bash: line 1: 3: Bad file descriptor Redirection to a descriptor that is not open exec 3> file first File Descriptors
find: ‘/root’: Permission denied No read access to a directory sudo, or 2>/dev/null Finding Files
find: paths must precede expression Unquoted -name pattern expanded by the shell Quote the pattern Finding Files
find: missing argument to -exec -exec not terminated End with {} \; or {} + Finding Files
tar: Refusing to read archive contents from terminal (missing -f option?) -f missing tar -xf <archive> Archiving and Compression
gzip: stdin: not in gzip format Not a gzip file, often an HTML page file <archive>; tar -xf without -z Archiving and Compression
tar: nosuch: Not found in archive Member path differs from the stored path tar -tf for the exact name Archiving and Compression
tar: Removing leading `/' from member names Informational: absolute paths stored as relative None; use -C / to avoid the message Archiving and Compression

Text Processing

Error Cause Fix Topic
sha256sum: WARNING: 1 computed checksum did NOT match File changed or download corrupt Download again; do not use the file Viewing and Comparing
gzip: plain.gz: not in gzip format zcat given data that is not gzip file <path>; use the matching tool Viewing and Comparing
tail: '/tmp/app.log' has become inaccessible The followed log was rotated away Normal with tail -F; it reopens the new file Viewing and Comparing
grep: /usr/bin/ls: binary file matches Input contains NUL bytes grep -a, or strings first grep and Regex
grep: Invalid regular expression Unbalanced bracket or brace Escape it or use grep -F grep and Regex
sed: -e expression #1, char 5: unterminated `s' command Missing final delimiter or an unescaped / Close the command or change the delimiter sed
sed: no input files sed -i used in a pipeline Pass a file name, or drop -i sed
unescaped newline inside substitute pattern (macOS) BSD sed -i took the script as the backup suffix sed -i '' ... or GNU gsed sed
awk: cmd. line:1: ... unexpected newline or end of string Unclosed brace or quote in the program Fix the braces; keep the program in single quotes awk
awk: fatal: cannot open file `nosuch.log' for reading Wrong input path Check the path awk
sort: access.log:14: disorder: Input not sorted in the current locale Sort with the same LC_ALL before comm or join Cut, Sort, Uniq and Tr
bash: bc: command not found bc not installed Install bc, or use awk Cut, Sort, Uniq and Tr
rm: missing operand from an xargs pipeline Empty input still ran the command xargs -r xargs and tee
xargs: sh: exited with status 255; aborting A command exited 255 Return 1 for ordinary failures xargs and tee
xargs: argument line too long One input item exceeds the command-line limit Pass the data through stdin or a file xargs and tee
jq: parse error: Unfinished JSON term at EOF Input is not valid JSON Check the raw input; curl -f JSON and YAML on the CLI
jq: error (at ...): Cannot iterate over null (null) .[] on a missing key Check the path; .key[]? JSON and YAML on the CLI
Error: bad file '-': yaml: line 2: did not find expected key Inconsistent YAML indentation Align keys; yamllint JSON and YAML on the CLI

Users and Access

Error Cause Fix Topic
useradd: user 'amor' already exists The name is taken getent passwd amor Users
userdel: user amor is currently used by process 1244 The user still has running processes Stop them (pkill -u amor), then delete Users
userdel: /home/amor not owned by amor, not removing Home directory belongs to another UID Check ownership; remove it manually if intended Users
groupdel: cannot remove the primary group of user 'amor' The group is a user's primary group usermod -g first Groups
groupadd: group 'amor' already exists Name taken, often by a user private group getent group amor Groups
su: User account has expired Account expiry date passed chage -E -1 <user> Passwords and Aging
You are required to change your password immediately (administrator enforced). chage -d 0 forced a change Set a new password at login Passwords and Aging
passwd: unrecognized option '--stdin' --stdin exists only in the RHEL build of passwd chpasswd Passwords and Aging
amor is not in the sudoers file. No sudoers rule matches the user Add to wheel or sudo, or a drop-in rule Sudo and Su
sudo: a password is required The command line does not match a NOPASSWD rule exactly Match arguments exactly, or use sudo -l Sudo and Su
/etc/sudoers.d/deploy:2:48: syntax error Invalid sudoers syntax visudo -cf <file> before installing Sudo and Su

