1IP Filter - What's this about ? 2============================ 3Web site: http://coombs.anu.edu.au/~avalon/ip-filter.html 4How-to: http://www.obfuscation.org/ipf/ipf-howto.txt 5 6 The idea behind this package is allow those who use Unix workstations as 7routers (a common occurance in Universities it appears) to apply packet 8filtering to packets going in and out of them. This package has been 9tested on all versions of SunOS 4.1 and Solaris 2.4/2.5, running on Sparcs. 10It is also quite possible for this small kernel extension to be installed 11and used effectively on Sun workstations which don't route IP, just for 12added security. It can also be integrated with the multicast patches. 13It has also been tested successfully on all of the modern free BSDs as 14well as BSDI, and SGI's IRIX 6.2. 15 16 The filter keeps a rule list for both inbound and outbound sides of 17the IP packet queue and a check is made as early as possible, aiming to 18stop the packet before it even gets as far as being checked for source 19route options. In the file "BNF", a set of rules for constructing filter 20rules understood by this package is given. The files in the directory 21"rules", "example.1" ... "example.sr" show example rules you might apply. 22 23 In practise, I've successfully isolated a workstation from all 24machines except the NFS file servers on its local subnets (yeah, ok, so 25this doesn't really increase security, because of NFS, but you get the 26drift on how it can be applied and used). I've also successfully 27setup and maintained my own firewalls using it with TIS's Firewall Toolkit, 28including using it on an mbone router. 29 30 When using it with multicast IP, the calls to fr_check() should be 31before the packet is unwrapped and after it is encapsulated. So the 32filter routines will see the packet as a UDP packet, protocol XYZ. 33Whether this is better or worse than having it filter on class D addresses 34is debateable, but the idea behind this package is to be able to 35discriminate between packets as they are on the 'wire', before they 36get routed anywhere, etc. 37 38 It is worth noting, that it is possible, using a small MTU and 39generating tiny fragmented IP packets to generate a TCP packet which 40doesn't contain enough information to filter on the "flags". Filtering 41on these types of packets is possible, but under the more general case 42of the packets being "short". ICMP and UDP packets which are too small 43(they don't contain a complete header) are dropped and logged, no questions 44asked. When filtering on fragmented packets, the last fragment will get 45through for TCP/UDP/ICMP packets. 46 47Bugs/Problems 48------------- 49If you have a problem with IP Filter on your operating system, please email 50a copy of the file "BugReport" with the details of your setup as required 51and email to darrenr@pobox.com. 52 53Some general notes. 54------------------- 55 To add/delete a rule from memory, access to the device in /dev is needed, 56allowing non-root maintenaince. The filter list in kernel memory is built 57from the kernel's heap. Each packet coming *in* or *out* is checked against 58the appropriate list, rejects dropped, others passed through. Thus this will 59work on an individual host, not just gateways. Presently there is only one 60list for all interfaces, the changes required to make it a per-interface list 61require more .o replacements for the kernel. When checking a packet, the 62packet is compared to the entire list from top to bottom, the last matching 63line being effective. 64 65 66What does what ? 67---------------- 68if_fil.o (Loadable kernel module) 69 - additional kernel routines to check an access list as to whether 70 or not to drop or pass a packet. It currently defaults to pass 71 on all packets. 72 73ipfstat 74 - digs through your kernel (need to check #define VMUNIX in fils.c) 75 and /dev/kmem for the access filter list and mini stats table. 76 Obviously needs to be run priviledged if required. 77 78ipf 79 - reads the files passed as parameters as input files containing new 80 filter rules to add/delete to the kernel list. The lines are 81 inserted in order; the first line is inserted first, and ends up 82 first on the list. Subsequent invocations append to the list 83 unless specified otherwise. 84 85ipftest 86 - test the ruleset given by filename. Reads in the ruleset and then 87 waits for stdin. 88 89 See the man pages (ipf.1, ipftest.1, ipfstat.8) for more detailed 90 information on what the above do. 91 92mkfilters 93 - suggests a set of filter rules to employ and suggests how to add 94 routes to back these up. 95 96BNF 97 - BNF rule set for the filter rules 98 99Darren Reed 100darrenr@pobox.com 101http://coombs.anu.edu.au/~avalon/ip-filter.html 102