Networking

Interfaces, routes, DNS, listening sockets and connectivity troubleshooting.

45 challenges · 14 easy · 20 medium · 11 pro

• Author resolv.confeasy · in-browser
  Write /etc/resolv.conf with two nameserver lines for 10.0.0.2 and 10.0.0.3, a search line for the domain corp.example.com, and an options line setting both timeout:2 and attempts:3. (Authored config, graded structurally — the engine isn't run here.)
• DHCP Ethernet Keyfileeasy · in-browser
  Write /etc/NetworkManager/system-connections/lan.nmconnection: a NetworkManager ethernet connection with a [connection] section setting id=lan and type=ethernet, and an [ipv4] section using method=auto so the interface gets its address by DHCP. The first line must be the [connection] section header. (Authored config, graded structurally — the engine isn't run here.)
• Default gatewayeasy
  Write the default-route gateway IP to /home/player/gw.txt.
• First nameservereasy
  Write the first nameserver IP from /etc/resolv.conf to /home/player/ns.txt.
• Interface counteasy
  Write the number of network interfaces to /home/player/n.txt.
• Loopback MTUeasy
  Write the MTU of lo to /home/player/mtu.txt.
• Loopback addresseasy
  Write the IPv4 address of the loopback interface to /home/player/ip.txt.
• Map a hostname in /etc/hostseasy
  Make the name db.internal resolve to 10.0.0.5 (add it to /etc/hosts).
• Multi-Alias Hosts Fileeasy · in-browser
  Write /etc/hosts mapping 127.0.0.1 to localhost, ::1 to localhost (the IPv6 loopback), and 10.20.0.15 to the canonical name app1.internal followed by the short alias app1 on the same line. (Authored config, graded structurally — the engine isn't run here.)
• Name the default-route NICeasy
  Write the network interface used by the default route to /home/player/iface.txt.
• Resolve localhosteasy
  Write the IPv4 address that 'localhost' resolves to on this box to /home/player/ip.txt.
• Set a DNS search domaineasy
  Add the DNS search domain lab.internal to /etc/resolv.conf (a `search` line listing lab.internal).
• Set the hostnameeasy
  Set this machine's hostname to lab-77.
• Socket Probe Scripteasy · in-browser
  Write /home/player/probe.sh, a bash script that lists all listening TCP sockets with numeric ports using `ss -tlnp`, and on a separate line filters that output through grep for the port :8095 (a grep ... :8095 pipeline). It must start with a bash shebang. (Authored config, graded structurally — the engine isn't run here.)
• Add a hosts entrymedium
  Add a static mapping so that 'db.internal' resolves to 10.0.0.5 (use /etc/hosts).
• Add a secondary IP to loopbackmedium
  Add the address 192.168.50.10/24 to the loopback interface 'lo' so it shows up in `ip addr show lo`.
• Add a static routemedium
  Add a route for 10.9.0.0/24 via the loopback interface (dev lo).
• Disable IPv6 (persistently)medium
  Turn off IPv6 globally now AND persist it: net.ipv6.conf.all.disable_ipv6 = 1 via a /etc/sysctl.d drop-in.
• Enable IP forwarding persistentlymedium
  Turn on IPv4 forwarding now AND make it survive a reboot (a sysctl drop-in under /etc/sysctl.d setting net.ipv4.ip_forward=1).
• Entries in /etc/hostsmedium
  Write the number of non-comment, non-blank lines in /etc/hosts to /home/player/n.txt.
• Established connectionsmedium
  Write the count of ESTABLISHED TCP connections, EXCLUDING SSH (port 22) connections, to /home/player/n.txt.
• Interface Bring-Up Scriptmedium · in-browser
  Write /home/player/setup-eth1.sh, a bash script that adds the address 10.0.5.10/24 to eth1 with `ip addr add`, brings the link up with `ip link set eth1 up`, and adds a default route via 10.0.5.1 with `ip route add default via`. It must start with a bash shebang. (Authored config, graded structurally — the engine isn't run here.)
• Loopback oper statemedium
  Write the operational state of lo (e.g. UP/UNKNOWN) to /home/player/s.txt.
• Name a Network in /etc/networksmedium · in-browser
  Write /etc/networks giving symbolic names to network numbers: a line naming the network corp-lan as 10.20.0.0, plus a line naming link-local as 169.254.0.0. Each line is a name followed by whitespace and the network number. (Authored config, graded structurally — the engine isn't run here.)
• NetworkManager Static IP Keyfilemedium · in-browser
  Write /etc/NetworkManager/system-connections/eth0.nmconnection: a NetworkManager keyfile connection with a [connection] section setting id=eth0 and type=ethernet, and an [ipv4] section using method=manual, address1=192.168.10.50/24,192.168.10.1 (address/prefix then gateway, comma-separated), and dns=192.168.10.53; (trailing semicolon). The first line must be the [connection] section header. (Authored config, graded structurally — the engine isn't run here.)
