Proxmox LAN Routing

proxmoxrouting

I am attempting to set up my first Proxmox hypervisor as a test server. I have managed to install Proxmox and set up some VMs however I can't get machines on my local LAN to connect to the Proxmox VMs.

Host Subnet: 10.11.11.0/24
Proxmox Host: 10.11.11.161

I have created two bridges one to act as a bridge to the WAN and the other as a private network.

Proxmox Host

auto lo
iface lo inet loopback

iface enp0s25 inet manual

auto vmbr0
iface vmbr0 inet static
    address  10.11.11.161
    netmask  255.255.255.0
    gateway  10.11.11.1
    bridge_ports enp0s25
    bridge_stp off
    bridge_fd 0

auto vmbr1
iface vmbr1 inet static
    address  10.11.12.1
    netmask  255.255.255.0
    bridge_ports none
    bridge_stp off
    bridge_fd 0

    post-up echo 1 > /proc/sys/net/ipv4/conf/vmbr0/proxy_arp
    post-up   iptables -t nat -A POSTROUTING -s '10.11.12.0/24' -o vmbr0 -j MASQUERADE
    post-down iptables -t nat -D POSTROUTING -s '10.11.12.0/24' -o vmbr0 -j MASQUERADE

Connecting to VMs from Office LAN

The VMs are set up to use network vmbr1 from the Proxmox host, I can connect into the VMs ok.

However from my machine on the office lan 10.11.11.141 I can't see the VMs. I have tried adding the following static route to pass traffic for 10.11.12.0/24 subnet to the Proxmox host.

ip route add 10.11.12.0/24 via 10.11.11.161 dev eth0

Can anyone please advice on how I can connect the office LAN to the VLAN of Proxmox.

Thanks in advance.

Best Answer

If you need direct networking access from different IP network a router (gateway) should be used by definition. The gateway should belong to both IP networks and you should be using it by means of routing table. All-in-all it can be even a VM that's run on same server or the server itself.

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You should only have one default gateway. If you remove the gateway line from eth1, it'll all just work (after networking is restarted).

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There are two problems with this setup:

The hosts on LAN1 know nothing about the LAN2 segment. When you ping a host on LAN1 (let's call it host1) from SRV-02, the packet will be routed through SRV-01 and will reach host1. However, host1 will send the reply to it's default gateway (ISP router) as it doesn't have a specific route to LAN2. (The ISP router will either a) also send it to it's default gateway as it also doesn't know about LAN2, or b) drop the packet as it comes from an unknown source not it's local LAN.)
When trying to reach WAN from LAN2, the packets will be routed through SRV-02 to ISP router where two situations are possible:
- The router will not NAT translate the packet as the source of the packet (LAN2) is not it's local LAN (this is the more probable situation), or
- The router will NAT translate the packet and send it to the Internet. However, when the reply comes and the destination is translated back to the LAN2 address, the packet will not be delivered as the ISP router doesn't have a route for that network. The packet will be sent incorrectly to the default gateway (ISP).

These issues could be fixed by adding a static route to LAN2 to ISP router and adding a source NAT configuration for LAN2 on SRV-01. However, that is not possible due to no admin access to the ISP router.

There are two solutions that get around it:

A. Make SRV-01 a full router for LAN1 and LAN2 hosts

Add another network adapter to SRV-01 (making it 3 in total)
Change the topology as follows:

WAN -> ISP router -> LAN1 -> SRV-01 +-> LAN3 (for hosts originally in LAN1)
                                    +-> LAN2 -> SRV-02

Basically, we're making SRV-01 a router for both LAN segments.

This will require moving hosts originally in LAN1 to a new subnet LAN3 - let's say we use 10.0.1.0/24
The network configuration of SRV-01 will need to be changed as follows:

/etc/network/interfaces:

# LAN1 - to ISP router
auto eth0
iface eth0 inet dhcp
# we can even use dhcp as the IP address is not really important
# - there are no more hosts on LAN1 apart from ISP router and SRV-01

# LAN3 - for hosts originally in LAN1
iface eth1
    address 10.0.1.1
    netmask 255.255.255.0

# LAN2
iface eth2
    address 10.0.2.1
    netmask 255.255.255.0

iptables rules to make WAN access work:

iptables -t nat -A POSTROUTING -o eth0 -s 10.0.1.0/24 -j MASQUERADE
iptables -t nat -A POSTROUTING -o eth0 -s 10.0.2.0/24 -j MASQUERADE

Alternatively, if you choose to keep the static IP address on SRV-01 on eth0 the rules could be changed (although MASQUERADE would still work):

iptables -t nat -A POSTROUTING -o eth0 -s 10.0.1.0/24 -j SNAT --to-source 192.168.5.8
iptables -t nat -A POSTROUTING -o eth0 -s 10.0.2.0/24 -j SNAT --to-source 192.168.5.8

DHCP will need to be configured on SRV-01 on eth1 (LAN3, for hosts originally on LAN1), and possibly on eth2 (LAN2) as well if required. (In both cases the gateway will be the local address of eth1 or eth2 respectively, but that goes without saying :)

This will make communication possible between LAN3 and LAN2 (via SRV-01 which is the default gateway for both). WAN access will also work from both LAN3 and LAN2 thanks to the double source NAT.

B. Make SRV-01 a DHCP server for LAN1

This approach is not as clean as above but is slightly simpler. It assumes you are able to disable DHCP on ISP router

Disable DHCP on ISP router
Set up DHCP for LAN1 on SRV-01 and make SRV-01 (192.168.5.8) the default gateway for LAN1
Set up source NAT translation for LAN2 on SRV-01 so that the WAN access works from LAN2:

iptables -t nat -A POSTROUTING -o eth0 -s 10.0.2.0/24 -d 192.168.5.4 -j SNAT --to-source 192.168.5.8
iptables -t nat -A POSTROUTING -o eth0 -s 10.0.2.0/24 ! -d 192.168.5.0/24 -j SNAT --to-source 192.168.5.8

The first line enables SNAT so that LAN2 hosts can access the ISP router itself and the second line disables SNAT for LAN2-LAN1 access.

Again, this approach is not as clean as the one above as there are two routers in the same subnet (SRV-01, ISP router). When I used this approach myself I noticed my second router (SRV-01 in this scenario) would send ICMP redirects to the ISP router as it would see that the client (host on LAN1) and the upstream router (ISP router) are on the same LAN. This might not be desired as network policies implemented on SRV-01 could be circumvented.

Hope that helps.