Inspirated

I needed to block some flows on OpenWRT from the Bro IDS. One option was to install the recent module for expiring iptables rules which sounded like an overkill. After some tinkering around I landed on using bash and at to expire the firewall rules after timeouts (luckily the at daemon was available on OpenWRT which made my job easier).

There are three parts to the process:

The bash script

First, a script which:

1. Constructs and adds the iptables rule to the FORWARD chain.
2. Constructs the corresponding deletion rule.
3. Creates a temporary bash script, writes the rule to it, makes the new script self-deleting.
4. Schedules a launch of the temporary script with at command.

Here’s the script:

#!/bin/sh
 
if [ $# -le 5 ] ; then
  echo "usage: $0 proto src sport dst dport timeout"
  exit 1
fi
 
proto=$1
src=$2
sport=$3
dest=$4
dport=$5
timeout=$6
 
echo "  proto: $1"
echo "    src: $2"
echo "  sport: $3"
echo "   dest: $4"
echo "  dport: $5"
echo "timeout: $6"
 
rule=""
 
if [ "$proto" != "any" ]; then
  rule="$rule --protocol $proto"
fi
 
if [ "$src" != "0.0.0.0" ]; then
  rule="$rule --source $src"
fi
 
if [ "$sport" != "0" ]; then
  rule="$rule --sport $sport"
fi
 
if [ "$dest" != "0.0.0.0" ]; then
  rule="$rule --destination $dest"
fi
 
if [ "$dport" != "0" ]; then
  rule="$rule --dport $dport"
fi
 
rule="$rule -j DROP"
 
echo "rule: $rule"
 
addcmd="iptables -I FORWARD $rule"
delcmd="iptables -D FORWARD $rule"
 
delscript=`mktemp`
echo "delscript: $delscript"
 
echo "#!/bin/sh" >>$delscript
echo $delcmd >>$delscript
echo "rm \"${delscript}\"" >>$delscript
chmod 755 $delscript
 
echo "adding iptable rule:"
echo $addcmd
`$addcmd`
 
atcmd="at -M -f $delscript now + $timeout min"
echo "creating at job for deletion:"
echo $atcmd
`$atcmd`

Given below is an example run. First, let’s print the default FORWARD chain:

# iptables -nL FORWARD

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             10.42.0.0/24         state RELATED,ESTABLISHED
ACCEPT     all  --  10.42.0.0/24         anywhere            
ACCEPT     all  --  anywhere             anywhere            
REJECT     all  --  anywhere             anywhere             reject-with icmp-port-unreachable
REJECT     all  --  anywhere             anywhere             reject-with icmp-port-unreachable
REJECT     all  --  anywhere             anywhere             reject-with icmp-host-prohibited

Block a flow for 2 minutes:

# sh blockflow.sh tcp 50.50.50.50 50 60.60.60.60 60 2

  proto: tcp
    src: 50.50.50.50
  sport: 50
   dest: 60.60.60.60
  dport: 60
timeout: 2
rule:  --protocol tcp --source 50.50.50.50 --sport 50 --destination 60.60.60.60 --dport 60 -j DROP
delscript: /tmp/tmp.SAREJvtsK0
adding iptable rule:
iptables -I FORWARD --protocol tcp --source 50.50.50.50 --sport 50 --destination 60.60.60.60 --dport 60 -j DROP
creating at job for deletion:
at -M -f /tmp/tmp.SAREJvtsK0 now + 2 min
job 79 at Sun Jun 30 14:37:00 2013

Let’s check if the new rule was added:

# iptables -nL FORWARD

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         
DROP       tcp  --  50.50.50.50          60.60.60.60          tcp spt:50 dpt:60
ACCEPT     all  --  anywhere             10.42.0.0/24         state RELATED,ESTABLISHED
ACCEPT     all  --  10.42.0.0/24         anywhere            
ACCEPT     all  --  anywhere             anywhere            
REJECT     all  --  anywhere             anywhere             reject-with icmp-port-unreachable
REJECT     all  --  anywhere             anywhere             reject-with icmp-port-unreachable
REJECT     all  --  anywhere             anywhere             reject-with icmp-host-prohibited

After 2 minutes, the temporary bash script shall remove the rule and then delete itself. To confirm:

# iptables -nL FORWARD

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             10.42.0.0/24         state RELATED,ESTABLISHED
ACCEPT     all  --  10.42.0.0/24         anywhere            
ACCEPT     all  --  anywhere             anywhere            
REJECT     all  --  anywhere             anywhere             reject-with icmp-port-unreachable
REJECT     all  --  anywhere             anywhere             reject-with icmp-port-unreachable
REJECT     all  --  anywhere             anywhere             reject-with icmp-host-prohibited

The Bro module

A simple module which exports just one function, i.e., BlockFlow::block which takes a conn_id and a count and calls the bash script with appropriate parameters:

module BlockFlow;
 
export {
  global block: function(id: conn_id, t: count);
}
 
function block(id: conn_id, t: count)
{
  print fmt("blocking %s:%d -> %s:%d for %d minutes", id$orig_h, id$orig_p, id$resp_h, id$resp_p, t);
 
  local protocol = get_port_transport_proto(id$resp_p);
  print fmt("protocol is: %s", protocol);
 
  local cmd: string = fmt("sh blockflow.sh %s %s %d %s %d %d", protocol
                                                             , id$orig_h, id$orig_p
                                                             , id$resp_h, id$resp_p, t);
  print fmt("executing: %s", cmd);
  system(cmd);
}

Bro module usage

And finally, using the module from a Bro script:

@load ./blockflow
 
event bro_init()
  {
    local id: conn_id;
    id$orig_h = 10.10.10.10;
    id$orig_p = 10/tcp;
    id$resp_h = 20.20.20.20;
    id$resp_p = 20/tcp;
    BlockFlow::block(id, 2);
  }

And the flow will be blocked for 2 minutes. Unfortunately, due to the way at command works the granularity of timeouts is limited to minutes. If you really want to block flows for only a few seconds a quick solution would be to use sleep in place of at before expiring the rule.