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5. Miscellaneous.

This section contains all the information and FAQs that I couldn't fit inside the structure above.

5.1 How to Organize Your Firewall Rules

This question requires some thought. You can try to organize them to optimize speed (minimize the number of rule-checks for the most common packets) or to increase manageability.

If you have an intermittent link, say a PPP link, you might want to set the first rule in the input chain to be set to `-i ppp0 -j DENY' at boot time, then have something like this in your ip-up script:

# Re-create the `ppp-in' chain.
ipchains-restore -f < ppp-in.firewall

# Replace DENY rule with jump to ppp-handling chain.
ipchains -R input 1 -i ppp0 -j ppp-in

Your ip-down script would look like:

ipchains -R input 1 -i ppp0 -j DENY

5.2 What Not To Filter Out

There are some things you should be aware of before you start filtering out everything you don't want.

ICMP packets

ICMP packets are used (among other things) to indicate failure for other protocols (such as TCP and UDP). `destination-unreachable' packets in particular. Blocking these packets means that you will never get `Host unreachable' or `No route to host' errors; any connections will just wait for a reply that never comes. This is irritating, but rarely fatal.

A worse problem is the role of ICMP packets in MTU discovery. All good TCP implementations (Linux included) use MTU discovery to try to figure out what the largest packet that can get to a destination without being fragmented (fragmentation slows performance, especially when occasional fragments are lost). MTU discovery works by sending packets with the "Don't Fragment" bit set, and then sending smaller packets if it gets an ICMP packet indicating "Fragmentation needed but DF set" (`fragmentation-needed'). This is a type of `destination-unreachable' packet, and if it is never received, the local host will not reduce MTU, and performance will be abysmal or non-existent.

Note that it is common to block all ICMP redirect messages (type 5); these can be used to manipulate routing (although good IP stacks have safeguards), and so are often seen as slightly risky.

TCP Connections to DNS (nameservers)

If you're trying to block outgoing TCP connections, remember that DNS doesn't always use UDP; if the reply from the server exceeds 512 bytes, the client uses a TCP connection (still going to port number 53) to get the data.

This can be a trap because DNS will `mostly work' if you disallow such TCP transfers; you may experience strange long delays and other occasional DNS problems if you do.

If your DNS queries are always directed at the same external source (either directly by using the nameserver line in /etc/resolv.conf or by using a caching nameserver in forward mode), then you need only allow TCP connections to port domain on that nameserver from the local domain port (if using a caching nameserver) or from a high port (> 1023) if using /etc/resolv.conf.

FTP Nightmares

The classic packet filtering problem is FTP. FTP has two modes; the traditional one is called active mode and the more recent one is called passive mode. Web browsers usually default to passive mode, but command-line FTP programs usually default to active mode.

In active mode, when the remote end wants to send a file (or even the results of an ls or dir command) it tries to open a TCP connection to the local machine. This means you can't filter out these TCP connections without breaking active FTP.

If you have the option of using passive mode, then fine; passive mode makes data connections from client to server, even for incoming data. Otherwise, it is recommended that you only allow TCP connections to ports above 1024 and not between 6000 and 6010 (6000 is used for X-Windows).

5.3 Filtering out Ping of Death

Linux boxes are now immune to the famous Ping of Death, which involves sending an illegally-large ICMP packet which overflows buffers in the TCP stack on the receiver and causes havoc.

If you are protecting boxes which might be vulnerable, you could simply block ICMP fragments. Normal ICMP packets aren't large enough to require fragmentation, so you won't break anything except big pings. I have heard (unconfirmed) reports that some systems required only the last fragment of an oversize ICMP packet to corrupt them, so blocking only the first fragment is not recommended.

While the exploit programs I have seen all use ICMP, there is no reasons that TCP or UDP fragments (or an unknown protocol) could not be used for this attack, so blocking ICMP fragments is only a temporary solution.

5.4 Filtering out Teardrop and Bonk

Teardrop and Bonk are two attacks (mainly against Microsoft Windows NT machines) which rely on overlapping fragments. Having your Linux router do defragmentation, or disallowing all fragments to your vulnerable machines are the other options.

5.5 Filtering out Fragment Bombs

Some less-reliable TCP stacks are said to have problems dealing with large numbers of fragments of packets when they don't receive all the fragments. Linux does not have this problem. You can filter out fragments (which might break legitimate uses) or compile your kernel with `IP: always defragment' set to `Y' (only if your Linux box is the only possible route for these packets).

5.6 Changing Firewall Rules

There are some timing issues involved in altering firewall rules. If you are not careful, you can let packets through while you are half-way through your changes. A simplistic approach is to do the following:

# ipchains -I input 1 -j DENY
# ipchains -I output 1 -j DENY
# ipchains -I forward 1 -j DENY

... make changes ...

