Skip to main content

Networking

Comprehensive reference guide for Linux network configuration, troubleshooting, and management.

Network Configuration and Interfaces​

IP Command (Modern)​

# Interface management
ip link show                    # List all network interfaces
ip link show eth0               # Show specific interface
ip link set eth0 up             # Bring interface up
ip link set eth0 down           # Bring interface down
ip link set eth0 mtu 1500       # Set MTU size

# IP address management
ip addr show                    # Show all IP addresses
ip addr show eth0               # Show IP for specific interface
ip addr add 192.168.1.100/24 dev eth0    # Add IP address
ip addr del 192.168.1.100/24 dev eth0    # Remove IP address
ip addr flush dev eth0          # Remove all IPs from interface

# IPv6 addresses
ip -6 addr show                 # Show IPv6 addresses
ip -6 addr add 2001:db8::1/64 dev eth0   # Add IPv6 address

Legacy ifconfig Command​

# Interface information
ifconfig                        # Show all interfaces
ifconfig eth0                   # Show specific interface
ifconfig -a                     # Show all interfaces (including down)

# Interface configuration
ifconfig eth0 192.168.1.100 netmask 255.255.255.0    # Set IP and netmask
ifconfig eth0 up                # Bring interface up
ifconfig eth0 down              # Bring interface down
ifconfig eth0 mtu 1500          # Set MTU size

# Interface statistics
ifconfig eth0 | grep "RX packets"    # Show RX statistics
ifconfig eth0 | grep "TX packets"    # Show TX statistics

NetworkManager (nmcli)​

# Connection management
nmcli con show                  # List all connections
nmcli con show --active         # Show active connections
nmcli con up connection-name    # Activate connection
nmcli con down connection-name  # Deactivate connection

# Device management
nmcli dev show                  # Show all devices
nmcli dev show eth0             # Show specific device
nmcli dev status                # Show device status
nmcli dev wifi list             # List WiFi networks

# WiFi management
nmcli dev wifi connect SSID password PASSWORD    # Connect to WiFi
nmcli dev wifi disconnect       # Disconnect from WiFi
nmcli dev wifi rescan           # Rescan for networks

# Create connections
nmcli con add type ethernet con-name eth0-static ifname eth0 \
    ip4 192.168.1.100/24 gw4 192.168.1.1      # Static IP connection
nmcli con add type wifi con-name wifi-home ifname wlan0 \
    ssid "Home-WiFi" wifi-sec.key-mgmt wpa-psk wifi-sec.psk "password"

Network Configuration Files​

# Debian/Ubuntu - /etc/network/interfaces
auto eth0
iface eth0 inet static
    address 192.168.1.100
    netmask 255.255.255.0
    gateway 192.168.1.1
    dns-nameservers 8.8.8.8 8.8.4.4

# Red Hat/CentOS - /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=static
IPADDR=192.168.1.100
NETMASK=255.255.255.0
GATEWAY=192.168.1.1
DNS1=8.8.8.8
DNS2=8.8.4.4
ONBOOT=yes

# Apply configuration changes
sudo systemctl restart networking     # Debian/Ubuntu
sudo systemctl restart network        # Red Hat/CentOS

DNS Configuration and Troubleshooting​

DNS Configuration Files​

# /etc/resolv.conf - DNS resolver configuration
nameserver 8.8.8.8
nameserver 8.8.4.4
search example.com
options timeout:2

# /etc/hosts - Static hostname resolution
127.0.0.1       localhost
192.168.1.100   server.example.com server
::1             localhost ip6-localhost ip6-loopback

# /etc/nsswitch.conf - Name service switch configuration
hosts: files dns
networks: files dns

DNS Troubleshooting Tools​

# Basic DNS lookup
nslookup google.com             # Basic DNS lookup
nslookup google.com 8.8.8.8     # Query specific DNS server

# Advanced DNS queries with dig
dig google.com                  # Basic query
dig google.com A                # Query A record
dig google.com MX               # Query MX record
dig google.com NS               # Query nameserver records
dig google.com TXT              # Query TXT records
dig @8.8.8.8 google.com         # Query specific DNS server
dig +trace google.com           # Trace DNS resolution path
dig -x 8.8.8.8                  # Reverse DNS lookup