Permissions

Error Cause Fix Topic
chown: changing ownership of 'report.txt': Operation not permitted Only root changes owners sudo chown Basic Permissions
ls: cannot access 'd-rw/f': Permission denied with -????????? Directory has r without x Add x on the directory Basic Permissions
./run.sh: Permission denied (exit 126) No execute bit, or noexec mount chmod +x; findmnt -no OPTIONS -T <file> Basic Permissions
umask: 999: octal number out of range Non-octal digits Use 0 to 7 or symbolic form umask
rm: cannot remove '...': Operation not permitted in a shared directory Sticky bit; the caller owns neither file nor directory The owner removes it Special Permissions
rm: cannot remove 'resolv.conf': Operation not permitted as root Immutable or append-only attribute lsattr, then chattr -i File Attributes
chattr: Operation not permitted while setting flags Only root may set i and a sudo chattr File Attributes
lsattr: Operation not supported While reading flags Filesystem without attribute support Use ext4, XFS or btrfs File Attributes
setfacl: secret.conf: Operation not permitted Only the owner or root changes an ACL sudo setfacl ACL
setfacl: Option -m: Invalid argument near character 3 Unknown user or group, or malformed entry getent passwd <user> ACL

Package Management

Error Cause Fix Topic
Error: Unable to find a match: nosuchpackage Wrong name or disabled repository dnf search, dnf provides, dnf repolist --all rpm and dnf
Error: Failed to download metadata for repo 'broken' One repository unreachable; dnf stops Fix or disable it; skip_if_unavailable=True rpm and dnf
missing ... (Permission denied) from rpm -V Verification run without root sudo rpm -V rpm and dnf
Package hello-notes-1.0-1.el10.noarch.rpm is not signed / GPG check FAILED Unsigned package with gpgcheck=1 Sign it; do not disable checks Packaging Concepts
E: Unable to locate package nosuchpackage Stale package lists or wrong name sudo apt update; apt-cache search dpkg and apt
E: Could not get lock /var/lib/dpkg/lock-frontend Another package process runs Wait; DPkg::Lock::Timeout in scripts dpkg and apt
E: dpkg was interrupted, you must manually run 'sudo dpkg --configure -a' A previous install stopped halfway sudo dpkg --configure -a, sudo apt -f install dpkg and apt
NO_PUBKEY 7EA0A9C3F273FCD8 Keyring for a source missing or wrong Download the key into /etc/apt/keyrings/ Repositories
W: Failed to fetch ... Could not resolve APT repository unreachable; APT continues Fix DNS or remove the source Repositories
error: No remote refs found for ‘flathub’ Flatpak remote in the other scope Add the remote to the scope in use Flatpak and Snap
error while loading shared libraries: libgreet.so.1 Library missing or not in the search path Install it or ldconfig its directory Shared Libraries
version `GLIBC_2.42' not found Binary built for a newer glibc Build on the oldest target or link statically Shared Libraries
cannot execute: required file not found for an ELF file The ELF interpreter (for example musl's loader) is missing readelf -l; build for glibc Binary Won't Execute
error: externally-managed-environment Ubuntu 24.04 blocks system pip install Use a venv or pipx Other Install Methods

Processes

Error Cause Fix Topic
bash: fork: retry: Resource temporarily unavailable User or system process limit reached (ulimit -u, TasksMax=, pid_max) Find the owner of the tasks; fix the leak, then the limit Process Lifecycle
ls: cannot read symbolic link '/proc/2102/cwd': Permission denied The process belongs to another user sudo, or read status instead Process Fundamentals
error: list of process IDs must follow -p Empty command substitution after ps -p Check the pgrep match first Viewing Processes
kill: (1) - Operation not permitted Target belongs to another user (EPERM) sudo, or signal as the owner Signals
kill: (2251) - No such process PID gone (ESRCH); stale PID file pgrep -a, systemctl show -p MainPID Signals
sleep: no process found killall matched nothing; exits 1 Check the name with pgrep -a Signals
Killed reported only after a hung write returns SIGKILL pending on a D-state process Fix the resource in wchan Process States
bash: fg: current: no such job Empty job table in this shell pgrep -a; the job belongs to another shell Job Control
nice: cannot set niceness: Permission denied Negative nice without privilege sudo, limits.d, or Nice= in a unit Priority and Nice
renice: failed to set priority for 2815 (process ID): Permission denied User tried to lower a nice value sudo renice Priority and Nice
chrt: failed to set pid 0's policy: Operation not permitted Real-time policy needs CAP_SYS_NICE sudo chrt, or CPUSchedulingPolicy= Priority and Nice
sudo: effective uid is not 0, is /usr/bin/sudo on a file system with the 'nosuid' option set ... strace sudo drops the setuid bit sudo strace ... System Calls and Tracing

