I understood that the first time.If you’re using a TAP interface to read packets, you’d still need to re-inject them back into the kernel via a write to TAP.But it sounds to me like you should be using tc and iptables (at least if you’re using Linux).You don’t want to be doing this in user-space (unless you’re using DPDK).Either way, libevent is probably not the droid you’re looking for.On Nov 15, 2016, at 2:56 PM, Prasad <divaturi@xxxxxxxxx> wrote:I am not inserting any new data - only need to modify existing packet header in-place.On Tue, Nov 15, 2016 at 1:50 PM, Philip Prindeville <philipp_subx@redfish-solutions.com > wrote:Nothing in the description of evbuffer_pullup() suggests to me that this would need to happen.There’s no discussion about freeing the returned memory, for instance, which would need to happen if it were a separate buffer.But there’s a simple test: try calling evbuffer_pullup() on the same buffer twice before draining it, and see if the pointer returned both times is the same…-PhilipOn Nov 15, 2016, at 1:29 PM, Devi Prasad Ivaturi <divaturi@xxxxxxxxx> wrote:Also, isn't the buffer returned by pullup() is only for peeking and NOT modifying the buffer?I understand it that way from http://www.wangafu.net/~nickm/libevent-book/Ref7_evbuff er.html Regards,-deviOn Tue, Nov 15, 2016 at 12:27 PM, Devi Prasad Ivaturi <divaturi@xxxxxxxxx> wrote:In the front only, if you want to call it that way.Say, I want to just modify the TTL or TOS values of IP header of a network packet (accessed using evbuffer API)passing through my box: don't want to use pullup() which is costly, since, the IP header could be a few headersaway from the beginning (MAC + other enapsulations).I don't understand how add_reference() would meet the need: I want to modify the bytes in the packet headers, fromless than a byte upto 16 bytes max.Regards,-deviOn Tue, Nov 15, 2016 at 11:35 AM, Philip Prindeville <philipp_subx@redfish-solutions.com > wrote:Where in the buffer is the data? Is it near the front or not?If it’s near the front, then use evbuffer_pullup(). Otherwise, you could provide your own underlying storage with evbuffer_add_reference().On Nov 15, 2016, at 11:58 AM, Devi Prasad Ivaturi <divaturi@xxxxxxxxx> wrote:And I get the impression that these API are to insert data rather than modify existing data.
On Nov 15, 2016 10:43 AM, "Devi Prasad Ivaturi" <divaturi@xxxxxxxxx> wrote:BTW, I did consider reserve/commit space API, but, thought they might be an overkill. I prefer peeking the byte location and modifying it.
On Nov 15, 2016 10:12 AM, "Devi Prasad Ivaturi" <divaturi@xxxxxxxxx> wrote:How can I modify a byte or two directly in evbuffer data?