Entwicklung · SKILL.md

ubuntu-network-infra

ubuntu-network-infra

Use when configuring network infrastructure services on Ubuntu 24.04 LTS — BIND9 DNS server, dnsmasq, Kea/ISC DHCP server, NTP with chrony/timesyncd, VLAN routing, IP forwarding, NAT, and network troubleshooting tools. Part of the ubuntu-* skill family.

Independent GitHub snapshot · Automated classification · Original documentation governs

Jjoogy06 / agent-foundryMIT

Skill-Dokumentation

GitHub

name: ubuntu-network-infra description: Use when configuring network infrastructure services on Ubuntu 24.04 LTS — BIND9 DNS server, dnsmasq, Kea/ISC DHCP server, NTP with chrony/timesyncd, VLAN routing, IP forwarding, NAT, and network troubleshooting tools. Part of the ubuntu-* skill family. family: ubuntu applies_when: os_family == debian

Ubuntu Server 24.04 LTS — Network Infrastructure

Companion skill to ubuntu-server-admin covering DNS, DHCP, NTP, routing, and network diagnostics on Ubuntu Server 24.04.4 LTS (Noble Numbat). For core system administration (packages, users, SSH, firewall, disks), see the parent skill.

1. BIND9 DNS Server

Installation

sudo apt install bind9 bind9-utils bind9-dnsutils
sudo systemctl enable --now named

Config directory: /etc/bind/ Key files: named.conf, named.conf.options, named.conf.local, named.conf.default-zones

named.conf.options — Caching Forwarder with ACLs

# /etc/bind/named.conf.options
acl "trusted" {
    10.0.0.0/8;
    192.168.0.0/16;
    172.16.0.0/12;
    localhost;
};

options {
    directory "/var/cache/bind";

    recursion yes;
    allow-recursion { trusted; };
    allow-query { trusted; };
    allow-transfer { none; };

    forwarders {
        1.1.1.1;
        8.8.8.8;
    };
    forward only;

    dnssec-validation auto;
    listen-on { any; };
    listen-on-v6 { none; };

    querylog yes;
};

Forward Zone

Add to /etc/bind/named.conf.local:

zone "home.lab" {
    type master;
    file "/etc/bind/zones/db.home.lab";
    allow-update { none; };
};

Create /etc/bind/zones/db.home.lab:

sudo mkdir -p /etc/bind/zones

$TTL    604800
@       IN      SOA     ns1.home.lab. admin.home.lab. (
                        2024031501  ; Serial (YYYYMMDDNN)
                        3600        ; Refresh
                        1800        ; Retry
                        604800      ; Expire
                        86400 )     ; Negative Cache TTL

; Name servers
@       IN      NS      ns1.home.lab.

; A records
ns1     IN      A       10.0.1.10
gw      IN      A       10.0.1.1
web01   IN      A       10.0.1.50
db01    IN      A       10.0.1.60
app01   IN      A       10.0.1.70

; CNAME records
www     IN      CNAME   web01.home.lab.
mail    IN      CNAME   web01.home.lab.

; MX record
@       IN      MX  10  mail.home.lab.

Reverse Zone (PTR Records)

Add to /etc/bind/named.conf.local:

zone "1.0.10.in-addr.arpa" {
    type master;
    file "/etc/bind/zones/db.10.0.1";
    allow-update { none; };
};

Create /etc/bind/zones/db.10.0.1:

$TTL    604800
@       IN      SOA     ns1.home.lab. admin.home.lab. (
                        2024031501  ; Serial
                        3600        ; Refresh
                        1800        ; Retry
                        604800      ; Expire
                        86400 )     ; Negative Cache TTL

@       IN      NS      ns1.home.lab.

1       IN      PTR     gw.home.lab.
10      IN      PTR     ns1.home.lab.
50      IN      PTR     web01.home.lab.
60      IN      PTR     db01.home.lab.
70      IN      PTR     app01.home.lab.

