path: root/open_issues/bpf.mdwn

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[[!tag open_issue_gnumach open_issue_hurd]]

This is a collection of resources concerning *Berkeley Packet Filter*s.


# Documentation

  * Wikipedia: [[!wikipedia "Berkeley Packet Filter"]]

  * [The Packet Filter: An Efficient Mechanism for User-level Network
    Code](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.36.8755),
    1987, Jeffrey C. Mogul, Richard F. Rashid, Michael J. Accetta

  * [The BSD Packet Filter: A New Architecture for User-level Packet
    Capture](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.43.7849),
    1992, Steven Mccanne, Van Jacobson

  * [Protocol Service Decomposition for High-Performance
    Networking](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.30.8387),
    1993, Chris Maeda, Brian N. Bershad

  * [Efficient Packet Demultiplexing for Multiple Endpoints and Large
    Messages](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.46.44),
    1994, Masanobu Yuhara Fujitsu, Masanobu Yuhara, Brian N. Bershad, Chris
    Maeda, J. Eliot, B. Moss

  * ... and many more


# Implementation

  * [[community/HurdFr]]

      * <http://wiki.hurdfr.org/index.php/BPF>

      * <http://wiki.hurdfr.org/index.php/Reseau_dans_gnumach>

      * Git repository: <http://rcs-git.duckcorp.org/hurdfr/bpf.git/>

    The patch for [[GNU Mach|microkernel/mach/gnumach]] is expected to be
    complete and functional, the [[hurd/translator]] less so -- amongst others,
    there are unresolved issues concerning support of [[hurd/glibc/IOCTL]]s.

      * <http://lists.gnu.org/archive/html/bug-hurd/2006-03/msg00025.html>

  * [[zhengda]]

      * [[!GNU_Savannah_bug 25054]] -- Kernel panic with eth-multiplexer

      * [[!GNU_Savannah_patch 6619]] -- pfinet uses the virtual interface

      * [[!GNU_Savannah_patch 6620]] -- pfinet changes its filter rules with
        its IP address

      * [[!GNU_Savannah_patch 6621]] -- pfinet sets the mach device into the
        promiscuous mode

      * [[!GNU_Savannah_patch 6622]] -- pfinet uses the BPF filter

      * [[!GNU_Savannah_patch 6851]] -- fix a bug in BPF


# IRC

## IRC, freenode, #hurd, 2012-01-13

    <braunr> hm, i think the bpf code needs a complete redesign :p
    <braunr> unless it's actually a true hurdish way to do things
    <braunr> antrik: i need your help :)
    <braunr> antrik: I need advice on the bpf "architecture"
    <braunr> the current implementation uses a translator installed at /dev/bpf
    <braunr> which means packets from the kernel are copied to that translator
      and then to client applications
    <braunr> does that seem ok to you ?
    <braunr> couldn't the translator be used to set a direct link between the
      kernel and the client app ?
    <braunr> which approach seems the more Hurdish to you ? (<= this is what I
      need your help on)
    <pinotree> braunr: so there would be a roundtrip like kernel → bpf
      translator → pfinet?
    <antrik> braunr: TBH, I don't see why we need a BPF translator at all...
    <braunr> antrik: it handles the ioctls
    <braunr> pinotree: pfinet isn't involved (it was merely modified to use the
      "new" filter format to specify it used the old packet filter, and not
      bpf)
    <antrik> braunr: do we really need to emulate the ioctl()s? can't we assume
      that all packages using BPF will just use libpcap?
    <antrik> (and even if we *do* want to emulate ioctl()s, why can't we handle
      this is libc?)
    <braunr> antrik: that's what i'm wondering actually
    <braunr> even if assuming all packages use libpcap, i'd like our bpf
      interface to be close to what bsds have, and most importantly, what
      libpcap expects from a bpf interface
    <antrik> well, why? if we already have a library handling the abstraction,
      I don't see much point in complicating the design and use by adding
      another layer :-)
    <braunr> so you would advise adapting libpcap to include a hurd specific
      module ?
    <antrik> there are two reasons for adding translators: more robustness or
      more flexibility... so far I don't see how a BPF translator would add
      either
    <braunr> right
    <antrik> yes
    <braunr> so we'd end up with a bpf-like interface, the same instructions
      and format, with different control calls
    <antrik> right
    <antrik> note that I had more or less the same desicion to make for KGI
      (emulate Linux/BSD ioctl()s, or implement a backend in libggi for
      handling Hurd-specific RPC; and after much consideration, I decided on
      the latter)


