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

debug

tcpdump -i any port 27017
nmap -sV -p6379 127.0.0.1

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

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

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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