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Cisco3750にミラーポート(SPAN)を設定してみる。と、tcpdumpのプロミスキャスモードについて

■Cisco3750にミラーポート(SPAN)を設定してみる。と、tcpdumpのプロミスキャスモードについて

Switch#show version | include ^Cisco
Cisco IOS Software, C3750 Software (C3750-IPSERVICESK9-M), Version 12.2(55)SE10, RELEASE SOFTWARE (fc2)

■以下の続き

 Cisco C2970にローカルミラーポート(SPAN)を設定してみる。
 http://labunix.hateblo.jp/entry/20160209/1454996869

■tcpdumpコマンドはデフォルトでプロミスキャススモードが有効なのでスルーしてました。
 ブロードキャストパケット「255.255.255.255」で後ほど確認する方針とする。

 ネットワークスペシャリスト - SE娘の剣 -
 http://nw.seeeko.com/archives/cat_10022037.html

■C3750の初期化

Switch#delete vlan.dat
Delete filename [vlan.dat]? 
Delete flash:vlan.dat? [confirm]

Switch#erase startup-config 
Erasing the nvram filesystem will remove all configuration files! Continue? [confirm]
[OK]
Erase of nvram: complete

Switch#show startup-config 
startup-config is not present

Switch#reload 
Proceed with reload? [confirm]

■初期化後の確認

...
Would you like to enter the initial configuration dialog? [yes/no]: no
Switch>show interface status | include full
Fa1/0/1                      connected    1          a-full  a-100 10/100BaseTX
Fa1/0/2                      connected    1          a-full  a-100 10/100BaseTX

■構成は以下のとおり

$ echo "[Linux1-eth1] <--> [Fa1/0/1] <-- SPAN --> [Fa1/0/2] --> [Linux2-eth1]" | \
    graph-easy --dot | dot -o C3750mirror.png

■Linux1側の設定

$ echo "[Linux1]"; \
    ip a list eth1 | \
      sed -e 's/ [0-9a-e][0-9a-e]:[0-9a-e][0-9a-e]:[0-9a-e][0-9a-e]:[0-9a-e][0-9a-e]:/ XX:XX:XX:XX:/g'
[Linux1]
2: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether XX:XX:XX:XX:10:ca brd ff:ff:ff:ff:ff:ff
    inet 172.16.16.88/24 brd 172.16.16.255 scope global eth1
       valid_lft forever preferred_lft forever

■Cisco3750のIPアドレス設定

enable 
configure terminal 
  interface vlan 100
    ip address 172.16.16.254 255.255.255.0
    no shutdown 
  interface fa1/0/1
    switchport mode access 
    switchport access vlan 100
    no shutdown 
end

■Cisco3750のVLANの確認

Switch#show vlan id 100

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
100  VLAN0100                         active    Fa1/0/1

VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
100  enet  100100     1500  -      -      -        -    -        0      0   

Remote SPAN VLAN
----------------
Disabled

Primary Secondary Type              Ports
------- --------- ----------------- ------------------------------------------

■Cisco3750からのIP疎通確認

Switch#ping 172.16.16.254

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.16.254, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/8 ms
Switch#ping 172.16.16.88 

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.16.88, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/203/1007 ms

■Linux2側について、プロミスキャスモードの有効化方法は下記のとおり。

$ echo "[Linux2]"; \
    ip a list eth1 | \
      sed -e 's/ [0-9a-e][0-9a-e]:[0-9a-e][0-9a-e]:[0-9a-e][0-9a-e]:[0-9a-e][0-9a-e]:/ XX:XX:XX:XX:/g'
[Linux2]
6: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
    link/ether 00:0a:79:9a:c2:e3 brd ff:ff:ff:ff:ff:ff

