Linux Networking

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Linux Networking
​GUI tool​
​Basic​
​/etc/network/interfaces​
​/etc/netplan/*.yaml​
​Port listening​
​debug​
​Netlink​
​tuntap​
​Features​
​disable ipv6​
​nftables​
​iptables​
​table / chain​
​rules​
​NAT​
​Trace​
​Log manually​
​Transparent Proxy​
​iptables frontend​
​CentOS firewall-cmd​
​Ubuntu - ufw​
​ip rule​
​tc​
​LVS​
​Virtual routing and forwarding​
​DNS​
GUI tool
nm-connection-editor
/etc/init.d/network-manager restart
Basic
https://access.redhat.com/sites/default/files/attachments/rh_ip_command_cheatsheet_1214_jcs_print.pdf
ls -l /sys/class/net/   # E.g.: ens3 -> ../../devices/pci0000:00/0000:00:03.0/virtio0/net/ens3
ip addr show dev eth1
​
ifconfig ens7 10.99.0.10/16 up
ip addr add 192.168.6.13/24 dev eth0 && ip link set eth0 up
​
ip addr flush dev eth0
ifconfig eth0 0.0.0.0 0.0.0.0 && dhclient
​
ip route add default via 192.168.1.1
ip route show table all
/etc/network/interfaces
auto enp1s0
iface enp1s0 inet dhcp
/etc/netplan/*.yaml
network:
  version: 2
​
  ethernets:
    enp1s0: {}
    enp2s0: {}
    enp3s0: {}
    enp4s0: {}
    enp5s0: {}
    enp6s0:
      dhcp4: yes
​
  bridges:
    br0:
      interfaces: [enp1s0,enp2s0]
      dhcp4: yes
    br1: {}
    ovs-br0:
      openvswitch: {} # 
      dhcp4: yes
​
  vlans:
    vlan.10:
      id: 10
      link: enp4s0
      dhcp4: yes
setup ovs manually
netplan try # if error, run: netplan generate
Port listening
# netstat -lntup | grep 8888
tcp        0      0 0.0.0.0:8888            0.0.0.0:*               LISTEN      5119/python         
tcp6       0      0 :::8888                 :::*                    LISTEN      5119/python     
​
# ss -lntup | grep 8888
tcp   LISTEN  0       128                    0.0.0.0:8888         0.0.0.0:*      users:(("jupyter-noteboo",pid=5119,fd=7))                                      
tcp   LISTEN  0       128                       [::]:8888            [::]:*      users:(("jupyter-noteboo",pid=5119,fd=6))       
debug
tcpdump -i any port 27017
nmap -sV -p6379 127.0.0.1
Netlink
https://en.wikipedia.org/wiki/Netlink
https://github.com/shemminger/iproute2/blob/master/misc/ss.c
tuntap
ip tuntap add mode tap dev tap1
        tap1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
        link/ether a2:05:e8:7f:d9:e8 brd ff:ff:ff:ff:ff:ff
​
ip tuntap add mode tun dev tun1
        tun1: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 500
        link/none
Features
ethtool -k ens3 | grep offload              # list Features
ethtool -K ens3 gro off gso off tso off     # set tcp-segmentation-offload, generic-segmentation/receive-offload
disable ipv6
echo "net.ipv6.conf.all.disable_ipv6=1"  >> /etc/sysctl.conf
sysctl  -p
nftables
https://wiki.nftables.org/wiki-nftables/index.php/Main_Page
https://wiki.nftables.org/wiki-nftables/index.php/Moving_from_iptables_to_nftables
https://openwrt.org/docs/guide-user/firewall/misc/nftables
iptables
table / chain
iptables -t nat -L # filter(default), nat, mangle, raw and security
iptables -t nat -F ...  # empty chain rules
iptables -t nat -X ...  # delete empty chain
​
iptables -t nat -L -n -x -v # --numeric IP/Port --exact packet and byte counters --verbose
iptables -L --line-numbers
iptables -D INPUT 2
​
iptables-save
rules
iptables -I INPUT -i docker0 -j ACCEPT
iptables -I INPUT -s localhost -j ACCEPT
​
iptables -A INPUT --dport 81 -j DROP
iptables -A INPUT -p tcp -m multiport  --dport 3306,6379 -j DROP
iptables -A INPUT -p udp --dport 161 -j ACCEPT
NAT
http://www.netfilter.org/documentation/HOWTO//NAT-HOWTO-3.html
    _____                                     _____
    /     \                                   /     \
PREROUTING -->[Routing ]----------------->POSTROUTING----->
    \D-NAT/     [Decision]                    \S-NAT/
                    |                            ^
                    |                            |
                    --------> Local Process ------
Masquerading is a specialized form of SNAT
Port forwarding, load sharing, and transparent proxying are all forms of DNAT.
