Linux – What are the semaphore arrays on Linux

ipclinux

I understand semaphores, but what are these semaphore arrays being used on my Linux box?

$ ipcs

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x00000000 327681     root      644        80         2                       
0x00000000 360450     root      644        16384      2                       
0x00000000 393219     root      644        280        2                       
...

------ Semaphore Arrays --------
key        semid      owner      perms      nsems     
0x4172d4f4 290914305  lazer     660        104       
0x3b87b970 291045378  lazer     660        104       
0xa97eb380 293928963  lazer     660        104       
0x1fde2040 294191108  lazer     660        104       

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages    

$

Also, which OS resource are they guarding?

Best Answer

ipcs -i <SEMID> -s will give you more information on the specific sem array. E.g.

[me@home]$ ipcs -i 32769 -s

Semaphore Array semid=32769
uid=537  gid=85  cuid=537        cgid=85
mode=0600, access_perms=0600
nsems = 1
otime = Mon Sep 19 12:18:53 2011
ctime = Mon Sep 19 12:07:11 2011
semnum     value      ncount     zcount     pid
0          1          0          0          7548

Use the pid to figure out who's using it.

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Linux – How to run a server on port 80 as a normal user on Linux

Short answer: you can't. Ports below 1024 can be opened only by root. As per comment - well, you can, using CAP_NET_BIND_SERVICE, but that approach, applied to java bin will make any java program to be run with this setting, which is undesirable, if not a security risk.

The long answer: you can redirect connections on port 80 to some other port you can open as normal user.

Run as root:

# iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 8080

As loopback devices (like localhost) do not use the prerouting rules, if you need to use localhost, etc., add this rule as well (thanks @Francesco):

# iptables -t nat -I OUTPUT -p tcp -d 127.0.0.1 --dport 80 -j REDIRECT --to-ports 8080

NOTE: The above solution is not well suited for multi-user systems, as any user can open port 8080 (or any other high port you decide to use), thus intercepting the traffic. (Credits to CesarB).

EDIT: as per comment question - to delete the above rule:

# iptables -t nat --line-numbers -n -L

This will output something like:

Chain PREROUTING (policy ACCEPT)
num  target     prot opt source               destination         
1    REDIRECT   tcp  --  0.0.0.0/0            0.0.0.0/0           tcp dpt:8080 redir ports 8088
2    REDIRECT   tcp  --  0.0.0.0/0            0.0.0.0/0           tcp dpt:80 redir ports 8080

The rule you are interested in is nr. 2, so to delete it:

# iptables -t nat -D PREROUTING 2

Best Answer

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