tcpdumpのインストールと使い方 パケット解析をする

今回はネットワーク周りで思うようにサーバーが動作しない時に確実に役立つトラブルシューティングのツールを紹介します。

ツールはパケットをダンプする「tcpdump」コマンドです。

 

タマちゃん
ダンプするって何?
管理人さん
ダンプするとは「そのまま吐き出す」ことだよ。
例えば通常のドキュメントって綺麗に改行していたり、読みやすく成形しているけど、ダンプはそのままデータを吐き出しているから読みにくかったりするよ。

 

生のパケットの中身を確認できるようになると、今までなら諦めていたレンタルサーバー周りの設定だったり、自宅サーバーの設定、VPSや専用サーバーの設定が最後までできるようになります。

例えば、「ホームページが開けない」、「FTPでアクセスできない」、「名前解決ができない」などレンタルサーバーや自宅サーバー、VPSでホームページを運用しているといろんなトラブルが起きます。

しかし「tcpdump」というツールを使って調査することで原因の特定までできるようになります。

(すべてのトラブルを解決できるわけではありません)

 

tcpdumpコマンドとは

tcpdump コマンドは、指定したネットワークインターフエースを監視し、到達したパケットのデータを表示します。

tcpdumpコマンドは「理論」よりも「実践」です。

manコマンドで tcpdump コマンドを勉強しつつ、様々な例を試しつつ覚えるのが一番いいです。

tcpdumpインストール手順

tcpdumpコマンドはyumコマンドで簡単にインストールできます。

# yum install tcpdump

 

 

tcpdump コマンド使い方

単純に tcpdump コマンドを実行するとプロミスキャスモードで動作します。

プロミスキャスモードとは「自分宛以外のパケットも受け取る状態」のことです。

下記は単純な tcpdump コマンドのみ実行した例です。

# tcpdump

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode

listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes

23:08:07.844408 IP tk2-xxx-xxx.vs.sakura.ne.jp.ssh > softbank126099xxxxxxx.bbtec.net.6905: Flags [P.], seq 179977145:179977385, ack 86209917, win 757, length 240

23:08:07.844994 IP tk2-xxx-xxx.vs.sakura.ne.jp.55070 > google-public-dns-a.google.com.domain: 62091+ PTR? 202.231.99.126.in-addr.arpa. (45)

23:08:07.849756 IP softbank126099xxxxxxx.bbtec.net.6905 > tk2-xxx-xxx.vs.sakura.ne.jp.ssh: Flags [.], ack 240, win 16266, length 0

23:08:07.882443 IP google-public-dns-a.google.com.domain > tk2-xxx-xxx.vs.sakura.ne.jp.55070: 62091 1/0/0 PTR softbank126099xxxxxxx.bbtec.net. (89)

23:08:07.882699 IP tk2-xxx-xxx.vs.sakura.ne.jp.58348 > google-public-dns-a.google.com.domain: 49586+ PTR? 115.217.16.160.in-addr.arpa. (45)

23:08:07.927224 ARP, Request who-has tk2-xxx-xxx.vs.sakura.ne.jp tell gateway, length 46

tcpdump[オプション][条件式]

 

オプション説明
-i インターフェースキャプチャするインターフェースを指定します。
-qデータの表示を簡易化します。
-s バイト数パケットから取り出すバイト数を指定します。
-Xデータを16進数とASCII文字で表示します。
-nアドレスを名前解決せずに表示します。
-nnアドレスを名前解決せず、且つ、プロトコル名も表示せずにポート番号で表示します。
-Nホストのドメイン名を表示しません。
-l標準出力をバッファリングします。
-tキャプチャしたデータの時刻を表示しません。
-v詳細に出力します。
-w ファイル名キャプチャしたデータをファイル名で出力します。
-aネットワークアドレスの名前解決をして表示します。
-Aキャプチャ↓データをASCII文字で表示します。
-pプロミスキャスモードにしません。つまり、自ホスト宛のパケットのみキャプチャします。
-c COUNTCOUNT個のパケットを受信したら終了します。

 

条件式説明
portポート番号を指定します。
protoプロトコルを指定します。

 

 

オプション解説

-nn ← IPアドレスやプロトコルをそのまま数字で表示します。ヘタに文字にされるよりは見やすいです。

 

 

例1 特定のポートのアクセスを解析したい

この例では、53番ポートへのアクセスのコマンド実行例です。

クライアント側は160.16.xxx.xxxで任意のポートを使用し、DNSサーバー8.8.8.8へ名前解決の問い合わせをしていることがわかります。

オプション -nn をつけることでIPアドレスもプロトコルも「番号」で表示されるようになりました。

こちらの方が分かりやすいですよね。

# tcpdump -nn port 53

23:29:50.748082 IP 160.16.xxx.xxx.41333 > 8.8.8.8.53: 35511+ A? google.co.jp. (30)
23:29:50.786010 IP 8.8.8.8.53 > 160.16.xxx.xxx.41333: 35511 1/0/0 A 216.58.197.163 (46) ←Aレコードを返しています。

23:30:17.399857 IP 160.16.xxx.xxx.34166 > 8.8.8.8.53: 13182+ A? yahoo.co.jp. (29)
23:30:17.400986 IP 8.8.8.8.53 > 160.16.xxx.xxx.34166: 13182 2/0/0 A 183.79.135.206, A 182.22.59.229 (61) ←Yahoo!の場合はAレコードが2つ表示されています。

23:30:54.478579 IP 160.16.xxx.xxx.54041 > 8.8.8.8.53: 15820+ A? google.co.jp. (30)
23:30:54.519863 IP 8.8.8.8.53 > 160.16.xxx.xxx.54041: 15820 1/0/0 A 172.217.25.195 (46)
23:30:54.521821 IP 160.16.xxx.xxx.42915 > 8.8.8.8.53: 39623+ PTR? 195.25.217.172.in-addr.arpa. (45)
23:30:54.560808 IP 8.8.8.8.53 > 160.16.xxx.xxx.42915: 39623 4/0/0 PTR nrt12s13-in-f195.1e100.net., PTR nrt12s13-in-f195.1e100.net., PTR nrt12s13-in-f3.1e100.net., PTR nrt12s13-in-f3.1e100.net. (142)

 

ちなみに上記のパケットは下記のコマンドを実行した際に出力されました。

$ nslookup google.co.jp

Server:         8.8.8.8
Address:        8.8.8.8#53

Non-authoritative answer:
Name:   google.co.jp
Address: 216.58.197.163

$ nslookup yahoo.co.jp
Server:         8.8.8.8
Address:        8.8.8.8#53

Non-authoritative answer:
Name:   yahoo.co.jp
Address: 183.79.135.206
Name:   yahoo.co.jp
Address: 182.22.59.229

$ ping google.co.jp
PING google.co.jp (172.217.25.195) 56(84) bytes of data.
64 bytes from nrt12s13-in-f195.1e100.net (172.217.25.195): icmp_seq=1 ttl=56 time=1.37 ms
64 bytes from nrt12s13-in-f195.1e100.net (172.217.25.195): icmp_seq=2 ttl=56 time=1.30 ms
64 bytes from nrt12s13-in-f195.1e100.net (172.217.25.195): icmp_seq=3 ttl=56 time=1.13 ms
64 bytes from nrt12s13-in-f195.1e100.net (172.217.25.195): icmp_seq=4 ttl=56 time=1.12 ms
^C
--- google.co.jp ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3004ms
rtt min/avg/max/mdev = 1.125/1.233/1.374/0.112 ms
$

 

次は、80番ポートへのアクセスです。

# tcpdump -nn -i eth0 port 80

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
21:32:46.508344 IP 160.16.xxx.xxx.60644 > 183.90.xxx.xxx.80: Flags [S], seq 1796187407, win 29200, options [mss 1460,sackOK,TS val 2507857308 ecr 0,nop,wscale 6], length 0 ← ローカルからリモートの80番ポートへアクセス。SYNパケットをWebサーバーへ送っています。「Flags [S]」
21:32:46.517503 IP 183.90.xxx.xxx.80 > 160.16.xxx.xxx.60644: Flags [S.], seq 3097415759, ack 1796187408, win 28960, options [mss 1460,sackOK,TS val 2987040806 ecr 2507857308,nop,wscale 7], length 0 ← リモート(Webサーバー)からローカルへSYN+ACKが返ってきます。
21:32:46.517536 IP 160.16.xxx.xxx.60644 > 183.90.xxx.xxx.80: Flags [.], ack 1, win 457, options [nop,nop,TS val 2507857317 ecr 2987040806], length 0 ← ローカルからリモートへACKを送っています。 これで3wayハンドシェイクの完了です。
21:32:46.517620 IP 160.16.xxx.xxx.60644 > 183.90.xxx.xxx.80: Flags [P.], seq 1:80, ack 1, win 457, options [nop,nop,TS val 2507857317 ecr 2987040806], length 79
21:32:46.526682 IP 183.90.xxx.xxx.80 > 160.16.xxx.xxx.60644: Flags [.], ack 80, win 227, options [nop,nop,TS val 2987040815 ecr 2507857317], length 0
21:32:46.527817 IP 183.90.xxx.xxx.80 > 160.16.xxx.xxx.60644: Flags [P.], seq 1:422, ack 80, win 227, options [nop,nop,TS val 2987040817 ecr 2507857317], length 421
21:32:46.527852 IP 160.16.xxx.xxx.60644 > 183.90.xxx.xxx.80: Flags [.], ack 422, win 473, options [nop,nop,TS val 2507857327 ecr 2987040817], length 0
21:32:46.528069 IP 160.16.xxx.xxx.60644 > 183.90.xxx.xxx.80: Flags [F.], seq 80, ack 422, win 473, options [nop,nop,TS val 2507857327 ecr 2987040817], length 0
21:32:46.537165 IP 183.90.xxx.xxx.80 > 160.16.xxx.xxx.60644: Flags [F.], seq 422, ack 81, win 227, options [nop,nop,TS val 2987040826 ecr 2507857327], length 0
21:32:46.537186 IP 160.16.xxx.xxx.60644 > 183.90.xxx.xxx.80: Flags [.], ack 423, win 473, options [nop,nop,TS val 2507857337 ecr 2987040826], length 0