# Host command
host google.com                 # Simple DNS lookup
host -t MX google.com           # Query specific record type
host -a google.com              # Query all records

# DNS cache management
sudo systemctl flush-dns        # Ubuntu with systemd-resolved
sudo dscacheutil -flushcache    # macOS

DNS Testing and Validation​

# Test DNS resolution speed
dig google.com | grep "Query time"

# Test multiple DNS servers
for server in 8.8.8.8 1.1.1.1 208.67.222.222; do
    echo "Testing $server:"
    dig @$server google.com | grep "Query time"
done

# Check DNS propagation
dig google.com @8.8.8.8         # Google DNS
dig google.com @1.1.1.1         # Cloudflare DNS
dig google.com @208.67.222.222  # OpenDNS

Routing and IP Tables​

Routing Table Management​

# View routing table
ip route show                   # Show all routes
ip route show table main        # Show main routing table
route -n                        # Show routes (numeric)
netstat -rn                     # Show routing table

# Add/remove routes
ip route add 192.168.2.0/24 via 192.168.1.1 dev eth0    # Add route
ip route del 192.168.2.0/24                             # Delete route
ip route add default via 192.168.1.1 dev eth0           # Add default route
ip route del default                                     # Delete default route

# Policy routing
ip rule list                    # List routing rules
ip rule add from 192.168.1.0/24 table 100              # Add rule
ip rule del from 192.168.1.0/24 table 100              # Delete rule

IPTables Firewall​

# Basic iptables operations
iptables -L                     # List all rules
iptables -L -n                  # List rules (numeric)
iptables -L INPUT               # List INPUT chain rules
iptables -F                     # Flush all rules
iptables -F INPUT               # Flush INPUT chain

# Basic rules
iptables -A INPUT -p tcp --dport 22 -j ACCEPT          # Allow SSH
iptables -A INPUT -p tcp --dport 80 -j ACCEPT          # Allow HTTP
iptables -A INPUT -p tcp --dport 443 -j ACCEPT         # Allow HTTPS
iptables -A INPUT -i lo -j ACCEPT                       # Allow loopback
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Block specific IP
iptables -A INPUT -s 192.168.1.100 -j DROP            # Block IP
iptables -A INPUT -s 192.168.1.0/24 -j DROP           # Block subnet

# Port forwarding
iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 8080
iptables -t nat -A PREROUTING -p tcp --dport 443 -j REDIRECT --to-port 8443

# Save and restore rules
iptables-save > /etc/iptables/rules.v4                 # Save rules
iptables-restore < /etc/iptables/rules.v4              # Restore rules

Advanced Routing​

# Multiple routing tables
echo "100 custom" >> /etc/iproute2/rt_tables           # Add custom table
ip route add default via 192.168.2.1 table custom     # Add route to custom table
ip rule add from 192.168.2.0/24 table custom          # Use custom table for subnet

# Load balancing
ip route add default scope global \
    nexthop via 192.168.1.1 dev eth0 weight 1 \
    nexthop via 192.168.2.1 dev eth1 weight 1

# Source-based routing
ip rule add from 192.168.1.100 table 100
ip route add default via 192.168.1.1 table 100

Network Tools​

Connection Monitoring (netstat/ss)​

# netstat (legacy)
netstat -tuln                   # Show listening ports (TCP/UDP)
netstat -tuna                   # Show all connections
netstat -i                      # Show interface statistics
netstat -r                      # Show routing table
netstat -s                      # Show network statistics

# ss (modern replacement)
ss -tuln                        # Show listening ports
ss -tuna                        # Show all connections
ss -s                           # Show socket statistics
ss -i                           # Show interface info
ss -p                           # Show process information
ss -o                           # Show timer information

# Specific port monitoring
ss -tlnp sport = :80            # Show what's listening on port 80
ss -tlnp sport = :443           # Show what's listening on port 443
netstat -tlnp | grep :22        # Show SSH connections

