106 lines
3 KiB
Markdown
106 lines
3 KiB
Markdown
title: Blocking DHCP servers and router advertisements with nftables
|
|
---
|
|
pub_date: 2020-08-27
|
|
---
|
|
author: ungleich
|
|
---
|
|
twitter_handle: ungleich
|
|
---
|
|
_hidden: no
|
|
---
|
|
_discoverable: yes
|
|
---
|
|
abstract:
|
|
Blocking typical data center workloads with nftables
|
|
---
|
|
body:
|
|
|
|
## Motivation
|
|
|
|
Here at [ungleich](https://ungleich.ch) we are providing a variety of
|
|
hosting services in the [Data Center
|
|
Light](https://datacenterlight.ch). One of the workloads we offer is
|
|
VM hosting and we need to take some security measures to prevent one
|
|
customer abusing another customer.
|
|
|
|
## The problem
|
|
|
|
The virtual machines in our next generation uncloud hosting will be
|
|
using standard DHCP and IPv6 address assignments and not the
|
|
[OpenNebula](https://github.com/OpenNebula/addon-context-linux)
|
|
contextualisation scripts that read the networking information from an
|
|
attached ISO.
|
|
|
|
While this makes it easier to create VM images and VMs behave even
|
|
more like regular computers, this exposes the VMs to attacks where one
|
|
customer runs a DHCP server or IPv6 router advertisement daemon and
|
|
tricks the other VMs into sending traffic to it.
|
|
|
|
## The architecture
|
|
|
|
VMs are connected to a single shared network in which they get
|
|
their IP addresses (in uncloud usually only IPv6) and then they can
|
|
retrieve more information from a metadata server. So the main
|
|
protection that is required is preventing to trick other customers
|
|
into using a wrong IP address or route.
|
|
|
|
|
|
## Fixing it
|
|
|
|
So the easiest thing to do is to disallow IPv6 router advertisements
|
|
and IPv4 DHCP server answers. However as all the interfaces are put
|
|
into one bridge, we will need to filter on bridge and not ip level:
|
|
|
|
```
|
|
table bridge filter {
|
|
chain prerouting {
|
|
type filter hook prerouting priority 0;
|
|
policy accept;
|
|
|
|
|
|
}
|
|
```
|
|
|
|
Next we create a chain to drop the packets we dislike:
|
|
|
|
```
|
|
chain drop_ra_dhcp {
|
|
# Default blocks: router advertisements, dhcpv6, dhcpv4
|
|
icmpv6 type nd-router-advert drop
|
|
ip6 version 6 udp sport 547 drop
|
|
ip version 4 udp sport 67 drop
|
|
}
|
|
```
|
|
|
|
Now the only thing left is to correctly classify the traffic:
|
|
|
|
* Let's assume the bridge is named **br100**
|
|
* Let's assume the upstream interface that should allow RA/DHCP is
|
|
named **vxlan100**
|
|
|
|
Then we can connect the chains together:
|
|
|
|
```
|
|
table bridge filter {
|
|
chain prerouting {
|
|
type filter hook prerouting priority 0;
|
|
policy accept;
|
|
|
|
iifname != vxlan100 meta ibrname br100 jump drop_ra_dhcp
|
|
}
|
|
|
|
chain drop_ra_dhcp {
|
|
# Blocks: router advertisements, dhcpv6, dhcpv4
|
|
icmpv6 type nd-router-advert drop
|
|
ip6 version 6 udp sport 547 drop
|
|
ip version 4 udp sport 67 drop
|
|
}
|
|
}
|
|
```
|
|
|
|
This way we have a very simple filter to prevent router advertisements
|
|
or dhcp answers to come from customer VMs.
|
|
|
|
If something does not make sense,
|
|
you can ask on our [open chat](/u/projects/open-chat/) or
|
|
[consult the nftables reference](https://wiki.nftables.org/wiki-nftables/index.php/Quick_reference-nftables_in_10_minutes#Meta).
|