Systemd and Services

Error Cause Fix Topic
Failed to start nginx.service: Interactive authentication required. State change without root sudo systemctl ... systemctl
Failed to start nginx.service: Unit nginx.service is masked. Unit linked to /dev/null sudo systemctl unmask systemctl
Unit nosuch.service could not be found. Wrong name, missing package or no daemon-reload systemctl list-unit-files, daemon-reload systemctl
Warning: The unit file, source configuration file or drop-ins of nginx.service changed on disk. Edited without reloading sudo systemctl daemon-reload Unit Files
Unit nginx.service has a bad unit file setting. Parse error, often a second ExecStart= Empty ExecStart= first; systemd-analyze verify Unit Files
Service has more than one ExecStart= setting, which is only allowed for Type=oneshot services. Refusing. Drop-in added an ExecStart= without resetting Add ExecStart= with no value before it Unit Files
flaky.service: Start request repeated too quickly. Start limit reached Fix the cause, reset-failed Unit Files
A dependency job for app-web.service failed. A Requires= unit failed systemctl list-dependencies, --failed Unit Files
status=203/EXEC / Failed at step EXEC spawning ExecStart= not executable or missing Check the path, mode, shebang, noexec Writing a Service
Failed to spawn 'start' task: No such file or directory EnvironmentFile= missing Create it, or prefix the path with - Writing a Service
Read-only file system inside a service ProtectSystem=strict ReadWritePaths= or StateDirectory= Writing a Service
sudo: The "no new privileges" flag is set NoNewPrivileges=yes in the unit Remove sudo from the service's code path Writing a Service
State 'stop-sigterm' timed out. Killing. Process ignores SIGTERM longer than TimeoutStopSec= Handle SIGTERM; exec in wrappers Signals
Failed to load environment files: No such file or directory EnvironmentFile= missing; Result: resources Create it, or prefix with - Service Won't Start
Failed to determine user credentials: No such process / status=217/USER User= names no existing user Create the user or fix the name Service Won't Start
OSError: [Errno 98] Address already in use Another process owns the port ss -tlnp "sport = :<port>" Service Won't Start

Logging

Error Cause Fix Topic
tail: cannot open '/var/log/secure' for reading: Permission denied RHEL log files are root only; Ubuntu's need group adm sudo, or journalctl as wheel or adm Log Locations
Specifying boot ID or boot offset has no effect, no persistent journal was found. Volatile journal Create /var/log/journal, journalctl --flush journalctl
No journal files were opened due to insufficient permissions. User not in adm, systemd-journal or wheel Add the group, log in again journalctl
Suppressed 4725 messages from flood.service journald rate limit reached Lower the log volume or raise LogRateLimitBurst= journalctl
rsyslogd: error during parsing file /etc/rsyslog.d/30-payments.conf, on or before line 2 rsyslog syntax error Fix the line, rsyslogd -N1 rsyslog
error: skipping "/var/log/app/app.log" because parent directory has insecure permissions Log directory writable by a non-root group or everyone chmod 755 the directory, or su user group in the rule logrotate
warning: /tmp/bad.conf:4 unknown option 'copytruncte' -- ignoring line Misspelled logrotate directive Correct it; logrotate -d logrotate
error: /tmp/dup.conf:1 duplicate log entry for /var/log/app/app.log Two rules match one file Keep one rule logrotate