Traffic Analysis (tcpdump)​

# Basic packet capture
tcpdump -i eth0                 # Capture on interface
tcpdump -i any                  # Capture on all interfaces
tcpdump -n                      # Don't resolve hostnames
tcpdump -v                      # Verbose output
tcpdump -X                      # Show packet contents in hex/ASCII

# Protocol filtering
tcpdump -i eth0 tcp             # TCP packets only
tcpdump -i eth0 udp             # UDP packets only
tcpdump -i eth0 icmp            # ICMP packets only
tcpdump -i eth0 arp             # ARP packets only

# Port and host filtering
tcpdump -i eth0 port 80         # Port 80 traffic
tcpdump -i eth0 host 192.168.1.100    # Specific host
tcpdump -i eth0 src 192.168.1.100     # Source host
tcpdump -i eth0 dst 192.168.1.100     # Destination host
tcpdump -i eth0 net 192.168.1.0/24    # Network subnet

# Complex filters
tcpdump -i eth0 'tcp and port 80'                      # TCP on port 80
tcpdump -i eth0 'host 192.168.1.100 and port 22'      # SSH to specific host
tcpdump -i eth0 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0'  # SYN/FIN packets

# Save and read captures
tcpdump -i eth0 -w capture.pcap                        # Save to file
tcpdump -r capture.pcap                                # Read from file
tcpdump -i eth0 -c 100 -w capture.pcap                 # Capture 100 packets

Network Scanning (nmap)​

# Basic host discovery
nmap 192.168.1.0/24            # Scan subnet
nmap -sn 192.168.1.0/24        # Ping scan (no port scan)
nmap -sL 192.168.1.0/24        # List scan (no ping)

# Port scanning
nmap -sS 192.168.1.100         # SYN scan (stealth)
nmap -sT 192.168.1.100         # TCP connect scan
nmap -sU 192.168.1.100         # UDP scan
nmap -sA 192.168.1.100         # ACK scan

# Specific ports
nmap -p 80,443 192.168.1.100   # Scan specific ports
nmap -p 1-1000 192.168.1.100   # Scan port range
nmap -p- 192.168.1.100         # Scan all ports
nmap --top-ports 100 192.168.1.100  # Scan top 100 ports

# Service detection
nmap -sV 192.168.1.100         # Version detection
nmap -O 192.168.1.100          # OS detection
nmap -A 192.168.1.100          # Aggressive scan (OS, version, scripts)
nmap -sC 192.168.1.100         # Default scripts

# Output formats
nmap -oN output.txt 192.168.1.100     # Normal output
nmap -oX output.xml 192.168.1.100     # XML output
nmap -oG output.grep 192.168.1.100    # Greppable output
nmap -oA output 192.168.1.100         # All formats

Connectivity Testing​

# Basic connectivity
ping google.com                 # Test connectivity
ping -c 4 google.com            # Send 4 packets
ping -i 0.5 google.com          # Change interval
ping -s 1000 google.com         # Large packet size

# IPv6 ping
ping6 google.com                # IPv6 ping
ping6 -c 4 2001:4860:4860::8888 # Google IPv6 DNS

# Traceroute
traceroute google.com           # Trace route to destination
traceroute -n google.com        # Numeric output
traceroute -I google.com        # Use ICMP instead of UDP
traceroute6 google.com          # IPv6 traceroute

# MTU discovery
ping -M do -s 1472 google.com   # Test MTU size
tracepath google.com            # MTU discovery along path

# Port testing
telnet google.com 80            # Test TCP port
nc -zv google.com 80            # Test port with netcat
nc -zvu google.com 53           # Test UDP port

VPN and Tunneling​

OpenVPN​

# Server configuration (/etc/openvpn/server.conf)
port 1194
proto udp
dev tun
ca ca.crt
cert server.crt
key server.key
dh dh.pem
server 10.8.0.0 255.255.255.0
push "redirect-gateway def1 bypass-dhcp"
push "dhcp-option DNS 8.8.8.8"
keepalive 10 120
cipher AES-256-CBC
user nobody
group nogroup
persist-key
persist-tun
status /var/log/openvpn-status.log
verb 3

