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Multiple ISPs

Has anyone of you set up a NAT server connecting to more than one ISP?

We are switching isp's and I wanted to avoid downtime by using both networks
simultaneously for some time.

My preliminary findings:

I have two cards connected to cable modems and have read the brillant
Linux Advanced Routing and Traffic Control Howto, which explains how to
set up a linux machine to connect to more than one ISP, however, it
starts from scratch using the iproute package. I have handled the NAT
of my internal network with the ipmasq debian package and I like not
having to write every rule by hand (especially when a ppp link comes
up, which is the case for me with vpn connections). I'm not worried about load balancing or anything like that yet, I would only like to be visible on both networks and route all masqeraded traffic through one of the links.

So what do you suggest, should I
1) Write everything by hand, not using ipmasq,
2) Extend ipmasq's rules somehow to fire the routing tables up correctly
3) Just run a script after ipmasq to introduce

For your information, that's how far I came:

root# ip route show
xx.yy.zz.16/29 dev eth2  proto kernel  scope link  src xx.yy.zz.17
aa.bb.cc.160/28 dev eth0  proto kernel  scope link  src aa.bb.cc.162 dev eth1  proto kernel  scope link  src
default via aa.bb.cc.161 dev eth0

I tried introducing a second default route:

default via xx.yy.zz.22 dev eth2  src xx.yy.zz.17

The second default route is never reached, so my idea of avoiding
routing tables using a src for the gateway didn't work. Swapping the
default routes gets eth2 pinging but eth0 switches off. I can see the
pings on the card without a default gateway attached coming in (with
tcpdump) but the kernel seems to reply the ping on the default gateway.

root# grep eth0 /etc/ipmasq/rules/A01precompute.rul
export EXTERNAL="eth0 eth2"

[blue:~] root# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:04:75:AD:6B:C9
inet addr:aa.bb.cc.162 Bcast: aa.bb.cc.175 Mask:
          RX packets:3249800 errors:0 dropped:0 overruns:0 frame:0
          TX packets:3410039 errors:0 dropped:0 overruns:0 carrier:42
          RX bytes:1235492433 (1.1 GiB)  TX bytes:1549390655 (1.4 GiB)

eth1      Link encap:Ethernet  HWaddr 00:04:76:0E:C1:1D
          inet addr:  Bcast:  Mask:
          EtherTalk Phase 2 addr:65280/113
          RX packets:5240114 errors:0 dropped:0 overruns:1 frame:0
          TX packets:4930194 errors:0 dropped:0 overruns:0 carrier:1579
          RX bytes:2028758870 (1.8 GiB)  TX bytes:1876902432 (1.7 GiB)

eth2      Link encap:Ethernet  HWaddr 00:D0:B7:79:51:9F
inet addr: xx.yy.zz.17 Bcast: xx.yy.zz.23 Mask:
          RX packets:11565 errors:0 dropped:0 overruns:0 frame:0
          TX packets:3579 errors:0 dropped:0 overruns:0 carrier:0
          RX bytes:3456804 (3.2 MiB)  TX bytes:458136 (447.3 KiB)

lo        Link encap:Local Loopback
          inet addr:  Mask:
          EtherTalk Phase 2 addr:0/0
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:1083536 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1083536 errors:0 dropped:0 overruns:0 carrier:0
          RX bytes:139937541 (133.4 MiB)  TX bytes:139937541 (133.4 MiB)

Thank you for your help.

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