Estou usando 16.04 LTS amd64. Minha conexão pptp costumava ficar bem há alguns dias, mas de ontem sobre isso avarias. Ele se conecta sem nenhum problema, mas em pouco tempo dentro de 3 ou 4 minutos descarta as conexões.
Aqui está o syslog (informando exatamente no momento em que ele falha):
May 29 11:52:14 ABFORCE pptp[11219]: nm-pptp-service-11187 log[pptp_handle_timer:pptp_ctrl.c:1105]: closing control connection due to missing echo reply
May 29 11:52:14 ABFORCE pptp[11219]: nm-pptp-service-11187 log[ctrlp_rep:pptp_ctrl.c:259]: Sent control packet type is 12 'Call-Clear-Request'
May 29 11:52:14 ABFORCE pptp[11219]: nm-pptp-service-11187 log[pptp_conn_close:pptp_ctrl.c:461]: Closing PPTP connection
May 29 11:52:14 ABFORCE NetworkManager[1017]: Modem hangup
May 29 11:52:14 ABFORCE pptp[11219]: nm-pptp-service-11187 log[ctrlp_rep:pptp_ctrl.c:259]: Sent control packet type is 3 'Stop-Control-Connection-Request'
May 29 11:52:14 ABFORCE NetworkManager[1017]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 8 / phase 'network'
May 29 11:52:14 ABFORCE NetworkManager[1017]: Connect time 4.0 minutes.
May 29 11:52:14 ABFORCE NetworkManager[1017]: Sent 3646742980 bytes, received 2594786 bytes.
May 29 11:52:14 ABFORCE pptp[11219]: nm-pptp-service-11187 log[call_callback:pptp_callmgr.c:84]: Closing connection (call state)
May 29 11:52:14 ABFORCE pppd[11196]: Modem hangup
May 29 11:52:14 ABFORCE pppd[11196]: Connect time 4.0 minutes.
May 29 11:52:14 ABFORCE pppd[11196]: Sent 3646742980 bytes, received 2594786 bytes.
May 29 11:52:14 ABFORCE NetworkManager[1017]: <error> [1464506534.7838] platform-linux: do-add-ip4-route[19: 0.0.0.0/0 50]: failure 100 (Network is down)
May 29 11:52:14 ABFORCE NetworkManager[1017]: <warn> [1464506534.7838] default-route: failed to add default route 0.0.0.0/0 via 0.0.0.0 dev 19 metric 50 mss 0 src vpn with effective metric 50
May 29 11:52:14 ABFORCE NetworkManager[1017]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 5 / phase 'establish'
May 29 11:52:14 ABFORCE NetworkManager[1017]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 11 / phase 'disconnect'
May 29 11:52:14 ABFORCE NetworkManager[1017]: Connection terminated.
May 29 11:52:14 ABFORCE pppd[11196]: Connection terminated.
May 29 11:52:14 ABFORCE NetworkManager[1017]: <error> [1464506534.7852] platform-linux: do-change-link[19]: failure changing link: failure 19 (No such device)
May 29 11:52:14 ABFORCE NetworkManager[1017]: <warn> [1464506534.7852] device (ppp0): failed to disable userspace IPv6LL address handling
May 29 11:52:14 ABFORCE NetworkManager[1017]: ** Message: Terminated ppp daemon with PID 11196.
May 29 11:52:14 ABFORCE NetworkManager[1017]: <error> [1464506534.7856] platform-linux: do-add-ip4-route[19: 0.0.0.0/0 50]: failure 19 (No such device)
May 29 11:52:14 ABFORCE NetworkManager[1017]: <warn> [1464506534.7856] default-route: failed to add default route 0.0.0.0/0 via 0.0.0.0 dev 19 metric 50 mss 0 src vpn with effective metric 50
May 29 11:52:14 ABFORCE NetworkManager[1017]: <info> [1464506534.7893] vpn-connection[0x2a17200,26f48d34-ac2b-4db9-ba9e-ef146551580a,"Sharif VPN",19:(ppp0)]: VPN plugin: state changed: stopping (5)
Eu tive exatamente o mesmo problema. Eu procurei por um dia inteiro para entender por que isso aconteceu. No meu caso, a causa raiz foi capturada no syslog em uma entrada: nm-pptp-service-17364 warn [decaps_gre: pptp_gre.c: 347]: short read (-1): Mensagem muito longa . Aparentemente, a causa foi uma configuração de MTU no adaptador de WLAN que era muito grande. Por padrão, é definido como automático.
O que fiz foi configurá-lo para 1200 e funciona bem.
Espero que isso ajude,
Radu