我新安装了一台装有 Ubuntu 18.04 的计算机，并连接到了我家的 wifi。当我尝试arp -a使用命令扫描连接到同一网络的其他设备时，我看到一些非常奇怪的输出。
首先，通过检查连接是否正常ifconfig：
john@home:~$ ifconfig
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 246054 bytes 21958490 (21.9 MB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 246054 bytes 21958490 (21.9 MB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
wlo1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.14 netmask 255.255.255.0 broadcast 192.168.1.255
inet6 fe80::e3f:e0a5:2438:a96a prefixlen 64 scopeid 0x20<link>
inet6 240d:1a:6a5:c900:1420:b3cc:994b:1b7b prefixlen 64 scopeid 0x0<global>
inet6 240d:1a:6a5:c900:74f4:f504:3a41:bc12 prefixlen 64 scopeid 0x0<global>
ether 04:33:c2:c4:02:a2 txqueuelen 1000 (Ethernet)
RX packets 2452125 bytes 3302288691 (3.3 GB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 964749 bytes 117659686 (117.6 MB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
它的 IP 地址是 192.168.1.14 然后我尝试arp -a：
john@home:~$ arp -a
? (192.168.1.19) at 92:c4:78:3c:46:16 [ether] on wlo1
_gateway (192.168.1.1) at e4:7e:66:1f:bf:4c [ether] on wlo1
? (192.168.1.5) at 26:36:46:f9:69:83 [ether] on wlo1
这是有道理的，因为我可以确认我的 iPad 的 IP 地址是192.168.1.19，我的手机的 IP 地址是192.168.1.5
然而，过了一会儿我arp -a再次执行，输出的结果让我大吃一惊：
john@home:~$ arp -a
? (192.168.1.206) at <incomplete> on wlo1
? (192.168.1.183) at <incomplete> on wlo1
? (192.168.1.107) at <incomplete> on wlo1
? (192.168.1.8) at <incomplete> on wlo1
? (192.168.1.18) at <incomplete> on wlo1
? (192.168.1.165) at <incomplete> on wlo1
? (192.168.1.186) at <incomplete> on wlo1
? (192.168.1.110) at <incomplete> on wlo1
? (192.168.1.77) at <incomplete> on wlo1
? (192.168.1.33) at <incomplete> on wlo1
? (192.168.1.178) at <incomplete> on wlo1
? (192.168.1.123) at <incomplete> on wlo1
? (192.168.1.112) at <incomplete> on wlo1
? (192.168.1.96) at <incomplete> on wlo1
? (192.168.1.117) at <incomplete> on wlo1
? (192.168.1.74) at <incomplete> on wlo1
? (192.168.1.95) at <incomplete> on wlo1
? (192.168.1.84) at <incomplete> on wlo1
? (192.168.1.41) at <incomplete> on wlo1
? (192.168.1.62) at <incomplete> on wlo1
? (192.168.1.51) at <incomplete> on wlo1
? (192.168.1.8) at <incomplete> on wlo1
? (192.168.1.29) at <incomplete> on wlo1
? (192.168.1.18) at <incomplete> on wlo1
? (192.168.1.231) at <incomplete> on wlo1
? (192.168.1.252) at <incomplete> on wlo1
? (192.168.1.241) at <incomplete> on wlo1
? (192.168.1.198) at <incomplete> on wlo1
? (192.168.1.219) at <incomplete> on wlo1
? (192.168.1.208) at <incomplete> on wlo1
? (192.168.1.165) at <incomplete> on wlo1
? (192.168.1.186) at <incomplete> on wlo1
? (192.168.1.143) at <incomplete> on wlo1
? (192.168.1.132) at <incomplete> on wlo1
? (192.168.1.153) at <incomplete> on wlo1
? (192.168.1.110) at <incomplete> on wlo1
? (192.168.1.99) at <incomplete> on wlo1
? (192.168.1.120) at <incomplete> on wlo1
? (192.168.1.77) at <incomplete> on wlo1
? (192.168.1.66) at <incomplete> on wlo1
? (192.168.1.87) at <incomplete> on wlo1
? (192.168.1.44) at <incomplete> on wlo1
? (192.168.1.33) at <incomplete> on wlo1
? (192.168.1.54) at <incomplete> on wlo1
? (192.168.1.11) at <incomplete> on wlo1
? (192.168.1.21) at <incomplete> on wlo1
? (192.168.1.234) at <incomplete> on wlo1
? (192.168.1.244) at <incomplete> on wlo1
? (192.168.1.201) at <incomplete> on wlo1
? (192.168.1.222) at <incomplete> on wlo1
? (192.168.1.211) at <incomplete> on wlo1
? (192.168.1.168) at <incomplete> on wlo1
? (192.168.1.189) at <incomplete> on wlo1
? (192.168.1.178) at <incomplete> on wlo1
? (192.168.1.135) at <incomplete> on wlo1