 

フラグの見方

[S] → クライアントが「SYN」を送る

[S.] → サーバーが「SYN+ACK」を返す

[.] → クライアントが「ACK」を送る

[F.] → クライアントが送る「FIN+ACK」とサーバーが送る「FIN+ACK」

[P] → PUSHを送る

 

例2 SSHなど特定のポートを除外したい

例えば、teraterm でリモートるグインしている Linux サーバー上で tcpdump コマンドを実行すると、sshの通信でデータがいっぱいになり、本来調べたい通信の情報が埋もれてしまうことがあります。

その場合はssh(TCP/22)を除外しましょう。

not port [ポート番号]で除外できます。

# tcpdump -nn not port 22

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
20:45:13.658330 ARP, Request who-has 160.16.xxx.xxx tell 160.16.xxx.xxx, length 46
20:45:13.658443 ARP, Request who-has 160.16.xxx.xxx tell 160.16.xxx.xxx, length 46
20:45:13.757387 ARP, Request who-has 160.16.xxx.xxx tell 160.16.xxx.xxx, length 46
20:45:13.793743 IP6 fe80::1 > ff02::12: ip-proto-112 40
20:45:13.922963 ARP, Request who-has 160.16.xxx.xxx tell 160.16.xxx.xxx, length 46
20:45:13.923074 ARP, Request who-has 160.16.xxx.xxx tell 160.16.xxx.xxx, length 46
20:45:13.923126 ARP, Request who-has 160.16.xxx.xxx tell 160.16.xxx.xxx, length 46

 

例3 ARPとSSHなど複数の特定のポートを除外したい

複数のポート、プロトコルを組み合わせて除外したい場合は、下記のように「and」でつなげます。

# tcpdump -nn -i eth0 not arp and not port 22

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
21:03:16.902986 IP 160.16.xxx.xxx > 224.0.0.18: VRRPv2, Advertisement, vrid 11, prio 255, authtype simple, intvl 1s, length 20
21:03:17.228378 IP6 fe80::1 > ff02::12: ip-proto-112 40
21:03:17.903018 IP 160.16.xxx.xxx > 224.0.0.18: VRRPv2, Advertisement, vrid 11, prio 255, authtype simple, intvl 1s, length 20
21:03:18.228510 IP6 fe80::1 > ff02::12: ip-proto-112 40

 

例4 特定のプロトコルを指定したい

以下の各プロトコルを特定してキャプチャすることができます。

全部で16種類あります。

ether、fddi、mopdl、ip、ip6、arp、rarp、decnet、lat、sca、moprc、mopdl、icmp、icmp6、tcp、udpです。

使い方は「単純にプロトコルを書くだけ」です。

# tcpdump -nn -i eth0 icmp

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
21:24:06.489929 IP 173.231.189.68 > 160.16.xxx.xxx: ICMP host 173.231.189.68 unreachable - admin prohibited, length 70
21:26:15.863451 IP 126.99.231.202 > 160.16.xxx.xxx: ICMP echo request, id 2267, seq 1, length 64
21:26:15.863553 IP 160.16.xxx.xxx > 126.99.231.202: ICMP echo reply, id 2267, seq 1, length 64
21:26:16.861808 IP 126.99.231.202 > 160.16.xxx.xxx: ICMP echo request, id 2267, seq 2, length 64
21:26:16.861866 IP 160.16.xxx.xxx > 126.99.231.202: ICMP echo reply, id 2267, seq 2, length 64

キャプチャデータをWiresharkで見る

のちほどWiresharkでキャプチャデータを見たい場合は、オプション「-w [ファイル名]」を指定します。

# tcpdump -i eth0 -w test.cap

 

キャプチャしたデータを「Wireshark」で開くと下図のように見やすく成形されて表示されます。

下図は「tcp.port == 80」で80番ポートの通信のみが表示されるようにフィルタリングをしています。

 

例 特定のポートを絞り、ファイルとして出力したい

80番ポートだけをキャプチャし、ファイルとして出力したい場合です。

[root@SAKURA_VPS ~]# tcpdump port 80 -i eth0 -w test111.cap
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
^C4100 packets captured
6202 packets received by filter
2102 packets dropped by kernel
[root@SAKURA_VPS ~]#

 

 

3.3MBのファイルが出力されました。

[root@SAKURA_VPS ~]# ls -lh test11.cap
-rw-r--r-- 1 tcpdump tcpdump 3.3M 11月 23 08:55 test11.cap
[root@SAKURA_VPS ~]#

 

 

Wiresharkで内容を確認してみると80番ポートの通信のみキャプチャされています。

 

【補足】IPv6を使っていたので無効にする

tcpdumpを実行したらIPv6が有効になっており、tcpdumpの結果もIPv6で出力されました。

そのため見にくいため以下の手順でIPv6を無効にしています。

【変更前の設定】

# ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 160.16.xxx.xxx netmask 255.255.254.0  broadcast 160.16.xxx.xxx
        inet6 2001:e42:102:1810:160:16:xxx:xxx  prefixlen 64  scopeid 0x0<global>
        inet6 fe80::9ea3:xxx:xxx:xxx  prefixlen 64  scopeid 0x20<link>
        ether 9c:a3:ba:02:0e:cd  txqueuelen 1000  (Ethernet)
        RX packets 66569131  bytes 4352540231 (4.0 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 672161  bytes 76183023 (72.6 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

# cat ifcfg-eth0
DEVICE="eth0"
IPADDR="160.16.xxx.xxx"
NETMASK="255.255.254.0"
GATEWAY="160.16.xxx.xxx"
ONBOOT="yes"
TYPE="Ethernet"
DNS1="210.188.xxx.xxx"
DNS2="210.188.xxx.xxx"
DNS3="2001:xxx::1"
IPV6INIT="yes"
IPV6_ROUTER="no"
IPV6ADDR="2001:e42:102:1810:160:16:xxx:xxx/64"

 

IPv6を無効にする手順【CentOS7】

現在の設定を確認します。

「/etc/sysctl.conf」ファイルを見ます。

# cat /etc/sysctl.conf

# System default settings live in /usr/lib/sysctl.d/00-system.conf.
# To override those settings, enter new settings here, or in an /etc/sysctl.d/<name>.conf file
#
# For more information, see sysctl.conf(5) and sysctl.d(5).

# Do not accept RA
net.ipv6.conf.default.accept_ra=0
net.ipv6.conf.all.accept_ra=0
net.ipv6.conf.eth0.accept_ra=0

気になるのが「Do not accept RA」です。

これはなんでしょうか。

RAとは「Router Advertisement(ルーター広告)」の略です。

広告と言っても、「コマーシャルなどの宣伝広告」とは異なり「知らせること、告知すること」の意味です。

「Router Advertisement」とは、IPv6アドレスの自動設定を行う機能です。

通常、アドレスの自動設定というと「DHCP」を想像しますが、IPv6の場合は「RA」「RS(Router Solicitation、ルータ要請)をやり取りしてIPv6アドレスを生成します。

 

「/etc/sysctl.conf」ファイルにIPv6を無効にする設定を入れます。

# vi /etc/sysctl.conf

# System default settings live in /usr/lib/sysctl.d/00-system.conf.
# To override those settings, enter new settings here, or in an /etc/sysctl.d/<name>.conf file
#
# For more information, see sysctl.conf(5) and sysctl.d(5).

# Do not accept RA
net.ipv6.conf.default.accept_ra=0
net.ipv6.conf.all.accept_ra=0
net.ipv6.conf.eth0.accept_ra=0

net.ipv6.conf.all.disable_ipv6 = 1 ← 追加する。

net.ipv6.conf.default.disable_ipv6 = 1 ← 追加する。

 

設定を反映します。

sysctlコマンドを実行すると読み込まれて有効になった設定の行が表示されます。

# sysctl -p

net.ipv6.conf.default.accept_ra = 0
net.ipv6.conf.all.accept_ra = 0
net.ipv6.conf.eth0.accept_ra = 0
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
#

 

設定が反映されているか確認します。

inet6のアドレスが消えています。

# ifconfig

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 160.16.xxx.xxx  netmask 255.255.254.0  broadcast 160.16.xxx.xxx
        ether 9c:a3:ba:02:0e:cd  txqueuelen 1000  (Ethernet) ← inet6の設定が消えています。
        RX packets 68658631  bytes 4483811139 (4.1 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 695147  bytes 80506482 (76.7 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

 

man tcpdump(随時日本語化)

 

TCPDUMP(8)                                                                                              System Manager's Manual                                                                                              TCPDUMP(8)

NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       tcpdump [ -AbdDefhHIJKlLnNOpqRStuUvxX ] [ -B buffer_size ] [ -c count ]
               [ -C file_size ] [ -G rotate_seconds ] [ -F file ]
               [ -i interface ] [ -j tstamp_type ] [ -m module ] [ -M secret ]
               [ -P in|out|inout ]
               [ -r file ] [ -V file ] [ -s snaplen ] [ -T type ] [ -w file ]
               [ -W filecount ]
               [ -E spi@ipaddr algo:secret,...  ]
               [ -y datalinktype ] [ -z postrotate-command ] [ -Z user ]
               [ expression ]

DESCRIPTION
       Tcpdump  prints  out a description of the contents of packets on a network interface that match the boolean expression.  

