[PATCH] firewall: always allow outgoing DNS traffic to root servers

[ummeegge]

Hi all,


On Do, 2019-09-26 at 16:25 +0100, Michael Tremer wrote:
> Hi,
> 
> > On 25 Sep 2019, at 20:45, peter.mueller at ipfire.org wrote:
> > 
> > Allowing outgoing DNS traffic (destination port 53, both TCP
> > and UDP) to the root servers is BCP for some reasons. First,
> > RFC 5011 assumes resolvers are able to fetch new trust ancors
> > from the root servers for a certain time period in order to
> > do key rollovers.
> > 
> > Second, Unbound shows some side effects if it cannot do trust
> > anchor signaling (see RFC 8145) or fetch the current trust anchor,
> > resulting in SERVFAILs for arbitrary requests a few minutes.
> > 
> > There is little security implication of allowing DNS traffic
> > to the root servers: An attacker might abuse this for exfiltrating
> > data via DNS queries, but is unable to infiltrate data unless
> > he gains control over at least one root server instance. If
> > there is no firewall ruleset in place which prohibits any other
> > DNS traffic than to chosen DNS servers, this patch will not
> > have security implications at all.
> 
> I think we need to document this on the wiki before we merge this
> patch.
> 
> > Fixes #12183
> > 
> > Cc: Michael Tremer <michael.tremer at ipfire.org>
> > Suggested-by: Horace Michael <horace.michael at gmx.com>
> > Signed-off-by: Peter Müller <peter.mueller at ipfire.org>
> > ---
> > config/rootfiles/core/137/filelists/files |  1 +
> > src/initscripts/system/firewall           | 16 ++++++++++++++--
> > 2 files changed, 15 insertions(+), 2 deletions(-)
> > 
> > diff --git a/config/rootfiles/core/137/filelists/files
> > b/config/rootfiles/core/137/filelists/files
> > index ce4e51768..a02840d12 100644
> > --- a/config/rootfiles/core/137/filelists/files
> > +++ b/config/rootfiles/core/137/filelists/files
> > @@ -1,4 +1,5 @@
> > etc/system-release
> > etc/issue
> > +etc/rc.d/init.d/firewall
> > srv/web/ipfire/cgi-bin/credits.cgi
> > var/ipfire/langs
> > diff --git a/src/initscripts/system/firewall
> > b/src/initscripts/system/firewall
> > index ec396c708..ff63a2ede 100644
> > --- a/src/initscripts/system/firewall
> > +++ b/src/initscripts/system/firewall
> > @@ -6,10 +6,11 @@
> > eval $(/usr/local/bin/readhash /var/ipfire/ppp/settings)
> > eval $(/usr/local/bin/readhash /var/ipfire/ethernet/settings)
> > eval $(/usr/local/bin/readhash /var/ipfire/optionsfw/settings)
> > -IFACE=`/bin/cat /var/ipfire/red/iface 2> /dev/null | /usr/bin/tr
> > -d '\012'`
> > +ROOTHINTS="/etc/unbound/root.hints"
> > +IFACE=$( /bin/cat /var/ipfire/red/iface 2> /dev/null | /usr/bin/tr
> > -d '\012' )
> > 
> > if [ -f /var/ipfire/red/device ]; then
> > -	DEVICE=`/bin/cat /var/ipfire/red/device 2> /dev/null |
> > /usr/bin/tr -d '\012'`
> > +	DEVICE=$( /bin/cat /var/ipfire/red/device 2> /dev/null |
> > /usr/bin/tr -d '\012' )
> > fi
> 
> Why the added whitespace? Should have been an extra patch.
> 
> > 
> > function iptables() {
> > @@ -307,6 +308,17 @@ iptables_init() {
> > 	iptables -A INPUT -j TOR_INPUT
> > 	iptables -N TOR_OUTPUT
> > 	iptables -A OUTPUT -j TOR_OUTPUT
> > +
> > +	# Allow outgoing DNS traffic (TCP and UDP) to DNS root servers
> > +	ROOTSERVERIPS="$( awk '/\s+A\s+/ { print $4 }' ${ROOTHINTS} |
> > xargs )"
> > +	ipset -N root-servers iphash
> 
> ROOTSERVERIPS could have been an array and could have been local.
> 
> You do not need to call xargs. It is a rather expensive way to remove
> line breaks.
> 
> > +
> > +	for ip in ${ROOTSERVERIPS}; do
> > +		ipset add root-servers $ip
> > +	done
> 
> It is also interesting that ipset does not allow to add more IP
> addresses in one go. This looks like a very expensive loop for a lot
> of IP addresses. I think this is fine here for about a dozen
> addresses, but importing a blacklist of thousands or tens of
> thousands of IP addresses will take a long time.
there is the possiblity to speed this process significantly up via
'ipset restore' whereby the format from 'ipset save' can be used which
looks like this (if no counters has been set) -->

...
add ipset_setname 11.22.33.44 -exist
add ipset_setname 22.33.44.55 -exist
...

so if there is a vast list you can convert it e.g. via perl and pipe it
to 'ipset restore'.

Example:
IP list called 'vast_list' is formatted one per line

...
11.22.33.44
22.33.44.55
...

can be formatted and restored with a

perl -pe 'chomp; $_ = "add ipset_setname $_ -exist\n"' vast_list | ipset restore

Just as a little gimmick :-).


> 
> > +
> > +	iptables -A OUTPUT -m set --match-set root-servers dst -p tcp
> > --dport 53 -j ACCEPT
> > +	iptables -A OUTPUT -m set --match-set root-servers dst -p udp
> > --dport 53 -j ACCEPT
> > 	
> > 	# Jump into the actual firewall ruleset.
> > 	iptables -N INPUTFW
> > -- 
> > 2.16.4
> 
> -Michael
> 

Best,

Erik