Split-Horizon DNS (Internal vs External Views)

Replace zone declarations in /etc/bind/named.conf.local:

view "internal" {
    match-clients { trusted; };
    recursion yes;

    zone "example.com" {
        type master;
        file "/etc/bind/zones/db.example.com.internal";
    };
};

view "external" {
    match-clients { any; };
    recursion no;

    zone "example.com" {
        type master;
        file "/etc/bind/zones/db.example.com.external";
    };
};

Logging

Add to /etc/bind/named.conf.local:

logging {
    channel query_log {
        file "/var/log/named/query.log" versions 3 size 10m;
        severity info;
        print-time yes;
        print-category yes;
    };
    channel default_log {
        file "/var/log/named/default.log" versions 3 size 10m;
        severity warning;
        print-time yes;
    };
    category queries { query_log; };
    category default { default_log; };
};

sudo mkdir -p /var/log/named
sudo chown bind:bind /var/log/named

DNSSEC Basics (Signing a Zone)

# Generate zone-signing key (ZSK) and key-signing key (KSK)
cd /etc/bind/zones
sudo dnssec-keygen -a ECDSAP256SHA256 -n ZONE home.lab
sudo dnssec-keygen -a ECDSAP256SHA256 -n ZONE -f KSK home.lab

# Include keys in zone file — add at bottom of db.home.lab:
# $INCLUDE "Khome.lab.+013+NNNNN.key"
# $INCLUDE "Khome.lab.+013+MMMMM.key"

# Sign the zone
sudo dnssec-signzone -A -3 $(head -c 1000 /dev/urandom | sha1sum | cut -b 1-16) \
  -N INCREMENT -o home.lab -t db.home.lab

# Update named.conf.local to point to db.home.lab.signed

Management Commands

# Validate config before restart
sudo named-checkconf
sudo named-checkzone home.lab /etc/bind/zones/db.home.lab

# Reload without restart
sudo rndc reload
sudo rndc reload home.lab           # reload single zone
sudo rndc flush                     # clear cache
sudo rndc querylog on               # toggle query logging
sudo rndc status                    # server status

# Service management
sudo systemctl restart named
sudo systemctl status named
journalctl -u named -f              # follow logs

2. dnsmasq — Lightweight DNS/DHCP

Ideal for small networks, lab environments, and container hosts where a full BIND9 + Kea stack is overkill.

sudo apt install dnsmasq
sudo systemctl enable --now dnsmasq

Config: /etc/dnsmasq.conf (or drop-ins in /etc/dnsmasq.d/)

# /etc/dnsmasq.d/local.conf

# Listen only on LAN interface
interface=ens18
bind-interfaces

# Upstream DNS
server=1.1.1.1
server=8.8.8.8

# Local domain
domain=home.lab
local=/home.lab/
expand-hosts

# DNS cache size (default 150)
cache-size=1000

# Local host records (instead of editing /etc/hosts)
address=/nas.home.lab/10.0.1.20
address=/printer.home.lab/10.0.1.30

# DHCP range — 10.0.1.100-200, 12h lease
dhcp-range=10.0.1.100,10.0.1.200,12h

# Static reservations
dhcp-host=aa:bb:cc:dd:ee:01,server01,10.0.1.50
dhcp-host=aa:bb:cc:dd:ee:02,server02,10.0.1.60

# DHCP options
dhcp-option=option:router,10.0.1.1
dhcp-option=option:dns-server,10.0.1.10
dhcp-option=option:domain-search,home.lab
dhcp-option=option:ntp-server,10.0.1.10

# Logging
log-queries
log-dhcp
log-facility=/var/log/dnsmasq.log

# Validate and restart
dnsmasq --test
sudo systemctl restart dnsmasq

3. Kea DHCP Server

Kea is the modern replacement for ISC DHCP, actively developed and the recommended choice on Ubuntu 24.04.