       Tcpdumpは、ブール式と一致するネットワークインターフェイス上のパケットの内容の説明を出力します。
       It can also be run with the -w flag, which causes it to save the packet data to a file for later analysis, and/or with the -r flag, which causes it to read from a saved packet file rather than to read packets from a network interface.  

       後で解析するためにパケット・データをファイルに保存する-wフラグ、またはパケットを読み取るのではなく保存されたパケット・ファイルから読み取る-rフラグを使用して実行することもできます ネットワークインターフェイスから。
       It can also be run with the -V flag, which causes it to read a list of saved packet files.

       -Vフラグを付けて実行すると、保存されたパケットファイルのリストを読み取ることができます。
       In all cases, only packets that match expression will be processed by tcpdump.

       Tcpdump  will,  if  not run with the -c flag, continue capturing packets until it is interrupted by a SIGINT signal (generated, for example, by typing your interrupt character, typically control-C) or a SIGTERM signal (typically generated with the kill(1) command); if run with the -c flag, it will capture packets until it is interrupted by a SIGINT or SIGTERM signal or the specified number of packets have been processed.

       Tcpdumpは、-cフラグを付けて実行しないと、SIGINTシグナル(割り込み文字、通常はcontrol-Cをタイプすることによって生成される)またはSIGTERMシグナル(通常はkill (1)コマンド)。 -cフラグを付けて実行すると、SIGINTまたはSIGTERMシグナルによって中断されるか、または指定された数のパケットが処理されるまでパケットをキャプチャします。

       When tcpdump finishes capturing packets, it will report counts of:

       tcpdumpがパケットのキャプチャを終了すると、次の数が報告されます。

              packets ``captured'' (this is the number of packets that tcpdump has received and processed);

              ``捕獲された ''パケット(これはtcpdumpが受信して処理したパケットの数です)。

              packets "received by filter'' (the meaning of this depends on the OS on which you're running tcpdump, and possibly on the way the OS was configured - if a filter was specified on the command line,  on  some  OSes  it counts  packets regardless of whether they were matched by the filter expression and, even if they were matched by the filter expression, regardless of whether tcpdump has read and processed them yet, on other OSes it counts only packets that were matched by the filter expression regardless of whether tcpdump has read and processed them yet, and on other OSes it counts only packets that were matched by  the  filter  expression  and were processed by tcpdump);

              パケットが「フィルタで受信されました」(この意味は、tcpdumpを実行しているOSによって異なりますが、OSが設定されている方法によっても異なります)。コマンドラインでフィルタが指定された場合、 tcpdumpがまだそれらを読み込んで処理しているかどうかにかかわらず、フィルタ式によって一致したかどうかにかかわらず、フィルタ式によって一致したかどうかにかかわらず、フィルタ式によって一致したパケットだけを数えます tcpdumpはそれをまだ読み込んで処理していますが、他のOSではフィルタ式と一致したパケットだけがカウントされ、tcpdumpで処理されます。

              packets  "dropped by kernel'' (this is the number of packets that were dropped, due to a lack of buffer space, by the packet capture mechanism in the OS on which tcpdump is running, if the OS reports that information to applications; if not, it will be reported as 0).

              パケットが "カーネルによってドロップされた"(これは、バッファスペースが不足しているため、tcpdumpが実行されているOSのパケットキャプチャメカニズムによってドロップされたパケットの数です。 そうでない場合、0と報告されます)。

       On platforms that support the SIGINFO signal, such as most BSDs (including Mac OS X) and Digital/Tru64 UNIX, it will report those counts when it receives a SIGINFO signal (generated, for example, by  typing  your  "status'' character, typically control-T, although on some platforms, such as Mac OS X, the "status'' character is not set by default, so you must set it with stty(1) in order to use it) and will continue capturing packets.

      ほとんどのBSD(Mac OS Xを含む)やDigital / Tru64 UNIXなどのSIGINFOシグナルをサポートするプラットフォームでは、SIGINFOシグナルを受け取ったときにそれらのカウントを報告します(例えば、 "status" 文字的にはcontrol-Tですが、Mac OS Xのようないくつかのプラットフォームでは、 "status"文字はデフォルトでは設定されていないので、それを使用するにはstty(1)で設定しなければなりません)。

       Reading packets from a network interface may require that you have special privileges; see the pcap (3PCAP) man page for details.  Reading a saved packet file doesn't require special privileges.

       ネットワークインタフェースからパケットを読み取るには、特別な権限が必要です。 詳細については、pcap(3PCAP)のマニュアルページを参照してください。 保存されたパケットファイルを読み取るのに特別な特権は必要ありません。

OPTIONS
       -A     Print each packet (minus its link level header) in ASCII.  Handy for capturing web pages.

               各パケット(そのリンクレベルヘッダーを引いたもの)をASCIIで印刷します。 Webページをキャプチャするのに便利です。

       -b     Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN notation.

               BGPパケットのAS番号を、ASPLAIN表記ではなくASDOT表記で表示します。

       -B     Set the operating system capture buffer size to buffer_size, in units of KiB (1024 bytes).

               オペレーティングシステムのキャプチャバッファサイズをKiB(1024バイト)の単位でbuffer_sizeに設定します。

       -c     Exit after receiving count packets.

               指定した数のパケットを受信した後に終了します。

       -C     Before  writing  a  raw packet to a savefile, check whether the file is currently larger than file_size and, if so, close the current savefile and open a new one.  

               生のパケットを保存ファイルに書き込む前に、ファイルが現在ファイルサイズよりも大きいかどうかをチェックし、もしあれば、現在の保存ファイルを閉じて新しいファイルを開きます。
              Savefiles after the first savefile will have the name specified with the -w flag, with a number after it, starting at 1 and continuing upward.  

              ファイルの保存最初の保存ファイルの後に-wフラグで指定された名前が付いています。その後に1から始まり上向きの数字が続きます。
              The units of file_size are millions of bytes (1,000,000 bytes, not 1,048,576 bytes).

              file_sizeの単位は、ミリオンバイト(1,000,000バイト、1,048,576バイトではありません)です。

              Note that when used with -Z option (enabled by default), privileges are dropped before opening first savefile.

              -Zオプション(デフォルトで有効)と一緒に使用すると、最初のsavefileを開く前に特権が削除されることに注意してください。

       -d     Dump the compiled packet-matching code in a human readable form to standard output and stop.

               コンパイルされたパケットマッチングコードを人間が読める形式で標準出力にダンプして停止します。

       -dd    Dump packet-matching code as a C program fragment.

                パケットマッチングコードをCプログラムの断片としてダンプします。

       -ddd   Dump packet-matching code as decimal numbers (preceded with a count).

                 パケットマッチングコードを10進数(ダンプカウントの前に)としてダンプします。

       -D     Print the list of the network interfaces available on the system and on which tcpdump can capture packets.

               tcpdumpがパケットをキャプチャできる、システム上で利用可能なネットワークインタフェースのリストを出力します。
【例】

# tcpdump -D
1.eth0
2.nflog (Linux netfilter log (NFLOG) interface)
3.nfqueue (Linux netfilter queue (NFQUEUE) interface)
4.eth1
5.usbmon1 (USB bus number 1)
6.eth2
7.any (Pseudo-device that captures on all interfaces)
8.lo
              For each network interface, a number and an interface name, possibly followed by a text  description  of  the interface, is printed.  

              各ネットワークインターフェースには、番号とインターフェース名、場合によってはインターフェースのテキスト記述が続きます。
              The interface name or the number can be supplied to the -i flag to specify an interface on which to capture.

              インターフェイス名または番号を-iフラグに指定すると、キャプチャするインターフェイスを指定できます。

              This  can  be useful on systems that don't have a command to list them (e.g., Windows systems, or UNIX systems lacking ifconfig -a); the number can be useful on Windows 2000 and later systems, where the interface name is a somewhat complex string.

              これは、リストするコマンドがないシステム(たとえば、Windowsシステム、またはifconfig -aがないUNIXシステム)で役に立ちます。 この番号は、インターフェイス名がやや複雑な文字列であるWindows 2000以降のシステムで役立ちます。

              The -D flag will not be supported if tcpdump was built with an older version of libpcap that lacks the pcap_findalldevs() function.

              tcpdumpがpcap_findalldevs()関数を持たない古いバージョンのlibpcapで構築されている場合、-Dフラグはサポートされません。

       -e     Print the link-level header on each dump line.  This can be used, for example, to print MAC layer addresses for protocols such as Ethernet and IEEE 802.11.

