http://ru.wikipedia.org/wiki/Маршрутизатор

root@gate:~# sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 0

root@gate:~# sysctl net.ipv4.ip_forward=1
net.ipv4.ip_forward = 1

root@gate:~# cat /etc/sysctl.d/20-my-forward.conf

net.ipv4.ip_forward=1

root@gate:~# sysctl -p --system 

[gate:~] # sysctl net.inet.ip.forwarding

[gate:~] # sysctl net.inet.ip.forwarding=1

[gate:~] # cat /etc/rc.conf

...
gateway_enable="YES"
...

[gate:~] # /etc/rc.d/routing restart

# ip route help

gate.isp.un# ip r add 192.168.Y.0/24 via 172.16.1.Y

gate# 
ip r add 192.168.110.0/24 via 192.168.X.201
ip r add 192.168.120.0/24 via 192.168.X.202
ip r add 192.168.130.0/24 via 192.168.X.203

# ip r del 192.168.Y.0/24

ip route add|del blackhole|unreachable 192.168.100+X.201

root@gate:~# netstat -rn

root@gate:~# route -n

root@gate:~# route del default

root@gate:~# route add default gw 172.16.1.254

root@gate:~# route add -net 192.168.Y.0 netmask 255.255.255.0 gw 172.16.1.Y

root@gate:~# route del -net 192.168.Y.0 netmask 255.255.255.0

# route add -host 192.168.100+X.201 reject

# route del -host 192.168.100+X.201 reject

[gate:~] # netstat -rn -f inet

[gate:~] # route delete default

[gate:~] # route add default 172.16.1.254

[gate:~] # route add 192.168.Y/24 172.16.1.Y

[gate:~] # route delete 192.168.Y/24
delete net 192.168.Y.0

[gate:~] # route -n flush

• How to add static route with netplan

root@gate:~# cat /etc/network/interfaces

...
up route add -net 192.168.Y.0 netmask 255.255.255.0 gw 172.16.1.Y
up ip route add 192.168.Z.0/24 via 172.16.1.Z

# cat /etc/sysconfig/network-scripts/route-eth1

192.168.Y.0/24 via 172.16.1.Y dev eth1

[gate:~] # cat /etc/rc.conf

...
defaultrouter="172.16.1.254"
gateway_enable="YES"
static_routes="netY netZ"
route_netY="192.168.Y.0/24 172.16.1.Y"
route_netZ="192.168.Z.0/24 172.16.1.Z"
...

[gate:~] # /etc/rc.d/routing start

http://ru.wikipedia.org/wiki/OSPF

• https://docs.frrouting.org/en/latest/setup.html

# apt install frr

# cat /etc/frr/daemons

...
ospfd=yes
...

# systemctl restart frr

# ps ax | grep frr

# vtysh
...
# show run

root@gate:~# apt install quagga

root@gate:~# cd /etc/quagga/

gate# cat zebra.conf

hostname service_zebra
password zebra
enable password zebra

gate# cat ospfd.conf

hostname service_ospfd
password zebra
enable password zebra

[gate:~] # pkg install quagga

[gate:~] # cd /usr/local/etc/quagga/

# service zebra start

# service ospfd start

[gate:~] # cat /etc/rc.conf

...
quagga_enable="YES"
quagga_daemons="zebra ospfd"

[gate:~] # /usr/local/etc/rc.d/quagga start

gate# telnet localhost ospfd

gate_ospfd> en
gate_ospfd# conf term
gate_ospfd(config)# router ospf
gate_ospfd(config-router)# network 172.16.1.0/24 area 0
gate_ospfd(config-router)# redistribute connected
gate_ospfd(config-router)# end

gate_ospfd# show ip ospf neighbor

gate_ospfd# write

gate_ospfd# exit

gate# netstat -rn

gate# telnet localhost zebra

server_zebra> show ip route

server_zebra> exit

gate# netstat -rn

[gate:~] # cat /etc/rc.conf

...
router_enable="YES"
router_flags="-s"
...

[gate:~] # /etc/rc.d/routed start

[gate:~] # netstat -rn -f inet
...
192.168.Y         172.16.1.Y       UG          0        0   fxp0
...

• em0 - LAN (net 192.168.X/24 gw 192.168.1.1)
• em1 - ISP1 (net 172.16.1/24 gw 172.16.1.254)
• em3 - ISP2 (net 172.16.2/24 gw 172.16.2.254)

http://www.opennet.ru/openforum/vsluhforumID1/63044.html

# cat /etc/pf.conf

nat on em1 from 192.168.X/24 to any -> 172.16.1.X
nat on em3 from 192.168.X/24 to any -> 172.16.2.X

pass in quick on em0 from 192.168.X.0/24 to em0
pass in on em0 route-to (em1 172.16.1.254) from 192.168.X/25 to any
pass in on em0 route-to (em3 172.16.2.254) from 192.168.X.128/25 to any

• Роутинг и policy-routing в Linux при помощи iproute2

# ip route show table all  #Записи таблицы main отображаются без имени

# ip route show
# ip route show table main  #Все пакеты, для которых нет правил, обрабатываются маршрутами таблицы main

# cat /etc/iproute2/rt_tables  #Таблицам можно давать имена

# ip route add default via 172.16.1.254 table 101
# ip route add default via 172.16.2.254 table 102

# ip route show table 101
# ip route show table 102

# ip route show table all | grep 'table 10[12]'

# ip rule show

# ip rule add prio 100 from 192.168.X.0/24 to 192.168.X.0/24 table main
# ip rule add prio 200 from 192.168.X.0/25 table 101
# ip rule add prio 300 from 192.168.X.128/25 table 102

# ip rule show

# ip rule del prio 300
# ip rule del prio 200
# ip rule del prio 100

# ip route del default via 172.16.1.254 table 101
# ip route del default via 172.16.2.254 table 102

# ip route flush table 101
# ip route flush table 102

# cat nat.sh

iptables -t nat --flush

iptables -t nat -A POSTROUTING -o eth1 -s 192.168.X.0/24 -j SNAT --to-source 172.16.1.X
iptables -t nat -A POSTROUTING -o eth2 -s 192.168.X.0/24 -j SNAT --to-source 172.16.2.X

conntrack -F

# iptables-save > /etc/iptables.rules

# cat /etc/network/interfaces

...
auto eth0
iface eth0 inet static

        pre-up iptables-restore < /etc/iptables.rules
        
        post-up ip rule add prio 100 from 192.168.X.0/24 to 192.168.X.0/24 table main
        post-up ip rule add prio 200 from 192.168.X.0/25 table 101
        post-up ip rule add prio 300 from 192.168.X.128/25 table 102
        
        address 192.168.X.1
        netmask 255.255.255.0

auto eth1
iface eth1 inet static

        post-up ip route add default via 172.16.1.254 table 101

        address 172.16.1.X
        netmask 255.255.255.0

auto eth2
iface eth2 inet static

        post-up ip route add default via 172.16.2.254 table 102

        address 172.16.2.X
        netmask 255.255.255.0