# Start OpenVPN
sudo openvpn --config /etc/openvpn/server.conf
systemctl start openvpn@server
systemctl enable openvpn@server

# Client connection
openvpn --config client.ovpn
systemctl start openvpn@client

WireGuard​

# Install WireGuard
sudo apt install wireguard      # Debian/Ubuntu
sudo yum install wireguard-tools  # Red Hat/CentOS

# Generate keys
wg genkey | tee private.key | wg pubkey > public.key
wg genkey | tee server-private.key | wg pubkey > server-public.key

# Server configuration (/etc/wireguard/wg0.conf)
[Interface]
PrivateKey = <server-private-key>
Address = 10.0.0.1/24
ListenPort = 51820
PostUp = iptables -A FORWARD -i wg0 -j ACCEPT
PostDown = iptables -D FORWARD -i wg0 -j ACCEPT

[Peer]
PublicKey = <client-public-key>
AllowedIPs = 10.0.0.2/32

# Client configuration
[Interface]
PrivateKey = <client-private-key>
Address = 10.0.0.2/24
DNS = 8.8.8.8

[Peer]
PublicKey = <server-public-key>
Endpoint = server.example.com:51820
AllowedIPs = 0.0.0.0/0

# Start WireGuard
wg-quick up wg0                 # Start interface
wg-quick down wg0               # Stop interface
systemctl enable wg-quick@wg0   # Enable at boot
wg show                         # Show status

SSH Tunneling​

# Local port forwarding
ssh -L 8080:localhost:80 user@server    # Forward local 8080 to remote 80
ssh -L 3306:db.example.com:3306 user@server  # Forward to different host

# Remote port forwarding
ssh -R 8080:localhost:80 user@server    # Forward remote 8080 to local 80

# Dynamic port forwarding (SOCKS proxy)
ssh -D 1080 user@server                 # Create SOCKS proxy on port 1080

# Keep tunnels alive
ssh -N -L 8080:localhost:80 user@server # No command execution
ssh -f -N -L 8080:localhost:80 user@server  # Run in background

# SSH tunnel through multiple hops
ssh -L 8080:target:80 -o ProxyCommand="ssh -W %h:%p user@jumphost" user@server

Network File Systems​

NFS (Network File System)​

# NFS Server setup
sudo apt install nfs-kernel-server     # Install NFS server

# /etc/exports configuration
/srv/nfs    192.168.1.0/24(rw,sync,no_subtree_check)
/home/user  192.168.1.100(rw,sync,no_root_squash)

# NFS server management
sudo exportfs -a                       # Export all filesystems
sudo exportfs -r                       # Re-export filesystems
sudo exportfs -v                       # Show exported filesystems
sudo systemctl restart nfs-kernel-server

# NFS Client
sudo apt install nfs-common            # Install NFS client
sudo mount -t nfs 192.168.1.100:/srv/nfs /mnt/nfs    # Mount NFS share
sudo umount /mnt/nfs                    # Unmount NFS share

# /etc/fstab entry for automatic mounting
192.168.1.100:/srv/nfs /mnt/nfs nfs defaults 0 0

# NFS troubleshooting
showmount -e 192.168.1.100             # Show exports from server
rpcinfo -p 192.168.1.100               # Show RPC services

SMB/CIFS (Samba)​

# Install Samba
sudo apt install samba samba-common-bin    # Debian/Ubuntu
sudo yum install samba samba-client        # Red Hat/CentOS

# Samba server configuration (/etc/samba/smb.conf)
[global]
    workgroup = WORKGROUP
    security = user
    map to guest = bad user

[share]
    path = /srv/samba/share
    guest ok = yes
    read only = no
    browsable = yes

# Samba user management
sudo smbpasswd -a username              # Add Samba user
sudo smbpasswd -d username              # Disable user
sudo smbpasswd -e username              # Enable user
sudo pdbedit -L                         # List Samba users

