./Sam_Stelfox.sh

Network

3 min read
note linux networking
found top level header in "/notes/linux/network/" that didn't match meta title ("Network Configuration" != "Network")

Modern Linux networking is typically managed through ip commands (from iproute2), NetworkManager (nmcli), or systemd-networkd. This note covers common tasks using these tools.

Viewing Network State

List all interfaces and their addresses:

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$ ip addr show
$ ip link show

Show the routing table:

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$ ip route show

Show active connections and listening ports:

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$ ss -tulnp

Static IP Address

Using ip commands (temporary, lost on reboot)

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$ ip addr add 10.13.37.200/24 dev eth0
$ ip route add default via 10.13.37.1
$ ip link set eth0 up

Using NetworkManager (persistent)

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$ nmcli con add type ethernet con-name "static-lan" ifname eth0 \
    ipv4.method manual \
    ipv4.addresses 10.13.37.200/24 \
    ipv4.gateway 10.13.37.1 \
    ipv4.dns "8.8.8.8,8.8.4.4"
$ nmcli con up static-lan

To modify an existing connection:

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$ nmcli con mod "static-lan" ipv4.addresses 10.13.37.201/24
$ nmcli con up "static-lan"

Additional IP Addresses

Add a second address to an interface:

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$ ip addr add 10.13.37.201/24 dev eth0

With NetworkManager:

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$ nmcli con mod "static-lan" +ipv4.addresses 10.13.37.201/24
$ nmcli con up "static-lan"

Bridges

Create a bridge and add an interface to it:

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$ ip link add name br0 type bridge
$ ip link set eth0 master br0
$ ip link set br0 up

With NetworkManager:

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$ nmcli con add type bridge con-name br0 ifname br0
$ nmcli con add type bridge-slave con-name br0-port0 ifname eth0 master br0
$ nmcli con up br0

Spanning Tree

Disable spanning tree on a bridge (common for VM host bridges):

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$ ip link set br0 type bridge stp_state 0

NIC Bonding

Create a bond with two interfaces in active-backup mode:

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$ nmcli con add type bond con-name bond0 ifname bond0 \
    bond.options "mode=active-backup,miimon=100"
$ nmcli con add type ethernet con-name bond0-port0 ifname eth0 master bond0
$ nmcli con add type ethernet con-name bond0-port1 ifname eth1 master bond0
$ nmcli con up bond0

Common bond modes:

  • balance-rr (0) - Round-robin for load balancing
  • active-backup (1) - Only one active interface, failover
  • 802.3ad (4) - LACP, requires switch support

VLANs

Create a VLAN interface:

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$ ip link add link eth0 name eth0.100 type vlan id 100
$ ip link set eth0.100 up
$ ip addr add 10.100.0.1/24 dev eth0.100

With NetworkManager:

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$ nmcli con add type vlan con-name vlan100 ifname eth0.100 \
    dev eth0 id 100 \
    ipv4.method manual ipv4.addresses 10.100.0.1/24

Routing

Enable IP forwarding (required for any routing):

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$ sysctl -w net.ipv4.ip_forward=1

Make it persistent in /etc/sysctl.d/99-routing.conf:

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# Enable packet forwarding
net.ipv4.ip_forward = 1

# Log packets with impossible addresses
net.ipv4.conf.all.log_martians = 1

# Do not accept source routing
net.ipv4.conf.all.accept_source_route = 0

# Ignore ICMP broadcast requests
net.ipv4.icmp_echo_ignore_broadcasts = 1

# Reject bogus ICMP error responses
net.ipv4.icmp_ignore_bogus_error_responses = 1

# Enable SYN cookies for SYN flood protection
net.ipv4.tcp_syncookies = 1

Add a static route:

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$ ip route add 10.16.0.0/24 via 172.16.10.2 dev eth1

Useful Tools

  • ss - Socket statistics (replacement for netstat)
  • ip - Network interface and routing configuration
  • nmcli - NetworkManager command line interface
  • conntrack - View/manage connection tracking entries
  • tcpdump - Packet capture
  • mtr - Combines traceroute and ping for network path analysis