Scheduling

Error Cause Fix Topic
"/tmp/bad.cron":1: bad minute / Invalid crontab file, can't install. Field out of range Correct the schedule cron and at
/bin/sh: -c: line 2: unexpected EOF while looking for matching `)' Unescaped % in a crontab command \% cron and at
db-backup: command not found (in CMDOUT) Command outside cron's PATH Full path or PATH= in the crontab cron and at
/bin/sh: line 1: /opt/jobs/crlf.sh: cannot execute: required file not found CRLF line endings in the script sed -i 's/\r$//' cron and at
/bin/sh: line 1: /opt/reports/run.sh: Permission denied Script not executable chmod 755 Cron Job Not Running
(CRON) bad command (/etc/cron.d/report) cron.d line without a user field (RHEL) Add the user cron and at
Error: bad username; while reading /etc/cron.d/nouser cron.d line without a user field (Ubuntu) Add the user Cron Job Not Running
You (bob) are not allowed to use this program (crontab) cron.deny or cron.allow Update the access files cron and at
(CRON) info (No MTA installed, discarding output) Job output with no mail server (Ubuntu) Redirect output to a file Cron Job Not Running
You do not have permission to use at. at.deny or at.allow Update the access files cron and at
Exec failed for mail command: No such file or directory atd could not mail job output Redirect output inside the job cron and at
Timer unit lacks value setting. Refusing. Timer without OnCalendar= or On*Sec= Add a trigger Systemd Timers
orphan.timer: Refusing to start, unit orphan.service to trigger not loaded. No matching service Create it or set Unit= Systemd Timers
Failed to parse calendar specification 'Mon..Fri 25:00': Invalid argument Invalid calendar expression systemd-analyze calendar Systemd Timers

Kernel and Hardware

Error Cause Fix Topic
-bash: line 1: /proc/sys/vm/swappiness: Permission denied sudo echo ... > redirects as the user sudo tee or sysctl -w proc and sys
echo: write error: Input/output error Read-only proc file Use the matching tunable proc and sys
sysctl: permission denied on key "vm.swappiness" Not root sudo sysctl -w sysctl
sysctl: setting key "vm.swappiness": Invalid argument Non-numeric or out-of-range value Check the valid range sysctl
sysctl: cannot stat /proc/sys/net/sctp/rto_min: No such file or directory Typo, or the module is not loaded sysctl -a --pattern; load the module sysctl
modprobe: FATAL: Module sctpp not found in directory /lib/modules/6.1.167 Wrong name or missing modules for the kernel find /lib/modules/$(uname -r); install extra modules Kernel Modules
rmmod: ERROR: Module sctp is in use Use count above zero Stop the users, or reboot Kernel Modules
insmod: ERROR: could not insert module ...: File exists Already loaded lsmod Kernel Modules
modprobe: FATAL: Module libcrc32c is builtin. Built into the kernel Parameters go on the kernel command line Kernel Modules
Error running install command '/bin/false' for module nbd: retcode 1 Module blocked with install ... /bin/false Remove the rule if the module is needed Kernel Modules
/tmp/99-bad.rules:1 Invalid operator for KERNEL. = used on a udev match key == Devices and udev
parse-config[4369]: segfault at 0 ip ... error 6 in parse-config[401000+1000] NULL pointer write coredumpctl debug dmesg and Kernel Messages
Memory cgroup out of memory: Killed process 4383 (python3) cgroup memory limit reached Raise MemoryMax= or fix the leak dmesg and Kernel Messages
Buffer I/O error on dev dm-0, logical block 0, async page read Read error from the block device Check hardware, restore from backup dmesg and Kernel Messages
INFO: task sh:4577 blocked for more than 10 seconds. Task stuck in D state Fix the device or filesystem it waits on dmesg and Kernel Messages

