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Page 186 / 427 Scroll up to view Page 181 - 185
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Chapter 13 Firewalls
Figure 101
Smurf Attack
13.4.2.1
ICMP Vulnerability
ICMP is an error-reporting protocol that works in concert with IP. The following ICMP types
trigger an alert:
13.4.2.2
Illegal Commands (NetBIOS and SMTP)
The only legal NetBIOS commands are the following - all others are illegal.
All SMTP commands are illegal except for those displayed in the following tables.
Table 67
ICMP Commands That Trigger Alerts
5
REDIRECT
13
TIMESTAMP_REQUEST
14
TIMESTAMP_REPLY
17
ADDRESS_MASK_REQUEST
18
ADDRESS_MASK_REPLY
Table 68
Legal NetBIOS Commands
MESSAGE:
REQUEST:
POSITIVE:
VE:
RETARGET:
KEEPALIVE:
Table 69
Legal SMTP Commands
AUTH
DATA
EHLO
ETRN
EXPN
HELO
HELP
MAIL
NOOP
QUIT
RCPT
RSET
SAML
SEND
SOML
TURN
VRFY
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187
13.4.2.3
Traceroute
Traceroute is a utility used to determine the path a packet takes between two endpoints.
Sometimes when a packet filter firewall is configured incorrectly an attacker can traceroute
the firewall gaining knowledge of the network topology inside the firewall.
Often, many DoS attacks also employ a technique known as "
IP Spoofing
" as part of their
attack. IP Spoofing may be used to break into systems, to hide the hacker's identity, or to
magnify the effect of the DoS attack. IP Spoofing is a technique used to gain unauthorized
access to computers by tricking a router or firewall into thinking that the communications are
coming from within the trusted network. To engage in IP spoofing, a hacker must modify the
packet headers so that it appears that the packets originate from a trusted host and should be
allowed through the router or firewall. The ZyXEL Device blocks all IP Spoofing attempts.
13.5
Stateful Inspection
With stateful inspection, fields of the packets are compared to packets that are already known
to be trusted. For example, if you access some outside service, the proxy server remembers
things about your original request, like the port number and source and destination addresses.
This “remembering” is called
saving the state.
When the outside system responds to your
request, the firewall compares the received packets with the saved state to determine if they
are allowed in. The ZyXEL Device uses stateful packet inspection to protect the private LAN
from hackers and vandals on the Internet. By default, the ZyXEL Device’s stateful inspection
allows all communications to the Internet that originate from the LAN, and blocks all traffic to
the LAN that originates from the Internet. In summary, stateful inspection:
•
Allows all sessions originating from the LAN (local network) to the WAN (Internet).
•
Denies all sessions originating from the WAN to the LAN.
Figure 102
Stateful Inspection
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The previous figure shows the ZyXEL Device’s default firewall rules in action as well as
demonstrates how stateful inspection works. User A can initiate a Telnet session from within
the LAN and responses to this request are allowed. However other Telnet traffic initiated from
the WAN is blocked.
13.5.1
Stateful Inspection Process
In this example, the following sequence of events occurs when a TCP packet leaves the LAN
network through the firewall's WAN interface. The TCP packet is the first in a session, and the
packet's application layer protocol is configured for a firewall rule inspection:
1
The packet travels from the firewall's LAN to the WAN.
2
The packet is evaluated against the interface's existing outbound access list, and the
packet is permitted (a denied packet would simply be dropped at this point).
3
The packet is inspected by a firewall rule to determine and record information about the
state of the packet's connection. This information is recorded in a new state table entry
created for the new connection. If there is not a firewall rule for this packet and it is not an
attack, then the settings in the
Firewall General
screen determine the action for this
packet.
4
Based on the obtained state information, a firewall rule creates a temporary access list
entry that is inserted at the beginning of the WAN interface's inbound extended access
list. This temporary access list entry is designed to permit inbound packets of the same
connection as the outbound packet just inspected.
5
The outbound packet is forwarded out through the interface.
6
Later, an inbound packet reaches the interface. This packet is part of the connection
previously established with the outbound packet. The inbound packet is evaluated against
the inbound access list, and is permitted because of the temporary access list entry
previously created.
7
The packet is inspected by a firewall rule, and the connection's state table entry is updated
as necessary. Based on the updated state information, the inbound extended access list
temporary entries might be modified, in order to permit only packets that are valid for the
current state of the connection.
8
Any additional inbound or outbound packets that belong to the connection are inspected
to update the state table entry and to modify the temporary inbound access list entries as
required, and are forwarded through the interface.
9
When the connection terminates or times out, the connection's state table entry is deleted
and the connection's temporary inbound access list entries are deleted.
