在一些场景下（特别是云原生场景），Linux系统中的网络部署变得越来越复杂。一个TCP连接，从客户端到服务端，中间可能要经过复杂的NAT、GRE、IPVS等过程，网络报文在节点（主机）上的处理路径也变得越来越长。在发生网络故障（比如网络丢包）时，如何快速、有效地定位出网络问题成为了一个难题。目前常规的网络故障定位手段，如tcpdump、dropwatch、ftrace、kprobe等存在一定的短板：
tcpdump：只能在链路层抓包，无法定位内核协议栈中的问题，比如常规的内核丢包问题ftrace：只能跟踪内核函数，无法进行报文过滤，且入手较难，需要对内核协议栈有一定了解kprobe：临时编写内核模块，效率和安全性低BCC：功能单一，临时编写BCC程序跟踪效率低，需要对内核有一定了解，入手难dropwatch：功能单一，只能查看网络丢包问题，且无法得到丢包原因和解决方案在此背景下，笔者结合多年的Kernel网络协议栈故障定位经验，基于eBPF开发了Linux环境下网络故障定位工具集——nettrace。
nettrace是一款基于eBPF的集网络报文跟踪（故障定位）、网络故障诊断、网络异常监控于一体的网络工具集，旨在能够提供一种更加高效、易用的方法来解决复杂场景下的网络问题。目前，其实现的功能包括：
droptrace：用于跟踪、监控系统中的丢包事件的工具，点击这里查看详情介绍。该功能已被遗弃，可以使用nettrace --drop实现相同的功能。nettrace是采用C语言编写的基于eBPF（libbpf）的命令行工具，在使用和安装时可以用编译好的RPM包和二进制程序。本工具对各个版本的内核都进行了兼容，最低可支持centos7的3.10版本的内核（需要自己编译）。
对于支持BTF特性（内核版本 >= 5.3，并且配置了CONFIG_DEBUG_INFO_BTF=y内核配置项）的内核，可以直接下载releases中编译好的nettrace-xxx-1.btf.x86_64.rpm、nettrace-xxx-1.btf.x86_64.deb安装包进行安装使用；对于不支持BTF的低版本的内核，需要在对应的系统上手动编译后才能使用。对于OpenCloudOS/TencentOS系统，可以直接使用yum命令来进行在线安装：
sudo yum install nettrace也可以直接从releases中下载对应的RPM/DEB安装包，手动进行安装。
直接从releases下载编译好的二进制包也是可以的，releases中的tar.bz2格式的压缩包即为二进制程序。由于里面的工具采用的都是静态编译的方式，因此在内核版本支持的情况下，都是可以直接下载解压后运行的。再次提醒：对于不支持BTF的内核版本，需要手动编译才能使用。
下面来介绍下如何在Centos、ubuntu等环境上进行nettrace工具的手动编译和安装。本工具目前在3.10/4.14/4.15/5.4/5.15/6.4等版本的内核上均进行过适配和测试。由于本工具对于libbpf的版本要求比较高，因此建议使用基于docker的方式来进行编译。
本工具在编译的时候依赖于libelf、libbpf和bpftool组件，clang和gcc编译工具。对于不支持BTF的内核，还需要安装kernel-headers头文件，可以通过查看目录/lib/modules/$(uname -a)/build是否存在来判断headers是否已经被安装了。
注意事项：
请尽量使用较高版本的libbpf（v0.2以上版本）。如果当前发行版的libbpf库版本达不到要求，可以手动进行libbpf的编译安装：
wget https://github.com/libbpf/libbpf/archive/refs/tags/v1.1.0.tar.gz
tar -xf v1.1.0.tar.gz
cd libbpf-1.1/src
make installclang版本要在10+
对于ubuntu系统，使用以下命令安装依赖：
sudo apt install python3 python3-yaml libelf-dev libbpf-dev linux-headers-`uname -r` clang llvm gcc linux-tools-`uname -r` linux-tools-generic -y对于opencloudos/tencentos/centos用户，使用以下命令来安装依赖：
sudo yum install python3-yaml elfutils-devel elfutils-devel-static libbpf-devel libbpf-static kernel-headers kernel-devel clang llvm bpftool -y直接下载nettrace的源码即可进行编译安装：
git clone https://github.com/OpenCloudOS/nettrace.git
cd nettrace
make all支持的编译选项有以下几个：
NO_BTF：
注意：对于不支持BTF的内核（内核版本低于5.3），在编译的时候需要加参数NO_BTF=1，采用兼容模式进行编译，如下所示：
make NO_BTF=1 all启用该参数，eBPF程序会以BPF_PROBE_READ的方式来读取数据；否则，eBPF程序会以BPF_CORE_READ的方式来读取。
INLINE：
以完全内联的方便编译BPF程序，对于不支持subprog的内核（内核版本在4.15及以下）需要指定这个参数：
make NO_BTF=1 INLINE=1 all由于采用完全的内联，这种方式编译出来的nettrace工具的体积会比常规的较大。
INIT
让BPF程序初始化所有的event数据，而不是局部初始化。对于内核版较低的情况，如果出现bpf_perf_event_output调用失败的错误，可以通过制定这个参数为1来解决问题。INLINE=1涵盖了INIT=1。
NO_GLOBAL_DATA：
对于不支持global data的内核版本（内核版本在5.4以下）需要指定这个参数，从而禁用global data的一些特性：
make NO_BTF=1 NO_GLOBAL_DATA=1 allCOMPAT：
上面三种的联合。对于版本较低的内核，可以直接使用这个参数来使能上面的三个参数。
OUTPUT_WHOLE:
采用全量的方式输出事件（更多的开销）。当加载的时候出现以下报错，可以尝试添加这个编译参数来解决：
R5 min value is negative, either use unsigned or 'var &= const'KERNEL：手动指定要使用的内核源码（内核头文件）：
make KERNEL=/home/ubuntu/kernel NO_BTF=1 all需要和NO_BTF一起使用，因为基于BTF编译的话是不需要头文件的。
注意： 兼容模式编译出来的nettrace工具只能运行在和KERNEL内核版本相同的环境上。如果没有指定KERNEL，那采用的就是当前编译环境上的内核头文件，这就要求编译环境和运行环境所使用的内核要完全相同才能正常运行。否则，会发生意想不到的意外。
对于发行版版本较低，难以安装高版本clang的情况下，可以基于docker来进行代码的编译，具体可参考2.4章节来进行安装。
同时，对于ubuntu 16.04/ubuntu 18.04系统，其内核似乎存在BUG，即其使用的内核版本实际为4.15.18，uname看到的却是4.15.0。这导致了加载eBPF程序的时候内核版本不一致，无法加载。因此对于这种情况，可以使用KERN_VER参数来手动指定内核版本（计算方式为：(4<<16) + (15<<8) + 18）：
make KERN_VER=266002 COMPAT=1 all注意： 这里最后一个版本号如果过大，超过了255，那么就取255。例如，对于4.19.282，就要按照(4<<16) + (19<<8) + 255的方式来计算。
使用命令make rpm可制作rpm包；使用命令make pack可制作二进制包（二进制程序打包到压缩包中，默认存放路径为output文件夹）。
对于支持BTF的内核，无需安装任何依赖，可以直接使用以下命令来进行nettrace的编译：
docker run -it --rm --network=host --privileged -v $(pwd):$(pwd) -v /lib/modules/:/lib/modules/ -v /usr/src/:/usr/src/ imagedong/nettrace-build make -C $(pwd) all对于不支持BTF的系统，这需要先安装kernel-headers软件包，如上面的手动编译里面所说的。ubuntu系统使用命令apt install linux-headers-$(uname -r) -y进行安装；centos使用命令yum install kernel-headers kernel-devel -y进行安装。然后使用下面的命令进行编译：
docker run -it --rm --network=host --privileged -v $(pwd):$(pwd) -v /lib/modules/:/lib/modules/ -v /usr/src/:/usr/src/ imagedong/nettrace-build make -C $(pwd) NO_BTF=1 all注意： 兼容模式编译出来的nettrace工具只能运行在和KERNEL内核版本相同的环境上。如果没有指定KERNEL，那采用的就是当前编译环境上的内核头文件，这就要求编译环境和运行环境所使用的内核要完全相同才能正常运行。否则，会发生意想不到的意外。