Installation

sudo apt install kea-dhcp4-server kea-dhcp6-server kea-admin
sudo systemctl enable --now kea-dhcp4-server

DHCPv4 Configuration

Config: /etc/kea/kea-dhcp4.conf (JSON format)

{
    "Dhcp4": {
        "interfaces-config": {
            "interfaces": ["ens18"]
        },
        "lease-database": {
            "type": "memfile",
            "persist": true,
            "name": "/var/lib/kea/kea-leases4.csv",
            "lfc-interval": 3600
        },
        "valid-lifetime": 43200,
        "renew-timer": 21600,
        "rebind-timer": 37800,
        "option-data": [
            { "name": "domain-name-servers", "data": "10.0.1.10, 1.1.1.1" },
            { "name": "domain-name", "data": "home.lab" },
            { "name": "domain-search", "data": "home.lab" },
            { "name": "ntp-servers", "data": "10.0.1.10" }
        ],
        "subnet4": [
            {
                "id": 1,
                "subnet": "10.0.1.0/24",
                "pools": [
                    { "pool": "10.0.1.100 - 10.0.1.200" }
                ],
                "option-data": [
                    { "name": "routers", "data": "10.0.1.1" }
                ],
                "reservations": [
                    {
                        "hw-address": "aa:bb:cc:dd:ee:01",
                        "ip-address": "10.0.1.50",
                        "hostname": "server01"
                    },
                    {
                        "hw-address": "aa:bb:cc:dd:ee:02",
                        "ip-address": "10.0.1.60",
                        "hostname": "server02"
                    }
                ]
            },
            {
                "id": 2,
                "subnet": "10.0.100.0/24",
                "pools": [
                    { "pool": "10.0.100.100 - 10.0.100.200" }
                ],
                "option-data": [
                    { "name": "routers", "data": "10.0.100.1" }
                ]
            }
        ],
        "loggers": [
            {
                "name": "kea-dhcp4",
                "output-options": [
                    { "output": "/var/log/kea/kea-dhcp4.log" }
                ],
                "severity": "INFO"
            }
        ]
    }
}

Kea Management

# Validate config
kea-dhcp4 -t /etc/kea/kea-dhcp4.conf

# Service control
sudo systemctl restart kea-dhcp4-server
sudo systemctl status kea-dhcp4-server
journalctl -u kea-dhcp4-server -f

# View leases
cat /var/lib/kea/kea-leases4.csv

# Log directory
sudo mkdir -p /var/log/kea
sudo chown _kea:_kea /var/log/kea

Kea High Availability (Hot Standby)

Add to the Dhcp4 object in both primary and standby servers:

"hooks-libraries": [
    {
        "library": "/usr/lib/x86_64-linux-gnu/kea/hooks/libdhcp_lease_cmds.so"
    },
    {
        "library": "/usr/lib/x86_64-linux-gnu/kea/hooks/libdhcp_ha.so",
        "parameters": {
            "high-availability": [{
                "this-server-name": "primary",
                "mode": "hot-standby",
                "heartbeat-delay": 10000,
                "max-response-delay": 60000,
                "max-unacked-clients": 0,
                "peers": [
                    {
                        "name": "primary",
                        "url": "http://10.0.1.10:8000/",
                        "role": "primary"
                    },
                    {
                        "name": "standby",
                        "url": "http://10.0.1.11:8000/",
                        "role": "standby"
                    }
                ]
            }]
        }
    }
]

4. ISC DHCP Server (Legacy)

Minimal Legacy Config Reference

Config: /etc/dhcp/dhcpd.conf

sudo apt install isc-dhcp-server
# Set interface in /etc/default/isc-dhcp-server:
# INTERFACESv4="ens18"

default-lease-time 43200;
max-lease-time 86400;
authoritative;

option domain-name "home.lab";
option domain-name-servers 10.0.1.10, 1.1.1.1;

subnet 10.0.1.0 netmask 255.255.255.0 {
    range 10.0.1.100 10.0.1.200;
    option routers 10.0.1.1;

    host server01 {
        hardware ethernet aa:bb:cc:dd:ee:01;
        fixed-address 10.0.1.50;
    }
}

Migration Path to Kea

Export current leases: cat /var/lib/dhcp/dhcpd.leases
Convert dhcpd.conf subnet/host declarations to Kea JSON subnet4 and reservations format
Install Kea and configure — validate with kea-dhcp4 -t
Stop ISC DHCP, start Kea: sudo systemctl stop isc-dhcp-server && sudo systemctl start kea-dhcp4-server
Monitor /var/log/kea/kea-dhcp4.log for client assignments
Purge legacy: sudo apt purge isc-dhcp-server