Storage

Error Cause Fix Topic
/dev/vda: Unable to detect device type Virtual or hidden disk behind a controller smartctl -d <type>, or check health on the host Disks and Devices
losetup: cannot find an unused loop device All loop devices in use, or no /dev/loop-control losetup -l; detach stale ones Disks and Devices
Re-reading the partition table failed.: Invalid argument Kernel refused the whole-table re-read partprobe or partx -a Partitioning
Error: /dev/loop2: unrecognised disk label No partition table on the disk parted -s <disk> mklabel gpt Partitioning
mkfs.xfs: /dev/loop0p1 appears to contain an existing filesystem (ext4). Existing signature Confirm the device; wipefs -a or -f Filesystems
Filesystem must be larger than 300MB. xfsprogs 6.16 minimum XFS size Use a larger device or ext4 RAID and Encryption
e2fsck: need terminal for interactive repairs e2fsck -f without a terminal -p, -y or -n Filesystems
dumpe2fs: Bad magic number in super-block while trying to open /dev/loop0p3 Damaged primary superblock or another filesystem type blkid; e2fsck -b <backup> Filesystems
xfs_repair: /dev/loop1p2 contains a mounted and writable filesystem Repair attempted on a mounted XFS Unmount first Filesystems
xfs_growfs: XFS_IOC_FSGROWFSDATA xfsctl failed: Invalid argument Shrink below the last allocation group Back up, recreate, restore Filesystems
mount: /srv/app: wrong fs type, bad option, bad superblock on /dev/loop0p3, missing codepage or helper program, or other error. Wrong type, bad option, missing helper or damage dmesg, blkid Filesystems
touch: cannot touch '/srv/app/test': Read-only file system Mounted or remounted ro findmnt -no OPTIONS; check the kernel log Mounting and fstab
umount: /srv/app: target is busy. Open files, working directories or nested mounts fuser -vm, lsof +f -- Mounting and fstab
mount: /srv/data: can't find UUID=683f9429-0000-4267-a91e-79c8e5787e21. No device with that UUID blkid; fix the line or add nofail Mounting and fstab
[E] unreachable on boot required source: UUID=... findmnt --verify found a missing device Fix fstab before rebooting Mounting and fstab
A dependency job for srv-data.mount failed. See 'journalctl -xe' for details. Device did not appear before the timeout journalctl -b; fix fstab Mounting and fstab
cache.mount: Where= setting doesn't match unit name. Refusing. Mount unit named differently from its path systemd-escape -p --suffix=mount <path> Mounting and fstab
mount: /srv/secure: unknown filesystem type 'crypto_LUKS'. LUKS device mounted directly Open it and mount /dev/mapper/<name> RAID and Encryption
swapon: /swapfile: skipping - it appears to have holes. Sparse swap file Recreate with dd or fallocate Swap
swapon: /swapfile: insecure permissions 0644, 0600 suggested. World-readable swap file chmod 600 before mkswap Swap
swapon: /dev/loop0p1: read swap header failed No swap signature on the device blkid; mkswap Swap
Cannot use /dev/loop0: device is partitioned pvcreate on a partitioned disk Use a partition or wipefs -a LVM
Can't open /dev/loop0p3 exclusively. Mounted filesystem? Device is mounted or in use Pick an unused device LVM
Insufficient free space: 512 extents needed, but only 258 available Too few free extents in the VG vgextend, or -l +100%FREE LVM
File system reduce is required and not supported (xfs). lvreduce -r on XFS Back up, recreate, restore LVM
fsadm: Xfs filesystem shrinking is unsupported. Same, on LVM 2.03.16 (Ubuntu 24.04) Back up, recreate, restore LVM
device-mapper: snapshots: Invalidating snapshot: Unable to allocate exception. Snapshot ran out of space Size snapshots for the change rate LVM
Aborting. Manual intervention required. lvcreate -s on an already frozen filesystem fsfreeze -u, lvremove, dmsetup remove Backup and Restore
NOCHANGE: partition 1 is size 6289375. it cannot be grown No free space after the partition Check lsblk; rescan the disk Resizing and Cloud Disks
must supply partition-number growpart given the partition as one argument growpart <disk> <number> Resizing and Cloud Disks
No space left on device Blocks (including the reserve) or inodes exhausted df -h, df -i, lsof -a +L1 Disk Usage
bash: line 1: /usr/bin/rm: Argument list too long Glob expanded beyond the argument limit find ... -delete Disk Usage
dd: error writing '/srv/web/uploads/a.bin': Disk quota exceeded User reached the hard quota quota -s <user> Quotas
rsync: [Receiver] mkdir "/backup/daily/2026-09-16" failed: No such file or directory (2) Destination parent missing mkdir -p or --mkpath Backup and Restore