注意： docker镜像可能会更新，为了使用最新的镜像，建议先试用命令docker pull imagedong/nettrace-build来获取最新的容器镜像。
nettrace是用来跟踪内核报文和诊断网络故障的，在进行报文跟踪时可以使用一定的过滤条件来跟踪特定的报文。其基本命令行参数为：
$ nettrace -h
nettrace: a tool to trace skb in kernel and diagnose network problem
Usage:
-s, --saddr filter source ip/ipv6 address
-d, --daddr filter dest ip/ipv6 address
--addr filter source or dest ip/ipv6 address
-S, --sport filter source TCP/UDP port
-D, --dport filter dest TCP/UDP port
-P, --port filter source or dest TCP/UDP port
-p, --proto filter L3/L4 protocol, such as 'tcp', 'arp'
--netns filter by net namespace inode
--netns-current filter by current net namespace
--pid filter by current process id(pid)
--min-latency filter by the minial time to live of the skb in us
--pkt-len filter by the IP packet length (include header) in byte
--tcp-flags filter by TCP flags, such as: SAPR
--basic use 'basic' trace mode, don't trace skb's life
--diag enable 'diagnose' mode
--diag-quiet only print abnormal packet
--diag-keep don't quit when abnormal packet found
--drop skb drop monitor mode, for replace of 'droptrace'
--drop-stack print the kernel function call stack of kfree_skb
--sock enable 'sock' mode
--monitor enable 'monitor' mode
--rtt enable 'rtt' in statistics mode
--rtt-detail enable 'rtt' in detail mode
--filter-srtt filter by the minial first-acked rtt in ms
--filter-minrtt filter by the minial last-acked rtt in ms
--latency-show show latency between kernel functions
--latency-free account the latency of skb free
--latency enable 'latency' mode
--latency-summary
show latency by statistics
-t, --trace enable trace group or trace. Some traces are disabled by default, use "all" to enable all
--force skip some check and force load nettrace
--ret show function return value
--detail show extern packet info, such as pid, ifname, etc
--date print timestamp in date-time format
-c, --count exit after receiving count packets
--hooks print netfilter hooks if dropping by netfilter
--tiny-show set this option to show less infomation
--trace-stack print call stack for traces or group
--trace-matcher traces that can match packet(default all)
--trace-exclude traces that should be disabled
--trace-noclone don't trace skb clone
--trace-free custom the free functions
--func-stats only do the statistics for function call
--rate-limit limit the output to N/s, not valid in diag/default mode
--btf-path custom the path of BTF info of vmlinux
-v show log information
--debug show debug information
-h, --help show help information
-V, --version show nettrace version过滤类参数
参数s/d/addr/S/D/port/p/pid用于进行报文的过滤，可以通过IP地址（包括IPv6地址）、端口、协议等属性进行过滤。其他参数的用途包括：
netns：根据网络命名空间进行过滤，该参数后面跟的是网络命名空间的inode，可以通过ls -l /proc/<pid>/ns/net来查看对应进程的网络命名空间的inode号netns-current：仅显示当前网络命名空间的报文，等价于--netns 当前网络命名空间的inodepid：根据当前处理报文的进程的ID进行过滤min-latency：根据报文的寿命进行过滤，仅打印处理时长超过该值的报文，单位为us。该参数仅在默认/diag/latency模式下可用。pkt-len：根据IP报文总长度（包括报文头部）来进行过滤tcp-flags：根据TCP报文的flags进行过滤，支持的flag包括：SAPRF模式类参数
basic：启用basic跟踪模式。默认情况下，启用的是生命周期跟踪模式。启用该模式后，会直接打印出报文所经过的内核函数/tracepoint。diag：启用诊断模式diag-quiet：只显示出现存在问题的报文，不显示正常的报文diag-keep：持续跟踪。diag模式下，默认在跟踪到异常报文后会停止跟踪，使用该参数后，会持续跟踪下去。drop：进行系统丢包监控，取代原先的droptracedrop-stack: 打印kfree_skb内核函数的调用堆栈，等价于--trace-stack kfree_skbsock：启用套接口模式。这个模式下，不会再跟踪报文，而会跟踪套接口。monitor：启用监控模式。一种轻量化的实时监控系统中网络异常的模式（对内核版本有一定要求）。rtt：启用RTT统计模式，会统计TCP RTT的分布情况rtt-detail：启用RTT详细模式，输出符合过滤条件的每个报文的RTT数据filter-srtt：根据srtt来进行过滤，rtt/rtt-detail模式下可用，单位msfilter-minrtt：根据minrtt来进行过滤，rtt/rtt-detail模式下可用，单位mslatency-show：显示延迟（协议栈处理耗时）信息，basic/sock模式下不可用latency：启用延迟分析模式，可以高效分析每个报文协议栈处理耗时latency-summary：启用延迟分析统计模式，可以统计协议栈处理耗时的分布情况显示类参数
t/trace：要启用的跟踪模块，默认启用所有ret：跟踪和显示内核函数的返回值detail：显示跟踪详细信息，包括当前的进程、网口和CPU等信息date：以时间格式打印（以2022-10-24 xx:xx:xx.xxxxxx格式打印），而不是时间戳c/count：指定要跟踪的报文个数c，达到该个数后自动退出hooks：结合netfilter做的适配，详见下文tiny-show：精简显示，只显示第一个报文的内容，用于提升性能trace-stack：指定需要进行堆栈打印的内核函数，可以指定多个，用“,”分隔。trace-matcher：指定进行报文匹配的内核函数，默认所有的函数，用于提升性能trace-exclude：不进行跟踪的函数trace-noclone：不跟踪报文的克隆时间，即不把克隆出来的报文和当前报文放到一块跟踪func-stats：只统计内核函数被调用的次数，不打印具体的报文，可指定过滤条件rate-limit：进行限速，限制每秒事件输出的数量btf-path：手动指定BTF文件的路径默认情况下，nettrace会跟踪报文从进入到内核协议栈到离开（销毁）的过程。对于有一定内核网络经验的人来说，可以通过报文的内核路径来快速推断出当前的网络问题，达到快速定位的目的。
sudo ./nettrace -p icmp --saddr 169.254.128.15
begin trace...