? (192.168.1.156) at <incomplete> on wlo1
? (192.168.1.145) at <incomplete> on wlo1
? (192.168.1.102) at <incomplete> on wlo1
? (192.168.1.123) at <incomplete> on wlo1
? (192.168.1.112) at <incomplete> on wlo1
? (192.168.1.69) at <incomplete> on wlo1
? (192.168.1.90) at <incomplete> on wlo1
? (192.168.1.47) at <incomplete> on wlo1
? (192.168.1.36) at <incomplete> on wlo1
? (192.168.1.57) at <incomplete> on wlo1
? (192.168.1.3) at <incomplete> on wlo1
? (192.168.1.24) at <incomplete> on wlo1
? (192.168.1.237) at <incomplete> on wlo1
? (192.168.1.226) at <incomplete> on wlo1
? (192.168.1.247) at <incomplete> on wlo1
? (192.168.1.204) at <incomplete> on wlo1
? (192.168.1.193) at <incomplete> on wlo1
? (192.168.1.214) at <incomplete> on wlo1
? (192.168.1.171) at <incomplete> on wlo1
? (192.168.1.160) at <incomplete> on wlo1
? (192.168.1.181) at <incomplete> on wlo1
? (192.168.1.138) at <incomplete> on wlo1
? (192.168.1.159) at <incomplete> on wlo1
? (192.168.1.148) at <incomplete> on wlo1
? (192.168.1.105) at <incomplete> on wlo1
? (192.168.1.126) at <incomplete> on wlo1
? (192.168.1.115) at <incomplete> on wlo1
? (192.168.1.72) at <incomplete> on wlo1
? (192.168.1.93) at <incomplete> on wlo1
? (192.168.1.82) at <incomplete> on wlo1
? (192.168.1.39) at <incomplete> on wlo1
? (192.168.1.60) at <incomplete> on wlo1
? (192.168.1.49) at <incomplete> on wlo1
? (192.168.1.6) at <incomplete> on wlo1
? (192.168.1.27) at <incomplete> on wlo1
? (192.168.1.16) at <incomplete> on wlo1
? (192.168.1.229) at <incomplete> on wlo1
? (192.168.1.250) at <incomplete> on wlo1
? (192.168.1.207) at <incomplete> on wlo1
? (192.168.1.196) at <incomplete> on wlo1
? (192.168.1.217) at <incomplete> on wlo1
? (192.168.1.174) at <incomplete> on wlo1
? (192.168.1.163) at <incomplete> on wlo1
? (192.168.1.184) at <incomplete> on wlo1
? (192.168.1.141) at <incomplete> on wlo1
? (192.168.1.130) at <incomplete> on wlo1
? (192.168.1.151) at <incomplete> on wlo1
? (192.168.1.108) at <incomplete> on wlo1
? (192.168.1.97) at <incomplete> on wlo1
? (192.168.1.118) at <incomplete> on wlo1
? (192.168.1.75) at <incomplete> on wlo1
? (192.168.1.64) at <incomplete> on wlo1
? (192.168.1.85) at <incomplete> on wlo1
? (192.168.1.42) at <incomplete> on wlo1
? (192.168.1.63) at <incomplete> on wlo1
? (192.168.1.52) at <incomplete> on wlo1
? (192.168.1.9) at <incomplete> on wlo1
? (192.168.1.30) at <incomplete> on wlo1
? (192.168.1.19) at <incomplete> on wlo1
? (192.168.1.232) at <incomplete> on wlo1
? (192.168.1.253) at <incomplete> on wlo1
? (192.168.1.242) at <incomplete> on wlo1
? (192.168.1.199) at <incomplete> on wlo1
? (192.168.1.220) at <incomplete> on wlo1
? (192.168.1.209) at <incomplete> on wlo1
? (192.168.1.166) at <incomplete> on wlo1
? (192.168.1.187) at <incomplete> on wlo1
? (192.168.1.176) at <incomplete> on wlo1
? (192.168.1.133) at <incomplete> on wlo1
? (192.168.1.154) at <incomplete> on wlo1
? (192.168.1.111) at <incomplete> on wlo1
? (192.168.1.100) at <incomplete> on wlo1
? (192.168.1.121) at <incomplete> on wlo1
? (192.168.1.78) at <incomplete> on wlo1
? (192.168.1.67) at <incomplete> on wlo1
? (192.168.1.88) at <incomplete> on wlo1
? (192.168.1.45) at <incomplete> on wlo1
? (192.168.1.34) at <incomplete> on wlo1
? (192.168.1.55) at <incomplete> on wlo1
? (192.168.1.12) at <incomplete> on wlo1
_gateway (192.168.1.1) at e4:7e:66:1f:bf:4c [ether] on wlo1
? (192.168.1.22) at <incomplete> on wlo1
? (192.168.1.235) at <incomplete> on wlo1
? (192.168.1.224) at <incomplete> on wlo1
? (192.168.1.245) at <incomplete> on wlo1
