Impossibile stabilire le connessioni TCP quando il traffico segue percorsi asimmetrici
Introduzione
In questo documento viene descritto il problema che si verifica quando si utilizzano percorsi asimmetrici per l'inoltro del traffico nell'infrastruttura SD-WAN.
Problema
Non è possibile stabilire connessioni Secure Shell (SSH) all'host 2 (hostname - edgeclien2) dall'host 1 (hostname - edgeclien1), ma allo stesso tempo SSH funziona correttamente in direzione inversa.
[root@edgeclient2 user]# ssh user@192.168.40.21 user@192.168.40.21's password: Last login: Sun Feb 10 13:26:32 2019 from 192.168.60.20 [user@edgeclient1 ~]$
[root@edgeclient1 user]# ssh user@192.168.60.20 <nothing happens after that>
o
[user@edgeclient1 ~]$ ssh user@192.168.60.20 ssh_exchange_identification: Connection closed by remote host
Sia i daemon SSH edgeclient1 che edgeclient2 dispongono di configurazioni e connessioni valide che possono essere stabilite dal segmento LAN locale correttamente:
vedge4# request execute vpn 40 ssh user@192.168.60.20 user@192.168.60.20's password: Last login: Sun Feb 10 13:28:23 2019 from 192.168.60.7 [user@edgeclient2 ~]$
Tutte le altre applicazioni TCP (Transmission Control Protocol) hanno problemi simili.
Diagramma topologico
Diagnostica
Questi elenchi di controllo di accesso (ACL) sono stati configurati e applicati nelle direzioni corrispondenti sulle interfacce del lato servizio di vEdge1 e vEdge3:
policy
access-list SSH_IN
sequence 10
match
source-ip 192.168.40.21/32
destination-ip 192.168.60.20/32
!
action accept
count SSH_IN
!
!
default-action accept
!
access-list SSH_OUT
sequence 10
match
source-ip 192.168.60.20/32
destination-ip 192.168.40.21/32
!
action accept
count SSH_OUT
!
!
default-action accept
!
!
ACL con mirroring applicato a vEdge4:
policy
access-list SSH_IN
sequence 10
match
source-ip 192.168.60.20/32
destination-ip 192.168.40.21/32
!
action accept
count SSH_IN
!
!
default-action accept
!
access-list SSH_OUT
sequence 10
match
source-ip 192.168.40.21/32
destination-ip 192.168.60.20/32
!
action accept
count SSH_OUT
!
!
default-action accept
!
!
Anche la visibilità delle app è stata abilitata su tutti i router vEdge e i flussi sono stati controllati durante la fase di creazione della connessione SSH:
vedge1# show app cflowd flows | tab ; show policy access-list-counters
TCP TIME EGRESS INGRESS
SRC DEST IP CNTRL ICMP TOTAL TOTAL MIN MAX TO INTF INTF
VPN SRC IP DEST IP PORT PORT DSCP PROTO BITS OPCODE NHOP IP PKTS BYTES LEN LEN START TIME EXPIRE NAME NAME
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
40 192.168.40.21 192.168.60.20 47866 22 0 6 24 0 192.168.109.7 3 227 66 87 Sun Feb 17 14:13:25 2019 34 ge0/0 ge0/1
COUNTER
NAME NAME PACKETS BYTES
----------------------------------
SSH_IN SSH_IN 3 227
SSH_OUT SSH_OUT 2 140
vedge3# show app cflowd flows | tab ; show policy access-list-counters
TCP TIME EGRESS INGRESS
SRC DEST IP CNTRL ICMP TOTAL TOTAL MIN MAX TO INTF INTF
VPN SRC IP DEST IP PORT PORT DSCP PROTO BITS OPCODE NHOP IP PKTS BYTES LEN LEN START TIME EXPIRE NAME NAME
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
40 192.168.60.20 192.168.40.21 22 47866 0 6 18 0 192.168.40.21 8 480 60 60 Sun Feb 17 14:14:08 2019 51 ge0/1 ge0/0
COUNTER
NAME NAME PACKETS BYTES
----------------------------------
SSH_IN SSH_IN 0 0
SSH_OUT SSH_OUT 7 420
vedge4# show app cflowd flows | tab ; show policy access-list-counters
TCP TIME EGRESS INGRESS
SRC DEST IP CNTRL ICMP TOTAL TOTAL MIN MAX TO INTF INTF
VPN SRC IP DEST IP PORT PORT DSCP PROTO BITS OPCODE NHOP IP PKTS BYTES LEN LEN START TIME EXPIRE NAME NAME