***************** e8fbc700,e8fbdc00 ***************
[1273445.360831] [dev_gro_receive ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360844] [__netif_receive_skb_core] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360847] [ip_rcv ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360850] [ip_rcv_core ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360854] [skb_clone ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360861] [nf_hook_slow ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360868] [nft_do_chain ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360875] [ip_rcv_finish ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360878] [ip_route_input_slow ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360882] [fib_validate_source ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360887] [ip_local_deliver ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360890] [nf_hook_slow ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360895] [ip_local_deliver_finish] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360899] [icmp_rcv ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360903] [icmp_echo ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360908] [icmp_reply ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360922] [consume_skb ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360930] [packet_rcv ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754
[1273445.360933] [consume_skb ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 54754上面的*中间的表示当前所跟踪的skb的地址，由于当前的报文被克隆过，因此当前跟踪上下文存在两个报文。如果不想跟踪被克隆的报文（不想将被克隆的报文和原报文一起显示），那么可以加个--trace-noclone参数。
sudo ./nettrace -p icmp --saddr 169.254.128.15 --detail
begin trace...
***************** e8fbcd00,e8fbcc00 ***************
[1273732.110173] [e8fbcd00][dev_gro_receive ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110185] [e8fbcd00][__netif_receive_skb_core][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110189] [e8fbcd00][ip_rcv ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110192] [e8fbcd00][ip_rcv_core ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110196] [e8fbcd00][skb_clone ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110204] [e8fbcc00][nf_hook_slow ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110211] [e8fbcc00][nft_do_chain ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110222] [e8fbcc00][ip_rcv_finish ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110229] [e8fbcc00][ip_route_input_slow ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110234] [e8fbcc00][fib_validate_source ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110240] [e8fbcc00][ip_local_deliver ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110243] [e8fbcc00][nf_hook_slow ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110252] [e8fbcc00][ip_local_deliver_finish][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110255] [e8fbcc00][icmp_rcv ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110260] [e8fbcc00][icmp_echo ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110267] [e8fbcc00][icmp_reply ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110283] [e8fbcc00][consume_skb ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110291] [e8fbcd00][packet_rcv ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464
[1273732.110294] [e8fbcd00][consume_skb ][cpu:40 ][ens5 ][pid:0 ][swapper/40 ] ICMP: 169.254.128.15 -> 172.27.0.6 ping request, seq: 56464可以看到，每个报文的地址、所在CPU、网口和进程信息都被打印了出来。
在对报文进行跟踪时，一旦报文被跟踪起来（命中过滤条件），那么这个报文即使内容发生了变化也会持续被跟踪，直到报文被释放。下面是NAT场景下的跟踪，可以看到报文的源地址由192.168.122.8通过SNAT被修改成了9.135.224.89，但是报文依然被跟踪到了：
$ sudo ./nettrace -p icmp --addr 192.168.122.8
begin tracing......
<------------------- skb: 8f02f900 ---------------------->
463697.331957: [__netif_receive_skb_core]: ICMP: 192.168.122.8 -> 10.123.119.98, ping request , seq: 0
463697.331972: [nf_hook_slow ]: ICMP: 192.168.122.8 -> 10.123.119.98, ping request , seq: 0
463697.331985: [nf_hook_slow ]: ICMP: 192.168.122.8 -> 10.123.119.98, ping request , seq: 0
463697.331990: [__netif_receive_skb_core]: ICMP: 192.168.122.8 -> 10.123.119.98, ping request , seq: 0
463697.331994: [ip_rcv ]: ICMP: 192.168.122.8 -> 10.123.119.98, ping request , seq: 0
463697.331998: [ip_rcv_core ]: ICMP: 192.168.122.8 -> 10.123.119.98, ping request , seq: 0
463697.332001: [nf_hook_slow ]: ICMP: 192.168.122.8 -> 10.123.119.98, ping request , seq: 0
463697.332004: [ip_rcv_finish ]: ICMP: 192.168.122.8 -> 10.123.119.98, ping request , seq: 0
463697.332010: [ip_forward ]: ICMP: 192.168.122.8 -> 10.123.119.98, ping request , seq: 0
463697.332014: [nf_hook_slow ]: ICMP: 192.168.122.8 -> 10.123.119.98, ping request , seq: 0
463697.332024: [ip_output ]: ICMP: 192.168.122.8 -> 10.123.119.98, ping request , seq: 0
463697.332027: [nf_hook_slow ]: ICMP: 192.168.122.8 -> 10.123.119.98, ping request , seq: 0
463697.332037: [ip_finish_output ]: ICMP: 9.135.224.89 -> 10.123.119.98, ping request , seq: 0
463697.332039: [ip_finish_output2 ]: ICMP: 9.135.224.89 -> 10.123.119.98, ping request , seq: 0
463697.332042: [dev_queue_xmit ]: ICMP: 9.135.224.89 -> 10.123.119.98, ping request , seq: 0
463697.332046: [dev_hard_start_xmit ]: ICMP: 9.135.224.89 -> 10.123.119.98, ping request , seq: 0
463697.332060: [consume_skb ]: ICMP: 9.135.224.89 -> 10.123.119.98, ping request , seq: 0可以通过--trace-stack来指定需要进行内核堆栈打印的traces，使用方式与--trace完全一致。出于性能的考虑，尽量不要一次跟踪多个内核函数的堆栈。基本用法：
$ sudo ./nettrace -p icmp --trace-stack consume_skb,icmp_rcv
begin trace...
***************** 2cafd200,2cafdc00 ***************
[2846531.810609] [nf_hook_slow ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956 *ipv4 in chain: OUTPUT*
[2846531.810612] [ip_output ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810613] [nf_hook_slow ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956 *ipv4 in chain: POST_ROUTING*
[2846531.810615] [ip_finish_output ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810617] [ip_finish_output2 ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810619] [__dev_queue_xmit ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810621] [dev_hard_start_xmit ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956 *skb is successfully sent to the NIC driver*
[2846531.810623] [enqueue_to_backlog ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810630] [__netif_receive_skb_core.constprop.0] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810632] [ip_rcv ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810634] [ip_rcv_core ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810635] [nf_hook_slow ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956 *ipv4 in chain: PRE_ROUTING*
[2846531.810637] [ip_local_deliver ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810639] [nf_hook_slow ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956 *ipv4 in chain: INPUT*
[2846531.810640] [nft_do_chain ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956 *iptables table:filter, chain:INPUT*
[2846531.810642] [ip_local_deliver_finish] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810644] [skb_clone ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810649] [icmp_rcv ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
Call Stack:
-> icmp_rcv+0x1
-> ip_local_deliver_finish+0x7f
-> ip_local_deliver+0xea
-> ip_rcv+0x16d
-> __netif_receive_skb_one_core+0x89
-> process_backlog+0xa9
-> __napi_poll+0x2e
-> net_rx_action+0x28f
-> __do_softirq+0xfb
-> do_softirq+0xa7
-> __local_bh_enable_ip+0x79
-> ip_finish_output2+0x170
-> __ip_finish_output+0xae
-> ip_finish_output+0x36
-> ip_output+0x73
-> ip_push_pending_frames+0xab
-> raw_sendmsg+0x651
-> inet_sendmsg+0x6e
-> sock_sendmsg+0x60
-> __sys_sendto+0x10a
-> __x64_sys_sendto+0x24
-> do_syscall_64+0x3f
-> entry_SYSCALL_64_after_hwframe+0x72
[2846531.810651] [ping_rcv ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810653] [ping_lookup.isra.0 ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810654] [kfree_skb ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
[2846531.810659] [consume_skb ] ICMP: 127.0.0.1 -> 127.0.0.1 ping reply, seq: 3, id: 51956
Call Stack:
-> consume_skb+0xb8
-> consume_skb+0xb8
-> skb_free_datagram+0x11
-> raw_recvmsg+0xb2
-> inet_recvmsg+0x11d
-> sock_recvmsg+0x6e
-> ____sys_recvmsg+0x90
-> ___sys_recvmsg+0x7c
-> __sys_recvmsg+0x60
-> __x64_sys_recvmsg+0x1d
-> do_syscall_64+0x3f
-> entry_SYSCALL_64_after_hwframe+0x72默认情况下，每个被跟踪的函数都会尝试进行报文的解析和匹配，这个是比较产生较大的开销的。在网络带宽较大的情况下，会严重影响系统的性能。最坏的情况下，可能会导致性能下降80%。为了提升跟踪所产生的开销，可以指定进行报文匹配的函数。例如，对于收包阶段，可以指定__netif_receive_skb_core作为报文匹配函数。由于GRO相关的函数调用频率比较高，如果定位的问题和GRO无关，可以将GRO的两个函数从跟踪列表中排出掉：
./nettrace --trace-matcher __netif_receive_skb_core --trace-exclude napi_gro_receive_entry,dev_gro_receive -p tcp --port 12345 --tcp-flags S使用这种方式进行跟踪所产生的性能开销要小得多，预计在10-20%的样子。
精简显示模式下，除了第一个函数，其他函数上报的事件数据量都比较小，因此可以显著节省系统开销。在报文内容不被修改的情况下，可以使用这种方式：
$ ./nettrace --tiny-show -p tcp --port 9999
begin trace...
