Debian as gateway for subnet/2nd network

Hello, guys! How are you?
I'm facing a problem. I have 2 networks (192.168.1.0/24 and 192.168.2.0/24) and what happen is this: in the 1st network (192.168.1.0/24) there's a gateway (192.168.1.1) to the Internet, and the 2nd network needs to pass through the 1st network to reach Internet.
To connect both networks, I have 2 servers (Debian "Stretch" 9.6.0) working as MASTER and SLAVE, and the idea is to use them as gateway for the 2nd network, like this:
[INTERNET] --- [1st GATEWAY] --- [192.168.1.0/24] --- [2nd GATEWAY (Servers MASTER/SLAVE: Debian)] --- [192.168.2.0/24]
Both servers are using enp0s3 to connect to 192.168.1.0/24 and enp0s8 to connect to 192.168.2.0/24, and they are working as DNS and DHCP for 192.168.2.0/24
I tried to add a default route to 192.168.1.1 (my 1st gateway) like this:
route add 192.168.1.1/32 dev enp0s8 (interface connected to my 2nd network)
route add default gw 192.168.1.1
But when I try to install Debian in a client on 192.168.2.0/24, during the network auto detection, it says there's no default route to Internet. When I proceed the installation (pointing to the server 192.168.2.1) the installer tries to set a repository mirror but it fails, saying that it wasn't possible to get that mirror, either because the network (repository) is unreachable or because the repository is broken/offline.
About Windows client, when I am setting Primary and Secondary DNS, instead of use 192.168.2.1 and 192.168.2.2 as my DNS for 192.168.2.0/24, I need to set 192.168.1.1 as Primary DNS.
My network interfaces are set like this:
FILE: /etc/network/interfaces
source /etc/network/interfaces.d/* auto lo iface lo inet loopback auto enp0s3 iface enp0s3 inet static network 192.168.1.0 gateway 192.168.1.1 broadcast 192.168.1.255 netmask 255.255.255.0 address 192.168.1.181 // 2nd server: address 192.168.1.182 auto enp0s8 iface enp0s8 inet static network 192.168.2.0 broadcast 192.168.2.255 netmask 255.255.255.0 address 192.168.2.1 // 2nd server: address 192.168.2.2
Also, my DHCP is set like this (with failover setting):
cat /etc/dhcp/dhcpd.conf
// This is the Master Server, while the Slave Server is almost equal, except few details to make it work as FAILOVER server
authoritative; ddns-update-style interim; option domain-name "mynetwork.local"; option domain-name-servers ns1.mynetwork.local, ns2.mynetwork.local, 192.168.2.1, 192.168.2.2; default-lease-time 600; max-lease-time 7200; log-facility local7; failover peer "DHCP-FAILOVER" { primary; address 192.168.2.1; port 647; peer address 192.168.2.2; peer port 647; max-response-delay 30; max-unacked-updates 10; load balance max seconds 3; mclt 1800; split 128; } subnet 192.168.2.0 netmask 255.255.255.0 { option routers 192.168.2.1, 192.168.2.2; option subnet-mask 255.255.255.0; option domain-name "mynetwork.local"; option domain-name-servers 192.168.2.1, 192.168.2.2; option netbios-dd-server 192.168.2.1, 192.168.2.2; option netbios-name-servers 192.168.2.1, 192.168.2.2; option netbios-node-type 8; option nis-domain "mynetwork.local"; option nis-servers 192.168.2.1, 192.168.2.2; option nisplus-domain "mynetwork.local"; option nisplus-servers 192.168.2.1, 192.168.2.2; option ntp-servers 192.168.2.1, 192.168.2.2; option time-offset -18000; pool { failover peer "DHCP-FAILOVER"; range 192.168.2.3 192.168.2.254; } }
And, finally, my firewall script is this:
#!/bin/bash modprobe iptable_nat modprobe iptable_filter modprobe iptable_mangle modprobe ipt_MASQUERADE modprobe ip_tables modprobe nf_conntrack modprobe nf_conntrack_ipv4 modprobe nf_nat modprobe nf_tables modprobe nf_tables_ipv4 modprobe nft_masq modprobe nft_masq_ipv4 modprobe nft_nat modprobe nft_redir modprobe nft_redir_ipv4 iptables -t filter -F iptables -t mangle -F iptables -t nat -F iptables -P INPUT DROP iptables -P FORWARD DROP iptables -P OUTPUT ACCEPT EXTERNAL_NETWORK=192.168.1.0 # My 1st network EXTERNAL_INTERFACE=enp0s3 # Interface connected to 192.168.1.0/24 INTERNAL_NETWORK=192.168.2.0 # My 2nd network INTERNAL_INTERFACE=enp0s8 # Interface connected to 192.168.2.0/24 [[ $(hostname) = master ]] && SRV_IP_ADDR=192.168.2.1 || SRV_IP_ADDR=192.168.2.2 echo 1 > /proc/sys/net/ipv4/ip_forward iptables -t nat -A POSTROUTING -o $EXTERNAL_INTERFACE -j MASQUERADE iptables -A INPUT -i lo -j ACCEPT iptables -A OUTPUT -o lo -j ACCEPT iptables -A INPUT -i $EXTERNAL_INTERFACE -j ACCEPT # Interface enp0s3 iptables -A OUTPUT -o $EXTERNAL_INTERFACE -j ACCEPT iptables -A FORWARD -i $EXTERNAL_INTERFACE -j ACCEPT iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT // SSH Connections iptables -A INPUT -i $EXTERNAL_INTERFACE -p tcp --dport 1024 -j ACCEPT iptables -A INPUT -s $EXTERNAL_NETWORK/24 -p tcp --dport 1024 -j ACCEPT //If a package goes to the internet (external network), it must return to the internal network iptables -A FORWARD -d $INTERNAL_NETWORK/24 -p tcp -j ACCEPT iptables -A FORWARD -s $INTERNAL_NETWORK/24 -p tcp -j ACCEPT // Accepting WWW, WWWS, DHCP and DNS connections/queries iptables -A FORWARD -s $SRV_IP_ADDR -m multiport -p tcp --dport 80,443 -j ACCEPT iptables -A FORWARD -d $SRV_IP_ADDR -m multiport -p tcp --sport 80,443 -j ACCEPT // These 2 lines bellow are for TCP iptables -A FORWARD -s $INTERNAL_NETWORK/24 -m multiport -p tcp --dport 53,67 -j ACCEPT iptables -A FORWARD -d $INTERNAL_NETWORK/24 -m multiport -p tcp --sport 53,67 -j ACCEPT // These 2 lines bellow are for UDP iptables -A FORWARD -s $INTERNAL_NETWORK/24 -m multiport -p udp --dport 53,67 -j ACCEPT iptables -A FORWARD -d $INTERNAL_NETWORK/24 -m multiport -p udp --sport 53,67 -j ACCEPT
What do I need to do to make my servers work as gateway / Primary-Secondary DNS for 192.168.2.0/24?
And how do I set a default route from 192.168.2.0/24 ---> 192.168.1.0/24?
Thank you for your attention.