               各ダンプ行にリンクレベルのヘッダーを出力します。 これは、たとえば、イーサネットやIEEE 802.11などのプロトコルのMACレイヤアドレスを印刷するために使用できます。

       -E     Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that are addressed to addr and contain Security Parameter Index value spi. This combination may be repeated with comma or newline separation.

               addrにアドレス指定され、セキュリティパラメータインデックス値spiを含むIPsec ESPパケットを解読するには、spi @ ipaddr algo:secretを使用します。 この組み合わせは、カンマまたは改行で繰り返すことができます。

              Note that setting the secret for IPv4 ESP packets is supported at this time.

              この時点では、IPv4 ESPパケットの秘密の設定がサポートされています。

              Algorithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc, or none.  The default is des-cbc.  The ability to decrypt packets is only present if tcpdump was compiled with cryptography enabled.

              アルゴリズムは、des-cbc、3des-cbc、blowfish-cbc、rc3-cbc、cast128-cbc、またはnoneのいずれかです。 デフォルトはdes-cbcです。 パケットを解読する能力は、暗号化を有効にしてtcpdumpをコンパイルした場合にのみ存在します。

              secret is the ASCII text for ESP secret key.  If preceded by 0x, then a hex value will be read.

              secretは、ESP秘密鍵のASCIIテキストです。 0xが先行する場合、16進値が読み取られます。

              The option assumes RFC2406 ESP, not RFC1827 ESP.  The option is only for debugging purposes, and the use of this option with a true `secret' key is discouraged.  

              このオプションはRFC2406 ESPであり、RFC1827 ESPではありません。 このオプションはデバッグ目的にのみ使用され、このオプションを真の `secret 'キーで使用することはお勧めしません。
              By presenting IPsec secret key onto  command  line  you make it visible to others, via ps(1) and other occasions.

              コマンドラインにIPsec秘密鍵を提示することで、ps(1)や他の機会を通じて他の人に見えるようにすることができます。

              In  addition  to  the  above syntax, the syntax file name may be used to have tcpdump read the provided file in.

              上記の構文に加えて、tcpdumpが提供されたファイルを読み込むために構文ファイル名を使用することができます。
              The file is opened upon receiving the first ESP packet, so any special permissions that tcpdump may have been given should already have been given up.

              このファイルは、最初のESPパケットを受信したときに開かれるので、tcpdumpが与えられている可能性がある特別な権限は、すでに放棄されているはずです。

       -f     Print `foreign' IPv4 addresses numerically rather than symbolically (this option is intended to get around serious brain damage in Sun's NIS server ― usually it hangs forever translating non-local internet numbers).

              The test for `foreign' IPv4 addresses is done using the IPv4 address and netmask of the interface on which capture is being done.  

              シンボリックではなく数値的に `foreign 'IPv4アドレスを表示します(このオプションは、SunのNISサーバーで重大な脳障害を回避するためのものです。通常、非ローカルインターネット番号の翻訳は永久に停止します)。
              If that address or netmask are not available, available, either because the  interface on which capture is being done has no address or netmask or because the capture is being done on the Linux "any" interface, which can capture on more than one interface, this option will not work correctly.

              キャプチャが実行されているインタフェースにアドレスまたはネットマスクがないか、またはキャプチャが複数のインタフェースでキャプチャできるLinuxの「any」インタフェースで実行されているために、そのアドレスまたはネットマスクが使用可能でない場合は、 このオプションは正しく動作しません。

       -F     Use file as input for the filter expression.  An additional expression given on the command line is ignored.

               フィルター式の入力としてfileを使用します。 コマンドラインで与えられた追加の式は無視されます。

       -G     If  specified,  rotates  the dump file specified with the -w option every rotate_seconds seconds.  

               指定すると、-wオプションで指定されたダンプ・ファイルをrotate_seconds秒ごとにローテートします。
              Savefiles will have the name specified by -w which should include a time format as defined by strftime(3).  

              セーブファイルは、-wで指定された名前を持ちます。このファイルには、strftime(3)で定義されている時刻書式を含める必要があります。
              If no time format is specified, each new file will overwrite the previous.

              時刻形式が指定されていない場合、新しいファイルはそれぞれ前のファイルを上書きします。

              If used in conjunction with the -C option, filenames will take the form of `file<count>'.

              -Cオプションと組み合わせて使用すると、ファイル名は `file <count> 'の形式になります。

       -h     Print the tcpdump and libpcap version strings, print a usage message, and exit.

            tcpdumpとlibpcapのバージョン文字列を表示し、使用法のメッセージを出力して終了します。

       -H     Attempt to detect 802.11s draft mesh headers.

       802.11sドラフトメッシュヘッダーを検出しようと試みます。

       -i     Listen on interface.  If unspecified, tcpdump searches the system interface list for the lowest numbered, configured up interface (excluding loopback), which may turn out to be, for example, ``eth0''.

       インターフェイスをチェックします。指定されていない場合、tcpdumpはシステムインタフェースリスト内で最も番号の小さい、設定されたインタフェース(ループバックを除く)を検索します。例えば `` eth0 ''となります。

              On Linux systems with 2.2 or later kernels, an interface argument of ``any'' can be used to capture packets from all interfaces.  Note that captures on the ``any'' device will not be done in promiscuous mode.

       2.2以降のカーネルを持つLinuxシステムでは、 `` any ''というインタフェース引数を使用して、すべてのインタフェースからパケットを取得できます。 `` any ''デバイス上のキャプチャは、プロミスキャスモードでは行われないことに注意してください。

              If the -D flag is supported, an interface number as printed by that flag can be used as the interface argument.

       -Dフラグがサポートされている場合は、そのフラグが出力するインターフェイス番号をインターフェイスの引数として使用できます。

       -I     Put the interface in "monitor mode"; this is supported only on IEEE 802.11 Wi-Fi interfaces, and supported only on some operating systems.

       インタフェースを「モニタモード」にします。これはIEEE 802.11 Wi-Fiインターフェイスでのみサポートされており、一部のオペレーティングシステムでのみサポートされています。

              Note that in monitor mode the adapter might disassociate from the network with which it's associated, so that you will not be able to use any wireless networks with that adapter.  This could prevent accessing files on a network server, or resolving host names or network addresses, if you are capturing in monitor mode and are not connected to another network with another adapter.

       モニターモードでは、アダプターは、関連付けられているネットワークとの関連付けを解除することがあります。そのため、そのアダプターではワイヤレスネットワークを使用できなくなります。 これにより、モニター・モードでキャプチャーしていて、別のアダプターで別のネットワークに接続していない場合に、ネットワーク・サーバー上のファイルへのアクセスやホスト名またはネットワーク・アドレスの解決を妨げる可能性があります。

              This  flag will affect the output of the -L flag.  If -I isn't specified, only those link-layer types available when not in monitor mode will be shown; if -I is specified, only those link-layer types available when in monitor mode will be shown.

       このフラグは、-Lフラグの出力に影響します。 -Iが指定されていない場合、モニタモードではないときに利用可能なリンク層タイプのみが表示されます。 -Iを指定すると、モニタモード時に使用可能なリンクレイヤタイプのみが表示されます。

       -j     Set the time stamp type for the capture to tstamp_type.  The names to use for the time stamp types are given in pcap-tstamp-type(7); not all the types listed there will necessarily be valid for any given interface.

       キャプチャのタイムスタンプのタイプをtstamp_typeに設定します。 タイムスタンプタイプに使用する名前はpcap-tstamp-type(7)で与えられます。 そこにリストされているすべてのタイプが、特定のインタフェースに対して必ずしも有効であるとは限りません。

       -J     List the supported time stamp types for the interface and exit.  If the time stamp type cannot be set for the interface, no time stamp types are listed.

       インタフェースにサポートされているタイムスタンプのタイプをリストして終了します。 タイムスタンプタイプをインターフェースに設定できない場合は、タイムスタンプタイプはリストされません。

       --time-stamp-precision=tstamp_precision
              When capturing, set the time stamp precision for the capture to tstamp_precision.  Note that availability of high precision time stamps (nanoseconds) and their actual accuracy is platform and hardware dependent.  

       キャプチャを行うときは、キャプチャのタイムスタンプ精度をtstamp_precisionに設定します。 高精度タイムスタンプ(ナノ秒)の利用可能性とその実際の精度は、プラットフォームおよびハードウェアに依存することに注意してください。
              Also note  that  when  writing  captures made with nanosecond accuracy to a savefile, the time stamps are written with nanosecond resolution, and the file is written with a different magic number, to indicate that the time stamps are in seconds and nanoseconds; not all programs that read pcap savefiles will be able to read those captures.

       また、ナノ秒の精度でキャプチャを保存ファイルに書き込む場合、タイムスタンプはナノ秒の分解能で書き込まれ、ファイルには別のマジックナンバーが書き込まれ、タイムスタンプが秒とナノ秒であることを示します。 pcap savefilesを読み取るすべてのプログラムがこれらのキャプチャを読み取ることはできません。

              When reading a savefile, convert time stamps to the precision specified by timestamp_precision, and display them with that resolution.  
              If the precision specified is less than the precision of time stamps in the file, the conversion will lose precision.

       保存ファイルを読み込むときは、タイムスタンプをtimestamp_precisionで指定された精度に変換し、その解像度で表示します。
              指定された精度がファイル内のタイムスタンプの精度よりも小さい場合、変換の精度は低下します。

              The supported values for timestamp_precision are micro for microsecond resolution and nano for nanosecond resolution.  The default is microsecond resolution.

       timestamp_precisionでサポートされている値は、マイクロ秒の解像度ではマイクロ秒、ナノ秒の解像度ではナノ秒です。 デフォルトはマイクロ秒の解像度です。

       -K     Don't attempt to verify IP, TCP, or UDP checksums.  This is useful for interfaces that perform some or all of those checksum calculation in hardware; otherwise, all outgoing TCP checksums will be flagged as bad.