***************** 63618400,636184e8 ***************
[11485.683405] [__tcp_transmit_skb ] TCP: 127.0.0.1:39642 -> 127.0.0.1:9999 seq:1977307561, ack:0, flags:
[11485.683413] [skb_clone ]
[11485.683416] [__ip_queue_xmit ]
[11485.683418] [nf_hook_slow ]
[11485.683423] [nft_do_chain ]
[11485.683425] [ip_output ]
[11485.683427] [nf_hook_slow ]
[11485.683428] [nft_do_chain ]
[11485.683432] [ip_finish_output ]
[11485.683434] [ip_finish_output2 ]
[11485.683435] [__dev_queue_xmit ]
[11485.683437] [dev_hard_start_xmit ]
[11485.683438] [enqueue_to_backlog ]
[11485.683441] [__netif_receive_skb_core.constprop.0]
[11485.683442] [ip_rcv ]
[11485.683444] [ip_rcv_core ]
[11485.683445] [nf_hook_slow ]
[11485.683446] [ip_local_deliver ]
[11485.683447] [nf_hook_slow ]
[11485.683449] [ip_local_deliver_finish]
[11485.683450] [tcp_v4_rcv ]
[11485.683452] [__inet_lookup_listener]
[11485.683454] [tcp_v4_send_reset ]
[11485.683469] [kfree_skb ]
[11485.683486] [__kfree_skb ]使用方式与上面的一致，加个diag参数即可使用诊断模式。上文的生命周期模式对于使用者的要求比较高，需要了解内核协议栈各个函数的用法、返回值的意义等，易用性较差。诊断模式是在生命周期模式的基础上，提供了更加丰富的信息，使得没有网络开发经验的人也可进行复杂网络问题的定位和分析。
下面是使用诊断模式进行报文跟踪的用法，可以看出来相比于普通模式，诊断模式提供了更多的可供参考的信息，包括当前报文经过了iptables的哪些表和哪些链、报文发生了NAT、报文被克隆了等。诊断模式设置了三种提示级别：
INFO：正常的信息提示WARN：警告信息，该报文可能存在一定的问题，需要关注ERROR：异常信息，报文发生了问题（比如被丢弃）。./nettrace -p icmp --diag --saddr 192.168.122.8
begin trace...
***************** ad356200 ***************
[3445.575957] [__netif_receive_skb_core] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[3445.575978] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *ipv4 in chain: PRE_ROUTING*
[3445.575990] [nft_do_chain ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *iptables table:nat, chain:PREROUT* *packet is accepted*
[3445.576005] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *bridge in chain: PRE_ROUTING*
[3445.576014] [__netif_receive_skb_core] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[3445.576024] [ip_rcv ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[3445.576029] [ip_rcv_core ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[3445.576040] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *ipv4 in chain: PRE_ROUTING*
[3445.576044] [ip_rcv_finish ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[3445.576052] [ip_route_input_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[3445.576061] [fib_validate_source ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[3445.576080] [ip_forward ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[3445.576084] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *ipv4 in chain: FORWARD*
[3445.576087] [nft_do_chain ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *iptables table:filter, chain:FORWARD* *packet is accepted*
[3445.576107] [ip_output ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[3445.576113] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *ipv4 in chain: POST_ROUTING*
[3445.576116] [nft_do_chain ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *iptables table:nat, chain:POSTROU* *packet is accepted*
[3445.576131] [nf_nat_manip_pkt ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *NAT happens (packet address will change)*
[3445.576148] [ip_finish_output ] ICMP: 192.168.255.10 -> 10.123.119.98 ping request, seq: 0
[3445.576152] [ip_finish_output2 ] ICMP: 192.168.255.10 -> 10.123.119.98 ping request, seq: 0
[3445.576158] [__dev_queue_xmit ] ICMP: 192.168.255.10 -> 10.123.119.98 ping request, seq: 0
[3445.576165] [netdev_core_pick_tx ] ICMP: 192.168.255.10 -> 10.123.119.98 ping request, seq: 0
[3445.576177] [dev_hard_start_xmit ] ICMP: 192.168.255.10 -> 10.123.119.98 ping request, seq: 0
[3445.576215] [consume_skb ] ICMP: 192.168.255.10 -> 10.123.119.98 ping request, seq: 0 *packet is freed (normally)*
---------------- ANALYSIS RESULT ---------------------
[1] WARNING happens in nf_nat_manip_pkt(netfilter):
NAT happens (packet address will change)如果当前报文存在ERROR，那么工具会给出一定的诊断修复建议，并终止当前诊断操作。通过添加diag-keep可以在发生ERROR事件时不退出，继续进行跟踪分析。下面是发生异常时的日志：
./nettrace -p icmp --diag --saddr 192.168.122.8
begin trace...