13.5.2
Stateful Inspection on Your ZyXEL Device
Additional rules may be defined to extend or override the default rules. For example, a rule
may be created which will:
•
Block all traffic of a certain type, such as IRC (Internet Relay Chat), from the LAN to the
Internet.
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•
Allow certain types of traffic from the Internet to specific hosts on the LAN.
•
Allow access to a Web server to everyone but competitors.
•
Restrict use of certain protocols, such as Telnet, to authorized users on the LAN.
These custom rules work by evaluating the network traffic’s Source IP address, Destination IP
address, IP protocol type, and comparing these to rules set by the administrator.
Note:
The ability to define firewall rules is a very powerful tool. Using custom rules, it
is possible to disable all firewall protection or block all access to the Internet.
Use extreme caution when creating or deleting firewall rules. Test changes
after creating them to make sure they work correctly.
Below is a brief technical description of how these connections are tracked. Connections may
either be defined by the upper protocols (for instance, TCP), or by the ZyXEL Device itself (as
with the "virtual connections" created for UDP and ICMP).
13.5.3
TCP Security
The ZyXEL Device uses state information embedded in TCP packets. The first packet of any
new connection has its SYN flag set and its ACK flag cleared; these are "initiation" packets.
All packets that do not have this flag structure are called "subsequent" packets, since they
represent data that occurs later in the TCP stream.
If an initiation packet originates on the WAN, this means that someone is trying to make a
connection from the Internet into the LAN. Except in a few special cases (see "Upper Layer
Protocols" shown next), these packets are dropped and logged.
If an initiation packet originates on the LAN, this means that someone is trying to make a
connection from the LAN to the Internet. Assuming that this is an acceptable part of the
security policy (as is the case with the default policy), the connection will be allowed. A cache
entry is added which includes connection information such as IP addresses, TCP ports,
sequence numbers, etc.
When the ZyXEL Device receives any subsequent packet (from the Internet or from the LAN),
its connection information is extracted and checked against the cache. A packet is only
allowed to pass through if it corresponds to a valid connection (that is, if it is a response to a
connection which originated on the LAN).
13.5.4
UDP/ICMP Security
UDP and ICMP do not themselves contain any connection information (such as sequence
numbers). However, at the very minimum, they contain an IP address pair (source and
destination). UDP also contains port pairs, and ICMP has type and code information. All of
this data can be analyzed in order to build "virtual connections" in the cache.
For instance, any UDP packet that originates on the LAN will create a cache entry. Its IP
address and port pairs will be stored. For a short period of time, UDP packets from the WAN
that have matching IP and UDP information will be allowed back in through the firewall.
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A similar situation exists for ICMP, except that the ZyXEL Device is even more restrictive.
Specifically, only outgoing echoes will allow incoming echo replies, outgoing address mask
requests will allow incoming address mask replies, and outgoing timestamp requests will
allow incoming timestamp replies. No other ICMP packets are allowed in through the firewall,
simply because they are too dangerous and contain too little tracking information. For
instance, ICMP redirect packets are never allowed in, since they could be used to reroute
traffic through attacking machines.
13.5.5
Upper Layer Protocols
Some higher layer protocols (such as FTP and RealAudio) utilize multiple network
connections simultaneously. In general terms, they usually have a "control connection" which
is used for sending commands between endpoints, and then "data connections" which are used
for transmitting bulk information.
Consider the FTP protocol. A user on the LAN opens a control connection to a server on the
Internet and requests a file. At this point, the remote server will open a data connection from
the Internet. For FTP to work properly, this connection must be allowed to pass through even
though a connection from the Internet would normally be rejected.
In order to achieve this, the ZyXEL Device inspects the application-level FTP data.
Specifically, it searches for outgoing "PORT" commands, and when it sees these, it adds a
cache entry for the anticipated data connection. This can be done safely, since the PORT
command contains address and port information, which can be used to uniquely identify the
connection.
Any protocol that operates in this way must be supported on a case-by-case basis. You can use
the web configurator’s Custom Ports feature to do this.
13.6
Guidelines for Enhancing Security with Your Firewall
•
Change the default password.
•
Limit who can telnet into your router.
•
Don't enable any local service (such as SNMP or NTP) that you don't use. Any enabled
service could present a potential security risk. A determined hacker might be able to find
creative ways to misuse the enabled services to access the firewall or the network.
•
For local services that are enabled, protect against misuse. Protect by configuring the
services to communicate only with specific peers, and protect by configuring rules to
block packets for the services at specific interfaces.
•
Protect against IP spoofing by making sure the firewall is active.
•
Keep the firewall in a secured (locked) room.
13.6.1
Security In General
You can never be too careful! Factors outside your firewall, filtering or NAT can cause
security breaches. Below are some generalizations about what you can do to minimize them.

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