Re: [PATCH 03/11] firewall: Log and drop spoofed loopback packets

[Michael Tremer <michael.tremer@ipfire.org>]

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Hello,

> On 18 Dec 2021, at 13:48, Peter Müller <peter.mueller(a)ipfire.org> wrote:
> 
> Traffic from and to 127.0.0.0/8 must only appear on the loopback
> interface, never on any other interface. This ensures offending packets
> are logged, and the loopback interface cannot be abused for processing
> traffic from and to any other networks.
> 
> Signed-off-by: Peter Müller <peter.mueller(a)ipfire.org>
> ---
> src/initscripts/system/firewall | 24 ++++++++++++++++++------
> 1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/src/initscripts/system/firewall b/src/initscripts/system/firewall
> index cc5baa292..1c62c6e2c 100644
> --- a/src/initscripts/system/firewall
> +++ b/src/initscripts/system/firewall
> @@ -80,6 +80,14 @@ iptables_init() {
> 	fi
> 	iptables -A NEWNOTSYN  -j DROP -m comment --comment "DROP_NEWNOTSYN"
> 
> +	# Log and subsequently drop spoofed packets or "martians", arriving from sources
> +	# on interfaces where we don't expect them
> +	iptables -N SPOOFED_MARTIAN
> +	if [ "$DROPSPOOFEDMARTIAN" == "on" ]; then

DROP? Shouldn’t the variable be called LOGSPOOFEDMARTIAN?

You will always drop any packets sent to this chain, but you won’t always log them.

Is this what you intended?

> +		iptables -A SPOOFED_MARTIAN  -m limit --limit 10/second -j LOG  --log-prefix "DROP_SPOOFED_MARTIAN "
> +	fi
> +	iptables -A SPOOFED_MARTIAN -j DROP -m comment --comment "DROP_SPOOFED_MARTIAN"
> +
> 	# Chain to contain all the rules relating to bad TCP flags
> 	iptables -N BADTCP
> 
> @@ -177,14 +185,18 @@ iptables_init() {
> 	iptables -A INPUT -j ICMPINPUT
> 	iptables -A ICMPINPUT -p icmp --icmp-type 8 -j ACCEPT
> 
> -	# Accept everything on loopback
> +	# Accept everything on loopback if source/destination is loopback space...
> 	iptables -N LOOPBACK
> -	iptables -A LOOPBACK -i lo -j ACCEPT
> -	iptables -A LOOPBACK -o lo -j ACCEPT
> +	iptables -A LOOPBACK -i lo -s 127.0.0.0/8 -j ACCEPT
> +	iptables -A LOOPBACK -o lo -d 127.0.0.0/8 -j ACCEPT
> +
> +	# ... and drop everything else on the loopback interface, since no other traffic should appear there
> +	iptables -A LOOPBACK -i lo -j SPOOFED_MARTIAN
> +	iptables -A LOOPBACK -o lo -j SPOOFED_MARTIAN
> 
> -	# Filter all packets with loopback addresses on non-loopback interfaces.
> -	iptables -A LOOPBACK -s 127.0.0.0/8 -j DROP
> -	iptables -A LOOPBACK -d 127.0.0.0/8 -j DROP
> +	# Filter all packets with loopback addresses on non-loopback interfaces (spoofed)
> +	iptables -A LOOPBACK -s 127.0.0.0/8 -j SPOOFED_MARTIAN
> +	iptables -A LOOPBACK -d 127.0.0.0/8 -j SPOOFED_MARTIAN
> 
> 	for i in INPUT FORWARD OUTPUT; do
> 		iptables -A ${i} -j LOOPBACK
> -- 
> 2.26.2