***************** b3c64f00 ***************
[4049.295546] [__netif_receive_skb_core] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[4049.295566] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *ipv4 in chain: PRE_ROUTING*
[4049.295578] [nft_do_chain ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *iptables table:nat, chain:PREROUT* *packet is accepted*
[4049.295594] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *bridge in chain: PRE_ROUTING*
[4049.295612] [__netif_receive_skb_core] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[4049.295624] [ip_rcv ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[4049.295629] [ip_rcv_core ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[4049.295640] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *ipv4 in chain: PRE_ROUTING*
[4049.295644] [ip_rcv_finish ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[4049.295655] [ip_route_input_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[4049.295664] [fib_validate_source ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[4049.295683] [ip_forward ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[4049.295687] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *ipv4 in chain: FORWARD* *packet is dropped by netfilter (NF_DROP)*
[4049.295695] [nft_do_chain ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *iptables table:filter, chain:FORWARD* *packet is dropped by iptables/iptables-nft*
[4049.295711] [kfree_skb ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *packet is dropped by kernel*
---------------- ANALYSIS RESULT ---------------------
[1] ERROR happens in nf_hook_slow(netfilter):
packet is dropped by netfilter (NF_DROP)
fix advice:
check your netfilter rule
[2] ERROR happens in nft_do_chain(netfilter):
packet is dropped by iptables/iptables-nft
fix advice:
check your iptables rule
[3] ERROR happens in kfree_skb(life):
packet is dropped by kernel
location:
nf_hook_slow+0x96
drop reason:
NETFILTER_DROP
analysis finished!
end trace...从这里的日志可以看出，在报文经过iptables的filter表的forward链的时候，发生了丢包。在诊断结果里，会列出所有的异常事件，一个报文跟踪可能会命中多条诊断结果。这里的诊断建议是让用户检查iptables中的规则是否存在问题。
其中，kfree_skb这个跟踪点是对drop reason内核特性（详见droptrace中的介绍）做了适配的，可以理解为将droptrace的功能集成到了这里的诊断结果中，这里可以看出其给出的对包原因是NETFILTER_DROP。
网络防火墙是网络故障、网络不同发生的重灾区，因此netfilter工具对netfilter提供了完美适配，包括老版本的iptables-legacy和新版本的iptables-nft。诊断模式下，nettrace能够跟踪报文所经过的iptables表和iptables链，并在发生由于iptables导致的丢包时给出一定的提示，上面的示例充分展现出了这部分。出了对iptables的支持，nettrace对整个netfilter大模块也提供了支持，能够显示在经过每个HOOK点时对应的协议族和链的名称。除此之外，为了应对一些注册到netfilter中的第三方内核模块导致的丢包问题，nettrace还可以通过添加参数hooks来打印出当前HOOK上所有的的钩子函数，从而深入分析问题：
./nettrace -p icmp --diag --saddr 192.168.122.8 --hooks
begin trace...
***************** aa054500 ***************
[5810.702473] [__netif_receive_skb_core] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943
[5810.702491] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943 *ipv4 in chain: PRE_ROUTING*
[5810.702504] [nft_do_chain ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943 *iptables table:nat, chain:PREROUT* *packet is accepted*
[5810.702519] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943 *bridge in chain: PRE_ROUTING*
[5810.702527] [__netif_receive_skb_core] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943
[5810.702535] [ip_rcv ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943
[5810.702540] [ip_rcv_core ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943
[5810.702546] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943 *ipv4 in chain: PRE_ROUTING*
[5810.702551] [ip_rcv_finish ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943
[5810.702556] [ip_route_input_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943
[5810.702565] [fib_validate_source ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943
[5810.702579] [ip_forward ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943
[5810.702583] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943 *ipv4 in chain: FORWARD* *packet is dropped by netfilter (NF_DROP)*
[5810.702586] [nft_do_chain ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943 *iptables table:filter, chain:FORWARD* *packet is dropped by iptables/iptables-nft*
[5810.702599] [kfree_skb ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 943 *packet is dropped by kernel*
---------------- ANALYSIS RESULT ---------------------
[1] ERROR happens in nf_hook_slow(netfilter):
packet is dropped by netfilter (NF_DROP)
following hook functions are blamed:
nft_do_chain_ipv4
fix advice:
check your netfilter rule
[2] ERROR happens in nft_do_chain(netfilter):
packet is dropped by iptables/iptables-nft
fix advice:
check your iptables rule
[3] ERROR happens in kfree_skb(life):
packet is dropped by kernel
location:
nf_hook_slow+0x96
drop reason:
NETFILTER_DROP
analysis finished!
end trace...可以看出，上面following hook functions are blamed中列出了导致当前netfilter丢包的所有的钩子函数，这里只有iptables一个钩子函数。
由于对drop reason内核特性进行了适配，因此对于支持drop reason的系统，基于drop reason本工具可以诊断70+种丢包问题。nettrace通过将网络诊断经验翻译成规则存储到规则库的方式来进行诊断分析，通过扩充规则配置文件的方式能够不断增强其诊断功能。目前，本工具已经集成了20+典型网络故障诊断功能，并且在实践中不断完善知识库（规则库）。
端口未监听导致的丢包：
./nettrace --diag --diag-quiet
begin trace...
***************** 97730ee0 ***************
[365673.326016] [ip_output ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326026] [ip_finish_output ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326029] [ip_finish_output2 ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326032] [__dev_queue_xmit ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326039] [dev_hard_start_xmit ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326042] [enqueue_to_backlog ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326051] [__netif_receive_skb_core] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326059] [ip_rcv ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326061] [ip_rcv_core ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326068] [ip_rcv_finish ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326072] [ip_local_deliver ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326075] [ip_local_deliver_finish] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326078] [tcp_v4_rcv ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326084] [__inet_lookup_listener] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S *tcp port is not listened*
[365673.326090] [tcp_v4_send_reset ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S
[365673.326125] [kfree_skb ] TCP: 127.0.0.1:40392 -> 127.0.0.1:9999 seq:3067626996, ack:0, flags:S *packet is dropped by kernel*
---------------- ANALYSIS RESULT ---------------------
[1] WARNING happens in __inet_lookup_listener(tcp-in):
tcp port is not listened
fix advice:
check your target tcp port
[2] ERROR happens in kfree_skb(life):
packet is dropped by kernel
location:
tcp_v4_rcv+0x4aXDP导致的丢包（XDP转发会给提示）：
./nettrace -p icmp --diag --diag-quiet
begin trace...
