strace Use Example - The Networked Client

When an application serves as a network client, often tracing it can be used to identify networking problems. Lets see how a trace of a networked client looks:

execve("/usr/bin/ftp", ["ftp", "actcom.co.il"], [/* 37 vars */]) = 0
uname({sys="Linux", node="simey", ...}) = 0
brk(0)                                  = 0x8065b40
[.. snip ..]

Now comes the interesting part - network address lookup. First, the /etc/nsswitch.conf is opened. Interesting? read the man page to know what it does.

open("/etc/nsswitch.conf", O_RDONLY)    = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=1744, ...}) = 0
old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40013000
read(3, "#\n# /etc/nsswitch.conf\n#\n# An ex"..., 4096) = 1744
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x40013000, 4096)                = 0