5. NTP — Time Synchronization

chrony (Recommended for Servers)

sudo apt install chrony
sudo systemctl enable --now chronyd

Config: /etc/chrony/chrony.conf

# Use Ubuntu NTP pool
pool ntp.ubuntu.com        iburst maxsources 4
pool 0.ubuntu.pool.ntp.org iburst maxsources 2
pool 1.ubuntu.pool.ntp.org iburst maxsources 2

# Record rate of system clock drift
driftfile /var/lib/chrony/chrony.drift

# Allow NTP clients on LAN
allow 10.0.0.0/8
allow 192.168.0.0/16

# Serve time even when not synchronized (useful for isolated LANs)
local stratum 10

# Step the clock on startup if off by more than 1 second
makestep 1.0 3

# Enable kernel time synchronization
rtcsync

# Log tracking, measurements, and statistics
logdir /var/log/chrony
log tracking measurements statistics

# Management commands
chronyc sources -v           # show NTP sources with details
chronyc tracking             # current sync status
chronyc clients              # list NTP clients (if serving time)
chronyc makestep             # force immediate time correction

sudo systemctl restart chronyd

systemd-timesyncd (Simple Client-Only)

Already installed and active by default on Ubuntu 24.04. Suitable for machines that only need to sync time, not serve it.

# Check status
timedatectl status
timedatectl timesync-status

# Configure NTP servers: /etc/systemd/timesyncd.conf
[Time]
NTP=10.0.1.10
FallbackNTP=ntp.ubuntu.com 0.ubuntu.pool.ntp.org

# Apply
sudo systemctl restart systemd-timesyncd

When to use which: Use chrony on any server that provides NTP to LAN clients or requires precise time (database servers, log aggregators). Use timesyncd on simple workstations or VMs that just need to keep time accurate.

6. Routing, NAT, and VLANs

IP Forwarding

# Enable immediately (non-persistent)
sudo sysctl -w net.ipv4.ip_forward=1
sudo sysctl -w net.ipv6.conf.all.forwarding=1

# Persistent — /etc/sysctl.d/99-ip-forward.conf
net.ipv4.ip_forward = 1
net.ipv6.conf.all.forwarding = 1

# Apply
sudo sysctl --system

# Verify
sysctl net.ipv4.ip_forward

Static Routes via Netplan

# /etc/netplan/01-static.yaml
network:
  version: 2
  renderer: networkd
  ethernets:
    ens18:
      addresses:
        - 10.0.1.1/24
      routes:
        - to: default
          via: 10.0.0.1
        - to: 10.0.200.0/24
          via: 10.0.1.254
          metric: 100
        - to: 172.16.0.0/16
          via: 10.0.1.253

sudo netplan try     # apply with 120s rollback safety
sudo netplan apply
ip route show        # verify

NAT / Masquerade with nftables

For a gateway server that shares its internet connection with LAN clients.

# /etc/nftables.conf
#!/usr/sbin/nft -f
flush ruleset

table inet filter {
    chain input {
        type filter hook input priority 0; policy drop;
        iif "lo" accept
        ct state established,related accept
        ct state invalid drop
        tcp dport 22 accept                          # SSH
        udp dport { 67, 68 } accept                  # DHCP
        tcp dport 53 accept                          # DNS
        udp dport 53 accept                          # DNS
        udp dport 123 accept                         # NTP
        icmp type echo-request accept
    }

    chain forward {
        type filter hook forward priority 0; policy drop;
        iifname "ens19" oifname "ens18" accept       # LAN -> WAN
        ct state established,related accept           # return traffic
    }

    chain output {
        type filter hook output priority 0; policy accept;
    }
}

table ip nat {
    chain postrouting {
        type nat hook postrouting priority 100;
        oifname "ens18" masquerade                   # NAT outbound on WAN
    }

    chain prerouting {
        type nat hook prerouting priority -100;
        # Port forward: external:8080 -> internal web server
        iifname "ens18" tcp dport 8080 dnat to 10.0.1.50:80
    }
}

sudo systemctl enable --now nftables
sudo nft list ruleset                # verify

VLAN Routing Between Subnets

Gateway server with VLAN trunk on ens18, routing between VLAN 10, 20, and 30:

# /etc/netplan/02-vlans.yaml
network:
  version: 2
  renderer: networkd
  ethernets:
    ens18:
      dhcp4: false
  vlans:
    vlan10:
      id: 10
      link: ens18
      addresses:
        - 10.0.10.1/24
    vlan20:
      id: 20
      link: ens18
      addresses:
        - 10.0.20.1/24
    vlan30:
      id: 30
      link: ens18
      addresses:
        - 10.0.30.1/24

With IP forwarding enabled, the server will route between all three VLANs automatically. Use nftables to restrict inter-VLAN traffic:

# Add to nftables forward chain to block VLAN 30 (IoT) from reaching VLAN 10 (servers)
nft add rule inet filter forward iifname "vlan30" oifname "vlan10" drop

Policy-Based Routing Basics

Route traffic from VLAN 20 out a different WAN gateway:

# /etc/netplan/03-policy-routing.yaml
network:
  version: 2
  ethernets:
    ens19:
      addresses:
        - 203.0.113.2/24
      routes:
        - to: default
          via: 203.0.113.1
          table: 100
      routing-policy:
        - from: 10.0.20.0/24
          table: 100
          priority: 100

# Verify routing tables
ip rule show
ip route show table 100

7. Network Troubleshooting

DNS Diagnostics

# dig — most detailed DNS tool
dig home.lab                         # A record query
dig @10.0.1.10 home.lab              # query specific server
dig home.lab MX                      # MX records
dig home.lab ANY +noall +answer      # all record types, clean output
dig -x 10.0.1.50                     # reverse lookup (PTR)
dig home.lab +trace                  # trace delegation path
dig home.lab +short                  # just the answer
dig @10.0.1.10 home.lab AXFR        # zone transfer (test security)

# host — quick lookups
host web01.home.lab
host 10.0.1.50                       # reverse lookup

# nslookup — interactive or one-shot
nslookup web01.home.lab 10.0.1.10

# systemd-resolve integration
resolvectl query web01.home.lab
resolvectl status                    # show DNS config per interface
resolvectl statistics                # cache hit/miss stats

Packet Capture with tcpdump

# Install
sudo apt install tcpdump

# Capture DNS traffic
sudo tcpdump -i ens18 port 53 -n

# Capture DHCP traffic
sudo tcpdump -i ens18 port 67 or port 68 -n -v

# Capture all traffic to/from a host
sudo tcpdump -i ens18 host 10.0.1.50 -n

# Write to file for Wireshark analysis
sudo tcpdump -i ens18 -w /tmp/capture.pcap -c 1000

# Read capture file
tcpdump -r /tmp/capture.pcap -n

# Capture with packet content (ASCII)
sudo tcpdump -i ens18 port 80 -A -s 0

Socket and Connection Analysis

# ss — socket statistics (replaces netstat)
ss -tlnp                            # TCP listening sockets with PIDs
ss -ulnp                            # UDP listening sockets with PIDs
ss -tnp                             # established TCP connections
ss -s                               # summary statistics
ss -tnp state established '( dport = :443 )'   # filter by dest port

# Who is using a specific port
sudo ss -tlnp | grep :53
sudo lsof -i :53                    # alternative

Routing Diagnostics

# Show routing table
ip route show
ip -6 route show

# Show which route a destination will use
ip route get 8.8.8.8
ip route get 10.0.20.5

# Show all routing tables (policy routing)
ip rule show
ip route show table all

# Show ARP / neighbor cache
ip neigh show

# Show interface details
ip addr show
ip -s link show ens18                # interface statistics (errors, drops)

Path Analysis

# traceroute — trace path to destination
traceroute 8.8.8.8
traceroute -n 10.0.1.50             # skip DNS resolution (faster)
traceroute -T -p 443 example.com    # TCP traceroute to port 443

# mtr — combines ping + traceroute (live updating)
sudo apt install mtr-tiny
mtr 8.8.8.8                         # interactive mode
mtr -r -c 100 8.8.8.8               # report mode, 100 pings
mtr -r -c 50 -n 10.0.1.50           # numeric, no DNS