Networking

Error Cause Fix Topic
RTNETLINK answers: File exists Address or route already present ip -br addr; ip addr replace Interfaces and Addresses
Cannot find device "eth9" Wrong interface name ip -br link (includes altname) Interfaces and Addresses
RTNETLINK answers: Operation not permitted Changing links or routes without CAP_NET_ADMIN Run with sudo Interfaces and Addresses
From 172.16.0.2 icmp_seq=1 Destination Host Unreachable No ARP answer for the target or gateway on the local subnet ip neigh; check VLAN and prefix Interfaces and Addresses
Error: Failed to modify connection 'dmz': Error checking authorization nmcli without sudo and without polkit Run with sudo Network Configuration
Error in network definition: expected sequence netplan list written as a single value addresses: [172.16.0.2/24] Network Configuration
Invalid YAML: inconsistent indentation netplan key indented differently from its siblings Align it; sudo netplan generate Network Configuration
Permissions for /etc/netplan/50-bad.yaml are too open. netplan file readable by other users sudo chmod 600 /etc/netplan/*.yaml Network Configuration
ping: connect: Network is unreachable No route matches the destination ip route; add the default or static route Routing
Error: Nexthop has invalid gateway. via address not on a connected subnet Use a local gateway, or onlink Routing
ping: connect: Invalid argument A blackhole route matches ip route; delete the route Routing
curl: (6) Could not resolve host: api.shop.internal No nsswitch source knows the name getent hosts, resolvectl status, dig @server DNS Resolution
ping: api.shop.internal: Name or service not known Name lookup failed in getaddrinfo() As for Could not resolve host DNS Resolution
;; communications error to 172.16.1.99#53: timed out No DNS server there, or port 53 filtered nc -vzu <server> 53; firewalls DNS Resolution
;; WARNING: recursion requested but not available Authoritative-only server asked for another zone Use a recursive resolver or a routing domain DNS Resolution
zone shop.internal/IN: not loaded due to errors. Syntax error in the zone file; old data stays served named-checkzone, fix, reload DNS Not Resolving
nc: connect to 172.16.1.3 port 9000 (tcp) failed: Connection refused Nothing listens on that address, or bound to loopback sudo ss -tlpn 'sport = :9000' on the server Ports and Sockets
nc: Address already in use Port already has a listener sudo ss -tlpn 'sport = :<port>' Ports and Sockets
nc: Permission denied Unprivileged bind below port 1024 sudo, CAP_NET_BIND_SERVICE, or a high port Ports and Sockets
nf_conntrack: table full, dropping packet More tracked flows than nf_conntrack_max Raise the limit, shorten timeouts, notrack Sockets and TCP States
Cannot assign requested address Client out of ephemeral ports, often many TIME-WAIT Reuse connections; widen ip_local_port_range Sockets and TCP States
ping: local error: message too long, mtu=1400 Packet with DF set exceeds the known path MTU Smaller packets; tracepath Connectivity Testing
From 172.16.0.3 icmp_seq=1 Frag needed and DF set (mtu = 1400) A router on the path has a smaller MTU Align MTUs; let ICMP through Connectivity Testing
nc: connect to 172.16.1.3 port 9200 (tcp) timed out: Operation now in progress SYN or reply dropped on the way Firewalls, security groups, routing Connectivity Testing
curl: (28) Connection timed out after 3002 milliseconds Connection attempt dropped Capture on the server; walk the ladder Troubleshooting Ladder
tcpdump: eth0: You don't have permission to perform this capture on that device Capture without root sudo tcpdump Packet Capture
tcpdump: eth9: No such device exists Wrong interface name tcpdump -D; -i any Packet Capture
tcpdump: truncated dump file; tried to read 4 file header bytes, only got 0 Empty pcap file Write with -U; stop with -c or Ctrl-C Packet Capture
hwclock: Cannot access the Hardware Clock via any known method. VM without an RTC Nothing; time comes from the hypervisor and NTP Time and Timezones
506 Cannot talk to daemon chronyd not running sudo systemctl enable --now chronyd Time and Timezones
error 9 at 0 depth lookup: certificate is not yet valid Local clock earlier than notBefore Fix time sync; chronyc tracking Time and Timezones
connect() failed (111: Connection refused) while connecting to upstream Backend not listening Check the backend service and bind address Reverse Proxy and Load Balancing
no live upstreams while connecting to upstream Every upstream server marked failed Fix backends; retried after fail_timeout Reverse Proxy and Load Balancing
Server shop_api/client is DOWN, reason: Layer4 connection problem HAProxy health check cannot connect Check the backend; show stat Reverse Proxy and Load Balancing
ping: sendmsg: Required key not available No WireGuard peer's AllowedIPs covers the destination Add the range to the right peer VPN (WireGuard)