       IP、TCP、またはUDPのチェックサムを検証しないでください。 これは、ハードウェアでこれらのチェックサム計算の一部またはすべてを実行するインターフェイスで役に立ちます。 さもなければ、すべての発信TCPチェックサムは不良としてフラグが立てられます。

       -l     Make stdout line buffered.  Useful if you want to see the data while capturing it.  E.g.,
       stdout行をバッファします。 キャプチャ中にデータを表示したい場合に便利です。

       例えば
                     tcpdump -l | tee dat

              or

                     tcpdump -l > dat & tail -f dat

              Note that on Windows,``line buffered'' means ``unbuffered'', so that WinDump will write each character individually if -l is specified.

       Windowsでは、 ``バッファリングされた ''は ``バッファなし ''を意味するので、-lが指定されている場合、WinDumpは個々の文字を個別に書き込みます。

              -U  is  similar to -l in its behavior, but it will cause output to be ``packet-buffered'', so that the output is written to stdout at the end of each packet rather than at the end of each line; this is buffered on all platforms, including Windows.

       -Uはその動作において-lと似ていますが、出力が ``パケットバッファリング ''になるので、出力は各行の終わりではなく各パケットの終わりでstdoutに書き込まれます。 これはWindowsを含むすべてのプラットフォームでバッファリングされます。

       -L     List the known data link types for the interface, in the specified mode, and exit.  
              The list of known data link types may be dependent on the specified mode; for example, on some platforms,  a  Wi-Fi  interface  might
              support one set of data link types when not in monitor mode (for example, it might support only fake Ethernet headers, or might support 802.11 headers but not support 802.11 headers with radio information) and another set of data link types when in monitor mode (for example, it might support 802.11 headers, or 802.11 headers with radio information, only in monitor mode).

       -m     Load SMI MIB module definitions from file module.  This option can be used several times to load several MIB modules into tcpdump.

       -M     Use secret as a shared secret for validating the digests found in TCP segments with the TCP-MD5 option (RFC 2385), if present.

       -n     Don't convert host addresses to names.  This can be used to avoid DNS lookups.

       -nn    Don't convert protocol and port numbers etc. to names either.

       -N     Don't print domain name qualification of host names.  E.g., if you give this flag then tcpdump will print ``nic'' instead of ``nic.ddn.mil''.

       -O     Do not run the packet-matching code optimizer.  This is useful only if you suspect a bug in the optimizer.

       -p     Don't put the interface into promiscuous mode.  Note that the interface might be in promiscuous mode for some other reason; hence, `-p' cannot be used as an abbreviation for `ether host {local-hw-addr} or ether broadcast'.

       -P     Choose send/receive direction direction for which packets should be captured. Possible values are `in', `out' and `inout'. Not available on all platforms.

       -q     Quick (quiet?) output.  Print less protocol information so output lines are shorter.

       -R     Assume  ESP/AH  packets  to be based on old specification (RFC1825 to RFC1829).  If specified, tcpdump will not print replay prevention field.  Since there is no protocol version field in ESP/AH specification, tcpdump cannot deduce the version of ESP/AH protocol.

       -r     Read packets from file (which was created with the -w option).  Standard input is used if file is ``-''.

       -S     Print absolute, rather than relative, TCP sequence numbers.

       -s     Snarf snaplen bytes of data from each packet rather than the default of 65535 bytes.  
              Packets truncated because of a limited snapshot are indicated in the output with ``[|proto]'', where proto is the name of the  protocol  level at which the truncation has occurred.  
              Note that taking larger snapshots both increases the amount of time it takes to process packets and, effectively, decreases the amount of packet buffering.  
              This may cause packets to be lost.  
              You should limit snaplen to the smallest number that will capture the protocol information you're interested in.  
              Setting snaplen to 0 sets it to the default of 65535, for backwards compatibility with recent older versions of tcpdump.

       -T     Force  packets selected by "expression" to be interpreted the specified type.  
              Currently known types are aodv (Ad-hoc On-demand Distance Vector protocol), carp (Common Address Redundancy Protocol), cnfp (Cisco NetFlow protocol), lmp (Link Management Protocol), pgm (Pragmatic General Multicast), pgm_zmtp1 (ZMTP/1.0 inside PGM/EPGM), radius (RADIUS), rpc (Remote Procedure Call), rtp (Real-Time Applications protocol), rtcp  (Real-Time Applications  control  protocol),  snmp (Simple Network Management Protocol), tftp (Trivial File Transfer Protocol), vat (Visual Audio Tool), wb (distributed White Board), zmtp1 (ZeroMQ Message Transport Protocol 1.0) and vxlan (Virtual eXtensible Local Area Network).

              Note that the pgm type above affects UDP interpretation only, the native PGM is always recognised as IP protocol 113 regardless. UDP-encapsulated PGM is often called "EPGM" or "PGM/UDP".

              Note that the pgm_zmtp1 type above affects interpretation of both native PGM and UDP at once.
              During the native PGM decoding the application data of an ODATA/RDATA packet would be decoded as a ZeroMQ  datagram  with ZMTP/1.0 frames.  
              During the UDP decoding in addition to that any UDP packet would be treated as an encapsulated PGM packet.

       -t     Don't print a timestamp on each dump line.

       -tt    Print an unformatted timestamp on each dump line.

       -ttt   Print a delta (micro-second resolution) between current and previous line on each dump line.

       -tttt  Print a timestamp in default format proceeded by date on each dump line.

       -ttttt Print a delta (micro-second resolution) between current and first line on each dump line.

       -u     Print undecoded NFS handles.

       -U     If  the  -w option is not specified, make the printed packet output ``packet-buffered''; i.e., as the description of the contents of each packet is printed, it will be written to the standard output, rather than, when not writing to a terminal, being written only when the output buffer fills.

              If the -w option is specified, make the saved raw packet output ``packet-buffered''; i.e., as each packet is saved, it will be written to the output file, rather than being written only when the output buffer fills.

              The -U flag will not be supported if tcpdump was built with an older version of libpcap that lacks the pcap_dump_flush() function.

       -v     When parsing and printing, produce (slightly more) verbose output.  For example, the time to live, identification, total length and options in an IP packet are printed.  Also enables additional packet integrity checks such as verifying the IP and ICMP header checksum.

              When writing to a file with the -w option, report, every 10 seconds, the number of packets captured.

       -vv    Even more verbose output.  For example, additional fields are printed from NFS reply packets, and SMB packets are fully decoded.

       -vvv   Even more verbose output.  For example, telnet SB ... SE options are printed in full.  With -X Telnet options are printed in hex as well.

       -V     Read a list of filenames from file. Standard input is used if file is ``-''.

       -w     Write the raw packets to file rather than parsing and printing them out.  They can later be printed with the -r option.  Standard output is used if file is ``-''.

              This  output  will be buffered if written to a file or pipe, so a program reading from the file or pipe may not see packets for an arbitrary amount of time after they are received.  
              Use the -U flag to cause packets to be written as soon as they are received.

              The MIME type application/vnd.tcpdump.pcap has been registered with IANA for pcap files.
              The filename extension .pcap appears to be the most commonly used along with .cap and .dmp.
              Tcpdump itself doesn't check the extension when reading capture files and doesn't add an extension when writing them (it uses magic numbers in the file header instead).
              However, many operating systems and applications will use the extension if it is present and adding one (e.g. .pcap) is recommended.

              See pcap-savefile(5) for a description of the file format.

       -W     Used in conjunction with the -C option, this will limit the number of files created to the specified number, and begin overwriting files from the beginning, thus creating a 'rotating' buffer.   
              In  addition,  it  will name the files with enough leading 0s to support the maximum number of files, allowing them to sort correctly.

              Used  in  conjunction with the -G option, this will limit the number of rotated dump files that get created, exiting with status 0 when reaching the limit.
              If used with -C as well, the behavior will result in cyclical files per timeslice.

       -x     When parsing and printing, in addition to printing the headers of each packet, print the data of each packet (minus its link level header) in hex.  
              The smaller of the entire packet or snaplen bytes  will  be  printed.
              Note that this is the entire link-layer packet, so for link layers that pad (e.g. Ethernet), the padding bytes will also be printed when the higher layer packet is shorter than the required padding.

       -xx    When parsing and printing, in addition to printing the headers of each packet, print the data of each packet, including its link level header, in hex.

       -X     When parsing and printing, in addition to printing the headers of each packet, print the data of each packet (minus its link level header) in hex and ASCII.  This is very handy for analysing new protocols.

       -XX    When parsing and printing, in addition to printing the headers of each packet, print the data of each packet, including its link level header, in hex and ASCII.

       -y     Set the data link type to use while capturing packets to datalinktype.

       -z     Used  in  conjunction  with the -C or -G options, this will make tcpdump run " command file " where file is the savefile being closed after each rotation.
              For example, specifying -z gzip or -z bzip2 will compress each savefile using gzip or bzip2.

              Note that tcpdump will run the command in parallel to the capture, using the lowest priority so that this doesn't disturb the capture process.

              And in case you would like to use a command that itself takes flags or different arguments, you can always write a shell script that will take the savefile name as  the  only  argument,  make  the  flags  &  arguments arrangements and execute the command that you want.