# Start Samba services
sudo systemctl start smbd nmbd
sudo systemctl enable smbd nmbd

# SMB/CIFS client
sudo apt install cifs-utils             # Install client tools
sudo mount -t cifs //192.168.1.100/share /mnt/samba \
    -o username=user,password=pass       # Mount SMB share
sudo umount /mnt/samba                   # Unmount share

# /etc/fstab entry
//192.168.1.100/share /mnt/samba cifs username=user,password=pass,uid=1000,gid=1000 0 0

# SMB troubleshooting
smbclient -L 192.168.1.100              # List shares
smbclient //192.168.1.100/share -U user # Connect to share
testparm                                # Test Samba configuration

Network Bonding and Bridging​

Network Bonding​

# Install bonding module
sudo modprobe bonding
echo "bonding" >> /etc/modules

# Create bonding interface
sudo ip link add bond0 type bond mode 802.3ad
sudo ip link set bond0 up

# Add interfaces to bond
sudo ip link set eth0 master bond0
sudo ip link set eth1 master bond0

# Configure bonding (/etc/network/interfaces - Debian/Ubuntu)
auto bond0
iface bond0 inet static
    address 192.168.1.100
    netmask 255.255.255.0
    gateway 192.168.1.1
    bond-slaves eth0 eth1
    bond-mode 802.3ad
    bond-lacp-rate 1
    bond-miimon 100

# Red Hat/CentOS bonding configuration
# /etc/sysconfig/network-scripts/ifcfg-bond0
DEVICE=bond0
BONDING_OPTS="mode=802.3ad miimon=100 lacp_rate=1"
BOOTPROTO=static
IPADDR=192.168.1.100
NETMASK=255.255.255.0
GATEWAY=192.168.1.1
ONBOOT=yes

# /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
MASTER=bond0
SLAVE=yes
ONBOOT=yes

# Bond monitoring
cat /proc/net/bonding/bond0              # Show bond status

Network Bridging​

# Create bridge
sudo ip link add br0 type bridge
sudo ip link set br0 up

# Add interfaces to bridge
sudo ip link set eth0 master br0
sudo ip link set eth1 master br0

# Configure bridge (/etc/network/interfaces)
auto br0
iface br0 inet static
    address 192.168.1.100
    netmask 255.255.255.0
    gateway 192.168.1.1
    bridge-ports eth0 eth1
    bridge-stp on
    bridge-fd 0
    bridge-maxwait 0

# Bridge management
bridge link show                        # Show bridge links
bridge fdb show                         # Show forwarding database
bridge vlan show                        # Show VLAN information
brctl show                              # Show bridge information (legacy)

Network Security and Port Scanning​

Port Security​

# Check open ports
ss -tuln                                # Show listening ports
netstat -tuln                          # Legacy method
lsof -i                                 # Show network connections

# Port scanning detection
sudo netstat -tuln | grep LISTEN        # Local listening ports
sudo lsof -i -n | grep LISTEN           # Processes listening on ports

# Firewall rules for port security
iptables -A INPUT -p tcp --dport 22 -m state --state NEW -m recent \
    --set --name SSH -j ACCEPT          # Rate limit SSH
iptables -A INPUT -p tcp --dport 22 -m state --state NEW -m recent \
    --update --seconds 60 --hitcount 4 --rttl --name SSH -j DROP

Intrusion Detection​

# Monitor failed login attempts
sudo grep "Failed password" /var/log/auth.log
sudo grep "authentication failure" /var/log/auth.log

# Monitor network connections
sudo netstat -tuln | grep ESTABLISHED   # Current connections
sudo lsof -i | grep ESTABLISHED         # Process connections

