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3.3.2.6 Ensure net.ipv6.conf.default.accept_source_route is configured

Information

The accept_source_route option causes network interfaces to accept packets with the Strict Source Route (SSR) or Loose Source Routing (LSR) option set.

net.ipv6.conf.default.accept_source_route controls accepting of all IPv6 packets with the SSR or LSR option set on newly added network interfaces.

In networking, source routing allows a sender to partially or fully specify the route packets take through a network. In contrast, non-source routed packets travel a path determined by routers in the network. In some cases, systems may not be routable or reachable from some locations (e.g. private addresses vs. Internet routable), and so source routed packets would need to be used.

Note: If IPv6 has been disabled this recommendation is not applicable.

Setting net.ipv6.conf.default.accept_source_route to 0 disables the system from accepting source routed packets. Assume this system was capable of routing packets to Internet routable addresses on one interface and private addresses on another interface. Assume that the private addresses were not routable to the Internet routable addresses and vice versa. Under normal routing circumstances, an attacker from the Internet routable addresses could not use the system as a way to reach the private address systems. If, however, source routed packets were allowed, they could be used to gain access to the private address systems as the route could be specified, rather than rely on routing protocols that did not allow this routing.

Solution

- Review all files being used by systemd sysctl and comment out or remove all net.ipv6.conf.default.accept_source_route lines that are not net.ipv6.conf.default.accept_source_route=0.

Example script:

#!/usr/bin/env bash

{
 l_option="net.ipv6.conf.default.accept_source_route" l_value="0"
 l_grep="${l_option//./(\\.|\\/)}" a_files=()
 l_systemdsysctl="$(readlink -e /lib/systemd/systemd-sysctl \
 || readlink -e /usr/lib/systemd/systemd-sysctl)"
 l_ufw_file="$([ -f /etc/default/ufw ] && \
 awk -F= '/^\s*IPT_SYSCTL=/ {print $2}' /etc/default/ufw)"
 [ -f "$(readlink -e "$l_ufw_file")" ] && \
 a_files+=("$l_ufw_file"); a_files+=("/etc/sysctl.conf")
 while IFS= read -r l_fname; do
 l_file="$(readlink -e "${l_fname//# /}")"
 [ -n "$l_file" ] && ! grep -Psiq -- '(^|\h+)'"$l_file"'\b' \
 <<< "${a_files[*]}" && a_files+=("$l_file")
 done < <("$l_systemdsysctl" --cat-config | tac | \
 grep -Pio -- '^\h*#\h*\/[^#\n\r\h]+\.conf\b')
 for l_file in "${a_files[@]}"; do
 grep -Poi -- '\h*'"$l_grep"'\h*=\h*\H+\b' "$l_file" \
 | grep -Pivq -- '^\h*'"$l_grep"'\h*=\h*'"$l_value"'\b' && \
 sed -ri '/^\s*'"$l_grep"'\s*=\s*(1[0-9]*)/s/^/# /' "$l_file"
 done
}
 - Create or edit a file in the /etc/sysctl.d/ directory ending in .conf and edit or add the following line:

net.ipv6.conf.default.accept_source_route = 0

Example:

# [ ! -d "/etc/sysctl.d/" ] && mkdir -p /etc/sysctl.d/
# printf '%s\n' "" "net.ipv6.conf.default.accept_source_route = 0" \
>> /etc/sysctl.d/60-ipv6_sysctl.conf

Note: If the UFW file was the first file listed in the audit, the entry will be commented out as part of the first step, however updating Uncomplicated Firewall (UFW) may update this change. In this case the updated entry will supersede the entry being created as part of this step.

 - Run the following command to load all sysctl configuration filles:

# sysctl --system

See Also

https://workbench.cisecurity.org/benchmarks/24008

Item Details

Category: CONFIGURATION MANAGEMENT

References: 800-53|CM-6b., CSCv7|9.2

Plugin: Unix

Control ID: 2696124aac004a42899e1c8278e404faa1b85c02ea296fdc42298d1d72803d0f