## IRC, freenode, #hurd, 2012-01-16

    <braunr> antrik: is there an existing facility to easily give a send right
      to the device master port to a task ?
    <braunr> another function of the bpf translator is to handle the /dev/bpf
      node, and most importantly its permissions
    <braunr> so that users which have read/write access to the node have access
      to the packet filter
    <braunr> i guess the translator could limit itself to that functionality
    <braunr> and then provide a device port on which libpcap operates directly
      by means of device_{g,s}et_status/device_set_filter
    <antrik> braunr: I don't see the point in seperating permissions for filter
      from permissions from general network device access...
    <antrik> as for device master port, all root tasks can obtain it from proc
      IIRC
    <braunr> antrik: yes, but how do we allow non-root users to access that
      facility ?
    <braunr> on a unix like system, it's a matter of changing the permissions
      of /dev/bpf
    <antrik> with devnode, non-root users can get access to specific device
      nodes, including network devices
    <braunr> i can't imagine the hurd being less flexible for that
    <braunr> ah devnode
    <braunr> good
    <antrik> so we can for example make /dev/eth0 accessible by users of some
      group
    <braunr> what's devnode exactly ?
    <antrik> it's a very simple translator that implements an FS node that
      looks somewhat like a file, but the only operation it supports is
      obtaining a pseudo device master port, giving access to a specific Mach
      device
    <braunr> is it already part of the hurd ?
    <braunr> or hurdextras maybe ?
    <antrik> it's only in zhengda's branch
    <braunr> ah
    <antrik> needed for both eth-multipexer and DDE
    <braunr> and bpf soon i guess
    <antrik> indeed :-)
    <braunr> "obtaining a pseudo device master port", i believe you meant a
      pseudo device port
    <antrik> I must admit that I don't remember exactly whether devnode proxies
      device_open(), so clients direct get a port to the device in question, or
      whether it implements a pseudo device master port...
    <antrik> but definitely not a pseudo device port :-)
    <braunr> i'm almost positive it gives the target device port, otherwise i
      don't see the point
    <braunr> i don't understand the user of the "pseudo" word here either
    <braunr> s/user/use/
    <braunr> aiui, devnode should be started as root (or in any way which gives
      it the device master port)
    <antrik> the point is that the client doesn't need to know the Mach device
      name, and also is not bound to actual kernel devices
    <braunr> and when started, implement the required permissions before giving
      clients a device port to the specific device it was installed for
    <braunr> right
    <braunr> but it mustn't be a proxy
    <antrik> yes, devnode needs access to either the real master device port
      (for kernel devices), or one provided by eth-multiplexer or the DDE
      network driver
    <braunr> well, a very simple proxy for deviceopen
    <braunr> ok
    <braunr> that seems exactly what i wanted to do
    <braunr> we now need to see if we can integrate it separately
    <braunr> create a separate branch that works for the current gnumach code,
      and merge dde/other specific code later on
    <antrik> you mean independent of eth-multiplexer or DDE? yes, it was
      generally agreed that devnode is a good idea in any case. I have no idea
      why there are no device nodes for network devices on other UNIX
      systems...
    <braunr> i've been wondering that for years too :)
    <antrik> zhengda's branch has a pfinet modified to a) use devnode, and b)
      use BPF
    <braunr> why bpf ?
    <braunr> for more specific filters maybe ?
    <antrik> hm... don't remember whether there was any technical reason for
      going with BPF; I guess it just seemed more reasonable to invest new work
      in BPF rather than obsolete Mach-specific NPF...
    <braunr> cspf could be removed altogether, i agree
    <antrik> another plus side of his modified pfinet is that it actually sets
      an appropriate filter for TCP/IP and the IP in use, rather than just
      setting a dummy filter catching app packets (including those irrelevant
      to the specific pfinet instance)
    <antrik> err... catching all packets
    <braunr> that's what i meant by "for more specific filters maybe ?"
    <braunr> he was probably more comfortable with the bpf interface to write
      his filter rules
    <antrik> well, it would probably be doable with NPF too :-) so by itself
      it's not a reason for switching to BPF...
    <antrik> it's rather the other way around: as it was necessary to implement
      filters in eth-multiplexer, and implementing BPF seemed more reasoable,
      pfinet had to be changed to use BPF...
    <braunr> antrik: where is zhengda's branch btw ?
    <antrik> (I guess using proper filters with eth-multiplexer is not strictly
      necessary; but it would be a major performance hit not to)
    <antrik> it's in incubator.git
    <antrik> but it's very messy
    <braunr> ok
    <antrik> at some point I asked him to provide cleaned up branches, and I'm
      pretty sure he said he did, but I totally fail to remember where he
      published them :-(
    <braunr> hm, i don't like how devnode is "architectured" :/
    <braunr> but it makes things a little more easy to get working i guess
    <LarstiQ> antrik: any idea what to grep the logs on for that?
    <braunr> ok never mind, devnode is fine
    <braunr> exactly what i need
    <braunr> i wonder however if it shouldn't be improved to better handle
      permissions
    <braunr> ok, never mind either, permission handling is fine
    <braunr> so what are we waiting for ? :)
    <antrik> I remember that there were some issues with permission handling,
      but I don't remember whether all were fixed :-(
    <antrik> LarstiQ: hm... good question...
    <braunr> ah ?
    <braunr> hm actually, there could be issues for packet filters, yes
    <braunr> i guess we want to allow e.g. read-only opens for capture only
    <antrik> braunr: that would have to be handled by the actual BPF
      implementation I'd say
    <braunr> it should already be the case
    <antrik> what's the problem then?
    <braunr> but when the actual device_open() is performed, the appropriate
      permissions must be provided
    <braunr> and checking those is the responsibility of the proxy, devnode in
      this case
    <antrik> and it doesn't do that?
    <braunr> apparently not
    <braunr> the only check is against the device name
    <braunr> i'll begin playing with that first
    <antrik> I vaguely remember that there has been discussion about the
      relation of underlying device open mode and devnode open mode... but I
      don't remember the outcome. in fact it was probably one of the
      discussions I never got around to follow up on... :-(
    <antrik> before you begin playing, take a look at the relevant messages in
      the ML archive :-)
    <antrik> must have been around two years ago
    <braunr> ok
    <antrik> some thread with me and scolobb (Sergiu Ivanov +- spelling) and
      probably zhengda
    <antrik> there might also be some outstanding patch(es) from scolobb, not
      sure