***************** 015acc00 ***************
[18490.607809] [__netif_receive_skb_core] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[18490.607828] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *ipv4 in chain: PRE_ROUTING*
[18490.607840] [nft_do_chain ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *iptables table:nat, chain:PREROUT* *packet is accepted*
[18490.607855] [nf_hook_slow ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *bridge in chain: PRE_ROUTING*
[18490.607874] [__netif_receive_skb_core] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0
[18490.607882] [netif_receive_generic_xdp] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *packet is dropped by XDP program*
[18490.607888] [kfree_skb ] ICMP: 192.168.122.8 -> 10.123.119.98 ping request, seq: 0 *packet is dropped by kernel*
---------------- ANALYSIS RESULT ---------------------
[1] ERROR happens in netif_receive_generic_xdp(link-in):
packet is dropped by XDP program
fix advice:
check your XDP eBPF program
[2] ERROR happens in kfree_skb(life):
packet is dropped by kernel
location:
netif_receive_generic_xdp+0x259
drop reason:
NOT_SPECIFIED
analysis finished!使用命令nettrace --drop可以对系统中的丢包事件进行监控，对于支持内核特性skb drop reason的内核，这里还会打印出丢包原因。可以通过查看/tracing/events/skb/kfree_skb/format来判断当前系统是否支持该特性：
cat /tracing/events/skb/kfree_skb/format
name: kfree_skb
ID: 1524
format:
field:unsigned short common_type; offset:0; size:2; signed:0;
field:unsigned char common_flags; offset:2; size:1; signed:0;
field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
field:int common_pid; offset:4; size:4; signed:1;
field:void * skbaddr; offset:8; size:8; signed:0;
field:void * location; offset:16; size:8; signed:0;
field:unsigned short protocol; offset:24; size:2; signed:0;
field:enum skb_drop_reason reason; offset:28; size:4; signed:0;
print fmt: "skbaddr=%p protocol=%u location=%p reason: %s", REC->skbaddr, REC->protocol, REC->location, __print_symbolic(REC->reason, { 1, "NOT_SPECIFIED" }, { 2, "NO_SOCKET" }, { 3, "PKT_TOO_SMALL" }, { 4, "TCP_CSUM" }, { 5, "SOCKET_FILTER" }, { 6, "UDP_CSUM" }, { 7, "NETFILTER_DROP" }, { 8, "OTHERHOST" }, { 9, "IP_CSUM" }, { 10, "IP_INHDR" }, { 11, "IP_RPFILTER" }, { 12, "UNICAST_IN_L2_MULTICAST" }, { 13, "XFRM_POLICY" }, { 14, "IP_NOPROTO" }, { 15, "SOCKET_RCVBUFF" }, { 16, "PROTO_MEM" }, { 17, "TCP_MD5NOTFOUND" }, { 18, "TCP_MD5UNEXPECTED" }, { 19, "TCP_MD5FAILURE" }, { 20, "SOCKET_BACKLOG" }, { 21, "TCP_FLAGS" }, { 22, "TCP_ZEROWINDOW" }, { 23, "TCP_OLD_DATA" }, { 24, "TCP_OVERWINDOW" }, { 25, "TCP_OFOMERGE" }, { 26, "TCP_RFC7323_PAWS" }, { 27, "TCP_INVALID_SEQUENCE" }, { 28, "TCP_RESET" }, { 29, "TCP_INVALID_SYN" }, { 30, "TCP_CLOSE" }, { 31, "TCP_FASTOPEN" }, { 32, "TCP_OLD_ACK" }, { 33, "TCP_TOO_OLD_ACK" }, { 34, "TCP_ACK_UNSENT_DATA" }, { 35, "TCP_OFO_QUEUE_PRUNE" }, { 36, "TCP_OFO_DROP" }, { 37, "IP_OUTNOROUTES" }, { 38, "BPF_CGROUP_EGRESS" }, { 39, "IPV6DISABLED" }, { 40, "NEIGH_CREATEFAIL" }, { 41, "NEIGH_FAILED" }, { 42, "NEIGH_QUEUEFULL" }, { 43, "NEIGH_DEAD" }, { 44, "TC_EGRESS" }, { 45, "QDISC_DROP" }, { 46, "CPU_BACKLOG" }, { 47, "XDP" }, { 48, "TC_INGRESS" }, { 49, "UNHANDLED_PROTO" }, { 50, "SKB_CSUM" }, { 51, "SKB_GSO_SEG" }, { 52, "SKB_UCOPY_FAULT" }, { 53, "DEV_HDR" }, { 54, "DEV_READY" }, { 55, "FULL_RING" }, { 56, "NOMEM" }, { 57, "HDR_TRUNC" }, { 58, "TAP_FILTER" }, { 59, "TAP_TXFILTER" }, { 60, "ICMP_CSUM" }, { 61, "INVALID_PROTO" }, { 62, "IP_INADDRERRORS" }, { 63, "IP_INNOROUTES" }, { 64, "PKT_TOO_BIG" }, { 65, "MAX" })该模式下使用的效果与原先的droptrace完全相同，如下所示：
nettrace --drop
begin trace...
[142.097193] TCP: 162.241.189.135:57022 -> 172.27.0.6:22 seq:299038593, ack:3843597961, flags:AR, reason: NOT_SPECIFIED, tcp_v4_rcv+0x81
[142.331798] TCP: 162.241.189.135:57022 -> 172.27.0.6:22 seq:299038593, ack:3843597961, flags:A, reason: NOT_SPECIFIED, tcp_v4_do_rcv+0x83
[142.331857] TCP: 162.241.189.135:57022 -> 172.27.0.6:22 seq:299038593, ack:3843597961, flags:AP, reason: NOT_SPECIFIED, tcp_v4_do_rcv+0x83
[146.136576] TCP: 127.0.0.1:43582 -> 127.0.0.1:9999 seq:3819454691, ack:0, flags:S, reason: NO_SOCKET, tcp_v4_rcv+0x81
[146.220414] TCP: 169.254.0.138:8186 -> 172.27.0.6:40634 seq:8486084, ack:2608831141, flags:A, reason: TCP_INVALID_SEQUENCE, tcp_validate_incoming+0x126
[146.533728] TCP: 127.0.0.1:36338 -> 127.0.0.1:56100 seq:1110580666, ack:1951926207, flags:A, reason: TCP_INVALID_SEQUENCE, tcp_validate_incoming+0x126
[147.255946] TCP: 20.44.10.122:443 -> 192.168.255.10:42878 seq:2950381253, ack:211751623, flags:A, reason: NOT_SPECIFIED, tcp_rcv_state_process+0xe9同样可以使用man dropreason命令来查看对应的丢包原因的详细解释。对于不支持skb drop reason特性的内核，该模式下将不会打印丢包原因字段，效果如下所示：
nettrace --drop
begin trace...
[2016.965295] TCP: 162.241.189.135:45432 -> 172.27.0.6:22 seq:133152310, ack:2529234288, flags:AR, tcp_v4_rcv+0x50
[2017.201315] TCP: 162.241.189.135:45432 -> 172.27.0.6:22 seq:133152310, ack:2529234288, flags:A, tcp_v4_do_rcv+0x70
[2019.041344] TCP: 176.58.124.134:37441 -> 172.27.0.6:443 seq:1160140493, ack:0, flags:S, tcp_v4_rcv+0x50
[2021.867340] TCP: 127.0.0.1:34936 -> 127.0.0.1:9999 seq:1309795878, ack:0, flags:S, tcp_v4_rcv+0x50
[2024.997146] TCP: 162.241.189.135:46756 -> 172.27.0.6:22 seq:1304582308, ack:1354418612, flags:AR, tcp_v4_rcv+0x50
[2025.235953] TCP: 162.241.189.135:46756 -> 172.27.0.6:22 seq:1304582308, ack:1354418612, flags:A, tcp_v4_do_rcv+0x70
[2025.235967] TCP: 162.241.189.135:46756 -> 172.27.0.6:22 seq:1304582308, ack:1354418612, flags:AP, tcp_v4_do_rcv+0x70套接口跟踪在原理上与skb的basic模式很类似，只不过跟踪对象从skb换成了sock。常规的过滤参数，如ip、端口等，在该模式下都可以直接使用，基本用法如下所示：
sudo ./nettrace -p tcp --port 9999 --sock
begin trace...
