strace ย่อมากจาก system call trace เป็นคำสั่งที่เอาไว้ติดตามดูว่า process นั้นๆ มีการเรียกใช้งาน system call อะไรบ้าง
ซึ่งเป็นคำสั่งที่มีประโยชน์มาก โดยเฉพาะสำหรับผู้ที่ทำงานเกี่ยวกับ computer forensic
บทความนี้เป็นการประยุกต์ใช้งาน strace เพื่อตรวจสอบว่าโปรแกรมที่เรารันมีปัญหาอะไรเกิดขึ้น
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[SIZE=2]5) Why the **** can't I connect to that server?</span>
Debugging why some process isn't connecting to a remote server can be exceedingly frustrating. DNS can fail, connect can hang, the server might send something unexpected back etc. You can use tcpdump to analyze a lot of that, and that too is a very nice tool, but a lot of the time strace will give you less chatter, simply because it will only ever return data related to the syscalls generated by "your" process. If you're trying to figure out what one of hundreds of running processes connecting to the same database server does for example (where picking out the right connection with tcpdump is a nightmare), strace makes life a lot easier.
This is an example of a trace of "nc" connecting to www.news.com on port 80 without any problems:
[code]
$ strace -e poll,select,connect,recvfrom,sendto nc www.news.com 80
sendto(3, "\24\0\0\0\26\0\1\3\255\373NH\0\0\0\0\0\0\0\0", 20, 0, {sa_family=AF_NETLINK, pid=0, groups=00000000}, 12) = 20
connect(3, {sa_family=AF_FILE, path="/var/run/nscd/socket"}, 110) = -1 ENOENT (No such file or directory)
connect(3, {sa_family=AF_FILE, path="/var/run/nscd/socket"}, 110) = -1 ENOENT (No such file or directory)
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("62.30.112.39")}, 28) = 0
poll([{fd=3, events=POLLOUT, revents=POLLOUT}], 1, 0) = 1
sendto(3, "\213\321\1\0\0\1\0\0\0\0\0\0\3www\4news\3com\0\0\34\0\1", 30, MSG_NOSIGNAL, NULL, 0) = 30
poll([{fd=3, events=POLLIN, revents=POLLIN}], 1, 5000) = 1
recvfrom(3, "\213\321\201\200\0\1\0\1\0\1\0\0\3www\4news\3com\0\0\34\0\1\300\f"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("62.30.112.39")}, [16]) = 153
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("62.30.112.39")}, 28) = 0
poll([{fd=3, events=POLLOUT, revents=POLLOUT}], 1, 0) = 1
sendto(3, "k\374\1\0\0\1\0\0\0\0\0\0\3www\4news\3com\0\0\1\0\1", 30, MSG_NOSIGNAL, NULL, 0) = 30
poll([{fd=3, events=POLLIN, revents=POLLIN}], 1, 5000) = 1
recvfrom(3, "k\374\201\200\0\1\0\2\0\0\0\0\3www\4news\3com\0\0\1\0\1\300\f"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("62.30.112.39")}, [16]) = 106
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("62.30.112.39")}, 28) = 0
poll([{fd=3, events=POLLOUT, revents=POLLOUT}], 1, 0) = 1
sendto(3, "\\\2\1\0\0\1\0\0\0\0\0\0\3www\4news\3com\0\0\1\0\1", 30, MSG_NOSIGNAL, NULL, 0) = 30
poll([{fd=3, events=POLLIN, revents=POLLIN}], 1, 5000) = 1
recvfrom(3, "\\\2\201\200\0\1\0\2\0\0\0\0\3www\4news\3com\0\0\1\0\1\300\f"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("62.30.112.39")}, [16]) = 106
connect(3, {sa_family=AF_INET, sin_port=htons(80), sin_addr=inet_addr("216.239.122.102")}, 16) = -1 EINPROGRESS (Operation now in progress)
select(4, NULL, [3], NULL, NULL)