# Fail2ban configuration
sudo apt install fail2ban
sudo systemctl enable fail2ban
sudo systemctl start fail2ban

# /etc/fail2ban/jail.local
[DEFAULT]
bantime = 600
findtime = 600
maxretry = 3

[ssh]
enabled = true
port = ssh
filter = sshd
logpath = /var/log/auth.log

Network Hardening​

# Disable unnecessary services
sudo systemctl disable telnet
sudo systemctl disable rsh
sudo systemctl disable rlogin

# Network parameter tuning (/etc/sysctl.conf)
net.ipv4.ip_forward = 0                 # Disable IP forwarding
net.ipv4.conf.all.send_redirects = 0    # Disable redirects
net.ipv4.conf.default.send_redirects = 0
net.ipv4.conf.all.accept_source_route = 0  # Disable source routing
net.ipv4.conf.default.accept_source_route = 0
net.ipv4.conf.all.accept_redirects = 0  # Disable ICMP redirects
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.all.secure_redirects = 0
net.ipv4.conf.default.secure_redirects = 0
net.ipv4.conf.all.log_martians = 1      # Log martian packets
net.ipv4.conf.default.log_martians = 1
net.ipv4.icmp_echo_ignore_broadcasts = 1  # Ignore ICMP broadcasts
net.ipv4.icmp_ignore_bogus_error_responses = 1  # Ignore bogus ICMP responses

# Apply sysctl settings
sudo sysctl -p

Bandwidth Monitoring and Traffic Shaping​

Bandwidth Monitoring​

# Real-time monitoring
iftop                                   # Interface traffic monitoring
iftop -i eth0                          # Monitor specific interface
nethogs                                # Process bandwidth usage
nload                                   # Network load monitor
slurm -i eth0                          # Simple network monitor

# Interface statistics
cat /proc/net/dev                      # Network interface statistics
watch -n 1 cat /proc/net/dev           # Real-time statistics
vnstat -i eth0                         # Interface statistics history
vnstat -l -i eth0                      # Live statistics

# Bandwidth testing
iperf3 -s                              # Start iperf3 server
iperf3 -c server_ip                    # Connect to iperf3 server
iperf3 -c server_ip -t 30              # Run test for 30 seconds
iperf3 -c server_ip -P 4               # Use 4 parallel streams

Traffic Shaping with tc​

# Create traffic control qdisc
sudo tc qdisc add dev eth0 root handle 1: htb default 30

# Create classes
sudo tc class add dev eth0 parent 1: classid 1:1 htb rate 1mbit
sudo tc class add dev eth0 parent 1:1 classid 1:10 htb rate 800kbit ceil 1mbit
sudo tc class add dev eth0 parent 1:1 classid 1:20 htb rate 200kbit ceil 400kbit

# Add filters
sudo tc filter add dev eth0 parent 1: protocol ip prio 1 u32 \
    match ip dport 80 0xffff flowid 1:10      # HTTP traffic
sudo tc filter add dev eth0 parent 1: protocol ip prio 1 u32 \
    match ip dport 443 0xffff flowid 1:10     # HTTPS traffic
sudo tc filter add dev eth0 parent 1: protocol ip prio 2 u32 \
    match ip dport 22 0xffff flowid 1:20      # SSH traffic

# Show traffic control configuration
sudo tc qdisc show dev eth0
sudo tc class show dev eth0
sudo tc filter show dev eth0

# Remove traffic control
sudo tc qdisc del dev eth0 root

Network Monitoring Scripts​

# Monitor bandwidth usage
#!/bin/bash
INTERFACE="eth0"
while true; do
    RX_BYTES=$(cat /sys/class/net/$INTERFACE/statistics/rx_bytes)
    TX_BYTES=$(cat /sys/class/net/$INTERFACE/statistics/tx_bytes)
    echo "RX: $RX_BYTES bytes, TX: $TX_BYTES bytes"
    sleep 1
done

# Monitor connection count
#!/bin/bash
while true; do
    CONNECTIONS=$(ss -tun | wc -l)
    echo "Active connections: $CONNECTIONS"
    sleep 5
done

Network Troubleshooting Methodologies​

Systematic Troubleshooting Approach​

# Layer 1 - Physical
ethtool eth0                           # Check physical link
ethtool eth0 | grep "Link detected"    # Check link status
ip link show eth0                      # Interface status
dmesg | grep eth0                      # Check for hardware errors

# Layer 2 - Data Link
arp -a                                 # Show ARP table
ip neighbor show                       # Show neighbor table
tcpdump -i eth0 arp                    # Monitor ARP traffic
bridge fdb show                        # Show MAC address table