## IRC, freenode, #hurd, 2012-01-17

    <braunr> antrik: i think i found the thread you mentioned about devnode
    <braunr> neither sergiu nor zhengda considered the use of a read-only
      device for packet filtering
    <braunr> leading to assumptions such as "only receiving packets
    <braunr> is not terribly useful, in view of the fact that you have to at
      least
    <braunr> request them, which implies *sending* packets :-)
    <braunr> "
    <braunr> IMO, devnode should definitely check its node permissions to build
      the device open flags
    <braunr> good news is that it doesn't depend on anything specific to other
      incubator projects
    <braunr> making it almost readily mergeable in the hurd
    <braunr> i'm not sure devnode is an appropriate name though
    <braunr> maybe something like device, or devproxy
    <braunr> proxy-devopen maybe
    <antrik> braunr: well, I don't remember the details of the disucssion; but
      as I mentioned in some mail, I did actually want to write a followup,
      just didn't get around to it... so I was definitely not in agreement with
      some of the statements made by others. I just don't remember on which
      point :-)
    <antrik> which thread was it?
    <antrik> anyways, this should in no way be specific to network
      devices... the idea is simply that if the client has only read
      permissions on the device node, it should only get to open the underlying
      device for read. it's up to the kernel to handle the read-only status for
      the device once it's opened
    <antrik> as for the naming, the idea is that devnode simply makes Mach
      devices accessible through FS nodes... so the name seemed appropriate
    <antrik> you may be right though that just "device" might be more
      straightforward... I don't agree on the other variants
    <braunr> antrik:
      http://lists.gnu.org/archive/html/bug-hurd/2009-12/msg00155.html
    <braunr> antrik: i agree with the general idea behind permission handling,
      i was just referring to their thoughts about it, which probably led to
      the hard coded READ | WRITE flags
    <antrik> braunr: unfortunately, I don't remember the context of the
      discussion... would take me a while to get into this again :-(
    <antrik> the discussion seems to be about eth-multiplexer as much as about
      devnode (if not more), and I don't remember the exact interaction


