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CERT® Coordination Center

Packet Filtering for Firewall Systems

If your site isn't filtering certain TCP/IP packets, it may not be as secure as you think.

When the CERT Coordination Center started in 1988, it was our opinion that security was the responsibility of the system and not the network. While we still believe it is important for system managers to be aware of security issues and to continue to be diligent in securing their systems, we realize that this effort will not protect your site from the exploitation of flawed protocols.

The CERT staff encourages system managers, site network managers, and regional network providers to take the time to understand packet filtering issues. Because of the flaws in several TCP/IP services, a site must be able to restrict external access to these services. Sites should consider purchasing programmable routers. Network providers should offer packet filtering as a service option.

Because of flaws in the protocol or chronic system administration problems, we recommend that the following services be filtered:

DNS zone transfers socket 53 (TCP)
tftpd socket 69 (UDP)
link socket 87 (TCP) (commonly used by intruders)
SunRPC & NFS socket 111 and 2049 (UDP and TCP)
BSD UNIX "r" cmds sockets 512, 513, and 514 (TCP)
lpd socket 515 (TCP)
uucpd socket 540 (TCP)
openwindows socket 2000 (UDP and TCP)
X windows socket 6000+ (UDP and TCP)
We suggest that sites filter socket 53 (TCP) to prevent domain name service zone transfers. Permit access to socket 53 (TCP) only from known secondary domain name servers. This prevents intruders from gaining additional knowledge about the systems connected to your local network.

We have handled incidents that involved automated TFTP attempts. Many of the systems affected were using the TFTP daemon to boot other devices. Filtering TFTP connections would have protected the sites from this attack.

The X windows sockets range from socket 6000 to 6000 plus the highest number of X terminals on the same host.

If your site does not need to provide other services to external users, those other services should be filtered. For example, filter telnet connections when all staff members are in the office, and filter FTP connections to all systems except to public information servers.

In addition to filtering specific services, we recommend that sites also filter based on the source address field of the packets to prevent IP spoofing. More information on this technique can be found in CERT advisory CA-95:01, "IP Spoofing Attacks and Hijacked Terminal Connections," available by anonymous FTP from

info.cert.org:/pub/cert_advisories
To prevent denial of service attacks based on ICMP bombs, filter ICMP redirect and ICMP destination unreachable packets. In addition, sites should filter source routed packets.
This document is available from: http://www.cert.org/tech_tips/packet_filtering.html