       -Z     If tcpdump is running as root, after opening the capture device or input savefile, but before opening any savefiles for output, change the user ID to user and the group ID to the primary group of user.

              This behavior is enabled by default (-Z tcpdump), and can be disabled by -Z root.

        expression
              selects which packets will be dumped.  If no expression is given, all packets on the net will be dumped.  Otherwise, only packets for which expression is `true' will be dumped.

              For the expression syntax, see pcap-filter(7).

              The  expression  argument  can be passed to tcpdump as either a single Shell argument, or as multiple Shell arguments, whichever is more convenient.  
              Generally, if the expression contains Shell metacharacters, such as backslashes used to escape protocol names, it is easier to pass it as a single, quoted argument rather than to escape the Shell metacharacters.  
              Multiple arguments are concatenated with spaces before being parsed.

EXAMPLES
       To print all packets arriving at or departing from sundown:
              tcpdump host sundown

       To print traffic between helios and either hot or ace:
              tcpdump host helios and \( hot or ace \)

       To print all IP packets between ace and any host except helios:
              tcpdump ip host ace and not helios

       To print all traffic between local hosts and hosts at Berkeley:
              tcpdump net ucb-ether

       To print all ftp traffic through internet gateway snup: (note that the expression is quoted to prevent the shell from (mis-)interpreting the parentheses):
              tcpdump 'gateway snup and (port ftp or ftp-data)'

       To print traffic neither sourced from nor destined for local hosts (if you gateway to one other net, this stuff should never make it onto your local net).
              tcpdump ip and not net localnet

       To print the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a non-local host.
              tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'

       To print all IPv4 HTTP packets to and from port 80, i.e. print only packets that contain data, not, for example, SYN and FIN packets and ACK-only packets.  (IPv6 is left as an exercise for the reader.)
              tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'

       To print IP packets longer than 576 bytes sent through gateway snup:
              tcpdump 'gateway snup and ip[2:2] > 576'

       To print IP broadcast or multicast packets that were not sent via Ethernet broadcast or multicast:
              tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'

       To print all ICMP packets that are not echo requests/replies (i.e., not ping packets):
              tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'

OUTPUT FORMAT
       The output of tcpdump is protocol dependent.  The following gives a brief description and examples of most of the formats.

       Link Level Headers

       If the '-e' option is given, the link level header is printed out.  On Ethernets, the source and destination addresses, protocol, and packet length are printed.

       On FDDI networks, the  '-e' option causes tcpdump to print the `frame control' field,  the source and destination addresses, and the packet length.  
       (The `frame control' field governs the interpretation of the  rest  of  the packet.   Normal  packets  (such  as  those containing IP datagrams) are `async' packets, with a priority value between 0 and 7; for example, `async4'.  
       Such packets are assumed to contain an 802.2 Logical Link Control (LLC) packet; the LLC header is printed if it is not an ISO datagram or a so-called SNAP packet.

       On Token Ring networks, the '-e' option causes tcpdump to print the `access control' and `frame control' fields, the source and destination addresses, and the packet length.  As on FDDI networks, packets are assumed to  contain an LLC packet.
       Regardless of whether the '-e' option is specified or not, the source routing information is printed for source-routed packets.

       On 802.11 networks, the '-e' option causes tcpdump to print the `frame control' fields, all of the addresses in the 802.11 header, and the packet length.  As on FDDI networks, packets are assumed to contain an LLC packet.

       (N.B.: The following description assumes familiarity with the SLIP compression algorithm described in RFC-1144.)

       On  SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound), packet type, and compression information are printed out.  
       The packet type is printed first.  The three types are ip, utcp, and ctcp.  
       No further link information is printed for ip packets.  For TCP packets, the connection identifier is printed following the type.  
       If the packet is compressed, its encoded header is printed out.  The special cases are  printed  out  as *S+n  and  *SA+n,  where  n is the amount by which the sequence number (or sequence number and ack) has changed.  
       If it is not a special case, zero or more changes are printed.  A change is indicated by U (urgent pointer), W (window), A (ack), S (sequence number), and I (packet ID), followed by a delta (+n or -n), or a new value (=n).  
       Finally, the amount of data in the packet and compressed header length are printed.

       For example, the following line shows an outbound compressed TCP packet, with an implicit connection identifier; the ack has changed by 6, the sequence number by 49, and the packet ID by 6; there are 3 bytes of  data  and  6 bytes of compressed header:
              O ctcp * A+6 S+49 I+6 3 (6)

       ARP/RARP Packets

       Arp/rarp output shows the type of request and its arguments.  
       The format is intended to be self explanatory.  Here is a short sample taken from the start of an `rlogin' from host rtsg to host csam:
              arp who-has csam tell rtsg
              arp reply csam is-at CSAM
       The first line says that rtsg sent an arp packet asking for the Ethernet address of internet host csam.  
       Csam replies with its Ethernet address (in this example, Ethernet addresses are in caps and internet addresses in lower case).

       This would look less redundant if we had done tcpdump -n:
              arp who-has 128.3.254.6 tell 128.3.254.68
              arp reply 128.3.254.6 is-at 02:07:01:00:01:c4

       If we had done tcpdump -e, the fact that the first packet is broadcast and the second is point-to-point would be visible:
              RTSG Broadcast 0806  64: arp who-has csam tell rtsg
              CSAM RTSG 0806  64: arp reply csam is-at CSAM
       For the first packet this says the Ethernet source address is RTSG, the destination is the Ethernet broadcast address, the type field contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.

       TCP Packets

       (N.B.:The following description assumes familiarity with the TCP protocol described in RFC-793.  If you are not familiar with the protocol, neither this description nor tcpdump will be of much use to you.)

       The general format of a tcp protocol line is:
              src > dst: flags data-seqno ack window urgent options
       Src and dst are the source and destination IP addresses and ports.  
       Flags are some combination of S (SYN), F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or `.' (ACK), or `none' if no flags  are  set.   
       Dataseqno  describes  the portion of sequence space covered by the data in this packet (see example below).  
       Ack is sequence number of the next data expected the other direction on this connection.  
       Window is the number of bytes of receive buffer space available the other direction on this connection.  
       Urg indicates there is `urgent' data in the packet.  
       Options are tcp options enclosed in angle brackets (e.g., <mss 1024>).

       Src, dst and flags are always present.  
       The other fields depend on the contents of the packet's tcp protocol header and are output only if appropriate.

       Here is the opening portion of an rlogin from host rtsg to host csam.
              rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
              csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
              rtsg.1023 > csam.login: . ack 1 win 4096
              rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
              csam.login > rtsg.1023: . ack 2 win 4096
              rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
              csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
              csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
              csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1
       The first line says that tcp port 1023 on rtsg sent a packet to port login on csam.  
       The S indicates that the SYN flag was set.   
       The  packet  sequence  number  was  768512  and  it  contained  no  data.  
        (The  notation  is `first:last(nbytes)' which means `sequence numbers first up to but not including last which is nbytes bytes of user data'.)  
        There was no piggy-backed ack, the available receive window was 4096 bytes and there was a max-seg-ment-size option requesting an mss of 1024 bytes.

       Csam replies with a similar packet except it includes a piggy-backed ack for rtsg's SYN.  Rtsg then acks csam's SYN.  
       The `.' means the ACK flag was set.  The packet contained no data so there is  no  data  sequence  number.
       Note  that  the ack sequence number is a small integer (1).  The first time tcpdump sees a tcp `conversation', it prints the sequence number from the packet.  
       On subsequent packets of the conversation, the difference between the current packet's sequence number and this initial sequence number is printed.  
       This means that sequence numbers after the first can be interpreted as relative byte positions in the conversation's data  stream  (with  the first data byte each direction being `1').  
       `-S' will override this feature, causing the original sequence numbers to be output.

       On  the  6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20 in the rtsg → csam side of the conversation).  The PUSH flag is set in the packet.  
       On the 7th line, csam says it's received data sent by rtsg up to but not including byte 21.  
       Most of this data is apparently sitting in the socket buffer since csam's receive window has gotten 19 bytes smaller.  Csam also sends one byte of data to rtsg in this packet.   
       On  the  8th  and  9th lines, csam sends two bytes of urgent, pushed data to rtsg.

       If  the  snapshot was small enough that tcpdump didn't capture the full TCP header, it interprets as much of the header as it can and then reports ``[|tcp]'' to indicate the remainder could not be interpreted.  
       If the header contains a bogus option (one with a length that's either too small or beyond the end of the header), tcpdump reports it as ``[bad opt]'' and does not interpret any further options (since it's impossible to  tell  where  they start).  
       If the header length indicates options are present but the IP datagram length is not long enough for the options to actually be there, tcpdump reports it as ``[bad hdr length]''.

       Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)

       There are 8 bits in the control bits section of the TCP header:

              CWR | ECE | URG | ACK | PSH | RST | SYN | FIN

       Let's  assume that we want to watch packets used in establishing a TCP connection.  
       Recall that TCP uses a 3-way handshake protocol when it initializes a new connection; the connection sequence with regard to the TCP control bits is

              1) Caller sends SYN
              2) Recipient responds with SYN, ACK
              3) Caller sends ACK

       Now we're interested in capturing packets that have only the SYN bit set (Step 1).  
       Note that we don't want packets from step 2 (SYN-ACK), just a plain initial SYN.  
       What we need is a correct filter expression for tcpdump.