## IRC, freenode, #hurd, 2012-01-18

    <braunr> so, does anyone have an objection to getting devnode into the hurd
      and calling it something else like e.g. device ?
    <youpi> braunr: it's Zhengda's work, right?
    <braunr> yes
    <youpi> I'm completely for it, it just perhaps needs some cleanup
    <braunr> i have a few changes to add to what already exists
    <braunr> ok
    <braunr> well i'm assigning myself to the task
    <antrik> braunr: I'm still not convinced just "device" is preferable
    <antrik> perhaps machdevice ;-)
    <antrik> but otherwise, I'd LOVE to see it in :-)
    <braunr> i don't know .. what if the device is actually eth-multiplexer or
      a dde one ?
    <braunr> it's not really "mach", is it ?
    <braunr> or do we only refer to the interface ?
    <youpi> that translator is only for mach devices
    <braunr> so you consider dde devices as being mach devices too ?
    <braunr> it's a simple proxy for device_open really
    <youpi> will these devices use that translator?
    <youpi> ah
    <youpi> I thought it was using a mach-specific RPC
    <braunr> so we can consider whatever we want
    <antrik> braunr: yes, the translator is for Mach device interface only. it
      might be provided by other servers, but it's still Mach devices
    <youpi> then drop the mach, yes
    <braunr> i'd tend to agree with antrik
    <youpi> antrik: I'd say the device interface is part of the hur dinterfaces
    <braunr> then machdev :p
    <braunr> no, it's really part of the mach interface
    <youpi> it's part of the mach interface, yes
    <youpi> but also of the Hurd, no?
    <antrik> DDE network servers also use the Mach device interface
    <braunr> no
    <youpi> can't we say it's part of it?
    <youpi> I mean
    <youpi> even if we change the kernel
    <braunr> dde is the only thing that implements it besides the kernel that i
      know of
    <youpi> we will probably want to keep the same interface
    <braunr> yes but that's a mach thing
    <youpi> what we have now is not necessarily a reason
    <antrik> as for other DDE drivers, I for my part believe they should export
      proper Hurd (UNIX) device nodes directly... but for some reason zhengda
      insisted on implementing it as Mach devices too :-(
    <braunr> antrik: i agree with you on that too
    <braunr> i was a bit surprised to see the same interface was reused
    <braunr> youpi: we can, we just have to agree on what we'll do
    <braunr> what do you mean by "even if we change the kernel" ?
    <antrik> the problem with "machdev" is that it might suggest the translator
      actually implements the device... not sure whether this would cause
      serious confusion
    <antrik> "devopen" might be another option
    <antrik> or "machdevopen" to be entirely verbose ;-)
    <braunr> an option i suggested earlier which you disagreed on :p
    <braunr> but devopen is the one i'd choose
    <antrik> youpi: as I already mentioned in the libburn thread, I don't
      actually think the Mach device interface is very nice; IMHO we should get
      rid of it as soon as we can, rather than port it to other
      architectures...
    <antrik> but even *if* we decided to reuse it after all, it would still be
      the Mach device interface :-)
    <braunr> actually, zheng da already suggested that name a long time ago
    <braunr> http://lists.gnu.org/archive/html/bug-hurd/2008-08/msg00005.html
    <braunr> no actually antrik did eh
    <braunr> ok let's use devopen
    <antrik> braunr: you suggested proxy-devopen, which I didn't like because
      of the "proxy" part :-)
    <braunr> not only, but i don't have the logs any more :p
    <antrik> oh, I already suggested devopen once? didn't expect myself to be
      that consistent... ;-)
    <antrik> braunr: you suggested device, devproxy or proxy-devopen
    <braunr> ah, ok
    <braunr> devopen is better
    <antrik> I wonder whether it's more important for clarity to have "mach" in
      there or "open"... or whether it's really too unweildy to have both


## IRC, freenode, #hurd, 2012-01-21

    <braunr> oh btw, i made devopen run today, it shouldn't be hard getting it
      in properly
    <braunr> patching libpcap will be somewhat trickier
    <braunr> i don't even really need it, but it allows having user access to
      mach devices, which is nice for the libpcap patch and tcpdump tests
    <braunr> permission checking is actually its only purpose
    <braunr> well, no, not really, it also allows opening devices implemented
      by user space servers transparently


