CentOS - Nov 2014 - outside ssh connection from two different ISP's

I have both cable and dsl service.  The router attached to the cable service is
192.168.0.1 and the router attached to the dsl service is 192.168.0.254.  I
determine which service my computers communicate through by setting the gateway
assignment to either of those addresses and it just works.

I can connect to my computers from the outside by port forwarding from my
routers to the computer that I want to connect to.

However, the outside ssh connection works only if the computer's gateway
assignment matches the router that I'm connecting to.  I suspect that I have
to set up some sort of routing assignment to tell the computer to reply to the
gateway that the ssh connection request came in on if it doesn't match the
current gateway assignment, but I have no idea how to do that.

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On 11/10/2014 02:11 PM, Frank Cox wrote:
> However, the outside ssh connection works only if the computer's
gateway assignment matches the router that I'm connecting to.  I suspect
that I have to set up some sort of routing assignment to tell the computer to
reply to the gateway that the ssh connection request came in on if it
doesn't match the current gateway assignment, but I have no idea how to do
that.
What you're describing is multi-homing, and it's fairly complex.
You'll
need to 1) create routing tables for each ISP, 2) mark incoming NEW 
connections based on the MAC of the router that forwarded the new 
connection, and 3) create ip rules to associate a connection with the 
correct routing table.

This article partially covers what you need to do, and might get you 
started:
https://blogs.oracle.com/networking/entry/advance_routing_for_multi_homed

You might also consider using Shorewall to manage the tables and rules 
rather than handling that yourself:
http://shorewall.net/MultiISP.html

On 11/10/2014 2:11 PM, Frank Cox wrote:
> I have both cable and dsl service.  The router attached to the cable
service is 192.168.0.1 and the router attached to the dsl service is
192.168.0.254.  I determine which service my computers communicate through by
setting the gateway assignment to either of those addresses and it just works.
>
> I can connect to my computers from the outside by port forwarding from my
routers to the computer that I want to connect to.
>
> However, the outside ssh connection works only if the computer's
gateway assignment matches the router that I'm connecting to.  I suspect
that I have to set up some sort of routing assignment to tell the computer to
reply to the gateway that the ssh connection request came in on if it
doesn't match the current gateway assignment, but I have no idea how to do
that.
>
I also have two ISPs at home.  I have a Peplink to manage this now, but 
before that I some commands in rc.local.  I had two networks and two 
NICs on my workstation.  This used to work on CentOS 6.

##############################
# symetric routing           #
##############################

NIC1=eth0
IP1=192.168.1.6
GW1=192.168.1.1
NET1=192.168.1.0/24

NIC2=eth1
IP2=192.168.2.6
GW2=192.168.2.1
NET2=192.168.2.0/24

DEFGW=$GW2

cat << EOF > /etc/iproute2/rt_tables
#
# reserved values
#
255     local
254     main
253     default
0       unspec
#
# local
#
#1      inr.ruhep

# symetric routing
101     T1
102     T2
EOF

# routing and default gateway for each interface
ip route add $NET1 dev $NIC1 src $IP1 table T1
ip route add default via $GW1 table T1
ip route add $NET2 dev $NIC2 src $IP2 table T2
ip route add default via $GW2 table T2

# regular routes
ip route add $NET1 dev $NIC1 src $IP1
ip route add $NET2 dev $NIC2 src $IP2

# preference for default route
route delete default
ip route add default via $DEFGW

# rules
ip rule add from $IP1 table T1
ip rule add from $IP2 table T2

# enable routing
echo 1 > /proc/sys/net/ipv4/ip_forward

Good luck,
Jack