Tracing a telnet session with strace

Sometime you just need to know if a port is open on a remote system. The simplest way to find out if the port is open is to just telnet into the host and the port number.
This should look like:

shell$ telnet localhost  23
Trying 127.0.0.1...
telnet: connect to address 127.0.0.1: Connection refused
telnet: Unable to connect to remote host: Connection refused

# let's redo it with the strace enabled.
strace -vo strace.telnet telnet localhost  23
Trying 127.0.0.1...
telnet: connect to address 127.0.0.1: Connection refused
telnet: Unable to connect to remote host: Connection refused
# the error is the same but now we do have the strace.telnet file to find more info

shell$ cat strace.telnet
execve("/usr/kerberos/bin/telnet", ["telnet", "localhost", "23"], ["HOSTNAME=localhost.localdomain", "TERM=xterm-color", "SHELL=/bin/bash", "HISTSIZE=1000", "SSH_CLIENT=10.211.55.2 62489 22", "SSH_TTY=/dev/pts/0", "USER=root", "LS_COLORS=no=00:fi=00:di=01;34:l", "MAIL=/var/spool/mail/root", "PATH=/usr/kerberos/sbin:/usr/ker", "INPUTRC=/etc/inputrc", "PWD=/root", "LANG=en_US.UTF-8", "SHLVL=1", "HOME=/root", "LOGNAME=root", "SSH_CONNECTION=10.211.55.2 62489", "LESSOPEN=|/usr/bin/lesspipe.sh %", "G_BROKEN_FILENAMES=1", "_=/usr/bin/strace", "OLDPWD=/usr/src"]) = 0
brk(0)                                  = 0x9c8d000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat64(3, {st_dev=makedev(3, 1), st_ino=129997, st_mode=S_IFREG|0644, st_nlink=1, st_uid=0, st_gid=0, st_blksize=4096, st_blocks=56, st_size=26551, st_atime=2011/04/21-14:46:38, st_mtime=2011/04/21-07:09:59, st_ctime=2011/04/21-07:09:59}) = 0
mmap2(NULL, 26551, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7fe5000
close(3)                                = 0
open("/usr/lib/libkrb4.so.2", O_RDONLY) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0pB(\0004\0\0\0"..., 512) = 512
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7fe4000
fstat64(3, {st_dev=makedev(3, 1), st_ino=239248, st_mode=S_IFREG|0755, st_nlink=1, st_uid=0, st_gid=0, st_blksize=4096, st_blocks=208, st_size=100960, st_atime=2011/04/21-14:46:38, st_mtime=2010/01/12-19:22:52, st_ctime=2011/04/08-04:02:50}) = 0
mmap2(NULL, 117948, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x6d3000
mmap2(0x6ea000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x17) = 0x6ea000
mmap2(0x6eb000, 19644, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x6eb000
close(3)                                = 0
open("/usr/lib/libdes425.so.3", O_RDONLY) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\300\354)\0004\0\0\0"..., 512) = 512
fstat64(3, {st_dev=makedev(3, 1), st_ino=236616, st_mode=S_IFREG|0755, st_nlink=1, st_uid=0, st_gid=0, st_blksize=4096, st_blocks=32, st_size=12816, st_atime=2011/04/21-14:46:38, st_mtime=2010/01/12-19:22:52, st_ctime=2011/04/08-04:02:41}) = 0
mmap2(NULL, 13868, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x1d9000
mmap2(0x1dc000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2) = 0x1dc000
close(3)                                = 0
.......
.......
fstat64(1, {st_dev=makedev(0, 12), st_ino=2, st_mode=S_IFCHR|0620, st_nlink=1, st_uid=0, st_gid=5, st_blksize=4096, st_blocks=0, st_rdev=makedev(136, 0), st_atime=2011/04/21-14:46:38, st_mtime=2011/04/21-14:46:38, st_ctime=2011/04/21-04:36:50}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7feb000
write(1, "Trying 127.0.0.1...\r\n", 21) = 21
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3
setsockopt(3, SOL_IP, IP_TOS, [16], 4)  = 0
connect(3, {sa_family=AF_INET, sin_port=htons(23), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 ECONNREFUSED (Connection refused)
write(2, "telnet: connect to address 127.0"..., 57) = 57
close(3)                                = 0
write(2, "telnet: Unable to connect to rem"..., 61) = 61
exit_group(1) 

# as you can see there is a lot of information and some i replaced with ..... 
# the line of interest will be 

connect(3, {sa_family=AF_INET, sin_port=htons(23), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 ECONNREFUSED (Connection refused)

# this tells when the socket is actually trying to connect onto the remote host and had a return code of -1, all after it is just output from the telnet program that formats it very carefully.