## IRC, freenode, #hurd, 2012-01-27

    <braunr> hmm, bpf needs more work :(
    <braunr> or we can use the userspace bpf filter in libpcap, so that it
      works with both gnumach and dde drivers
    <antrik> braunr: there is a userspace BPF implementation in libpcap? I'm
      surprised that zhengda didn't notice it, and ported the one from gnumach
      instead...
    <antrik> what is missing in the kernel implementation?
    <braunr> antrik: filling the bpf header
    <braunr> frankly, i'm not sure we want to bother with the kernel
      implementation
    <braunr> i'd like it to work with both gnumach and dde drivers
    <braunr> and in the long run, we'll be using userspace drivers anyway
    <braunr> the bpf header was one of the things the defunct translator did
    <braunr> which involved ugly memcpy()s :p
    <antrik> braunr: well, if you want to get rid of the kernel implementation,
      basically you would have to take up eth-multiplexer and get it into
      mainline
    <antrik> (and make sure it's used by default in Debian)
    <antrik> I frankly believe it's the better design anyways... but quite a
      major change :-)
    <braunr> not that major to me
    <braunr> in the meantime i'll use the libpcap embedded implementation
    <braunr> we'll have something useful faster, with minimum work when
      eth-multiplexer is available
    <antrik> eth-multiplexer is ready for use, it just needs to go upstream
    <antrik> though it's probably desirable to switch it to the BPF
      implementation from libpcap
    <braunr> using the libpcap implementation in libpcap and in eth-multiplexer
      are two different things
    <braunr> the latter is preferrable
    <braunr> (and yes, by available, i meant upstream ofc)
    <antrik> eth-mulitplexer is already using libpcap anyways (for compiling
      the filters); I'm sure zhengda just didn't realize it has an actual BPF
      implementation too...
    <braunr> we want the filter implementation as close to the packet source as
      possible
    <antrik> I have been using eth-multiplexer for at least two years now
    <braunr> hm, there is a "snoop" source type, using raw sockets
    <braunr> too far from the packet source, but i'll try it anyway
    <braunr> hm wrong, snoop was the solaris packet filter fyi


## IRC, freenode, #hurd, 2012-01-28

    <braunr> nice, i have tcpdump working :)
    <braunr> let's see if it's as simple with wireshark
    <pinotree> \o/
    <braunr> pinotree: it was actually very simple
    <pinotree> heh, POV ;)
    <braunr> yep, wireshark works too
    <braunr> promiscuous mode is harder to test :/
    <braunr> but that's a start


## IRC, freenode, #hurd, 2012-01-30

    <braunr> ok so next step: get tcpreplay working
    <antrik> braunr: BTW, when you checked the status of the kernel BPF code,
      did you take zhengda's enhancements/fixes into account?...
    <braunr> no
    <braunr> when did i check it ?
    <antrik> braunr: well, you said the kernel BPF code has serious
      shortcomings. did you take zhengda's changes into account?
    <braunr> antrik: ah, when i mention the issues, i considered the userspace
      translator only
    <braunr> antrik: and stuff like non blocking io, exporting a selectable
      file descriptor
    <braunr> antrik: deb http://ftp.sceen.net/debian-hurd experimental/
    <braunr> antrik: this is my easy to use repository with a patched
      libpcap0.8
    <braunr> and a small and unoptimized pcap-hurd.c module
    <braunr> it doesn't use devopen yet
    <braunr> i thought it would be better to have packet filtering working
      first as a debian patch, then get the new translator+final patch upstream
    <jkoenig> braunr, tcpdump works great here (awesome!). I'm probably using
      exactly the same setup and "hardware" as you do, though :-P


## IRC, freenode, #hurd, 2012-01-31

    <braunr> antrik: i tend to think we need a bpf translator, or anything
      between the kernel and libpcap to provide selectable file descriptors
    <braunr> jkoenig: do you happen to know how mach_msg (as called in a
      hello.c file without special macros or options) deals with signals ?
    <braunr> i mean, is it wrapped by the libc in a version that sets errno ?
    <jkoenig> braunr: no idea.
    <pinotree> braunr: what's up with it? (not that i have an idea about your
      actual question, just curious)
    <braunr> pinotree: i'm improving signal handling in my pcap-hurd module
    <braunr> i guess checking for MACH_RCV_INTERRUPTED will dio
    <braunr> -INFO is correctly handled :)
    <braunr> ok new patch seems fine
    <antrik> braunr: selectable file descriptors?
    <braunr> antrik: see pcap_fileno() for example
    <braunr> it returns a file descriptor matching the underlying object
      (usually a socket) that can be multiplexed in a select/poll call
    <braunr> obviously a mach port alone can't do the job
    <braunr> i've upgraded the libpcap0.8 package with improved signal handling
      for tests
    <antrik> braunr: no idea what you are talking about :-(