       Recall the structure of a TCP header without options:

        0                            15                              31
       -----------------------------------------------------------------
       |          source port          |       destination port        |
       -----------------------------------------------------------------
       |                        sequence number                        |
       -----------------------------------------------------------------
       |                     acknowledgment number                     |
       -----------------------------------------------------------------
       |  HL   | rsvd  |C|E|U|A|P|R|S|F|        window size            |
       -----------------------------------------------------------------
       |         TCP checksum          |       urgent pointer          |
       -----------------------------------------------------------------

       A TCP header usually holds 20 octets of data, unless options are present.  
       The first line of the graph contains octets 0 - 3, the second line shows octets 4 - 7 etc.

       Starting to count with 0, the relevant TCP control bits are contained in octet 13:

        0             7|             15|             23|             31
       ----------------|---------------|---------------|----------------
       |  HL   | rsvd  |C|E|U|A|P|R|S|F|        window size            |
       ----------------|---------------|---------------|----------------
       |               |  13th octet   |               |               |

       Let's have a closer look at octet no. 13:

                       |               |
                       |---------------|
                       |C|E|U|A|P|R|S|F|
                       |---------------|
                       |7   5   3     0|

       These are the TCP control bits we are interested in.  We have numbered the bits in this octet from 0 to 7, right to left, so the PSH bit is bit number 3, while the URG bit is number 5.

       Recall that we want to capture packets with only SYN set.  Let's see what happens to octet 13 if a TCP datagram arrives with the SYN bit set in its header:

                       |C|E|U|A|P|R|S|F|
                       |---------------|
                       |0 0 0 0 0 0 1 0|
                       |---------------|
                       |7 6 5 4 3 2 1 0|

       Looking at the control bits section we see that only bit number 1 (SYN) is set.

       Assuming that octet number 13 is an 8-bit unsigned integer in network byte order, the binary value of this octet is

              00000010

       and its decimal representation is

          7     6     5     4     3     2     1     0
       0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2  =  2

       We're almost done, because now we know that if only SYN is set, the value of the 13th octet in the TCP header, when interpreted as a 8-bit unsigned integer in network byte order, must be exactly 2.

       This relationship can be expressed as
              tcp[13] == 2

       We can use this expression as the filter for tcpdump in order to watch packets which have only SYN set:
              tcpdump -i xl0 tcp[13] == 2

       The expression says "let the 13th octet of a TCP datagram have the decimal value 2", which is exactly what we want.

       Now, let's assume that we need to capture SYN packets, but we don't care if ACK or any other TCP control bit is set at the same time.  Let's see what happens to octet 13 when a TCP datagram with SYN-ACK set arrives:

            |C|E|U|A|P|R|S|F|
            |---------------|
            |0 0 0 1 0 0 1 0|
            |---------------|
            |7 6 5 4 3 2 1 0|

       Now bits 1 and 4 are set in the 13th octet.  The binary value of octet 13 is

                   00010010

       which translates to decimal

          7     6     5     4     3     2     1     0
       0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2   = 18

       Now we can't just use 'tcp[13] == 18' in the tcpdump filter expression, because that would select only those packets that have SYN-ACK set, but not those with only SYN set.  Remember that we don't care if ACK  or  any  other control bit is set as long as SYN is set.

       In order to achieve our goal, we need to logically AND the binary value of octet 13 with some other value to preserve the SYN bit.  We know that we want SYN to be set in any case, so we'll logically AND the value in the 13th octet with the binary value of a SYN:

                 00010010 SYN-ACK              00000010 SYN
            AND  00000010 (we want SYN)   AND  00000010 (we want SYN)
                 --------                      --------
            =    00000010                 =    00000010

       We see that this AND operation delivers the same result regardless whether ACK or another TCP control bit is set.  
       The decimal representation of the AND value as well as the result of this operation is 2  (binary  00000010), so we know that for packets with SYN set the following relation must hold true:

              ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )

       This points us to the tcpdump filter expression
                   tcpdump -i xl0 'tcp[13] & 2 == 2'

       Some  offsets  and  field values may be expressed as names rather than as numeric values.
       For example tcp[13] may be replaced with tcp[tcpflags].
       The following TCP flag field values are also available: tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-act, tcp-urg.

       This can be demonstrated as:
                   tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'

       Note that you should use single quotes or a backslash in the expression to hide the AND ('&') special character from the shell.

       UDP Packets

       UDP format is illustrated by this rwho packet:
              actinide.who > broadcast.who: udp 84
       This says that port who on host actinide sent a udp datagram to port who on host broadcast, the Internet broadcast address.  
       The packet contained 84 bytes of user data.

       Some UDP services are recognized (from the source or destination port number) and the higher level protocol information printed.  
       In particular, Domain Name service requests (RFC-1034/1035) and Sun RPC  calls  (RFC-1050)  to NFS.

       UDP Name Server Requests

       (N.B.:The following description assumes familiarity with the Domain Service protocol described in RFC-1035.  
       If you are not familiar with the protocol, the following description will appear to be written in greek.)

       Name server requests are formatted as
              src > dst: id op? flags qtype qclass name (len)
              h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
       Host h2opolo asked the domain server on helios for an address record (qtype=A) associated with the name ucbvax.berkeley.edu.  
       The query id was `3'.  The `+' indicates the recursion desired flag was set.  
       The query length was 37 bytes, not including the UDP and IP protocol headers.  
       The query operation was the normal one, Query, so the op field was omitted.  
       If the op had been anything else, it would have been printed between the `3' and the `+'.
       Similarly, the qclass was the normal one, C_IN, and omitted.  
       Any other qclass would have been printed immediately after the `A'.

       A  few  anomalies  are  checked and may result in extra fields enclosed in square brackets:  If a query contains an answer, authority records or additional records section, ancount, nscount, or arcount are printed as `[na]', `[nn]' or  `[nau]' where n is the appropriate count.  
       If any of the response bits are set (AA, RA or rcode) or any of the `must be zero' bits are set in bytes two and three, `[b2&3=x]' is printed, where x is the hex value of header bytes two and three.

       UDP Name Server Responses

       Name server responses are formatted as
              src > dst:  id op rcode flags a/n/au type class data (len)
              helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
              helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
       In  the  first  example,  helios  responds  to  query  id  3  from  h2opolo with 3 answer records, 3 name server records and 7 additional records.  
       The first answer record is type A (address) and its data is internet address 128.32.137.3.  The total size of the response was 273 bytes, excluding UDP and IP headers.  
       The op (Query) and response code (NoError) were omitted, as was the class (C_IN) of the A record.

       In the second example, helios responds to query 2 with a response code of non-existent domain (NXDomain) with no answers, one name server and no authority records.  
       The `*' indicates that the  authoritative  answer  bit  was set.  
       Since there were no answers, no type, class or data were printed.

       Other flag characters that might appear are `-' (recursion available, RA, not set) and `|' (truncated message, TC, set).  If the `question' section doesn't contain exactly one entry, `[nq]' is printed.

       SMB/CIFS decoding

       tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on UDP/137, UDP/138 and TCP/139.  Some primitive decoding of IPX and NetBEUI SMB data is also done.

       By default a fairly minimal decode is done, with a much more detailed decode done if -v is used.  
       Be warned that with -v a single SMB packet may take up a page or more, so only use -v if you really want all the gory details.

       For  information  on  SMB  packet  formats  and  what  all  the  fields  mean  see  www.cifs.org  or  the  pub/samba/specs/  directory  on your favorite samba.org mirror site.  
       The SMB patches were written by Andrew Tridgell (tridge@samba.org).

       NFS Requests and Replies

       Sun NFS (Network File System) requests and replies are printed as:
              src.xid > dst.nfs: len op args
              src.nfs > dst.xid: reply stat len op results
              sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
              wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
              sushi.201b > wrl.nfs:
                   144 lookup fh 9,74/4096.6878 "xcolors"
              wrl.nfs > sushi.201b:
                   reply ok 128 lookup fh 9,74/4134.3150
       In the first line, host sushi sends a transaction with id 6709 to wrl (note that the number following the src host is a transaction id, not the source port).  
       The request was 112 bytes, excluding the UDP and IP headers.  
       The operation  was  a readlink (read symbolic link) on file handle (fh) 21,24/10.731657119.  
       (If one is lucky, as in this case, the file handle can be interpreted as a major,minor device number pair, followed by the inode number and generation number.)  
       Wrl replies `ok' with the contents of the link.

       In the third line, sushi asks wrl to lookup the name `xcolors' in directory file 9,74/4096.6878.  
       Note that the data printed depends on the operation type.  The format is intended to be self explanatory if read  in  conjunction with an NFS protocol spec.

       If the -v (verbose) flag is given, additional information is printed.  For example:
              sushi.1372a > wrl.nfs:
                   148 read fh 21,11/12.195 8192 bytes @ 24576
              wrl.nfs > sushi.1372a:
                   reply ok 1472 read REG 100664 ids 417/0 sz 29388
       (-v  also  prints  the IP header TTL, ID, length, and fragmentation fields, which have been omitted from this example.)  
       In the first line, sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte offset 24576.  
       Wrlreplies `ok'; the packet shown on the second line is the first fragment of the reply, and hence is only 1472 bytes long (the other bytes will follow in subsequent fragments, but these fragments do not have NFS  or  even  UDP headers and so might not be printed, depending on the filter expression used).  
       Because the -v flag is given, some of the file attributes (which are returned in addition to the file data) are printed: the file type (``REG'', for regular file), the file mode (in octal), the uid and gid, and the file size.

