Disable Kernel Parameter for Sending ICMP Redirects on all IPv4 Interfaces
Forces net.ipv4.conf.all.send_redirects to 0 so the host never emits ICMP redirect messages on any IPv4 interface.
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
ICMP redirect messages tell other hosts that a more direct route exists for a destination. By emitting them, a host leaks portions of its routing table, revealing network topology, and offers an attacker a primitive to influence neighbours' routing. Only systems acting as routers have a legitimate reason to send redirects; an ordinary host should keep this at 0.
What Pavois checks
Pavois reads the live runtime value of net.ipv4.conf.all.send_redirects from the kernel (InSpec kernel_parameter, equivalent to sysctl), not a /etc/sysctl.d/ file. A drop-in could say 0 while a later file or a runtime sysctl -w re-enabled sending. Auditing the effective parameter shows whether the kernel will actually emit redirects; reading a file misses such overrides.
describe kernel_parameter('net.ipv4.conf.all.send_redirects') do
its('value') { should cmp 0 }
end
describe command("grep -hsE '^[[:space:]]*net.ipv4.conf.all.send_redirects[[: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.all.send_redirects. Expected output:
net.ipv4.conf.all.send_redirects = 0
Inspect & investigate
There is no dedicated event log for sending redirects. Inspect the state with sysctl net.ipv4.conf.all.send_redirects and review all per-interface values with sysctl -a | grep send_redirects. To observe whether redirects are emitted on the wire, capture with tcpdump -ni any icmp[icmptype] == icmp-redirect.
Remediation
Pavois's harden plan uses the sysctl resource to set net.ipv4.conf.all.send_redirects 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.all.send_redirects |
|---|---|
| resource | sysctl |
| value | 0 |
pavois harden plan localwhere the target is local, a user@host SSH alias, or a container , Docs
Impact & precautions
Safe on any host that is not a router. The only systems that should keep send_redirects enabled are deliberate IPv4 routers that rely on ICMP redirects to off-load suboptimal first-hop choices onto neighbours, an uncommon design. Disabling it does not affect a host's ability to receive traffic or remote access; it only stops the host advising others about better routes.
Standards mapping
| Standard | Reference | Type | Version | Confidence |
|---|---|---|---|---|
| ANSSI BP-028 | R12 | direct | 2.0 | high |
| CIS | 1.4.5, 3.3.2, 3.3.1.4 | direct | per OS, see the benchmark table | high |
| NIST | 3.1.20, CM-6(a), CM-7(a), CM-7(b), SC-5, SC-7(a) | supporting | 800-53 Rev 5 · 800-171 Rev 2 (pinned) | medium |
| PCI DSS | 1.4.5 | supporting | 4.0.1 | 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.