[2157947.050509] [inet_listen ] TCP: 0.0.0.0:9999 -> 0.0.0.0:0 info:(0 0)
[2157958.364842] [__tcp_transmit_skb ] TCP: 127.0.0.1:36562 -> 127.0.0.1:9999 info:(1 0)
[2157958.364875] [tcp_rcv_state_process] TCP: 0.0.0.0:9999 -> 0.0.0.0:0 info:(0 0)
[2157958.364890] [tcp_rcv_state_process] TCP: 127.0.0.1:36562 -> 127.0.0.1:9999 info:(1 0) timer:(retrans, 1.000s)
[2157958.364896] [tcp_ack ] TCP: 127.0.0.1:36562 -> 127.0.0.1:9999 info:(1 0) timer:(retrans, 1.000s)
[2157958.364906] [__tcp_transmit_skb ] TCP: 127.0.0.1:36562 -> 127.0.0.1:9999 info:(0 0)
[2157958.364917] [tcp_rcv_state_process] TCP: 127.0.0.1:9999 -> 127.0.0.1:36562 info:(0 0)
[2157958.364921] [tcp_ack ] TCP: 127.0.0.1:9999 -> 127.0.0.1:36562 info:(0 0)
[2157959.365240] [tcp_write_timer_handler] TCP: 127.0.0.1:36562 -> 127.0.0.1:9999 info:(0 0)其中，info里显示的内容分别是：报文在外数量、报文重传数量。timer显示的为当前套接口上的定时器和超时时间。目前，信息还在不断完善中。
常规的网络定位手段，包括上面的报文跟踪、诊断等方式，由于开销过大，不适合在生产环境中部署和常态化运行。监控模式能够提供一种更加轻量级别的网络异常、丢包监控。由于这种模式是基于TRACING类型的BPF，因此其对于内核版本有较高的要求。以下是内核版本要求：
| TencentOS | 开源版本 | BPF特性 | monitor |
|---|---|---|---|
| 5.4.119-19.0009 | 5.5 | TRACING | 可用，不可监控内核模块中的函数和参数个数超过6的内核函数 |
| 开发中 | 5.11 | BTF_MODULES | 可用，不可监控参数个数超过6的内核函数 |
| 开发中 | upstream | TRACING支持6+参数 | 完全可用 |
其中，“TRACING支持6+参数”内核特性已经合入到upstream：bpf, x86: allow function arguments up to 12 for TRACING
基本用法（在内核特性完全支持的情况下）：
$ nettrace --monitor
begin trace...
[25.167980] [nft_do_chain ] ICMP: 192.168.122.1 -> 192.168.122.9 ping request, seq: 1, id: 1523 *iptables table:filter, chain:INPUT* *packet is dropped by iptables/iptables-nft*
[25.167996] [kfree_skb ] ICMP: 192.168.122.1 -> 192.168.122.9 ping request, seq: 1, id: 1523, reason: NETFILTER_DROP, nf_hook_slow+0xa8
[25.168000] [nf_hook_slow ] ICMP: 192.168.122.1 -> 192.168.122.9 ping request, seq: 1, id: 1523 *ipv4 in chain: INPUT* *packet is dropped by netfilter (NF_DROP)*监控模式下，也可以使用普通模式的下各种参数，如报文过滤、--detail详情显示等。默认情况下，monitor模式下不跟踪rtt。但是如果指定了--filter-minrtt或者--filter-srtt参数，那么就会跟踪rtt事件：
./src/nettrace --monitor --filter-minrtt 10
begin trace...
[2651830.434898] [tcp_ack_update_rtt.isra.51] TCP: 127.0.0.1:14275 -> 127.0.0.1:62522 ESTABLISHED CA_Open info:(0 0) mem:(w0 r0) *srtt:8ms, rtt:40ms*
[2651830.435267] [tcp_ack_update_rtt.isra.51] TCP: 10.37.80.82:22 -> 10.85.114.159:53493 ESTABLISHED CA_Open info:(0 0) mem:(w0 r0) *srtt:38ms, rtt:41ms*
[2651830.484520] [tcp_ack_update_rtt.isra.51] TCP: 10.37.80.82:22 -> 10.85.114.159:53493 ESTABLISHED CA_Open info:(0 0) mem:(w0 r0) *srtt:39ms, rtt:37ms*
[2651830.529385] [tcp_ack_update_rtt.isra.51] TCP: 10.37.80.82:22 -> 10.85.114.159:53493 ESTABLISHED CA_Open info:(0 0) mem:(w0 r0) *srtt:38ms, rtt:36ms*
[2651830.578473] [tcp_ack_update_rtt.isra.51] TCP: 10.37.80.82:22 -> 10.85.114.159:53493 ESTABLISHED CA_Open info:(0 0) mem:(w0 r0) *srtt:38ms, rtt:41ms*
[2651830.752827] [tcp_retransmit_timer] TCP: 10.37.80.82:100 -> 10.154.16.12:8603 SYN_SENT CA_Open info:(0 0) mem:(w0 r0) *TCP retransmission timer out*本工具目前支持的性能分析方法有两种，一种是采集主机上的tcp连接的rtt情况，从而判断当前的网络处理延迟情况；一种是采集当前主机协议栈处理网络报文的耗时情况，从而可以判断当前主机引入性能问题的环节。
在特定场景下，如网络时延问题诊断的时候，我们可能要关注处理时长协议栈哪个环节处理比较耗时。此时，就需要根据报文的处理时长进行输出过滤。目前，是根据报文从被跟踪到，直到被释放的前一个函数来作为报文的处理时长的。这里没有考虑报文被释放的延迟，是因为内核里会存在“延迟批量销毁skb”的行为，影响延迟测量的准确性。下面的命令会过滤处理时长超过1ms的报文：
$ sudo ./nettrace -p icmp --latency-show
begin trace...
