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I have a linux box with a bank of modems and need to create demand-dial (outbound) PPP connections to a very large set of remote machines. Since I have far more remotes than local modems, I'd like to launch one or more instances of pppd in demand mode, but not allocate a particular modem until a connection is requested. When the connection becomes idle, I'd like pppd to release the modem so it can be used for a connection to a different remote.
I've been through the pppd man pages and understand the concepts of the call and connect options, as well as the options.DEVICE files and the /etc/ppp/peers directory. Lock files will tell me which modems are in use or available. And I'm able to connect to multiple remote peers as long as I hard-code the modem assignments. But I haven't found a way to assign them dynamically, say by calling a get_available_modem script when pppd needs to dial. Google doesn't seem to know, either.
A work-around might be to detect the need to demand-dial with an external mechanism, and then launch pppd to service it, but it seems cleaner to leverage the facility within pppd itself.
Any pointers or ideas are appreciated. Thanks for your thoughts!
This is the solution I'm going with:
Enumerate the modems as /dev/modem0, /dev/modem1, ...
Start an instance of pppd in demand-dial mode for each peer, but instead of a "real" modem, attach it to /dev/modem.peer, which the "ppp-on-dialer" script soft-links to the first available modem dynamically.
When the connection times out and the modem goes offline, soft link /dev/modem.peer to /dev/null so the modem can be re-used for a connection to a different peer.
Thanks for your comments!
Does it have to be directly driven by the Linux box, or could you use something like the old Lucent Portmaster boxen?
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Let's say I have a 100G disk which i want to expose as an ISCSI-target and I have configured 2-ISCSI-portals(IP1, IP2) which can be used to access the ISCSI-target
Note: I have used more than one portal for my High-Availability Use-cases
Let's say, From the Host, I have used IP1 for the login to the ISCSI-Target and am able to connect to the target successfully. After some time Say, because of some reason IP1 is down.is there a config/way to specify the ISCSIADM to fall back to IP2 for connecting to ISCSI-target
As stark says in the comments above, the answer is to use dm-multipath. There are numerous articles on how to set this up, but the short answer is that it'll likely "just work".
First, install multipathd on your system. Then, when you use iscsiadm in discovery mode, so long as your iSCSI target reports both portals, Linux is going to connect to both portals. You'll get two block devices, both with the same SCSI WWN. Multipathd wakes up, sees the two devices with the same WWN, and bundles them into a /dev/dm-X device for your use. From that point forward, multipathd manages the paths according to how you've configured its policy. The default may be fine for your use.
The key point here is that iscsiadm and iSCSI are kind of "out of the way". You'll have a session for each path. The sessions may come and go. DM-Multipath manages which sessions are involved in providing access to your LUN.
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Background Info:
My office is closing down due to COVID19 outbreak policies.
Unfortunately we do not have a VPN in place, and currently one of my coworkers has to going in every day to make commits for everyone who is working remotely--our repos are stored inside the office internal network.
The Problem:
I need to be able to get inside the internal network remotely using SSH, without having to make any changes to network configuration (I do not have access to the network devices). This means I would not be opening ports, or making firewall exceptions for example.
I was thinking of using a raspberry pi with a fresh linux image (any supported distro would be fine, I'm open to suggestions).
My goal is to plug the pi3 to my workstation ethernet port, and leave it there for the time being. I would not be able to do any physical reboots , etc.
I need a suggestion for a free tunneling solution that would be easy to set up and use for 1-10 people to be able to connect using a secure connection (SSH) and gain access to internal resources remotely.
Question:
I need suggestions for which distro would be nice and light enough for the pi3 to handle, and SSH server software to use.
I looked into OpenSSh which has been suggested in other similar threads, but it would require configuring firewall and opening port 22, which I cannot do.
Other
I've used Google Chrome's remote desktop in the past and it has worked wonderfully. I would set up the "host" machine with it, and then any other machine with Chrome (with the same account) could remote in without any sort of network configuration.
I guess I just need something similar to this, but instead of remote desktop I need an SSH solution. I would create a few root-enabled accounts to allow other devs to connect to it at the same time if possible.
I found a solution to my problem and will pass it along to my IT team for proper approval prior to putting in place. However I wanted to share the answer here in case anyone else ever has a similar situation.
