Disable Kernel Parameter for Accepting Source-Routed Packets on IPv4 Interfaces by Default
Forces net.ipv4.conf.default.accept_source_route to 0 so every newly created IPv4 interface refuses source-routed packets by default.
Checked against the resolved running state (e.g. sshd -T, sysctl, systemctl show), catches drop-ins and Includes a file read would miss. Caveat: runtime ≠ persistence; a value correct now may not survive a reboot.
Pavois asserts the effective configuration, the live, resolved state, not a file. File-based scanners (OVAL/SCAP, Lynis) miss Includes, drop-ins and runtime defaults; this check sees what is actually applied.
A mapping is a cross-reference to where each standard places this requirement, anchored and cross-validated, not a claim of equivalence. A passing check is evidence toward these references, how to read it.
Why this rule matters
Source-routed packets carry an explicit path chosen by the sender, instructing routers to forward them along a route other than the one the routing table would pick. Attackers use this to bypass firewalls and routing-based controls, reach otherwise unreachable hosts, and spoof return paths. Legitimate uses are essentially limited to deliberate routers; an ordinary host should set the default template to 0 so every new interface refuses source-routed packets.
What Pavois checks
Pavois reads the live runtime value of net.ipv4.conf.default.accept_source_route from the kernel (InSpec kernel_parameter, equivalent to sysctl), not a /etc/sysctl.d/ file. The default key sets the template for interfaces created later, so only the running value reflects the posture future interfaces will inherit. A drop-in might declare 0 while a later file or sysctl -w overrode it; the effective parameter is authoritative.
describe kernel_parameter('net.ipv4.conf.default.accept_source_route') do
its('value') { should cmp 0 }
end
describe command("grep -hsE '^[[:space:]]*net.ipv4.conf.default.accept_source_route[[:space:]]*=[[:space:]]*0([[:space:]]|$)' /etc/sysctl.conf /etc/sysctl.d/*.conf /run/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /lib/sysctl.d/*.conf 2>/dev/null") do
its('stdout') { should match(/\S/) }
endHow to verify it is applied
Run sysctl net.ipv4.conf.default.accept_source_route. Expected output:
net.ipv4.conf.default.accept_source_route = 0
Inspect & investigate
No dedicated log exists. With log_martians enabled, source-routed packets that are dropped surface via dmesg | grep -i martian and journalctl -k. Inspect the state with sysctl net.ipv4.conf.default.accept_source_route and all related values with sysctl -a | grep accept_source_route.
Remediation
Pavois's harden plan uses the sysctl resource to set net.ipv4.conf.default.accept_source_route to 0, applying it to the running kernel and persisting it in a Pavois-managed drop-in under /etc/sysctl.d/ so it survives reboot. Apply it with pavois harden apply.
Pavois applies this with its own harden engine, the plan below, not a shell script:
| key | net.ipv4.conf.default.accept_source_route |
|---|---|
| resource | sysctl |
| value | 0 |
pavois harden plan localwhere the target is local, a user@host SSH alias, or a container , Docs
Impact & precautions
Safe for essentially all hosts; source routing is obsolete and almost universally undesirable. The only legitimate exception is a system functioning as a router with IPv4 forwarding enabled that must honour source-routed packets, a very rare requirement. This is the default template; pair it with the all.accept_source_route rule for full coverage. No risk of losing remote access from setting this to 0.
Standards mapping
| Standard | Reference | Type | Version | Confidence |
|---|---|---|---|---|
| ANSSI BP-028 | R12 | direct | 2.0 | high |
| CIS | 3.3.8, 3.3.1.15 | direct | per OS, see the benchmark table | high |
| NIST | 3.1.20, CM-7(a), CM-7(b), SC-5, SC-7(a) | supporting | 800-53 Rev 5 · 800-171 Rev 2 (pinned) | medium |
Each reference is a cross-reference anchored in the upstream benchmark and cross-validated against the SCAP Security Guide and ansible-lockdown, not a claim of equivalence. Direct = a prescriptive, line-level requirement; supporting = an abstract control family (NIST) the check provides evidence toward. How to read a mapping.