SSH and Remote Access

Error Cause Fix Topic
Permission denied (publickey). No offered key is accepted by the server Check ssh -v and the server log; install the key or fix modes SSH Troubleshooting
ssh: connect to host X port 22: Connection refused Nothing listens on the port Start sshd; check the port with ss -tlnp SSH Troubleshooting
ssh: connect to host X port 22: Connection timed out A firewall or the route drops the packets Open the port; check routes and security groups SSH Troubleshooting
WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! The stored host key differs Verify the new fingerprint, then ssh-keygen -R host SSH Troubleshooting
Authentication refused: bad ownership or modes for directory Home or .ssh writable by group or others chmod 755 ~, 700 ~/.ssh, 600 authorized_keys SSH Troubleshooting
Could not open user 'X' authorized keys ...: Permission denied Wrong SELinux label on authorized_keys restorecon -Rv ~/.ssh SSH Troubleshooting
Received disconnect ... 2: Too many authentication failures The agent offered more keys than MaxAuthTries IdentitiesOnly yes with one IdentityFile SSH Troubleshooting
kex_exchange_identification: read: Connection reset by peer Server closed before key exchange (penalty, MaxStartups) Read the server log for penalty; wait or fix the client SSH Troubleshooting
Your account has expired; please contact your system administrator. Account expiry date passed chage -E -1 user SSH Troubleshooting
Bad owner or permissions on /home/X/.ssh/config Client config writable by others chmod 600 ~/.ssh/config SSH Client
Bad configuration option: PasswordAuthentcation Misspelled or unsupported sshd option Fix the line, then sshd -t sshd Server
bad ownership or modes for chroot directory "..." SFTP chroot path not root-owned or group-writable chown root:root and chmod 755 the chroot sshd Server
channel N: open failed: administratively prohibited: open failed AllowTcpForwarding no on the server Allow forwarding, or check PermitOpen SSH Tunnels
bind [127.0.0.1]:PORT: Address already in use The local forward port is taken Free the port; use -o ExitOnForwardFailure=yes SSH Tunnels
scp: dest open "...": Permission denied The remote user cannot write the destination Copy to /tmp, then sudo install File Transfer
bash: line 1: rsync: command not found rsync missing on the remote host Install rsync, or use tar over ssh File Transfer

Security

Error Cause Fix Topic
curl: (7) Failed to connect ... Couldn't connect to server Nothing listens, or firewalld rejected the packet Check ss -tlnp and firewall-cmd --list-all firewalld and ufw
nc: connect to X port P (tcp) failed: Connection timed out A drop rule (ufw, security group, nft drop) Check ufw status and nft list ruleset firewalld and ufw
nc: connect to X port P (tcp) failed: No route to host firewalld rejected with ICMP host-prohibited Add the port or service to the zone firewalld and ufw
avc: denied { read } for ... comm="nginx" ... tcontext=...:default_t SELinux: wrong file label semanage fcontext -a then restorecon SELinux
nginx: [emerg] bind() to 0.0.0.0:PORT failed (13: Permission denied) SELinux: port has no matching label semanage port -a -t http_port_t -p tcp PORT SELinux
avc: denied { name_connect } ... comm="nginx" SELinux boolean off (proxy to a backend) setsebool -P httpd_can_network_connect on SELinux
ping: sendmsg: Operation not permitted (setcap gone) File capability lost on copy Reapply setcap, or grant it in the unit Capabilities
curl: (60) SSL certificate problem: unable to get local issuer certificate Issuing CA not trusted, or missing intermediate Add the CA to the trust store; serve the full chain OpenSSL and Trust Store
curl: (60) SSL: no alternative certificate subject name matches target host name Requested name not in the certificate SAN Use a listed name, or reissue with the name OpenSSL and Trust Store
curl: (60) SSL certificate problem: certificate has expired Certificate expired, or the clock is wrong Renew the certificate; fix time sync OpenSSL and Trust Store
Failed to restart auditd.service: Operation refused ... auditd refuses manual restart service auditd restart, or augenrules --load for rules auditd
gpg: BAD signature from ... The signed content changed, or the wrong key Re-download; verify the signer's key by fingerprint GPG

