tcpdump: out.pcap: 权限被拒绝

Question

[root@localhost ~]# cat /etc/issue
Fedora release 17 (Beefy Miracle)
Kernel \r on an \m (\l)
[root@localhost ~]# uname -a
Linux localhost.localdomain 3.6.10-2.fc17.i686 #1 SMP Tue Dec 11 18:33:15 UTC 2012 i686 i686 i386 GNU/Linux
[root@localhost ~]# tcpdump -i p3p1 -n -w out.pcap -C 16
tcpdump: out.pcap: Permission denied

为什么我得到错误？

我该怎么办？

Answer 1

我在 Centos 5 上尝试过，即使在 tmp 或根文件夹上仍然相同。从 tcpdump 手册页中，在打开第一个保存文件之前与 -Z 选项（默认启用）一起使用时，权限会被删除。因为你指定了“-C 1”，权限被拒绝是因为文件大小已经达到1，当创建新文件时会引发权限被拒绝错误。所以只需指定 -Z 用户

# strace tcpdump -i eth0 -n -w out.pcap -C 1
fstat(4, {st_mode=S_IFREG|0644, st_size=903, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2aea31934000
lseek(4, 0, SEEK_CUR)                   = 0
read(4, "root:x:0:root\nbin:x:1:root,bin,d"..., 4096) = 903
read(4, "", 4096)                       = 0
close(4)                                = 0
munmap(0x2aea31934000, 4096)            = 0
setgroups(1, [77])                      = 0
setgid(77)                              = 0
setuid(77)                              = 0
setsockopt(3, SOL_SOCKET, SO_ATTACH_FILTER, "\1\0\0\0\0\0\0\0\310\357k\0\0\0\0\0", 16) = 0
fcntl(3, F_GETFL)                       = 0x2 (flags O_RDWR)
fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK)    = 0

recvfrom(3, 0x7fff9563d35f, 1, 32, 0, 0) = -1 EAGAIN (Resource temporarily unavailable)
fcntl(3, F_SETFL, O_RDWR)               = 0
setsockopt(3, SOL_SOCKET, SO_ATTACH_FILTER, "\1\0\17\0\0\0\0\0P\327\233\7\0\0\0\0", 16) = 0
open("out.pcap", O_WRONLY|O_CREAT|O_TRUNC, 0666) = -1 EACCES (Permission denied)
write(2, "tcpdump: ", 9tcpdump: )                = 9
write(2, "out.pcap: Permission denied", 27out.pcap: Permission denied) = 27
write(2, "\n", 1
)                       = 1
exit_group(1)                           = ?

你可以看到上面的 strace 结果，tcpdump 将权限放入用户和组 pcap (77)。

# grep 77 /etc/group
pcap:x:77:
# grep 77 /etc/passwd
pcap:x:77:77::/var/arpwatch:/sbin/nologin

从 tcpdump 手册页，-C

# man tcpdump
       -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.  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.  The units of file_size are mil-
              lions of bytes (1,000,000 bytes, not 1,048,576 bytes).

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


# tcpdump --help
tcpdump version 3.9.4
libpcap version 0.9.4
Usage: tcpdump [-aAdDeflLnNOpqRStuUvxX] [-c count] [ -C file_size ]
                [ -E algo:secret ] [ -F file ] [ -i interface ] [ -M secret ]
                [ -r file ] [ -s snaplen ] [ -T type ] [ -w file ]
                [ -W filecount ] [ -y datalinktype ] [ -Z user ]
                [ expression ]

使用 -Z user 指定特定用户

# tcpdump -i eth0 -n -w out.pcap -C 1 -Z root
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
35 packets captured
35 packets received by filter
0 packets dropped by kernel     

Answer 2

尝试从/tmp或任何其他世界可写目录运行命令。我记得在不是世界可写的目录中 tcpdump 有问题，我不知道为什么 -:)

         cd /tmp
         tcpdump -i p3p1 -n -w out.pcap -C 16 

Answer 3

当我遇到这个Permission denied问题时，结果是我.cap在文件上放置了扩展名而不是.pcap. 正如 RichL 在评论中指出的那样，Ubuntu 上的 AppArmor 配置文件/etc/apparmor.d/usr.sbin.tcpdump导致了这种情况。

  # uname -a ; lsb_release -a
  Linux bidder-lb4 3.2.0-76-virtual #111-Ubuntu SMP Tue Jan 13 22:33:42 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
  No LSB modules are available.
  Distributor ID: Ubuntu
  Description:    Ubuntu 12.04.5 LTS
  Release:        12.04
  Codename:       precise

Answer 4

您的 tcpdump 正在放弃对 'tcpdump' 用户的权限，请检查手册页（“-Z tcpdump”是默认值，并且 tcpdump 用户无权写入 root 的 homedir）。正如 Daniel T. 告诉您的那样，在像 /tmp 这样的世界可写目录中运行您的捕获，或者至少在您已授予用户或组“tcpdump”写入权限的目录中运行。