$ sudo ifconfig eth1 promisc
$ ip a list eth1 | \
    sed -e 's/ [0-9a-e][0-9a-e]:[0-9a-e][0-9a-e]:[0-9a-e][0-9a-e]:[0-9a-e][0-9a-e]:/ XX:XX:XX:XX:/g'
6: eth1: <BROADCAST,MULTICAST,PROMISC> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
    link/ether XX:XX:XX:XX:c2:e3 brd ff:ff:ff:ff:ff:ff

■Linux2側について、プロミスキャスモードの解除方法は下記のとおり。

$ sudo ifconfig eth1 -promisc
$ ip a list eth1 | \
>     sed -e 's/ [0-9a-e][0-9a-e]:[0-9a-e][0-9a-e]:[0-9a-e][0-9a-e]:[0-9a-e][0-9a-e]:/ XX:XX:XX:XX:/g'
6: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
    link/ether XX:XX:XX:XX:c2:e3 brd ff:ff:ff:ff:ff:ff

■Cisco3750のローカルSPANを設定する。

configure terminal 
  monitor session 1 source interface fa1/0/1 both
  monitor session 1 destination interface fa1/0/2
end

■プロミスキャスモードを設定した状態で確認

$ sudo ifconfig eth1 promisc up

Switch#ping ip 172.16.16.255 repeat 1

Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 172.16.16.255, timeout is 2 seconds:
.
$ sudo tcpdump -i eth1 -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 262144 bytes
23:02:26.713238 IP 172.16.16.254 > 255.255.255.255: ICMP echo request, id 7, seq 0, length 80

■「255.255.255.255」を発信してみる。
 tcpdumpのプロミスキャスモードにより、結果に差が無い。

$ sudo ping -b 255.255.255.255 -c 2 -I eth1
WARNING: pinging broadcast address
PING 255.255.255.255 (255.255.255.255) from 172.16.16.88 eth1: 56(84) bytes of data.
64 bytes from 172.16.16.254: icmp_seq=1 ttl=255 time=1.81 ms
64 bytes from 172.16.16.254: icmp_seq=2 ttl=255 time=0.818 ms

--- 255.255.255.255 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1000ms
rtt min/avg/max/mdev = 0.818/1.318/1.819/0.501 ms

$ sudo ifconfig eth1 0.0.0.0 promisc
$ sudo tcpdump -i eth1 -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 262144 bytes
23:26:16.282436 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 4939, seq 1, length 64
23:26:16.284111 IP 172.16.16.254 > 172.16.16.88: ICMP echo reply, id 4939, seq 1, length 64
23:26:17.283370 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 4939, seq 2, length 64
23:26:17.284071 IP 172.16.16.254 > 172.16.16.88: ICMP echo reply, id 4939, seq 2, length 64

$ sudo ping -b 255.255.255.255 -c 2 -I eth1
WARNING: pinging broadcast address
PING 255.255.255.255 (255.255.255.255) from 172.16.16.88 eth1: 56(84) bytes of data.
64 bytes from 172.16.16.254: icmp_seq=1 ttl=255 time=0.742 ms
64 bytes from 172.16.16.254: icmp_seq=2 ttl=255 time=2.48 ms

--- 255.255.255.255 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1000ms
rtt min/avg/max/mdev = 0.742/1.612/2.483/0.871 ms

$ sudo ifconfig eth1 0.0.0.0 -promisc
$ sudo tcpdump -i eth1 -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 262144 bytes
23:27:40.889469 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 4944, seq 1, length 64
23:27:40.890100 IP 172.16.16.254 > 172.16.16.88: ICMP echo reply, id 4944, seq 1, length 64
23:27:41.889673 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 4944, seq 2, length 64
23:27:41.892162 IP 172.16.16.254 > 172.16.16.88: ICMP echo reply, id 4944, seq 2, length 64

■tcpdumpのプロミスキャスモードを「-p」で無効にしてみる。
 この場合、NICのプロミスキャスモード設定の有無で差が出る。
 自分宛で無いパケットを無視された。