***************** 723c4700 ***************
[37898.357352] [__netif_receive_skb_core.constprop.0] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573
[37898.357368] [enqueue_to_backlog ] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.016ms
[37898.357374] [__netif_receive_skb_core.constprop.0] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.005ms
[37898.357378] [ip_rcv ] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.003ms
[37898.357381] [ip_rcv_core ] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.003ms
[37898.357384] [nf_hook_slow ] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.003ms *ipv4 in chain: PRE_ROUTING*
[37898.357391] [ip_route_input_slow ] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.006ms
[37898.357398] [fib_validate_source ] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.006ms
[37898.357401] [ip_local_deliver ] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.003ms
[37898.357404] [nf_hook_slow ] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.002ms *ipv4 in chain: INPUT*
[37898.357408] [ip_local_deliver_finish] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.003ms
[37898.357415] [icmp_rcv ] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.007ms
[37898.357419] [icmp_echo ] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.004ms
[37898.357424] [icmp_reply ] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.004ms
[37898.357476] [consume_skb ] ICMP: 192.168.122.9 -> 192.168.122.1 ping request, seq: 3, id: 9573 latency: 0.052ms *packet is freed (normally)*
total latency: 0.071ms可以使用--min-latency来根据协议栈处理耗时对报文进行过滤，单位是us，例如：nettrace -p icmp --min-latency 1000，可过滤处理耗时超过1ms的报文。指定该参数的时候，会默认显示延迟信息。
上面的这种方式可以显示协议栈各个处理环节的详细延迟信息，但是效率是比较低的，特别是在网络流量很大的时候。这时因为上面的模式下，是将每个匹配到的报文事件上送到用户态，在用户态程序中进行的延迟过滤。为了提升性能，可以使用延迟跟踪模式。这种模式产生的性能开销比较小，缺点是只能跟踪报文开始（第一次匹配到）到截止（释放前的函数）的耗时，不能查看各个环节的耗时。
收包阶段
跟踪报文被放到收包队列 -> 用户将报文从收包队列中取走的延迟。这部分延迟主要是由于用户态程序收包不及时导致的：
nettrace -p tcp --latency -t tcp_queue_rcv,tcp_data_queue_ofo --trace-matcher tcp_queue_rcv,tcp_data_queue_ofo --latency-free --min-latency 1000跟踪网卡驱动收包报文 -> 放到套接口收包队列的延迟。这部分如果存在延迟，那说明是CPU处理的延迟：
nettrace -p tcp --latency -t __netif_receive_skb_core,tcp_queue_rcv,tcp_data_queue_ofo --trace-matcher __netif_receive_skb_core --trace-free tcp_queue_rcv,tcp_data_queue_ofo --min-latency 1000发包阶段
跟踪报文放到发包队列 -> 报文开始发送，一般由nagle算法引发的报文聚合延迟。下面的命令会输出该阶段延迟超过1ms的报文：
nettrace -p tcp --latency -t skb_entail,tcp_skb_entail,__tcp_transmit_skb,__tcp_retransmit_skb --trace-matcher skb_entail,tcp_skb_entail --trace-free __tcp_transmit_skb,__tcp_retransmit_skb --min-latency 1000跟踪报文从传输层（TCP层）到网卡驱动层的延迟。这个中间有个qdisc，因此如果有延迟的话，可能是这块导致的：
nettrace -p tcp --latency -t __ip_queue_xmit,dev_hard_start_xmit --trace-matcher __ip_queue_xmit --trace-free dev_hard_start_xmit --min-latency 1000跟踪报文从放到发送队列到收到ack之间的延迟，这个延迟会受到nagle和延迟ACK的双重影响：
nettrace -p tcp --latency -t skb_entail,tcp_skb_entail,tcp_rate_skb_delivered --trace-matcher skb_entail,tcp_skb_entail --trace-free tcp_rate_skb_delivered --min-latency 1000默认情况下，是不会将报文被释放的延迟当作协议栈处理耗时的，可以通过加上--latency-free来将报文释放的耗时也考虑进去。这对于收报过程中的网络延迟分析比较有用，比如我们可以通过报文被释放的延迟来分析出来收包队列中的数据被用户态取走而释放的延迟。
除此之外，还可以通过指定--latency-summary来进行协议栈处理延迟的统计，如下所示：
./nettrace --latency -p tcp --latency -t skb_entail,tcp_skb_entail,__tcp_transmit_skb,__tcp_retransmit_skb --trace-matcher skb_entail,tcp_skb_entail --trace-free __tcp_transmit_skb,__tcp_retransmit_skb --latency-summary
latency distribution: 21
0 - 1us: 0 0.0000
2 - 3us: 1 0.0476
4 - 7us: 6 0.2857
8 - 15us: 3 0.1428
16 - 31us: 11 0.5238
32 - 63us: 0 0.0000
64 - 127us: 0 0.0000
128 - 255us: 0 0.0000
256 - 511us: 0 0.0000
latency distribution: 21
0 - 1us: 0 0.0000
2 - 3us: 1 0.0476
4 - 7us: 6 0.2857
8 - 15us: 3 0.1428
16 - 31us: 11 0.5238
32 - 63us: 0 0.0000
64 - 127us: 0 0.0000
128 - 255us: 0 0.0000
256 - 511us: 0 0.0000
end trace...这里的统计信息会每秒刷新。通过这个分布，可以分析出来当前协议栈处理耗时的一个大体情况。
rtt模式下可以去分析连接的RTT变化，支持根据rtt和srtt来进行过滤。这里是通过跟踪tcp_ack_update_rtt内核函数的调用来获取套接口的rtt更新事件的，默认情况下是统计RTT的分布情况的，使用方式如下：
./nettrace --rtt
begin trace...
rtt distribution: 29
0 - 1ms: 12 0.4137
2 - 3ms: 0 0.0000
4 - 7ms: 0 0.0000
8 - 15ms: 0 0.0000
16 - 31ms: 0 0.0000
32 - 63ms: 0 0.0000
64 - 127ms: 1 0.0344
128 - 255ms: 4 0.1379
256 - 511ms: 12 0.4137上面的信息分别代表：加上--rtt-detail参数，即可查看每个报文的rtt情况。在需要监控系统中超过一定阈值的RTT的情况下，比较适合使用这种方式。例如，下面是监控系统中数据传输延迟超过10ms的报文情况：
./nettrace --sock -t tcp_ack_update_rtt --filter-srtt 10
begin trace...
[11281.589357] [tcp_ack_update_rtt ] TCP: 192.168.0.111:42890 -> 43.129.25.208:38000 ESTABLISHED CA_Open out:(p0 r0) unack:196997740 mem:(w0 r0) timer:(loss_probe, 0.176s) *rtt:152ms, rtt_min:152ms*
[11281.802090] [tcp_ack_update_rtt ] TCP: 192.168.0.111:42890 -> 43.129.25.208:38000 ESTABLISHED CA_Open out:(p0 r0) unack:196998022 mem:(w0 r0) timer:(loss_probe, 0.124s) *rtt:209ms, rtt_min:209ms*
[11282.201314] [tcp_ack_update_rtt ] TCP: 192.168.0.111:42890 -> 43.129.25.208:38000 ESTABLISHED CA_Open out:(p0 r0) unack:196998304 mem:(w0 r0) timer:(loss_probe, 0.032s) *rtt:308ms, rtt_min:308ms*
[11282.408500] [tcp_ack_update_rtt ] TCP: 192.168.0.111:42882 -> 43.129.25.208:38000 ESTABLISHED CA_Open out:(p0 r0) unack:2248556186 mem:(w0 r0) timer:(loss_probe, 0.376s) *rtt:131ms, rtt_min:131ms*
[11282.408513] [tcp_ack_update_rtt ] TCP: 192.168.0.111:42914 -> 43.129.25.208:38000 ESTABLISHED CA_Open out:(p0 r0) unack:564521627 mem:(w0 r0) timer:(loss_probe, 0.376s) *rtt:128ms, rtt_min:128ms*
[11282.408518] [tcp_ack_update_rtt ] TCP: 192.168.0.111:42884 -> 43.129.25.208:38000 ESTABLISHED CA_Open out:(p0 r0) unack:106249688 mem:(w0 r0) timer:(loss_probe, 0.376s) *rtt:128ms, rtt_min:128ms*rtt代表的是经过平滑处理的RTT，而rtt_min代表的是本次发送的报文被确认过程中实际的RTT。
启动程序报如下错误：
map 'kprobe.rodata': failed to create: Invalid argument(-22)这个是由于当前的内核不支持BPF_GLOBAL_DATA导致的，需要重新编译nettrace，在编译的时候加上NO_GLOBAL_DATA=1即可：
make NO_GLOBAL_DATA=1 all启动程序报错：
unknown func bpf_jiffies64#118这个是当前内核对于DEAD CODE的检查机制不完善导致的，需要重新编译nettrace，在编译的时候加上NO_GLOBAL_DATA=1即可：
make NO_GLOBAL_DATA=1 all启动程序报错：
R5 min value is negative, either use unsigned or 'var &= const'为了提升效率，nettrace在输出事件的时候尽量只输出部分需要的数据，在低版本内核上，clang有的时候编译出来的会有问题。编译的时候加上COMPAT=1 OUTPUT_WHOLE=1参数可解决该问题。