Infrastructure Port Scanning

# Install
sudo apt install nmap

# Scan your own subnet for active hosts
nmap -sn 10.0.1.0/24

# Scan a host for open TCP ports
nmap -sT 10.0.1.50

# Service version detection
nmap -sV 10.0.1.50

# Scan specific ports
nmap -p 22,53,80,443 10.0.1.50

# UDP scan (requires root, slow)
sudo nmap -sU -p 53,67,123,161 10.0.1.50

# OS detection
sudo nmap -O 10.0.1.50

Quick Connectivity Checks

# Ping with count
ping -c 4 10.0.1.1

# Check if a TCP port is open (no nmap needed)
timeout 3 bash -c 'echo > /dev/tcp/10.0.1.50/80' && echo "open" || echo "closed"

# curl for HTTP services
curl -I http://10.0.1.50              # headers only
curl -sv https://web01.home.lab 2>&1 | head -30  # verbose with TLS info

# Test DNS resolution explicitly
dig +short @10.0.1.10 web01.home.lab

Anti-Patterns

Anti-Pattern	Why It Fails	Correct Approach
Editing /etc/resolv.conf directly on systems using systemd-resolved	systemd-resolved overwrites manual changes; DNS configuration reverts on next network event	Configure DNS through netplan or `resolvectl`; manage upstream DNS in systemd-resolved.conf
Running BIND9 as an open resolver	Becomes a DNS amplification attack vector; abused for DDoS; consumes bandwidth and resources	Restrict recursion to internal networks (`allow-recursion { trusted; };`); block external recursive queries
No NTP monitoring on infrastructure servers	Clock drift causes Kerberos auth failures, certificate validation errors, and log timestamp mismatches	Monitor chrony/timesyncd sync status; alert on offset > 100ms; verify NTP source availability regularly
Static routes everywhere instead of proper VLAN configuration	Route tables become unmaintainable at scale; topology changes require touching every server	Use VLAN interfaces via netplan; define proper gateway per VLAN; document VLAN assignments and purpose
Running DHCP server without IP conflict detection	Rogue DHCP servers or static IP conflicts cause intermittent connectivity; difficult to diagnose	Enable DHCP conflict detection; monitor lease table; use DHCP snooping on managed switches; reserve IPs for infrastructure

Related Skills

Workload	Skill
Core system admin (packages, SSH, firewall, disks)	`ubuntu-server-admin`
Web servers (Nginx, Apache, Caddy)	`ubuntu-web-servers`
Databases (PostgreSQL, MySQL, Redis)	`ubuntu-databases`
Docker / containers	`ubuntu-docker-host`
File sharing (NFS, Samba, ZFS)	`ubuntu-file-storage`
Prometheus, Grafana, logging	`ubuntu-monitoring`
NVIDIA GPU, Ollama, CUDA	`ubuntu-ollama-nvidia`

MAKE IT PART OF YOUR WORKFLOW

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Quellcode

ubuntu-network-infra

Skill-Dokumentation

Ubuntu Server 24.04 LTS — Network Infrastructure

1. BIND9 DNS Server

Installation

named.conf.options — Caching Forwarder with ACLs

Forward Zone

Reverse Zone (PTR Records)

Split-Horizon DNS (Internal vs External Views)

Logging

DNSSEC Basics (Signing a Zone)

Management Commands

2. dnsmasq — Lightweight DNS/DHCP

3. Kea DHCP Server

Installation

DHCPv4 Configuration

Kea Management

Kea High Availability (Hot Standby)

4. ISC DHCP Server (Legacy)

Minimal Legacy Config Reference

Migration Path to Kea

5. NTP — Time Synchronization

chrony (Recommended for Servers)

systemd-timesyncd (Simple Client-Only)

6. Routing, NAT, and VLANs

IP Forwarding

Static Routes via Netplan

NAT / Masquerade with nftables

VLAN Routing Between Subnets

Policy-Based Routing Basics

7. Network Troubleshooting

DNS Diagnostics

Packet Capture with tcpdump

Socket and Connection Analysis

Routing Diagnostics

Path Analysis

Infrastructure Port Scanning

Quick Connectivity Checks

Anti-Patterns

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