## IRC, freenode, #hurd, 2012-02-01

    <braunr> antrik: you do know about select/poll
    <braunr> antrik: you know they work with multiple selectable/pollable file
      descriptors
    <braunr> on most unix systems, packet capture sources are socket
      descriptors
    <braunr> they're selectable/pollable
    <antrik> braunr: what are packet capture sources?
    <braunr> antrik: objects that provide applications with packets :)
    <braunr> antrik: a PF_PACKET socket on Linux for example, or a Mach device,
      or a BPF file descriptor on BSD
    <antrik> for a single network device? or all of them?
    <antrik> AIUI the userspace BPF implementation in libpcap opens this
      device, waits for packets, and if any arrive, decides depending on the
      rules whether to pass them to the main program?
    <braunr> antrik: that's it, but it's not the point
    <braunr> antrik: the point is that, if programs need to include packet
      sources in select/poll calls, they need file descriptors
    <braunr> without a translator, i can't provide that
    <braunr> so we either decide to stick with the libpcap patch only, and keep
      this limitation, or we write a translator that enables this feature
    <pinotree> braunr: are the two options exclusive?
    <braunr> pinotree: unless we implement a complete bpf translator like i did
      years ago, we'll need a patch in libpcap
    <braunr> pinotree: the problem with my early translator implementation is
      that it's buggy :(
    <braunr> pinotree: and it's also slower, as packets are small enough to be
      passed through raw copies
    <antrik> braunr: I'm not sure what you mean when talking about "programs
      including packet sources". programs only interact with packet sources
      through libpcap, right?
    <antrik> braunr: or are you saying that programs somehow include file
      descriptors for packet sources (how do they obtain them?) in their main
      loop, and explicitly pass control to libpcap once something arrives on
      the respecitive descriptors?
    <braunr> antrik: that's the idea, yes
    <antrik> braunr: what is the idea?
    <braunr> 20:38 < antrik> braunr: or are you saying that programs somehow
      include file descriptors for packet sources (how do they obtain them?) in
      their main loop, and explicitly pass control to libpcap once something
      arrives on the respecitive descriptors?
    <antrik> braunr: you didn't answer my question though :-)
    <antrik> braunr: how do programs obtain these FDs?
    <braunr> antrik: using pcap_fileno() for example


## IRC, freenode, #hurd, 2012-02-02

    <antrik> braunr: oh right, you already mentioned that one...
    <antrik> braunr: so you want some entity that exposes the device as
      something more POSIXy, so it can be used in standard FS calls, unlike the
      Mach devices used for pfinet
    <antrik> this is probably a good sentiment in general... but I'm not in
      favour of a special solution only for BPF. rather I'd take this as an
      indication that we probably should expose network interfaces as something
      file-like in general after all, and adapt pfinet, eth-multiplexer, and
      DDE accordingly
    <braunr> antrik: i agree
    <braunr> antrik: eth-multiplexer would be the right place


## IRC, freenode, #hurd, 2012-04-24

    <gnu_srs> braunr: Is  BPF fully supported by now? Can it be used for
      isc-dhcp?
    <braunr> gnu_srs: bpf isn't supported at all
    <braunr> gnu_srs: instead of emulating it, i added a hurd-specific module
      in libpcap
    <braunr> if isc-dhcp can use libpcap, then fine
    <braunr> (otherwise we could create a hurd-specific patch for dhcp that
      uses the in-kernel bpf filter implementation)
    <braunr> gnu_srs: can't it use a raw socket ?
    <youpi> it can
    <youpi> it's just that the shape of the patch to do so wasn't exactly how
      they needed it
    <youpi> so they have to rework it a bit
    <youpi> and that takes time
    <braunr> ok
    <braunr> antrik: for now, we prefer encapsulating the system specific code
      in libpcap, and let users of that library benefit from it
    <braunr> instead of implementing the low level bpf interface, which
      nonetheless has some system-specific variants ..


## IRC, freenode, #hurd, 2012-08-03

In context of the [[select]] issue.

    <braunr> i understand now why my bpf translator was so buggy
    <braunr> the condition_timedwait i wrote at the time was .. incomplete :)