Efficiently test if a port is open on Linux

Question

From a bash script how can I quickly find out whether a port 445 is open listening on a server   I have tried a couple of options  but I want something quick  1  lsof -i  445   Takes seconds  2  netstat -an  grep 445  grep LISTEN  Takes seconds  3  telnet  it doesn t return  4  nmap  netcat are not available on the server    It will be nice to know of a way that doesn t enumerate first and greps after that

User · Answer

There s a very short with  fast answer  here   How to test if remote TCP port is opened from Shell script   nc -z  lt host gt   lt port gt   echo      I use it with 127 0 0 1 as  remote  address   this returns  0  if the port is open and  1  if the port is closed  e g   nc -z 127 0 0 1 80  echo         -z        Specifies that nc should just scan for listening daemons    without sending any data to them   It is an error to use this option   in conjunc-                tion with the -l option

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You can use netstat this way for much faster results   On Linux   netstat -lnt   awk   6     LISTEN   amp  amp   4      445      On Mac   netstat -anp tcp   awk   6     LISTEN   amp  amp   4      445      This will output a list of processes listening on the port  445 in this example  or it will output nothing if the port is free

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nc -l 8000   Where 8000 is the port number  If the port is free  it will start a server that you can close easily  If it isn t it will throw an error   nc  Address already in use

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A surprise I found out recently is that Bash natively supports tcp connections as file descriptors  To use   exec 6 lt  gt  dev tcp ip addr of server 445 echo -e  GET   HTTP 1 0 n   gt  amp 6 cat  lt  amp 6   I m using 6 as the file descriptor because 0 1 2 are stdin  stdout  and stderr  5 is sometimes used by Bash for child processes  so 3 4 6 7 8  and 9 should be safe   As per the comment below  to test for listening on a local server in a script   exec 6 lt  gt  dev tcp 127 0 0 1 445    echo  No one is listening   exec 6 gt  amp -   close output connection exec 6 lt  amp -   close input connection   To determine if someone is listening  attempt to connect by loopback  If it fails  then the port is closed or we aren t allowed access  Afterwards  close the connection    Modify this for your use case  such as sending an email  exiting the script on failure  or starting the required service

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ss -tl4    sport    22      2ms is quick enough    Add the colon and this works on Linux

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Based on Spencer Rathbun s answer  using bash   true  amp  gt  dev null  lt  dev tcp 127 0 0 1  PORT  amp  amp  echo open    echo closed

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I wanted to check if a port is open on one of our linux test servers   I was able to do that by trying to connect with telnet from my dev machine to the test server  On you dev machine try to run     telnet test2 host com 8080 Trying 05 066 137 184    Connected to test2 host com   In this example I want to check if port 8080 is open on host test2 host com

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tcping is a great tool with a very low overhead It also has a timeout argument to make it quicker    root centos f831dfb3     tcping 10 86 151 175 22 -t 1 10 86 151 175 port 22 open   root centos f831dfb3     tcping 10 86 150 194 22 -t 1 10 86 150 194 port 22 user timeout   root centos f831dfb3     tcping 1 1 1 1 22 -t 1 1 1 1 1 port 22 closed

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If you re using iptables try   iptables -nL   or  iptables -nL   grep 445

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they re listed in  proc net tcp   it s the second column  after the      in hex    gt  cat  proc net tcp   sl  local address rem address   st tx queue rx queue tr tm- gt when retrnsmt   uid  timeout inode                                                         0  00000000 0016 00000000 0000 0A 00000000 00000000 00 00000000 00000000     0        0 10863 1 ffff88020c785400 99 0 0 10 -1                         1  0100007F 0277 00000000 0000 0A 00000000 00000000 00 00000000 00000000     0        0 7983 1 ffff88020eb7b3c0 99 0 0 10 -1                          2  0500010A 948F 0900010A 2328 01 00000000 00000000 02 00000576 00000000  1000        0 10562454 2 ffff88010040f7c0 22 3 30 5 3                       3  0500010A E077 5F2F7D4A 0050 01 00000000 00000000 02 00000176 00000000  1000        0 10701021 2 ffff880100474080 41 3 22 10 -1                     4  0500010A 8773 16EC97D1 0050 01 00000000 00000000 02 00000BDC 00000000  1000        0 10700849 2 ffff880104335440 57 3 18 10 -1                     5  0500010A 8772 16EC97D1 0050 01 00000000 00000000 02 00000BF5 00000000  1000        0 10698952 2 ffff88010040e440 46 3 0 10 -1                      6  0500010A DD2C 0900010A 0016 01 00000000 00000000 02 0006E764 00000000  1000        0 9562907 2 ffff880104334740 22 3 30 5 4                        7  0500010A AAA4 6A717D4A 0050 08 00000000 00000001 02 00000929 00000000  1000        0 10696677 2 ffff880106cc77c0 45 3 0 10 -1     so i guess one of those  50 in the third column must be stackoverflow  o   look in man 5 proc for more details   and picking that apart with sed etc is left as an exercise for the gentle reader

User · Answer

nmap is the right tool   Simply use nmap example com -p 80  You can use it from local or remote server   It also helps you identify if a firewall is blocking the access

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Here s one that works for both Mac and Linux   netstat -aln   awk   6     LISTEN   amp  amp   4           445

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You can use netcat command as well   location of netcat  netcat -zv  ip   port    or   nc -zv  ip   port    -z     sets nc to simply scan for listening daemons  without actually sending any data to them  -v     enables verbose mode

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You can use netcat for this   nc ip port  lt   dev null   connects to the server and directly closes the connection again  If netcat is not able to connect  it returns a non-zero exit code  The exit code is stored in the variable     As an example   nc ip port  lt   dev null  echo      will return 0 if and only if netcat could successfully connect to the port

[linux] Efficiently test if a port is open on Linux?

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