On Jul 4, 2025, at 8:55 AM, Bill Fenner <fen...@gmail.com> wrote: > We may know that we will be using this code on a kernel that > requires BPF_SPECIAL_VLAN_HANDLING, and so I'd like to be able to set that > flag on a "dead" handle. Obviously, the current mechanism is to test this > dynamically when the handle is live. > > Does anyone have any opinions about how to implement this, through the > generic pcap interface? I can think of two possibilities: > > 1. implement set_special_vlan_handling_op, and have it implemented by dead > and linux, and have the default version return an error > 2. implement set_bpf_codegen_flags_op, and have a generic implementation, > assuming that the caller completely knows what they are doing. Then move > the definition of BPF_SPECIAL_VLAN_HANDLING from pcap-int.h to the public > header.
The underlying problem is that we have a compiler with multiple targets, and the target is indicated by a pcap_t *. My inclination might be to have a version of pcap_compile() that takes, as additional arguments, a link-layer type, a snapshot, and a set of BPF_SPECIAL_ flags. That might be slightly cleaner than the bpf_open_dead() hack. Yes, it means that the caller has to know what they're doing, but they'd have to know that *anyway* - is the code intended for: * live captures with a Linux kernel without VLAN tag fetching support in BPF, in which case you want BPF_SPECIAL_BASIC_HANDLING; * live captures with a Linux kernel *with* VLAN tag fetching support in BPF, in which case you want BPF_SPECIAL_BASIC_HANDLING|BPF_SPECIAL_VLAN_HANDLING; * live captures with Npcap, in which case you want BPF_SPECIAL_VLAN_HANDLING; * anything else, in which case you want 0. While we're at it, a new pcap_dump_open() routine that takes a link-layer type and a snapshot length might be worth introducing, rendering pcap_open_dead() mostly, if not completely, redundant. _______________________________________________ tcpdump-workers mailing list -- tcpdump-workers@lists.tcpdump.org To unsubscribe send an email to tcpdump-workers-le...@lists.tcpdump.org %(web_page_url)slistinfo%(cgiext)s/%(_internal_name)s
