[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
为什么我会得到错误？
我该怎么办？
解决方法
我在Centos 5上试过,即使在tmp或root文件夹上也是如此.从tcpdump手册页中,在打开第一个savefile之前,与-Z选项(默认启用)一起使用时,将删除权限.因为您指定了“-C 1”,因为文件大小已经达到1而发生权限被拒绝,并且在创建新文件时它将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,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) = -1 EAGAIN (Resource temporarily unavailable)
fcntl(3,O_RDWR) = 0
setsockopt(3,"\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 ]
# 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