As stated, be sure to check your company's policy and consult the proper channels before doing this sort of thing, as it can cause security risks and could get you fired... so be advised, do this at your own risk!
With that said, a "Reverse SSH" could be the solution to the problem. It allows you to connect to a machine inside of a network without having to open ports or firewall exceptions.
This isn't a good long term solution for most cases, but might be the workaround you need to do the trick in a pinch.
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Closed 5 years ago.
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I'm on Linux Mint, never connected to the internet for weeks, and still not connected, doing some python programming (Python 3.6).
I happened to look at the firewall program in it 'report' section and see this
(please not that the firewall was actually set to ALLOW outgoing until I changed it just before this screenshot (it was only set to block incoming). Again, to my knowledge I am not, and have not, been connected to the internet at all and my networking is turned off.:
I'm not a network programmer, but this seems to be showing that these applications have been accessing network ports, correct?
Why would python 3.6 (which I was using) be on this list or ever need to connect to any port at all?? Am I looking at nefarious activity here?
I've heard that software can be placed directly on the hardware of a system. IS it possible that my network interface card could be activated and used without giving any indication?
And again, why would my python 3.6 application be on a firewall report at all?
I'm very concerned about this. Can someone enlighten me?
What you are looking at are mostly servers that are listening for incoming connections. That certainly covers ntpd, cups, ntpd.
It looks like some python programs have also been listening for connections, maybe as part of your programming exercises.
The fact that programs are listening for incoming connections doesn't mean anything has actually connected to them, or even that anything could.
An address of "*" indicates that it's a potential connection, not an actual connection that happened.
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Motivation: I run wpa_supplicant without a network managing tool because I don't want to force new DHCP leases when my old ones work. This way I can get onto an old network very fast (as soon as wpa_supplicant establishes the lower network layers) if the DHCP lease has not expired.
Problem: But when the DHCP lease has expired, or if my IP address is from another network, I do need to call upon DHCP. I currently use dhclient, but dhcpcd is another option here. The problem is, even though these DHCP clients run in the background, they don't automatically realize that I need a new IP address, since I've got an old, invalid one.
Question: Is there a way to very quickly determine if I'm in need of DHCP? E.g. as a reaction to wpa_supplicant or as a response to contacting the standard gateway and realizing a connection isn't available. I can easily run sudo dhclient wlan0, and it gives me the address in one second. But what can I do to automate this process, when necessary?
Making unnecessary DHCP requests is not out of the question, but flooding the DHCP server is.
Elaborate question: For those of you who use wpa_supplicant directly, how do you DHCP?
Run dhcpcd again whenever you attach to a new network. If you have an existing lease it should be confirmed quickly.
If you object to typing in "dhclient" or "dhcpcd" everytime, you could automate it with a shell script. It could look something like
#!/bin/sh
dhcpcd wlan0 &
wpa_supplicant -d -c/etc/wpa_supplicant.conf -iwlan0
dhcpcd will say it is "waiting for carrier" until wpa_supplicant brings up the wireless link.
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I work with embedded Linux systems that sometimes want to get their IP address from a DHCP server. The DHCP Client client we use (dhcpcd) has limited retry logic. If our device starts up without any DHCP server available and times out, dhcpcd will exit and the device will never get an IP address until it's rebooted with a DHCP server visible/connected. I can't be the only one that has this problem. The problem doesn't even seem to be specific to embedded systems (though it's worse there). How do you handle this? Is there a more robust client available?
The reference dhclient from the ISC should run forever in the default configuration, and it should acquire a lease later if it doesn't get one at startup.
I am using the out of the box dhcp client on FreeBSD, which is derived from OpenBSD's and based on the ISC's dhclient, and this is the out of the box behavior.
See http://www.isc.org/index.pl?/sw/dhcp/
You have several options:
While you don't have an IP address, restart dhcpcd to get more retries.
Have a backup static IP address. This was quite successful in the embedded devices I've made.
Use auto-IP as a backup. Windows does this.
Add to rc.local a check to see if an IP has been obtained. If no setup an 'at' job in the near future to attempt again. Continue scheduling 'at' jobs until an IP is obtained.