From nobody Fri Mar 03 13:52:25 2023 X-Original-To: freebsd-net@mlmmj.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mlmmj.nyi.freebsd.org (Postfix) with ESMTP id 4PSqD43Kw2z3vxGq for ; Fri, 3 Mar 2023 13:52:36 +0000 (UTC) (envelope-from freebsd-rwg@gndrsh.dnsmgr.net) Received: from gndrsh.dnsmgr.net (br1.CN84in.dnsmgr.net [69.59.192.140]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 4PSqD35ptPz4P9G; Fri, 3 Mar 2023 13:52:35 +0000 (UTC) (envelope-from freebsd-rwg@gndrsh.dnsmgr.net) Authentication-Results: mx1.freebsd.org; none Received: from gndrsh.dnsmgr.net (localhost [127.0.0.1]) by gndrsh.dnsmgr.net (8.13.3/8.13.3) with ESMTP id 323DqQwF066349; Fri, 3 Mar 2023 05:52:26 -0800 (PST) (envelope-from freebsd-rwg@gndrsh.dnsmgr.net) Received: (from freebsd-rwg@localhost) by gndrsh.dnsmgr.net (8.13.3/8.13.3/Submit) id 323DqPjh066348; Fri, 3 Mar 2023 05:52:25 -0800 (PST) (envelope-from freebsd-rwg) From: "Rodney W. Grimes" Message-Id: <202303031352.323DqPjh066348@gndrsh.dnsmgr.net> Subject: Re: BPF to filter/mod ARP In-Reply-To: <93749673-2939-4A38-BA37-2AEBEF5764D0@lurchi.franken.de> To: Michael Tuexen Date: Fri, 3 Mar 2023 05:52:25 -0800 (PST) CC: "Rodney W. Grimes" , "Scheffenegger, Richard" , "freebsd-net@freebsd.org" X-Mailer: ELM [version 2.4ME+ PL121h (25)] List-Id: Networking and TCP/IP with FreeBSD List-Archive: https://lists.freebsd.org/archives/freebsd-net List-Help: List-Post: List-Subscribe: List-Unsubscribe: Sender: owner-freebsd-net@freebsd.org MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=US-ASCII X-Rspamd-Queue-Id: 4PSqD35ptPz4P9G X-Spamd-Bar: ---- X-Spamd-Result: default: False [-4.00 / 15.00]; REPLY(-4.00)[]; ASN(0.00)[asn:13868, ipnet:69.59.192.0/19, country:US] X-Rspamd-Pre-Result: action=no action; module=replies; Message is reply to one we originated X-ThisMailContainsUnwantedMimeParts: N > > On 2. Mar 2023, at 18:20, Rodney W. Grimes wrote: > > > >>> On 2. Mar 2023, at 02:24, Rodney W. Grimes wrote: > >>> > >>>> Hi group, > >>>> > >>>> Maybe someone can help me with this question - as I am usually only > >>>> looking at L4 and the top side of L3 ;) > >>>> > >>>> In order to validate a peculiar switches behavior, I want to adjust some > >>>> fields in gracious arps sent out by an interface, after a new IP is > >>>> assigned or changed. > >>> > >>> Gracious or Gratuitous? > >>> > >>>> > >>>> I believe BPF can effectively filter on arbitrary bit patterns and > >>>> modify packets on the fly. > >>> > >>> It can. > >>> > >>>> > >>>> However, as ARP doesn't seem to be accessible in the ipfw > >>>> infrastructure, I was wondering how to go about setting up an BPF to > >>>> tweak (temporarily) some of these ARPs to validate how the switch will > >>>> behave. > >>> > >>> ipfw is IP firewall, a layer 3 function. Arp is a layer 2 protocol, > >>> so very hard to do much with it in ipfw, but perhaps the layer2 > >>> keyword, and some use of mac-type can get it to match an arp > >>> packet. Arp is ethernet type 0x806. > >>> > >>> ipfw add 111 count log all from any to any layer2 mac-type arp > >>> That does seem to work > >>> ipfw -a list 111 > >>> 00111 4 0 count log ip from any to any layer2 mac-type 0x0806 > >>> > >>> Also normally ipfw does NOT pick packets up early enough to see > >>> them, to get the layer2 option to work you need: > >>> sysctl net.link.ether.ipfw=1 so that the filters at ether_demux > >>> get turned on. > >>> > >>> So perhaps use a divert rule and send them to a socket where > >>> a program can mangle them, and then return them to ipfw > >>> and hopefully the kernel does what you want after that... > >> I thought that you receive/send