Whenever we route a packet from the Client to a DNS resource, we now
also capture the domain name. If this is the first packet and we are
thus creating a new flow, we'll save that domain in it. Later packets
for the same IP are rolled up under the same flow and thus don't need to
re-set the domain.
Resolves: #10691
Network flow logs are a common feature of VPNs. Due to the nature of a
shared exit node, it is of great interest to a network analyst, which
TCP connections are getting routed through the tunnel, who is initiating
them, for long do they last and how much traffic is sent across them.
With this PR, the Firezone Gateway gains the ability of detecting the
TCP and UDP flows that are being routed through it. The information we
want to attach to these flows is spread out over several layers of the
packet handling code. To simplify the implementation and not complicate
the APIs unnecessarily, we chose to rely on TLS (thread-local storage)
for gathering all the necessary data as a packet gets passed through the
various layers. When using a const initializer, the overhead of a TLS
variable over an actual local variable is basically zero. The entire
routing state of the Gateway is also never sent across any threads,
making TLS variables a particularly good choice for this problem.
In its MVP form, the detected flows are only emitted on stdout and also
that only if `flow_logs=trace` is set using `RUST_LOG`. Early adopters
of this feature are encouraged to enable these logs as described and
then ingest the Gateway's logs into the SIEM of their choice for further
analysis.
Related: #8353
Currently, the eBPF module can translate from channel data messages to
UDP packets and vice versa. It can even do that across IP stacks, i.e.
translate from an IPv6 UDP packet to an IPv4 channel data messages.
What it cannot do is handle packets to itself. This can happen if both -
Client and Gateway - pick the same relay to make an allocation. When
exchanging candidates, ICE will then form pairs between both relay
candidates, essentially requiring the relay to loop packets back to
itself.
In eBPF, we cannot do that. When sending a packet back out with
`XDP_TX`, it will actually go out on the wire without an additional
check whether they are for our own IP.
Properly handling this in eBPF (by comparing the destination IP to our
public IP) adds more cases we need to handle. The current module
structure where everything is one file makes this quite hard to
understand, which is why I opted to create four sub-modules:
- `from_ipv4_channel`
- `from_ipv4_udp`
- `from_ipv6_channel`
- `from_ipv6_udp`
For traffic arriving via a data-channel, it is possible that we also
need to send it back out via a data-channel if the peer address we are
sending to is the relay itself. Therefore, the `from_ipX_channel`
modules have four sub-modules:
- `to_ipv4_channel`
- `to_ipv4_udp`
- `to_ipv6_channel`
- `to_ipv6_udp`
For the traffic arriving on an allocation port (`from_ipX_udp`), we
always map to a data-channel and therefore can never get into a routing
loop, resulting in only two modules:
- `to_ipv4_channel`
- `to_ipv6_channel`
The actual implementation of the new code paths is rather simple and
mostly copied from the existing ones. For half of them, we don't need to
make any adjustments to the buffer size (i.e. IPv4 channel to IPv4
channel). For the other half, we need to adjust for the difference in
the IP header size.
To test these changes, we add a new integration test that makes use of
the new docker-compose setup added in #10301 and configures masquerading
for both Client and Gateway. To make this more useful, we also remove
the `direct-` prefix from all tests as the test script itself no longer
makes any decisions as to whether it is operating over a direct or
relayed connection.
Resolves: #7518