       If the -v flag is given more than once, even more details are printed.

       Note that NFS requests are very large and much of the detail won't be printed unless snaplen is increased.  Try using `-s 192' to watch NFS traffic.

       NFS reply packets do not explicitly identify the RPC operation.  Instead, tcpdump keeps track of ``recent'' requests, and matches them to the replies using the transaction ID.  If a reply does not closely follow  the  corresponding request, it might not be parsable.

       AFS Requests and Replies

       Transarc AFS (Andrew File System) requests and replies are printed as:

              src.sport > dst.dport: rx packet-type
              src.sport > dst.dport: rx packet-type service call call-name args
              src.sport > dst.dport: rx packet-type service reply call-name args
              elvis.7001 > pike.afsfs:
                   rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
                   new fid 536876964/1/1 ".newsrc"
              pike.afsfs > elvis.7001: rx data fs reply rename
       In the first line, host elvis sends a RX packet to pike.  
       This was a RX data packet to the fs (fileserver) service, and is the start of an RPC call.  
       The RPC call was a rename, with the old directory file id of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory file id of 536876964/1/1 and a new filename of `.newsrc'.  
       The host pike responds with a RPC reply to the rename call (which was successful, because  it  was  a  data packet and not an abort packet).

       In general, all AFS RPCs are decoded at least by RPC call name.  Most AFS RPCs have at least some of the arguments decoded (generally only the `interesting' arguments, for some definition of interesting).

       The format is intended to be self-describing, but it will probably not be useful to people who are not familiar with the workings of AFS and RX.

       If the -v (verbose) flag is given twice, acknowledgement packets and additional header information is printed, such as the RX call ID, call number, sequence number, serial number, and the RX packet flags.

       If the -v flag is given twice, additional information is printed, such as the RX call ID, serial number, and the RX packet flags.  The MTU negotiation information is also printed from RX ack packets.

       If the -v flag is given three times, the security index and service id are printed.

       Error codes are printed for abort packets, with the exception of Ubik beacon packets (because abort packets are used to signify a yes vote for the Ubik protocol).

       Note that AFS requests are very large and many of the arguments won't be printed unless snaplen is increased.  Try using `-s 256' to watch AFS traffic.

       AFS  reply packets do not explicitly identify the RPC operation.  Instead, tcpdump keeps track of ``recent'' requests, and matches them to the replies using the call number and service ID.  
       If a reply does not closely follow the corresponding request, it might not be parsable.

       KIP AppleTalk (DDP in UDP)

       AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated and dumped as DDP packets (i.e., all the UDP header information is discarded).  
       The file /etc/atalk.names is used to translate AppleTalk  net  and  node numbers to names.  Lines in this file have the form
              number    name

              1.254          ether
              16.1      icsd-net
              1.254.110 ace
       The  first two lines give the names of AppleTalk networks.  
       The third line gives the name of a particular host (a host is distinguished from a net by the 3rd octet in the number - a net number must have two octets and a host number must have three octets.)  
       The number and name should be separated by whitespace (blanks or tabs).  
       The /etc/atalk.names file may contain blank lines or comment lines (lines starting with a `#').

       AppleTalk addresses are printed in the form
              net.host.port

              144.1.209.2 > icsd-net.112.220
              office.2 > icsd-net.112.220
              jssmag.149.235 > icsd-net.2
       (If the /etc/atalk.names doesn't exist or doesn't contain an entry for some AppleTalk host/net number, addresses are printed in numeric form.)  
       In the first example, NBP (DDP port 2) on net 144.1 node 209 is sending to whatever is listening on port 220 of net icsd node 112.  
       The second line is the same except the full name of the source node is known (`office').  
       The third line is a send from port 235 on net jssmag node 149 to broadcast on the icsd-net NBP port (note that the broadcast address (255) is indicated by a net name with no host number - for this reason it's a good idea to keep node names and net names distinct in /etc/atalk.names).

       NBP (name binding protocol) and ATP (AppleTalk transaction protocol) packets have their contents interpreted.  
       Other protocols just dump the protocol name (or number if no name is registered  for  the  protocol)  and  packet size.

       NBP packets are formatted like the following examples:
              icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
              jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
              techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
       The  first  line  is a name lookup request for laserwriters sent by net icsd host 112 and broadcast on net jssmag.  
       The nbp id for the lookup is 190.  The second line shows a reply for this request (note that it has the same id) from host jssmag.209 saying that it has a laserwriter resource named "RM1140" registered on port 250.  
       The third line is another reply to the same request saying host techpit has laserwriter "techpit" registered on  port 186.

       ATP packet formatting is demonstrated by the following example:
              jssmag.209.165 > helios.132: atp-req  12266<0-7> 0xae030001
              helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
              jssmag.209.165 > helios.132: atp-req  12266<3,5> 0xae030001
              helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
              jssmag.209.165 > helios.132: atp-rel  12266<0-7> 0xae030001
              jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
       Jssmag.209 initiates transaction id 12266 with host helios by requesting up to 8 packets (the `<0-7>').  
       The hex number at the end of the line is the value of the `userdata' field in the request.

       Helios  responds with 8 512-byte packets.  
       The `:digit' following the transaction id gives the packet sequence number in the transaction and the number in parens is the amount of data in the packet, excluding the atp header.
       The `*' on packet 7 indicates that the EOM bit was set.

       Jssmag.209 then requests that packets 3 & 5 be retransmitted.  Helios resends them then jssmag.209 releases the transaction.  
       Finally, jssmag.209 initiates the next  request.   The  `*'  on  the  request  indicates  that  XO (`exactly once') was not set.

       IP Fragmentation

       Fragmented Internet datagrams are printed as
              (frag id:size@offset+)
              (frag id:size@offset)
       (The first form indicates there are more fragments.  The second indicates this is the last fragment.)

       Id is the fragment id.  Size is the fragment size (in bytes) excluding the IP header.  Offset is this fragment's offset (in bytes) in the original datagram.

       The  fragment  information  is  output for each fragment.  The first fragment contains the higher level protocol header and the frag info is printed after the protocol info.  
       Fragments after the first contain no higher level protocol header and the frag info is printed after the source and destination addresses.  
       For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa over a CSNET connection that doesn't appear to handle  576  byte datagrams:
              arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
              arizona > rtsg: (frag 595a:204@328)
              rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
       There  are  a  couple  of  things  to  note  here:  First, addresses in the 2nd line don't include port numbers.  
       This is because the TCP protocol information is all in the first fragment and we have no idea what the port or sequence numbers are when we print the later fragments.  
       Second, the tcp sequence information in the first line is printed as if there were 308 bytes of user data when, in fact, there are 512 bytes (308 in the first frag and 204 in the second).  
       If you are looking for holes in the sequence space or trying to match up acks with packets, this can fool you.

       A packet with the IP don't fragment flag is marked with a trailing (DF).

       Timestamps

       By default, all output lines are preceded by a timestamp.  The timestamp is the current clock time in the form
              hh:mm:ss.frac
       and is as accurate as the kernel's clock.  
       The timestamp reflects the time the kernel first saw the packet.  
       No attempt is made to account for the time lag between when the Ethernet interface removed the packet from the wire and when the kernel serviced the `new packet' interrupt.

SEE ALSO
       stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5), pcap-filter(7), pcap-tstamp-type(7)

              http://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap

AUTHORS
       The original authors are:

       Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.

       It is currently being maintained by tcpdump.org.

       The current version is available via http:

              http://www.tcpdump.org/

       The original distribution is available via anonymous ftp:

              ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z

       IPv6/IPsec support is added by WIDE/KAME project.  This program uses Eric Young's SSLeay library, under specific configurations.

BUGS
       Please send problems, bugs, questions, desirable enhancements, patches etc. to:

              tcpdump-workers@lists.tcpdump.org

       NIT doesn't let you watch your own outbound traffic, BPF will.  We recommend that you use the latter.

       On Linux systems with 2.0[.x] kernels:

              packets on the loopback device will be seen twice;

              packet filtering cannot be done in the kernel, so that all packets must be copied from the kernel in order to be filtered in user mode;

              all of a packet, not just the part that's within the snapshot length, will be copied from the kernel (the 2.0[.x] packet capture mechanism, if asked to copy only part of a packet to userland, will not report the  true length of the packet; this would cause most IP packets to get an error from tcpdump);

              capturing on some PPP devices won't work correctly.

       We recommend that you upgrade to a 2.2 or later kernel.

       Some attempt should be made to reassemble IP fragments or, at least to compute the right length for the higher level protocol.

       Name  server  inverse queries are not dumped correctly: the (empty) question section is printed rather than real query in the answer section.  
       Some believe that inverse queries are themselves a bug and prefer to fix the program generating them rather than tcpdump.

       A packet trace that crosses a daylight savings time change will give skewed time stamps (the time change is ignored).

       Filter expressions on fields other than those in Token Ring headers will not correctly handle source-routed Token Ring packets.

       Filter expressions on fields other than those in 802.11 headers will not correctly handle 802.11 data packets with both To DS and From DS set.

       ip6 proto should chase header chain, but at this moment it does not.  ip6 protochain is supplied for this behavior.

       Arithmetic expression against transport layer headers, like tcp[0], does not work against IPv6 packets.  It only looks at IPv4 packets.

                                                                                                              12 July 2012                                                                                                   TCPDUMP(8)

 

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