# Layer 3 - Network
ping -c 4 192.168.1.1                 # Test default gateway
ping -c 4 8.8.8.8                     # Test external connectivity
traceroute 8.8.8.8                    # Trace network path
ip route show                          # Check routing table

# Layer 4 - Transport
telnet google.com 80                   # Test TCP connectivity
nc -zv google.com 80                   # Test port connectivity
ss -tuln | grep :80                    # Check if service is listening

# Layer 7 - Application
curl -I http://google.com              # Test HTTP connectivity
wget --spider http://google.com        # Test web connectivity

Common Network Issues and Solutions​

# DNS resolution issues
nslookup google.com                    # Test DNS resolution
dig google.com                         # Detailed DNS query
cat /etc/resolv.conf                   # Check DNS configuration
systemctl restart systemd-resolved     # Restart DNS resolver

# IP configuration issues
ip addr show                           # Check IP configuration
dhclient eth0                          # Renew DHCP lease
systemctl restart networking           # Restart networking service

# Routing issues
ip route show                          # Check routing table
ping -c 4 $(ip route | grep default | awk '{print $3}')  # Test gateway
traceroute 8.8.8.8                    # Trace routing path

# Firewall issues
iptables -L                            # Check firewall rules
iptables -F                            # Flush firewall rules (temporary)
systemctl status ufw                   # Check UFW status

Performance Testing and Optimization​

# Network performance testing
ping -c 100 -i 0.1 google.com         # Latency testing
iperf3 -c iperf.he.net                # Bandwidth testing
mtr google.com                         # Combined ping/traceroute

# Network optimization
echo 'net.core.rmem_max = 16777216' >> /etc/sysctl.conf
echo 'net.core.wmem_max = 16777216' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_rmem = 4096 87380 16777216' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_wmem = 4096 65536 16777216' >> /etc/sysctl.conf
sysctl -p                              # Apply changes

# Interface optimization
ethtool -G eth0 rx 4096 tx 4096        # Increase ring buffer size
ethtool -K eth0 tso on                 # Enable TCP segmentation offload
ethtool -K eth0 gso on                 # Enable generic segmentation offload

Network Monitoring and Logging​

# Enable network logging
echo 'net.netfilter.nf_log_all_netns = 1' >> /etc/sysctl.conf
iptables -A INPUT -j LOG --log-prefix "INPUT: "
iptables -A OUTPUT -j LOG --log-prefix "OUTPUT: "

# Monitor network logs
tail -f /var/log/syslog | grep "INPUT:"
tail -f /var/log/syslog | grep "OUTPUT:"
journalctl -u networking -f            # Follow networking service logs

# Network event monitoring
ip monitor                             # Monitor network events
ip monitor route                       # Monitor routing changes
ip monitor link                        # Monitor link changes

Quick Reference Commands​

Most Common Network Commands​

# Interface management
ip addr show                           # Show IP addresses
ip link show                           # Show network interfaces
ip route show                          # Show routing table

# Connectivity testing
ping google.com                        # Test connectivity
traceroute google.com                  # Trace network path
telnet google.com 80                   # Test port connectivity

# DNS troubleshooting
nslookup google.com                    # Basic DNS lookup
dig google.com                         # Advanced DNS query
host google.com                        # Simple DNS lookup

# Network monitoring
ss -tuln                               # Show listening ports
netstat -tuln                          # Show network connections
lsof -i                                # Show network connections by process

# Firewall management
iptables -L                            # List firewall rules
iptables -F                            # Flush firewall rules
ufw status                             # Check UFW firewall status

Emergency Network Recovery​

# Reset network configuration
sudo systemctl restart networking      # Restart networking service
sudo systemctl restart NetworkManager  # Restart NetworkManager
sudo dhclient eth0                     # Renew DHCP lease
sudo ifdown eth0 && sudo ifup eth0     # Restart interface

# Emergency connectivity
sudo ip addr add 192.168.1.100/24 dev eth0    # Manual IP assignment
sudo ip route add default via 192.168.1.1      # Manual default route
echo "nameserver 8.8.8.8" | sudo tee /etc/resolv.conf  # Manual DNS