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Como depuro um servidor que não libera seu IP DHCP?

tag-icon • tag-icon networking debian dhcp
Como depuro um servidor que não libera seu IP DHCP?

Parece que meu servidor continua solicitando um endereço DHCP, mas não libera os antigos. Anteriormente, ele estava conectado diretamente ao meu ISP e mantinha milhares de concessões de DHCP (isso pode ter causado um DoS em toda a cidade).

O servidor ainda pode acessar a Internet. O problema não causa problemas de conectividade para o servidor, mas todos os outros não podem usar os endereços IP reivindicados.

Na tentativa de isolar o problema, conectei-o à LAN. Não é novidade que isso esgotou o pool de IP do roteador. Todos os endereços IP foram acessíveis a partir do host local (posso fazer ping, <1 ms de atraso), mas não de fora.

Não tenho muita experiência dhclientou como um servidor lida com a aquisição/liberação de endereços IP. /etc/network/interfacesnão foi modificado, excetoallow-hotplug ens5

O problema pode ter começado quando eu

  • substituído sendmailpor postfix( apt-get install postfix, nada mais)
  • correu apt-get upgrade(atualizou o kernel e alguns módulos PHP, nada mais)

Não tenho nenhum script personalizado no servidor. dmesgestá limpo. É um servidor web, e a maior parte do que faço é no nginxlocal ou no local (Wordpress).

Como posso começar a depurar isso? Eu não encontrei tal coisa no passado.

root@placeholder-not-actual-hostname:~# uname -a
Linux placeholder-not-actual-hostname 4.9.0-5-amd64 #1 SMP Debian 4.9.65-3+deb9u2 (2018-01-04) x86_64 GNU/Linux

Saída de ip a:

root@placeholder-not-actual-hostname:~# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: ens5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether e0:cb:4e:48:4e:28 brd ff:ff:ff:ff:ff:ff
    inet 192.168.249.2/24 brd 255.255.255.255 scope global ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.3/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.4/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.5/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.6/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.7/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.8/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.9/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.10/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.11/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.12/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.13/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.14/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.15/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.16/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.17/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.18/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.19/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.20/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.21/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.22/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.23/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.24/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.25/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.26/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.27/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.28/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.29/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.30/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.31/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.32/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.33/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.34/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.35/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.36/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.37/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.38/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.39/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.40/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.41/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.42/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.43/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.44/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.45/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.46/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.47/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.48/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.49/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.50/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.51/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.52/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.53/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.54/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.55/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.56/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.57/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.58/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.59/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.60/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.61/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.62/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.63/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.64/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.65/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.66/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.67/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.68/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.69/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.70/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.72/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.73/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.74/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.75/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.76/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.77/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.78/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.79/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.80/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.81/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.82/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.83/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.84/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.85/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.86/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.87/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.88/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.89/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.90/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.91/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.92/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.93/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.94/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.95/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.96/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.97/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.98/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.99/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.100/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.101/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.102/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.103/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.104/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.105/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.106/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.107/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.108/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.109/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.110/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.111/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.112/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.113/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.114/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.115/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.116/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.117/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.118/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.119/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.120/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.121/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.122/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.123/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.124/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.125/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.126/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.127/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.128/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.129/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.130/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.131/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.132/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.133/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.134/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.135/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.136/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.137/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.138/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.139/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.140/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.141/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.142/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.143/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.144/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.145/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.146/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.147/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.148/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.149/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.150/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.151/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.152/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.153/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.154/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.155/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.156/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.157/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.158/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.159/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.160/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.161/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.162/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.163/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.164/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.165/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.166/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.167/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.168/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.169/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.170/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.171/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.172/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.173/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.174/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.175/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.176/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.177/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.178/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.179/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.180/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.181/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.182/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.183/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.184/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.185/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.186/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.187/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.188/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.189/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.190/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.191/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.192/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.193/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.194/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.195/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.196/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.197/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.198/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.199/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.200/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.254/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet 192.168.249.71/24 brd 255.255.255.255 scope global secondary ens5
       valid_lft forever preferred_lft forever
    inet6 fe80::e2cb:4eff:fe48:4e28/64 scope link
       valid_lft forever preferred_lft forever
3: wls1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 00:25:d3:cd:e0:4f brd ff:ff:ff:ff:ff:ff
4: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 100
    link/none
    inet 10.8.0.1/24 brd 10.8.0.255 scope global tun0
       valid_lft forever preferred_lft forever
    inet6 fe80::f3c8:a059:15d5:5ec7/64 scope link flags 800
       valid_lft forever preferred_lft forever

Conteúdo completo de /etc/network/interfaces. /etc/network/interfaces.d/está vazia.

root@placeholder:~# cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto ens5
allow-hotplug ens5
iface ens5 inet dhcp

Responder1

Eu já resolvi o problema.

Observei em minha resposta que adquirir endereços IP funciona bem, mas liberar concessões não funciona.

Eu verifiquei /etc/dhcp/e encontrei dois diretórios interessantes:

root@placeholder:~# ls -lah /etc/dhcp/
total 744K
drwxr-xr-x   4 root root 4.0K Feb 17 08:00 .
drwxr-xr-x 104 root root 720K Feb 18 11:57 ..
-rw-r--r--   1 root root 1.4K Nov 26  2016 debug
drwxr-xr-x   2 root root 4.0K Aug 21  2010 dhclient-enter-hooks.d
drwxr-xr-x   2 root root 4.0K Feb 17 13:46 dhclient-exit-hooks.d
-rw-r--r--   1 root root 1.7K Dec 25 22:41 dhclient.conf

Acabei de descobrir que dhclientna verdade existem ganchos para quando 1.) A concessão de DHCP é adquirida e 2.) A concessão de DHCP é liberada. Os .ddiretórios contêm scripts bash que são executados quando os eventos mencionados são acionados.

Como o servidor estava tendo problemas para liberar concessões de DHCP, verifiquei o arquivo dhclient-exit-hooks.d. Com certeza, havia um script defeituoso. Havia um sendmailscript lá que sempre falhava, apesar de retornar 0 (diretório ausente, pois sendmailfoi desinstalado).

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