• Persistent Static Route Filemedium · in-browser
  Write /etc/sysconfig/network-scripts/route-eth0 in iproute2 command syntax: a single line adding a route for the network 10.50.0.0/16 via gateway 192.168.1.254 out dev eth0. (Authored config, graded structurally — the engine isn't run here.)
• Read ip_forwardmedium
  Write the current net.ipv4.ip_forward value to /home/player/v.txt.
• Read somaxconnmedium
  Write the current net.core.somaxconn to /home/player/v.txt.
• Register a Routing Tablemedium · in-browser
  Write /etc/iproute2/rt_tables registering a custom policy-routing table: a line mapping numeric table id 200 to the name uplink2 (the numeric id first, then whitespace, then the name). Keep the standard reserved mappings 255 local and 254 main present too. (Authored config, graded structurally — the engine isn't run here.)
• Route countmedium
  Write the number of routes in the main table to /home/player/n.txt.
• Router Forwarding Profilemedium · in-browser
  Write /etc/sysctl.d/95-router.conf turning the host into a router: enable net.ipv4.ip_forward (value 1), raise the connection backlog net.core.somaxconn to 1024, and disable ICMP redirect acceptance with net.ipv4.conf.all.accept_redirects set to 0. (Authored config, graded structurally — the engine isn't run here.)
• Strict Reverse-Path Filteringmedium · in-browser
  Write /etc/sysctl.d/80-rpfilter.conf enabling strict reverse-path filtering: one key=value line setting net.ipv4.conf.all.rp_filter to 1, and another setting net.ipv4.conf.default.rp_filter to 1. (Authored config, graded structurally — the engine isn't run here.)
• TCP read buffer defaultmedium
  Write the default value of net.ipv4.tcp_rmem (middle number) to /home/player/v.txt.
• Tune the connection backlogmedium
  Raise net.core.somaxconn to 1024, applied now and persisted in /etc/sysctl.d.
• Active-Backup Bond Keyfilepro · in-browser
  Write /etc/NetworkManager/system-connections/bond0.nmconnection defining a bond: a [connection] section with id=bond0, type=bond and interface-name=bond0; a [bond] section with mode=active-backup and miimon=100; and an [ipv4] section with method=auto. The first line must be the [connection] section header. (Authored config, graded structurally — the engine isn't run here.)
• Bridge Connection Keyfilepro · in-browser
  Write /etc/NetworkManager/system-connections/br0.nmconnection defining a software bridge: a [connection] section with id=br0, type=bridge and interface-name=br0, a [bridge] section setting stp=false, and an [ipv4] section with method=manual and address1=172.16.5.1/24. The first line must be the [connection] section header. (Authored config, graded structurally — the engine isn't run here.)
• Bring up a listener on tcp/8088pro
  Start any TCP service LISTENing on port 8088 (all interfaces) and keep it running while you press Check.
• Count listening TCP portspro
  Write the number of distinct TCP ports in LISTEN state to /home/player/n.txt.
• Extra Routes in a Keyfilepro · in-browser
  Write /etc/NetworkManager/system-connections/wan.nmconnection: an ethernet connection with id=wan whose [ipv4] section uses method=manual and address1=203.0.113.10/24,203.0.113.1, adds a static route route1=10.10.0.0/16,203.0.113.254, and disables DHCP-supplied DNS with ignore-auto-dns=true. The first line must be the [connection] section header. (Authored config, graded structurally — the engine isn't run here.)
• Loopback IPv6 presentpro
  Write 1 if ::1/128 is configured on lo, else 0, to /home/player/v.txt.
• Neighbour table sizepro
  Write the number of entries in the ARP/neighbour table to /home/player/n.txt.
• Persist a kernel settingpro
  Enable IPv4 forwarding now AND make it persist across reboots (net.ipv4.ip_forward=1 via /etc/sysctl.d).
• Resolve a port conflictpro
  Service 'legacy' owns port 80 and must stay there. Get nginx running on port 8080 instead (serving "NGINX8080") without stopping legacy.
• Tune TCP Memory Bufferspro · in-browser
  Write /etc/sysctl.d/90-tcp-buffers.conf tuning socket buffers: set net.core.rmem_max and net.core.wmem_max each to 16777216, and set net.ipv4.tcp_rmem to the three-value triple 4096 87380 16777216 (whitespace-separated, in min/default/max order). (Authored config, graded structurally — the engine isn't run here.)
• UDP listener countpro
  Write the number of UDP sockets in LISTEN/UNCONN state to /home/player/n.txt.