From 52b84706664ae19f80310e64fe00936d8b964b56 Mon Sep 17 00:00:00 2001 From: Duncan Roe Date: Sun, 27 Oct 2019 19:38:04 +1100 Subject: src: doc: Document nfq_nlmsg_verdict_put_mark() and nfq_nlmsg_verdict_put_pkt() This completes the "Verdict helpers" module. Signed-off-by: Duncan Roe Signed-off-by: Florian Westphal --- src/nlmsg.c | 45 ++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 44 insertions(+), 1 deletion(-) (limited to 'src') diff --git a/src/nlmsg.c b/src/nlmsg.c index d06e6db..c40a9e4 100644 --- a/src/nlmsg.c +++ b/src/nlmsg.c @@ -31,7 +31,7 @@ */ /** - * nfq_nlmsg_verdict_put - Put a verdict into a Netlink header + * nfq_nlmsg_verdict_put - Put a verdict into a Netlink message * \param nlh Pointer to netlink message * \param id ID assigned to packet by netfilter * \param verdict verdict to return to netfilter (NF_ACCEPT, NF_DROP) @@ -50,6 +50,15 @@ void nfq_nlmsg_verdict_put(struct nlmsghdr *nlh, int id, int verdict) mnl_attr_put(nlh, NFQA_VERDICT_HDR, sizeof(vh), &vh); } +/** + * nfq_nlmsg_verdict_put_mark - Put a packet mark into a netlink message + * \param nlh Pointer to netlink message + * \param mark Value of mark to put + * + * The mark becomes part of the packet's metadata, and may be tested by the *nft + * primary expression* **meta mark** + * \sa __nft__(1) + */ EXPORT_SYMBOL void nfq_nlmsg_verdict_put_mark(struct nlmsghdr *nlh, uint32_t mark) { @@ -57,6 +66,40 @@ void nfq_nlmsg_verdict_put_mark(struct nlmsghdr *nlh, uint32_t mark) } EXPORT_SYMBOL +/** + * nfq_nlmsg_verdict_put_pkt - Put replacement packet content into a netlink + * message + * \param nlh Pointer to netlink message + * \param pkt Pointer to start of modified IP datagram + * \param plen Length of modified IP datagram + * + * There is only ever a need to return packet content if it has been modified. + * Usually one of the nfq_*_mangle_* functions does the modifying. + * + * This code snippet uses nfq_udp_mangle_ipv4. See nf-queue.c for + * context: + * \verbatim +// main calls queue_cb (line 64) to process an enqueued packet: + // Extra variables + uint8_t *payload, *rep_data; + unsigned int match_offset, match_len, rep_len; + + // The next line was commented-out (with payload void*) + payload = mnl_attr_get_payload(attr[NFQA_PAYLOAD]); + // Copy data to a packet buffer (allow 255 bytes for mangling). + pktb = pktb_alloc(AF_INET, payload, plen, 255); + // (decide that this packet needs mangling) + nfq_udp_mangle_ipv4(pktb, match_offset, match_len, rep_data, rep_len); + // Update IP Datagram length + plen += rep_len - match_len; + + // Eventually nfq_send_verdict (line 39) gets called + // The received packet may or may not have been modified. + // Add this code before nfq_nlmsg_verdict_put call: + if (pktb_mangled(pktb)) + nfq_nlmsg_verdict_put_pkt(nlh, pktb_data(pktb), plen); +\endverbatim + */ void nfq_nlmsg_verdict_put_pkt(struct nlmsghdr *nlh, const void *pkt, uint32_t plen) { -- cgit v1.2.3