# ipchains -D input 1
# ipchains -D output 1
# ipchains -D forward 1

This drops all packets for the duration of the changes.

If your changes are restricted to a single chain, you might want to create a new chain with the new rules, and then replace (`-R') the rule that pointed to the old chain with one that points to the new chain: then you can delete the old chain. This replacement will occur atomically.

5.7 How Do I Set Up IP Spoof Protection?

IP spoofing is a technique where a host sends out packets which claim to be from another host. Since packet filtering makes decisions based on this source address, IP spoofing is uses to fool packet filters. It is also used to hide the identity of attackers using SYN attacks, Teardrop, Ping of Death and the like (don't worry if you don't know what they are).

The best way to protect from IP spoofing is called Source Address Verification, and it is done by the routing code, and not firewalling at all. Look for a file called /proc/sys/net/ipv4/conf/all/rp_filter. If this exists, then turning on Source Address Verification at every boot is the right solution for you. To do that, insert the following lines somewhere in your init scripts, before any network interfaces are initialized:

# This is the best method: turn on Source Address Verification and get
# spoof protection on all current and future interfaces.
if [ -e /proc/sys/net/ipv4/conf/all/rp_filter ]; then
  echo -n "Setting up IP spoofing protection..."
  for f in /proc/sys/net/ipv4/conf/*/rp_filter; do
      echo 1 > $f
  echo "done."
  echo "CONTROL-D will exit from this shell and continue system startup."
  # Start a single user shell on the console
  /sbin/sulogin $CONSOLE

If you cannot do this, you can manually insert rules to protect every interface. This requires knowledge of each interface. The 2.1 kernels automatically reject packets claiming to come from the 127.* addresses (reserved for the local loopback interface, lo).

For example, say we have three interfaces, eth0, eth1 and ppp0. We can use ifconfig to tell us the address and netmask of the interfaces. Say eth0 was attached to a network with netmask, eth1 was attached to a network with netmask, and ppp0 connected to the Internet (where any address except the reserved private IP addresses are allowed), we would insert the following rules:

# ipchains -A input -i eth0 -s ! -j DENY
# ipchains -A input -i ! eth0 -s -j DENY
# ipchains -A input -i eth1 -s ! -j DENY
# ipchains -A input -i ! eth1 -s -j DENY

This approach is not as good as the Source Address Verification approach, because if your network changes, you have to change your firewalling rules to keep up.

If you are running a 2.0 series kernel, you might want to protect the loopback interface as well, using a rule like this:

# ipchains -A input -i ! lo -s -j DENY

5.8 Advanced Projects

There is a userspace library I have written which is included with the source distribution called `libfw'. It uses the ability of IP Chains 1.3 and above to copy a packet to userspace (using the IP_FIREWALL_NETLINK config option).

The mark value can be used to specify the Quality of Service parameters for packets, or to specify how packets should be port-forwarded. I've never used either, but if you want to write about it, please contact me.

Things such as stateful inspection (I prefer the term dynamic firewalling) can be implemented in userspace using this library. Other nifty ideas include controlling packets on a per-user basis by doing a lookup in a userspace daemon. This should be pretty easy.

SPF: Stateful Packet Filtering is the site of Brian Murrell's SPF project, which does connection tracking in userspace. It adds significant security for low-bandwidth sites.

There's little documentation at present, but here's a post to the mailing list in which Brian answered some questions:

> I believe it does exactly what I want: Installing a temporary
> "backward"-rule to let packets in as a response to an
> outgoing request.

Yup, that is exactly what it does.  The more protocols it
understands, the more "backward" rules it gets right.  Right
now it has support for (from memory, please excuse any errors
or omissions) FTP (both active and passive, in and out), some
RealAudio, traceroute, ICMP and basic ICQ (inbound from the ICQ
servers, and direct TCP connections, but alas the secondary
direct TCP connections for things like file transfer, etc. are
not there yet)

> Is it a replacement for ipchains or a supplement?

It is a supplement.  Think of ipchains as the engine to allow
and prevent packets from travelling across a Linux box.  SPF is
the driver, constantly monitoring traffic and telling ipchains
how to change it's policies to reflect the changes in traffic

Michael Hasenstein's ftp-data hack

Michael Hasenstein of SuSE has written a kernel patch which adds ftp connection tracking to ipchains. It can currently be found at

5.9 Future Enhancements

Firewalling and NAT have being redesigned for 2.4. Plans and discussions are available on the netfilter list (see These enhancements should clear up many outstanding usability issues (really, firewalling and masquerading shouldn't be this hard), and allow growth for far more flexible firewalling.

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