These are examples for my blogpost about custom BPF firewalls for systemd services that I implemented in this commit. You can find more explanations and examples on how to use it when reading the blog post.

The port-firewall folder contains a small configurable packet filter that parses IP/IPv6 packets, ICMP, UDP ports, and TCP ports. The forward rule is a C expression passed as FILTER variable to the compiler with -D.

The expression can use the boolean variables udp, tcp, icmp, ip, and ipv6 denoting the packet type and the the integers dst_port and src_port for the UDP/TCP ports. If the expression evaluates to 0 (false), the packet will be dropped. Valid filters examples are FILTER='icmp || (udp && dst_port == 53) || (tcp && dst_port == 80)' or FILTER='!udp || dst_port == 53'.

The makefile requires to pass the filter to build the program: make FILTER='…'. With make load the bytecode is loaded to /sys/fs/bpf/port-firewall as pinned BPF program in the special BPF filesystem.

From there you can use it with the systemd options IP(Ingress|Egress)FilterPath= or attach it manually to a cgroup.

The folder also includes a bpf-make.service systemd unit file to configure and load the firewall and an example my-filtered-ping.service file that uses the loaded firewall. It includes an workaround you can use to not require systemd v243.

Instead of making a custom loader unit for every service you can also use the systemd unit template bpf-firewall@.service by as done in the service-with-filter@.service file. This is also a unit template used through systemctl start "service-with-filter@icmp || (udp && dst_port == 53).service" but it can also be a regular service without the @. The disadvantage is that you have to specify the filter more than once it the file but the advantage is that it's you don't use templates unnecessary and don't have the filter as (ugly) part of the service name. Ideally I would store the filter in a BPF map so that it can be set after BPF program loading, allowing it to be a simple ExecStartPre line in the service file.

The next section shows how to load and use a simple dropping filter as template for your own filters if you don't want to use this one.

In the bpf-program-only folder is a minimal BPF cgroup filter dropping all packets. You can build it with make and then run make load to /sys/fs/bpf/cgroup-sock-drop. This will load it with bpftool to /sys/fs/bpf/cgroup-sock-drop.

You can use this to specify an IPIngressFilterPath or IPEgressFilterPath for systemd services (>= 243). Here an example with ping running in a temporary systemd system scope (or service) with an ingress filter but no egress filter. You can also use user scopes without sudo by passing --user.

$ sudo systemd-run -p IPIngressFilterPath=/sys/fs/bpf/cgroup-sock-drop --scope ping 127.0.0.1
Running scope as unit: run-re62ba1c….scope
PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
^C # cancel since it will not get responses
--- 127.0.0.1 ping statistics ---
8 packets transmitted, 0 received, 100% packet loss, time 186ms

You can also find the example service my-ping.service configured with a filter that is loaded from its dependency service cgroup-sock-drop-filter.service (please note the LimitMEMLOCK=infinity entry in the unit that load the filter). When you want to use it for your service to start at boot you need to add, e.g., After=network.target and an [Install] section with WantedBy=multi-user.target.

Until systemd 243 is released you can also try the interactive MTU filter program from the next section below which has an option to attach the filter to a cgroup.

Use systemd-run to spawn a shell in a new cgroup either as system scope or user scope (a temporary service). -S can be replaced with a concrete binary if you don't want to start a shell.

$ sudo systemd-run --scope -S
$ # or:
$ systemd-run --user --scope -S
Running scope as unit: run-r63de6b74621b4ae3877d4fa86b54be75.scope

This will print out the unit name which is also the name of the cgroup. The full cgroup path for the system service shell is /sys/fs/cgroup/unified/system.slice/NAME. For the user service shell the path is /sys/fs/cgroup/unified/user.slice/user-1000.slice/user@1000.service/NAME depening on your UID not being 1000.

Then attach the BPF program to the cgroup:

$ sudo $(which bpftool) cgroup attach /sys/fs/cgroup/unified/user.slice/user-1000.slice/user@1000.service/run-rfaa93ac79de2482d8ef1870fd6b508cd.scope egress pinned /sys/fs/bpf/cgroup-sock-drop multi

You can either choose ingress or egress to filter incoming or outgoing packets. You can load the same filter for both ingress and egrees and you can load multiple different filters per ingress/egress (which also true when used through the systemd v243 option above). If you turn on IPAccounting in systemd-run you need to turn on Delegate as well to allow multiple BPF programs.

In the standalone folder is an interactive program that loads a BPF filter and controls its behavior.

From the source code comment: Loads a BPF cgroup ingress/egress filter bytecode that filters based on the packet size. It loads the BPF filter to a given location in /sys/fs/bpf/. Through the +/- keys the MTU can be changed interactively (changes values in the BPF map). Optionally the initial MTU value can be specified on startup. The program can also attach the BPF filter to a cgroup by specifying the cgroup by its path. The BPF filter stays loaded when the program exits and has to be deleted manually.

It does not use a BPF compiler but uses hardcoded BPF assembly instructions to include the BPF code in the final program. Not very accessible for hacking but for me it was interesting to see how BPF instructions work and what needs to be done to comply with the verifier.

In one terminal you can run the interactive filter:

$ sudo ./load_and_control_filter -m 100 -t ingress /sys/fs/bpf/ingressfilter
cgroup dropped 0 packets, forwarded 0 packets, MTU is 100 bytes (Press +/- to change)
… # keeps running

It loaded the BPF filter to /sys/fs/bpf/ingressfilter which you can use for a systemd service.

In another terminal you can, for example, again run ping as root in a temporary systemd scope and specify our filter as IPIngressFilterPath:

$ sudo systemd-run -p IPIngressFilterPath=/sys/fs/bpf/ingressfilter --scope ping 127.0.0.1
Running scope as unit: run-….scope
PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.086 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.069 ms
…

When you switch back to the first terminal and press -, the new MTU is 50 bytes and you can see the dropped packet count increase. In the ping terminal you will see no new responses because they are all dropped.

The load_and_control_filter program can be told to attach the filter to a cgroup of a systemd service.

Systemd uses a BPF filter for its IP accounting and firewalling based on IP addresses. If such a filter is present but no others, the flag to allow multiple BPF filters for a cgroup is missing. As workaround when, e.g., IP accounting is enabled, you can tell systemd that the cgroup management is done by externally. This means that systemd will use the flag to allow multiple BPF filters instead of loading the IP accounting BPF filter without this flag.

$ sudo systemd-run -p IPAccounting=yes -p Delegate=yes --scope ping 127.0.0.1
Running scope as unit: run-r9f31b3947f4c4a11a24babf5517fe025.scope
PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.086 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.069 ms
…

You can see the scope name in the first output line. This is also the last part of the cgroup path you have to use as argument in order to attach the filter to the cgroup.

$ sudo ./load_and_control_filter -m 100 -c /sys/fs/cgroup/unified/system.slice/run-r9f31b3947f4c4a11a24babf5517fe025.scope -t ingress /sys/fs/bpf/myfilter
cgroup dropped 0 packets, forwarded 4 packets, MTU is 100 bytes (Press +/- to change)
… # keeps running and increases the forward count

Now hit - to reduce the MTU and observe the packet drop count increasing while no ping responses can be seen.