an IP packet on a divert socket, not > >> an ethernet frame. Am I wrong? > > > > That is unclear to me, technically it should just be a binary > > blob and the kernel and userland just have to agree as to > > what it is. Understand that ipfw originally only had IP layer > > functionality. The ability to muck with layer2 was added > > later, so I suspect the documentation about what is sent > > over the divert socket may be out of date. Simple enough > > to test though, just setup as I show above only change > > to: > > ipfw add 111 divert 4444 all from any to any layer2 mac-type arp > > and write a program to dump what you get on the divert socket. > > I suspect you get an ethernet frame. > > > > And finally divert(4) says: NAME: divert kernel packet diversion mechanism > > That says packet, so again, IMHO, it should be arbitrary to what layer. > > It also later says "Divert sockets are similar to raw IP sockets", > > I think similar is the key aspect here, they are not identical. > I can confirm that using > sudo sysctl net.link.ether.ipfw=1 > sudo ipfw add 111 count log all from any to any layer2 mac-type arp > ... wait some time and observe ARP traffic via tcpdump > sudo ipfw show > 00111 22 0 count log logamount 5 ip from any to any layer2 mac-type 0x0806 > 65535 7892 849004 allow ip from any to any > So the rule is hit. > > However, now doing > sudo ipfw delete 111 > sudo ipfw add 111 divert 1234 all from any to any layer2 mac-type arp > ... wait some time and observe ARP traffic via tcpdump > tuexen@head:~ % sudo ipfw show > 00111 0 0 divert 1234 ip from any to any layer2 mac-type 0x0806 > 65535 10048 1000948 allow ip from any to any > So this time, rule 111 is not hit. I also ran Nice work, to me I would classify this behavior as some form of bug, the action verb of a rule in ipfw should in no way change what is matched by the rule filter. I am assuming you either had IPDIVERT compiled into your kernel, or you you had loaded the module, as you dont clearly state this. I am also uncertain on what the results are if you use the divert keyword without ipdivert.ko loaded, is it an error when the rule gets created, or is it silently ignored? > > #include > #include > #include > #include > #include > #include > > #define BUFFER_SIZE (1<<16) > #define PORT 1234 > > int > main(void) > { > char buffer[BUFFER_SIZE]; > struct sockaddr_in addr; > ssize_t n; > int fd; > > if ((fd = socket(PF_DIVERT, SOCK_RAW, 0)) < 0) { > perror("socket()"); > } > bzero(&addr, sizeof(addr)); > addr.sin_family = AF_INET; > addr.sin_len = sizeof(struct sockaddr_in); > addr.sin_addr.s_addr = INADDR_ANY; > addr.sin_port = htons(PORT); > > if (bind(fd, (struct sockaddr *)&addr, (socklen_t)sizeof(struct sockaddr_in)) < 0) { > perror("bind()"); > } > for (;;) { > n = recv(fd, buffer, sizeof(buffer), 0); > printf("Received %zd bytes.\n", n); > } > if (close(fd) < 0) { > perror("close()"); > } > return (0); > } > > but nothing was printed... > > Best regards > Michael > > > >> > >> Best regards > >> Michael > >>> > >>>> (I need to validate, if there is some difference when the target > >>>> hardware address doesn't conform to RFC5227 - which states it SHOULD be > >>>> zero and is ignored on the receiving side; i have reasons to believe > >>>> that the switch needs either a target hardware address of > >>>> ff:ff:ff:ff:ff:ff or the local interface MAC, to properly update it's > >>>> entries.) > >>>> > >>>> Thanks a lot! > >>>> > >>>> Richard > >>>> > >>> > >>> -- > >>> Rod Grimes rgrimes@freebsd.org > >>> > >> > >> > >> > > > > -- > > Rod Grimes rgrimes@freebsd.org > > > > -- Rod Grimes rgrimes@freebsd.org