http://www.netfilter.org/documentation/HOWTO//NAT-HOWTO-6.html
http://ipset.netfilter.org/iptables-extensions.man.html
Trace
http://backreference.org/2010/06/11/iptables-debugging/
modprobe nf_log_ipv4
sysctl net.netfilter.nf_log.2=nf_log_ipv4
iptables -t raw -A OUTPUT -p icmp -j TRACE
iptables -t raw -A PREROUTING -p icmp -j TRACE
vi /var/log/kern.log
Log manually
http://www.microhowto.info/troubleshooting/troubleshooting_iptables.html
iptables_log_INPUT_DROP() {
    iptables -N LOGGING
    iptables -A INPUT -j LOGGING
    iptables -A LOGGING -m limit --limit 2/min -j LOG --log-prefix "IPTables-Dropped: " --log-level 4
    iptables -A LOGGING -j DROP
}
Transparent Proxy
iptables -t nat -N TP
iptables -t nat -I TP -p tcp --dport 4433 -j RETURN  # bypass Port
​
# https://tools.ietf.org/html/rfc5735#page-6
iptables -t nat -A TP -d 0.0.0.0/8 -j RETURN
iptables -t nat -A TP -d 10.0.0.0/8 -j RETURN
iptables -t nat -A TP -d 127.0.0.0/8 -j RETURN
iptables -t nat -A TP -d 169.254.0.0/16 -j RETURN
iptables -t nat -A TP -d 172.16.0.0/12 -j RETURN
iptables -t nat -A TP -d 192.168.0.0/16 -j RETURN
iptables -t nat -A TP -d 224.0.0.0/4 -j RETURN
iptables -t nat -A TP -d 240.0.0.0/4 -j RETURN
​
# Anything else should be redirected to Dokodemo-door's local port
iptables -t nat -A TP -p tcp -j REDIRECT --to-ports 20088
iptables -t nat -I OUTPUT -p tcp -j TP
iptables -t nat -I PREROUTING -p tcp -j TP
​
# Add any UDP rules
iptables -t mangle -N TP
iptables -t mangle -A TP -p udp --dport 53 -j TPROXY --on-port 20088 --tproxy-mark 0x01/0x01
iptables -t mangle -A PREROUTING -j TP
​
iptables -t mangle -N TP_MARK
iptables -t mangle -A TP_MARK -p udp --dport 53 -j MARK --set-mark 1
iptables -t mangle -A OUTPUT -j TP_MARK
​
ip route add local default dev lo table 100
ip rule add fwmark 1 lookup 100
​
ip rule del fwmark 1 lookup 100 # disable UDP
iptables frontend
CentOS firewall-cmd
firewall-cmd --permanent --zone=public \
--add-rich-rule="rule family="ipv4" \
source address="1.2.3.4/32" \
port protocol="tcp" port="4567" accept"
​
firewall-cmd --zone=public --add-port=4433/tcp --permanent
firewall-cmd --zone=public --add-port=4433/udp--permanent
firewall-cmd --reload
firewall-cmd --list-all
​
service firewalld stop 
Ubuntu - ufw
sudo ufw allow 11200:11299/tcp
sudo ufw status verbose
sudo ufw disable
ip rule
Priority: 0, Selector: match anything, Action: lookup routing
        table local (ID 255).  The local table is a special routing
        table containing high priority control routes for local and
        broadcast addresses.
​
Priority: 32766, Selector: match anything, Action: lookup
        routing table main (ID 254).  The main table is the normal
        routing table containing all non-policy routes. This rule may
        be deleted and/or overridden with other ones by the
        administrator.
​
Priority: 32767, Selector: match anything, Action: lookup
        routing table default (ID 253).  The default table is empty.
        It is reserved for some post-processing if no previous default
        rules selected the packet.  This rule may also be deleted.
​
# cat /etc/iproute2/rt_tables
​
    #
    # reserved values
    #
    255	local
    254	main
    253	default
    0	unspec
    #
    # local
    #
    #1	inr.ruhep
tc
http://events.linuxfoundation.org/sites/events/files/slides/Linux_traffic_control.pdf
LVS
http://kb.linuxvirtualserver.org/wiki/IPVS
https://github.com/torvalds/linux/tree/master/net/netfilter/ipvs
IPVS - an advanced layer-4 load balancing solution， NAT/Direct Routing/IP Tunneling
Virtual routing and forwarding
https://docs.cumulusnetworks.com/display/DOCS/Virtual+Routing+and+Forwarding+-+VRF
multiple independent routing tables working simultaneously on the same router or switch Think of this feature as VLAN for layer 3
https://www.kernel.org/doc/Documentation/networking/vrf.txt
DNS
/etc/nsswitch.conf
    #hosts:          files mdns4_minimal [NOTFOUND=return] dns mdns4
    hosts:          files dns  # fix nslookup works but ping not work
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Networking
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GUI tool
Basic
/etc/network/interfaces
/etc/netplan/*.yaml
Port listening
debug
Netlink
tuntap
Features
disable ipv6
nftables
iptables
table / chain
rules
NAT
Trace
Log manually
Transparent Proxy
iptables frontend
CentOS firewall-cmd
Ubuntu - ufw
ip rule
LVS
Virtual routing and forwarding
DNS