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
40 192.168.40.21 192.168.60.20 47866 22 0 6 2 0 192.168.60.20 4 240 60 60 Sun Feb 17 14:17:44 2019 37 ge0/2 ge0/0
40 192.168.60.20 192.168.40.21 22 47866 0 6 18 0 192.168.110.6 8 592 74 74 Sun Feb 17 14:17:44 2019 49 ge0/0 ge0/2
COUNTER
NAME NAME PACKETS BYTES
----------------------------------
SSH_IN SSH_IN 8 592
SSH_OUT SSH_OUT 4 240
Come si può vedere da questi output, i flussi in entrata e in uscita sono asimmetrici. edgeclient1 (192.168.40.21) sta tentando di stabilire una sessione SSH con edgeclient2 (192.168.60.20), il traffico in entrata via vEdge1 e il traffico di ritorno via vEdge3. Dai contatori ACL è possibile verificare che il numero di pacchetti in entrata e in uscita su vEdge4 non corrisponde alla somma nelle direzioni corrispondenti su vEdge1 e vEdge3. Allo stesso tempo, non si verifica alcuna perdita di pacchetti durante il test con ping. :
[root@edgeclient1 user]# ping -f 192.168.60.20 -c 10000 PING 192.168.60.20 (192.168.60.20) 56(84) bytes of data. --- 192.168.60.20 ping statistics --- 10000 packets transmitted, 10000 received, 0% packet loss, time 3076ms rtt min/avg/max/mdev = 0.128/0.291/6.607/0.623 ms, ipg/ewma 0.307/0.170 ms
[root@edgeclient2 user]# ping -f 192.168.40.21 -c 10000 PING 192.168.40.21 (192.168.40.21) 56(84) bytes of data. --- 192.168.40.21 ping statistics --- 10000 packets transmitted, 10000 received, 0% packet loss, time 3402ms rtt min/avg/max/mdev = 0.212/0.318/2.766/0.136 ms, ipg/ewma 0.340/0.327 ms
Sappiamo anche che SSH funziona bene in direzione inversa e i file possono essere copiati su SCP/SFTP anche senza problemi.
Soluzione
Inizialmente si sospettava che fossero presenti una configurazione DPI (Deep Packet Inspection) o criteri dati, ma nessuno di essi è stato attivato:
vedge3# show policy from-vsmart % No entries found. vedge1# show policy from-vsmart % No entries found.
Ma alla fine è stato scoperto che l'ottimizzazione TCP era abilitata:
vedge1# show app tcp-opt active-flows
EGRESS INGRESS
SRC DEST INTF INTF TX RX UNOPT PROXY
VPN SRC IP DEST IP PORT PORT START TIME NAME NAME BYTES BYTES TCP STATE REASON IDENTITY
--------------------------------------------------------------------------------------------------------------------------------------------
40 192.168.40.21 192.168.60.20 47868 22 Sun Feb 17 14:18:13 2019 ge0_0 ge0_1 314 0 In-progress - Client-Proxy
vedge1# show app tcp-opt expired-flows
SRC DEST TX RX UNOPT PROXY
TIMESTAMP VPN SRC IP DEST IP PORT PORT START TIME END TIME BYTES BYTES TCP STATE REASON IDENTITY DELETE REASON
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1549819969608 40 192.168.40.21 192.168.60.7 22 56612 Sun Feb 10 18:32:49 2019 Sun Feb 10 18:36:03 2019 5649 4405 Optimized - Server-Proxy CLOSED
1549820055487 40 192.168.40.21 192.168.60.7 22 56613 Sun Feb 10 18:34:15 2019 Sun Feb 10 19:07:46 2019 5719 4669 Optimized - Server-Proxy CLOSED
1550408210511 40 192.168.40.21 192.168.60.20 47862 22 Sun Feb 17 13:56:50 2019 Sun Feb 17 13:56:58 2019 401 0 Optimized - Client-Proxy STATE-TIMEOUT
1550408981634 40 192.168.40.21 192.168.60.20 47864 22 Sun Feb 17 14:09:41 2019 Sun Feb 17 14:09:49 2019 401 0 Optimized - Client-Proxy STATE-TIMEOUT
1550409205399 40 192.168.40.21 192.168.60.20 47866 22 Sun Feb 17 14:13:25 2019 Sun Feb 17 14:13:33 2019 227 0 Optimized - Client-Proxy STATE-TIMEOUT
1550409493042 40 192.168.40.21 192.168.60.20 47868 22 Sun Feb 17 14:18:13 2019 Sun Feb 17 14:18:21 2019 401 0 Optimized - Client-Proxy STATE-TIMEOUT
Inoltre, nei debug ftm può essere visualizzato il messaggio tcpopt CONN_TEARDOWN.
