This lets a file system to be mounted and dismounted multiple times, all while the process is running. SuSv3 does not require these functions to return values than remain constant all through an entire process runtime. In the case of the former, the pathname needs to be specified, while for the latter, a previously opened file descriptor is required. The difference between pathconf() and fpathconf() is the method of specifying the file/directory. SuSv3 dictates that the value returned by the function has to be constant all through the calling. If the limit cannot be determined, or there is an error of any sort, the function will return a value of –1′. The getconfcommand can help you determine the limits in the UNIX implementation you are currently working on.ĭuring runtime, sysconf() can tell you the limits you need when you specify the name of the limit in the function’s argument. Limits can also be ascertained through shell commands. Limits on these pathnames could be determined using functions like fpathconf() and pathconf(). This function was also a great tool to determine runtime invariant and increasable values.Ī function like pathconf(), on the other hand, deals with the name of file paths. This made determining the limits of a system using sysconf() etc. However, that would result in a loss of functionality that higher limit values would bring in.
#Limit standard accounts ubuntu portable#
If an application could stick to the specified minimum limits, it would manage to be portable across most implementations in all likelihood. This particular limit would be set as a constant in the limits header file, with a name that begun with the _posix_ string. The most crucial part of this set range was a minimum limit. It also defined a limit range that a UNIX implementation could enforce. The Single UNIX Specification’s third version (SuSv3, in short) defined three discrete functions that an application could call to determine the limits of the system it was working on – fpathconf(), pathconf() andsysconf(). Since these limits effectively reduce the scope and abilities of a program, an application trying to be portable across different UNIX implementations needs to account for varying limit standards. This can range from simplistic models like the complete length of file’s path, to the kind of arguments and calls that a program can have. Match patterns are documented in ssh_config man page.Every UNIX based system like Linux has a certain set of limitations on the kind of feature sets and resources that can be used by a program or user. Match arguments and allowed conditional configuration options are documented in sshd_config man page. Other clients are still able to connect, but logins will fail because there is no available authentication methods.
Placing Match in the end of the file is important, since all the configuration lines after it are placed inside the conditional block until the next Match line. Then add desired authentication methods after a Match Address in the end of the file. If you only want to block other hosts from connecting, you should use iptables or TCP wrappers instead.įirst remove default authentication methods: PasswordAuthentication no You can configure ssh daemon in sshd_config to use different authentication method depending on the client address/hostname. For example to allow network 192.168.0.0/24 and localhost.
With TCP wrappers, in addition to IP addresses you can also use hostnames in rules. You can also configure which hosts can connect using TCP wrappers.
Note: this might not be an option on modern distributions, as support for tcpwrappers was removed from OpenSSH 6.7 Systems which have ssh listening to IPv6 address the necessary configuration can be done with ip6tables. You need to configure some mechanism to restore iptables on boot. Iptables are not persistent across reboots. There is also iprange module which allows using any arbitrary range of IP addresses. If you have a lot of networks or host addresses, you should use ipset module. You can add more rules before the drop rule to match more networks/hosts.
Iptables -A INPUT -p tcp -dport 22 -j DROP iptables -A INPUT -p tcp -dport 22 -source 192.168.0.0/24 -j ACCEPT The DROP rule is not required if your iptables default policy is configured to DROP. Iptables rules are evaluated in order, until first match.įor example, to allow traffic from 192.168.0.0/24 network and otherwise drop the traffic (to port 22). If you want to use different authentication methods depending on the client IP address, configure SSH daemon instead (option 3). You can limit which hosts can connect by configuring TCP wrappers or filtering network traffic (firewalling) using iptables.