? (192.168.1.202) at <incomplete> on wlo1
? (192.168.1.223) at <incomplete> on wlo1
? (192.168.1.212) at <incomplete> on wlo1
? (192.168.1.169) at <incomplete> on wlo1
? (192.168.1.190) at <incomplete> on wlo1
? (192.168.1.179) at <incomplete> on wlo1
? (192.168.1.136) at <incomplete> on wlo1
? (192.168.1.157) at <incomplete> on wlo1
? (192.168.1.146) at <incomplete> on wlo1
? (192.168.1.103) at <incomplete> on wlo1
? (192.168.1.124) at <incomplete> on wlo1
? (192.168.1.113) at <incomplete> on wlo1
? (192.168.1.70) at <incomplete> on wlo1
? (192.168.1.91) at <incomplete> on wlo1
? (192.168.1.80) at <incomplete> on wlo1
? (192.168.1.37) at <incomplete> on wlo1
? (192.168.1.58) at <incomplete> on wlo1
? (192.168.1.15) at <incomplete> on wlo1
? (192.168.1.4) at <incomplete> on wlo1
? (192.168.1.25) at <incomplete> on wlo1
? (192.168.1.238) at <incomplete> on wlo1
? (192.168.1.227) at <incomplete> on wlo1
? (192.168.1.248) at <incomplete> on wlo1
? (192.168.1.205) at <incomplete> on wlo1
? (192.168.1.194) at <incomplete> on wlo1
? (192.168.1.215) at <incomplete> on wlo1
? (192.168.1.172) at <incomplete> on wlo1
? (192.168.1.161) at <incomplete> on wlo1
? (192.168.1.182) at <incomplete> on wlo1
? (192.168.1.139) at <incomplete> on wlo1
? (192.168.1.128) at <incomplete> on wlo1
? (192.168.1.149) at <incomplete> on wlo1
? (192.168.1.106) at <incomplete> on wlo1
? (192.168.1.127) at <incomplete> on wlo1
? (192.168.1.116) at <incomplete> on wlo1
? (192.168.1.73) at <incomplete> on wlo1
? (192.168.1.94) at <incomplete> on wlo1
? (192.168.1.83) at <incomplete> on wlo1
? (192.168.1.40) at <incomplete> on wlo1
? (192.168.1.61) at <incomplete> on wlo1
? (192.168.1.50) at <incomplete> on wlo1
? (192.168.1.7) at <incomplete> on wlo1
? (192.168.1.28) at <incomplete> on wlo1
? (192.168.1.17) at <incomplete> on wlo1
? (192.168.1.230) at <incomplete> on wlo1
? (192.168.1.251) at <incomplete> on wlo1
? (192.168.1.240) at <incomplete> on wlo1
? (192.168.1.197) at <incomplete> on wlo1
? (192.168.1.218) at <incomplete> on wlo1
? (192.168.1.175) at <incomplete> on wlo1
? (192.168.1.164) at <incomplete> on wlo1
? (192.168.1.185) at <incomplete> on wlo1
? (192.168.1.142) at <incomplete> on wlo1
? (192.168.1.131) at <incomplete> on wlo1
? (192.168.1.152) at <incomplete> on wlo1
? (192.168.1.109) at <incomplete> on wlo1
? (192.168.1.98) at <incomplete> on wlo1
? (192.168.1.119) at <incomplete> on wlo1
? (192.168.1.76) at <incomplete> on wlo1
? (192.168.1.65) at <incomplete> on wlo1
? (192.168.1.86) at <incomplete> on wlo1
? (192.168.1.43) at <incomplete> on wlo1
? (192.168.1.32) at <incomplete> on wlo1
? (192.168.1.53) at <incomplete> on wlo1
? (192.168.1.10) at <incomplete> on wlo1
? (192.168.1.31) at <incomplete> on wlo1
? (192.168.1.20) at <incomplete> on wlo1
? (192.168.1.233) at <incomplete> on wlo1
? (192.168.1.254) at <incomplete> on wlo1
? (192.168.1.243) at <incomplete> on wlo1
? (192.168.1.200) at <incomplete> on wlo1
? (192.168.1.221) at <incomplete> on wlo1
john@home:~$
这是什么？刚刚发生了什么？我以为arp -a应该扫描并列出同一网络上的其他设备。这些结果是什么？
答案1
我认为
arp -a应该扫描并列出同一网络上的其他设备。
arp -a不进行扫描。它仅显示内核的 IPv4 网络邻居缓存。输出中的任何条目都存在，因为它已经在缓存中，而不是因为您运行了arp。
这些结果是什么？
不完整条目意味着已发送请求以查找给定 IP 地址的网络邻居的 MAC 地址，但尚未收到答复。要么马上就会有有效答复，因此条目将变为完整，并将在缓存中保留一段时间；要么不会有有效答复，不完整条目将很快从缓存中删除。
刚刚发生了什么？
您的操作系统中的某些程序刚刚尝试访问所有这些 IP 地址。它可能是一个实际的扫描仪，例如nmap 192.168.1.0/24。
未完成的条目最终会消失，除非您的操作系统定期尝试访问相关 IP 地址。新的访问尝试将使条目重新出现；或者它会阻止它消失。
如果您运行（或继续运行）扫描仪或类似程序，则无需担心。发布的输出arp -a正常。即使arp -a现在只打印几个条目，这将打印很多：
nmap 192.168.1.0/24; arp -a
如果不是你，那就值得调查一下。你的网络在你不知情的情况下被操作系统扫描，这可能表明操作系统已被入侵，也可能不表明操作系统已被入侵。