vedge1# show log /var/log/tmplog/vdebug tail "-f" local7.debug: Feb 17 13:56:50 vedge1 FTMD[662]: ftm_tcpopt_flow_add[268]: Created new tcpflow :- vrid-3 192.168.40.21/47862 192.168.60.20/22 local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpd_send_conn_tear_down[388]: Trying to pack and send the following message to TCPD local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpd_send_conn_tear_down[408]: Sending following CONN_TD msg local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpd_send_conn_tear_down[413]: 192.168.40.21:47862->192.168.60.20:22; vpn:40; syn_seq_num:4172167164; identity:0; cport_prime:0 local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpd_msgq_tx[354]: Transfering size = 66 bytes data local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpd_send_conn_tear_down[416]: Successfully sent conn_td msg to TCPD local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpopt_propagate_tear_down[1038]: Sent CONN_TEARDOWN msg to tcpd for existing tcpflow :- vrid-3 192.168.40.21/47862 192.168.60.20/22 ; identity:CLIENT_SIDE_PROXY . Send Successful ! local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpopt_append_expired_err_flow_tbl[958]: Appending flow vrid-3 192.168.40.21/47862 192.168.60.20/22 to the expired flow table at Sun Feb 17 13:56:58 2019 local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpopt_append_expired_err_flow_tbl[980]: Appending flow vrid-3 192.168.40.21/47862 192.168.60.20/22 to the error flow table at Sun Feb 17 13:56:58 2019 local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpopt_flow_delete[293]: Removing tcpflow :- vrid-3 192.168.40.21/47862 192.168.60.20/22 local7.debug: Feb 17 13:56:58 vedge1 TCPD[670]: handle_upstream_connect[538]: Error - BP NULL local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpd_msg_decode[254]: FTM-TCPD: Received FTM_TCPD__PB_FTM_TCPD_MSG__E_MSG_TYPE__CONN_CLOSED msg local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpd_handle_conn_closed[139]: FTM-TCPD: Received CONN_CLOSED for following C->S local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpd_handle_conn_closed[150]: 192.168.40.21:47862->192.168.60.20:22; vpn:40; syn_seq_num:4172167164; identity:0; cport_prime:47862; bind_port:0 local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpd_handle_conn_closed[184]: FTM-TCPD: Could not find entry in FT for following flow local7.debug: Feb 17 13:56:58 vedge1 FTMD[662]: ftm_tcpd_handle_conn_closed[185]: vrid-3 192.168.40.21/47862 192.168.60.20/22
A questo punto è possibile osservare un esempio di funzionamento corretto dell'ottimizzazione TCP (è possibile visualizzare il messaggio CONN_EST):
vedge3# show log /var/log/tmplog/vdebug tail "-f -n 0" local7.debug: Feb 17 15:41:13 vedge3 FTMD[657]: ftm_tcpd_msg_decode[254]: FTM-TCPD: Received FTM_TCPD__PB_FTM_TCPD_MSG__E_MSG_TYPE__CONN_CLOSED msg local7.debug: Feb 17 15:41:13 vedge3 FTMD[657]: ftm_tcpd_handle_conn_closed[139]: FTM-TCPD: Received CONN_CLOSED for following C->S local7.debug: Feb 17 15:41:13 vedge3 FTMD[657]: ftm_tcpd_handle_conn_closed[150]: 192.168.40.21:47876->192.168.60.20:22; vpn:40; syn_seq_num:2779178897; identity:0; cport_prime:47876; bind_port:0 local7.debug: Feb 17 15:41:15 vedge3 FTMD[657]: ftm_tcpd_msg_decode[258]: FTM-TCPD: Received FTM_TCPD__PB_FTM_TCPD_MSG__E_MSG_TYPE__CONN_EST msg local7.debug: Feb 17 15:41:15 vedge3 FTMD[657]: ftm_tcpd_handle_conn_est[202]: FTM-TCPD: Received CONN_EST for following C->S local7.debug: Feb 17 15:41:15 vedge3 FTMD[657]: ftm_tcpd_handle_conn_est[213]: 192.168.40.21:47878->192.168.60.20:22; vpn:40; syn_seq_num:2690847868; identity:0; cport_prime:47878; bind_port:0 local7.debug: Feb 17 15:41:15 vedge3 FTMD[657]: ftm_tcpopt_flow_add[268]: Created new tcpflow :- vrid-3 192.168.40.21/47878 192.168.60.20/22
Conclusioni
L'ottimizzazione TCP richiede che i flussi siano simmetrici, quindi per risolvere il problema è necessario disabilitare l'ottimizzazione TCP (senza ottimizzazione tcp vpn 40) o creare criteri dati per forzare i flussi TCP a seguire lo stesso percorso in entrambe le direzioni. Per ulteriori informazioni su questo argomento, consultare la sezione Traffic Symmetry for DPI della SD-WAN Design Guide, pagina 23.
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