# This is a pretty complicated set of filter rules. # (These are the rules I use myself.) # # I've divided the rules up into four sections. # TCP packets, UDP packets, ICMP packets and a general catch all rule # at the end. #------------------------------------------------------------------------------ # Rules for TCP packets. #------------------------------------------------------------------------------ # General comments on the rule set: # # In general we would like to treat only data on a TCP link as signficant # for timeouts. Therefore, we try to ignore packets with no data. # Since the shortest possible set of headers in a TCP/IP packet is 40 bytes. # Any packet with length 40 must have no data riding in it. # We may miss some empty packets this way (optional routing information # and other extras may be present in the IP header), but we should get # most of them. Note that we don't want to filter out packets with # tcp.live clear, since we use them later to speedup disconnects # on some TCP links. # # We also want to make sure WWW packets live even if the TCP socket # is shut down. We do this because WWW doesn't keep connections open # once the data has been transfered, and it would be annoying to have the link # keep bouncing up and down every time you get a document. # # Outside of WWW the most common use of TCP is for long lived connections, # that once they are gone mean we no longer need the network connection. # We don't neccessarily want to wait 10 minutes for the connection # to go down when we don't have any telnet's or rlogin's running, # so we want to speed up the timeout on TCP connections that have # shutdown. We do this by catching packets that do not have the live flag set. # --- start of rule set proper --- # When initiating a connection we only give the link 15 seconds initially. # The idea here is to deal with possibility that the network on the opposite # end of the connection is unreachable. In this case you don't really # want to give the link 10 minutes up time. With the rule below # we only give the link 15 seconds initially. If the network is reachable # then we will normally get a response that actually contains some # data within 15 seconds. If this causes problems because you have a slow # response time at some site you want to regularly access, you can either # increase the timeout or remove this rule. accept tcp 15 tcp.syn # Keep named xfers from holding the link up ignore tcp tcp.dest=tcp.domain ignore tcp tcp.source=tcp.domain # (Ack! SCO telnet starts by sending empty SYNs and only opens the # connection if it gets a response. Sheesh..) accept tcp 5 ip.tot_len=40,tcp.syn # keep empty packets from holding the link up (other than empty SYN packets) ignore tcp ip.tot_len=40,tcp.live # make sure http transfers hold the link for 2 minutes, even after they end. # NOTE: Your /etc/services may not define the tcp service www, in which # case you should comment out the following two lines or get a more # up to date /etc/services file. See the FAQ for information on obtaining # a new /etc/services file. accept tcp 120 tcp.dest=tcp.www accept tcp 120 tcp.source=tcp.www # SSL connections are usually for secure http so treat them like http. # NOTE: Your /etc/services may not define the tcp service https, in which # case you should comment out the following two lines or get a more # up to date /etc/services file. See the FAQ for information on obtaining # a new /etc/services file. keepup tcp 120 tcp.dest=tcp.https keepup tcp 120 tcp.source=tcp.https # Once the link is no longer live, we try to shut down the connection # quickly. Note that if the link is already down, a state change # will not bring it back up. keepup tcp 5 !tcp.live ignore tcp !tcp.live # an ftp-data or ftp connection can be expected to show reasonably frequent # traffic. accept tcp 120 tcp.dest=tcp.ftp accept tcp 120 tcp.source=tcp.ftp #NOTE: ftp-data is not defined in the /etc/services file provided with # the latest versions of NETKIT, so I've got this commented out here. # If you want to define it add the following line to your /etc/services: # ftp-data 20/tcp # and uncomment the following two rules. #accept tcp 120 tcp.dest=tcp.ftp-data #accept tcp 120 tcp.source=tcp.ftp-data # If we don't catch it above, give the link 10 minutes up time. accept tcp 600 any # Rules for UDP packets # # We time out domain requests right away, we just want them to bring # the link up, not keep it around for very long. # This is because the network will usually come up on a call # from the resolver library (unless you have all your commonly # used addresses in /etc/hosts, in which case you will discover # other problems.) # Note that you should not make the timeout shorter than the time you # might expect your DNS server to take to respond. Otherwise # when the initial link gets established there might be a delay # greater than this between the initial series of packets before # any packets that keep the link up longer pass over the link. # Don't bring the link up for rwho. ignore udp udp.dest=udp.who ignore udp udp.source=udp.who # Don't bring the link up for RIP. ignore udp udp.dest=udp.route ignore udp udp.source=udp.route # Don't bring the link up for NTP or timed. ignore udp udp.dest=udp.ntp ignore udp udp.source=udp.ntp ignore udp udp.dest=udp.timed ignore udp udp.source=udp.timed # Don't bring up on domain name requests between two running nameds. ignore udp udp.dest=udp.domain,udp.source=udp.domain # Bring up the network whenever we make a domain request from someplace # other than named. accept udp 30 udp.dest=udp.domain accept udp 30 udp.source=udp.domain # Do the same for netbios-ns broadcasts # NOTE: your /etc/services file may not define the netbios-ns service # in which case you should comment out the next three lines. ignore udp udp.source=udp.netbios-ns,udp.dest=udp.netbios-ns accept udp 30 udp.dest=udp.netbios-ns accept udp 30 udp.source=udp.netbios-ns # keep routed and gated transfers from holding the link up ignore udp tcp.dest=udp.route ignore udp tcp.source=udp.route # Anything else gest 2 minutes. accept udp 120 any # Ignore some background protocols. ignore igmp any ignore ospfigp any # Catch any packets that we didn't catch above and give the connection # 30 seconds of live time. accept any 30 any