Boot and Recovery

Error Cause Fix Topic
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0) Wrong root= or an initramfs missing the root driver Correct root=, rebuild the initramfs, or boot an older kernel Kernel Panic
You are in emergency mode A filesystem in /etc/fstab failed to mount Remount root rw, fix the entry, mount -a, reboot Recovery
mount: /data: can't find UUID=... The device or UUID in /etc/fstab is wrong or absent Correct the UUID, or add nofail for non-critical mounts Recovery
error: file '/vmlinuz-...' not found (GRUB) The kernel a menu entry references was removed Boot an older entry, reinstall the kernel, regenerate grub.cfg GRUB2
edits to /etc/default/grub have no effect grub.cfg was not regenerated after the change Run grub2-mkconfig -o or update-grub, then reboot GRUB2

Performance and Troubleshooting

Error Cause Fix Topic
[Errno 24] Too many open files (EMFILE) The process hit its per-process descriptor limit Raise LimitNOFILE (service) or limits.conf (login), or fix a leak Limits and File Descriptors
bash: fork: retry: Resource temporarily unavailable (EAGAIN) RLIMIT_NPROC or pid_max exhausted Kill the runaway with builtins; raise -u or pid_max Cannot Fork
Out of memory: Killed process N (name) Memory and swap exhausted; OOM killer ran Fix the leak or add RAM; protect a process with oom_score_adj Memory
container exits with code 137 and no app error Cgroup memory limit hit; OOM-killed (128 + 9) Raise the memory limit or reduce footprint; check dmesg Memory
CONFIG_TASK_DELAY_ACCT not enabled (iotop) Per-process I/O delay accounting is off echo 1 > /proc/sys/kernel/task_delayacct, or use pidstat -d Disk I/O
Command 'iostat' not found The sysstat package is not installed Install sysstat; vmstat/top/free work without it Methodology

Network Storage

Error Cause Fix Topic
clnt_create: RPC: Program not registered The NFS server is not running, or a firewall blocks it Start nfs-server; allow the nfs service (port 2049) NFS
mount.nfs: access denied by server while mounting The client is not in /etc/exports, or exports not re-applied Fix the client range; sudo exportfs -rav NFS
a process is stuck in D on an NFS mount, ignoring kill -9 Hard mount with the server unreachable; uninterruptible sleep Restore the server or network; lazy or force unmount NFS
mount error(13): Permission denied (CIFS) Wrong credentials or unsupported SMB version Check the credentials file; add vers=3.0 Samba and CIFS
mount error(112): Host is down (CIFS) The server refused the requested SMB version Specify a supported version such as vers=3.0 Samba and CIFS
iscsiadm: no records found The initiator IQN is not in the target ACL, or wrong portal Add the IQN to the ACL; rediscover the correct portal iSCSI and NBD

Containers

Error Cause Fix Topic
unshare: unshare failed: Operation not permitted Creating the namespace needs CAP_SYS_ADMIN Use sudo, or first unshare --user --map-root-user Namespaces
echo: write error: No such file or directory (cgroup file) The controller is not enabled in the parent cgroup.subtree_control echo +memory +cpu > <parent>/cgroup.subtree_control first Cgroups
container exits 137 with no application error Cgroup OOM killer hit memory.max (128 + SIGKILL) Raise the limit if legitimate, or fix the leak; check dmesg Cgroups
docker stop waits then kills the container PID 1 does not forward SIGTERM exec the app as PID 1, or run with --init Containers vs VMs
bind-mounted files are Permission denied in a container on RHEL SELinux blocks a host mount with no matching label Add :Z to the volume flag Podman and Quadlet

Virtualization and Provisioning

Error Cause Fix Topic
virt-host-validate: hardware virtualization FAIL No vmx/svm flag, or nested virt off; no /dev/kvm Enable virtualization in firmware or nested virt on the host KVM and libvirt
error: failed to connect to the hypervisor libvirtd not running, or user not in libvirt group Start libvirtd; add the user to libvirt; reconnect KVM and libvirt
WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED on a new clone The clone reused the template's SSH host keys Regenerate host keys (ssh-keygen -A); virt-sysprep does this VM Images and Cloning
a cloned VM ignored its cloud-config The instance-id was unchanged, so first-boot modules skipped cloud-init clean on the template before cloning Cloud-init and Kickstart