$ sudo ifconfig eth1 0.0.0.0 promisc
$ sudo tcpdump -i eth1 -n -p
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 262144 bytes
23:35:29.250623 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 5012, seq 1, length 64
23:35:29.250989 IP 172.16.16.254 > 172.16.16.88: ICMP echo reply, id 5012, seq 1, length 64
23:35:30.249666 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 5012, seq 2, length 64
23:35:30.250034 IP 172.16.16.254 > 172.16.16.88: ICMP echo reply, id 5012, seq 2, length 64

$ sudo ifconfig eth1 0.0.0.0 -promisc
$ sudo tcpdump -i eth1 -n -p
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 262144 bytes
23:35:05.544735 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 5010, seq 1, length 64
23:35:06.545450 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 5010, seq 2, length 64

■「他の理由でインターフェイスが無差別透過モードになってしまうこともある」が、
 「ifconfig eth1 promisc」のこと。

$ man tcpdump 2>/dev/null | grep -A 3 ' -p ' 
       -p     無差別透過モードを  利用しない。しかしながら、他の理由でインター
              フェイスが無差別透過モードになってしまうこともあることに注意する
              こと。  このため '-p' オプションは 'ether host {loca-lw-addr} or
              ether broadcast' の省略形としては使用できない。

■他の理由とは、「kernel」で有効、無効となるから。

$ sudo ifconfig eth1 promisc up ;dmesg | tail -1
[ 6955.477104] device eth1 entered promiscuous mode
$ sudo ifconfig eth1 -promisc up ;dmesg | tail -1
[ 7190.395117] device eth1 left promiscuous mode

■「tcpdump」をデフォルトで起動、終了時には以下の処理がある。

 「tcpdump」を起動した時に「entered promiscuous mode」
 「tcpdump」を終了した時に「left promiscuous mode」

$ sudo tcpdump -i eth1 -n -c2;dmesg | tail -2
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 262144 bytes
00:08:44.606684 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 5408, seq 1, length 64
00:08:44.607105 IP 172.16.16.254 > 172.16.16.88: ICMP echo reply, id 5408, seq 1, length 64
2 packets captured
2 packets received by filter
0 packets dropped by kernel
[ 8221.738872] device eth1 entered promiscuous mode
[ 8239.777312] device eth1 left promiscuous mode

■「tcpdump -p」で起動、終了時に、「promiscuous mode」に関する処理を行わない。

$ sudo tcpdump -i eth1 -n -p -c2;dmesg | tail -2
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 262144 bytes
00:09:16.423298 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 5410, seq 1, length 64
00:09:17.422298 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 5410, seq 2, length 64
2 packets captured
2 packets received by filter
0 packets dropped by kernel
[ 8221.738872] device eth1 entered promiscuous mode
[ 8239.777312] device eth1 left promiscuous mode

■dmesgのキャッシュをクリアすることで明確になる。

$ sudo dmesg -c > /dev/null;sudo tcpdump -i eth1 -n -p -c2;sudo dmesg -c | tail -2
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 262144 bytes
00:16:41.926525 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 5470, seq 1, length 64
00:16:42.925514 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 5470, seq 2, length 64
2 packets captured
2 packets received by filter
0 packets dropped by kernel

■プロミスキャスモードなら、以下のようにIPアドレスを設定しても問題ない。

$ sudo ifconfig eth1 172.16.16.89/24 promisc up
$ sudo tcpdump -i eth1 -n -p -c4
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 262144 bytes
00:21:57.796137 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 5504, seq 1, length 64
00:21:57.796478 IP 172.16.16.254 > 172.16.16.88: ICMP echo reply, id 5504, seq 1, length 64
00:21:58.796722 IP 172.16.16.88 > 255.255.255.255: ICMP echo request, id 5504, seq 2, length 64
00:21:58.797077 IP 172.16.16.254 > 172.16.16.88: ICMP echo reply, id 5504, seq 2, length 64
4 packets captured
4 packets received by filter
0 packets dropped by kernel