ab9dd18bec
See https://github.com/hashicorp/consul/issues/3977 While trying to improve furthermore #3948 (This pull request is still valid since we are not using Compression to compute the result anyway). I saw a strange behaviour of dns library. Basically, msg.Len() and len(msg.Pack()) disagree on Message len. Thus, calculation of DNS response is false consul relies on msg.Len() instead of the result of Pack() This is linked to miekg/dns#453 and a fix has been provided with miekg/dns#454 Would it be possible to upgrade miekg/dns to a more recent function ? Consul might for instance upgrade to a post 1.0 release such as https://github.com/miekg/dns/releases/tag/v1.0.4
83 lines
3.1 KiB
Go
83 lines
3.1 KiB
Go
// Copyright 2016 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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/*
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Package bpf implements marshaling and unmarshaling of programs for the
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Berkeley Packet Filter virtual machine, and provides a Go implementation
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of the virtual machine.
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BPF's main use is to specify a packet filter for network taps, so that
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the kernel doesn't have to expensively copy every packet it sees to
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userspace. However, it's been repurposed to other areas where running
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user code in-kernel is needed. For example, Linux's seccomp uses BPF
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to apply security policies to system calls. For simplicity, this
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documentation refers only to packets, but other uses of BPF have their
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own data payloads.
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BPF programs run in a restricted virtual machine. It has almost no
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access to kernel functions, and while conditional branches are
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allowed, they can only jump forwards, to guarantee that there are no
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infinite loops.
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The virtual machine
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The BPF VM is an accumulator machine. Its main register, called
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register A, is an implicit source and destination in all arithmetic
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and logic operations. The machine also has 16 scratch registers for
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temporary storage, and an indirection register (register X) for
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indirect memory access. All registers are 32 bits wide.
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Each run of a BPF program is given one packet, which is placed in the
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VM's read-only "main memory". LoadAbsolute and LoadIndirect
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instructions can fetch up to 32 bits at a time into register A for
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examination.
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The goal of a BPF program is to produce and return a verdict (uint32),
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which tells the kernel what to do with the packet. In the context of
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packet filtering, the returned value is the number of bytes of the
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packet to forward to userspace, or 0 to ignore the packet. Other
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contexts like seccomp define their own return values.
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In order to simplify programs, attempts to read past the end of the
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packet terminate the program execution with a verdict of 0 (ignore
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packet). This means that the vast majority of BPF programs don't need
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to do any explicit bounds checking.
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In addition to the bytes of the packet, some BPF programs have access
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to extensions, which are essentially calls to kernel utility
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functions. Currently, the only extensions supported by this package
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are the Linux packet filter extensions.
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Examples
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This packet filter selects all ARP packets.
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bpf.Assemble([]bpf.Instruction{
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// Load "EtherType" field from the ethernet header.
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bpf.LoadAbsolute{Off: 12, Size: 2},
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// Skip over the next instruction if EtherType is not ARP.
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bpf.JumpIf{Cond: bpf.JumpNotEqual, Val: 0x0806, SkipTrue: 1},
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// Verdict is "send up to 4k of the packet to userspace."
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bpf.RetConstant{Val: 4096},
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// Verdict is "ignore packet."
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bpf.RetConstant{Val: 0},
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})
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This packet filter captures a random 1% sample of traffic.
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bpf.Assemble([]bpf.Instruction{
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// Get a 32-bit random number from the Linux kernel.
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bpf.LoadExtension{Num: bpf.ExtRand},
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// 1% dice roll?
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bpf.JumpIf{Cond: bpf.JumpLessThan, Val: 2^32/100, SkipFalse: 1},
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// Capture.
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bpf.RetConstant{Val: 4096},
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// Ignore.
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bpf.RetConstant{Val: 0},
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})
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*/
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package bpf // import "golang.org/x/net/bpf"
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