Category: System Administration

Blocking Device Internet Access

We block Internet access for a lot of our smart devices. All of our control is done through the local server; and, short of updating firmware, the devices have no need to be chatting with the Internet. Unfortunately, our DSL modem/router does not have any sort of parental control, blocking, or filtering features. Fortunately, ISC DHCPD allows you to define per-host options. Setting the router to the device’s IP (0.0.0.0 may work as well) allows us to have devices that can communicate with anything on their subnet without allowing access out to other subnets or the Internet.

Viewing and recording packets using tshark

This time, I’m writing this down so I don’t have to keep looking it up. To display some packet info to the screen while writing a network capture to a file, include the -P option (older versions of tshark used -S)

2021-04-18 13:58:58 [lisa@server ~]# tshark -f "udp port 123" -w /tmp/ntpd.cap -P
Running as user "root" and group "root". This could be dangerous.
Capturing on 'enp0s25'
1 0.000000000 10.x.x.x → x.x.x.18 NTP 90 NTP Version 4, client
2 3.898916081 10.x.x.x → x.x.x.199 NTP 90 NTP Version 4, client
3 7.898948128 10.x.x.x → x.x.x.20 NTP 90 NTP Version 4, client
4 7.928749596 x.x.x.20 → 10.x.x.x NTP 90 NTP Version 4, server
5 9.898958577 10.x.x.x → x.x.x.76 NTP 90 NTP Version 4, client
6 9.949450324 x.x.x.76 → 10.x.x.x NTP 90 NTP Version 4, server
7 10.898981132 10.x.x.x → x.x.x.185 NTP 90 NTP Version 4, client
8 11.009163093 x.x.x.185 → 10.x.x.x NTP 90 NTP Version 4, server

Console access from virsh

I had a whole host of problems that were eventually resolved by rebooting the physical server … but, in the process of trying to figure out exactly what was wrong, I wanted to console into the virtual machines from the physical server. Using “virsh console vmname” should have worked … but it didn’t. Turns out you’ve got to enable a service on each guest before you’re able to console in from the physical server. To do so, run:

systemctl enable serial-getty@ttyS0.service

And, if you want to connect in *right now*, also start the service:

systemctl start serial-getty@ttyS0.service

Now, running “virsh console vmname” doesn’t appear to do much … but, if you hit the enter key, you’ll get a logon prompt for the VM.

Linux: Identifying Large Packages

The disk filled up on our primary server, and there wasn’t anything obvious like a decade worth of log files to clean up. I had to resort to uninstalling ‘stuff’ (it was, after all, installing ‘stuff’ that created the problem … tons of X11-related stuff for troubleshooting purposes). There is a way to list installed packages by size:

 

rpm -qia|awk '$1=="Name" { n=$3} $1=="Size" {s=$3} $1=="Description" {print s " " n }' |sort -n

MTU Probing

We’ve had a number of very strange network problems lately — Zoneminder cannot talk to cameras, clients veg out talking to Myth, Twonky is non-functional (even the web page — you get enough of the header to have a title, but the page just hangs, Scott cannot get to our Discourse site. And, more frustratingly, he cannot SSH to some of our hosts. Using “ssh -v” and throwing on a bunch of flags to not attempt key auth (-o PasswordAuthentication=yes -o PreferredAuthentications=keyboard-interactive,password -o PubkeyAuthentication=no) and his connection still hung. But, at least, I could see something. The last thing the SSH connection reported is:

debug1: expecting SSH2_MSG_KEX_ECDH_REPLY

Which I’ve seen before … fortunately when I had a great Unix support guy working in the same office building that I did. Who let me stop over and bounce really oddball problems off of him. He told me to enable mtu probing.

echo 1 >/proc/sys/net/ipv4/tcp_mtu_probing

And, if that doesn’t work, use “echo 2”. Which …. yeah, wouldn’t have been any of my first thirty guesses. Cloudflare published a good article on what exactly MTU path discovery is, and I can RTFM enough to figure out what I’ve set here. But no idea what’s got a smaller MTU than our computers.

 

tcp_mtu_probing - INTEGER
	Controls TCP Packetization-Layer Path MTU Discovery.  
	  0 - Disabled
	  1 - Disabled by default, enabled when an ICMP black hole detected
	  2 - Always enabled, use initial MSS of tcp_base_mss.

PIP SSL Error

Upgraded pip today, and I pretty quickly regretted it. SSL Error attempting to install anything from the Internet (and, amazingly, some things where I downloaded the wheel file). The answer is to downgrade PIP until you hit a version that doesn’t have the error. Annoying. Not sure what the latest rev I could have used was — going back one level and getting the error in loop was more time than I could devote to the project, so I just jumped back six months. Had success with 20.0.2 and left working alone.

Everything from 20.3.1 through 21.0.1 has this failure:

D:\tmp\5\pip>pip install basic_sftp
WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by ‘SSLError(SSLError(1, ‘[SSL: WRONG_VERSION_NUMBER] wrong version number (_ssl.c:1076)’))’: /simple/basic-sftp/
WARNING: Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by ‘SSLError(SSLError(1, ‘[SSL: WRONG_VERSION_NUMBER] wrong version number (_ssl.c:1076)’))’: /simple/basic-sftp/
WARNING: You are using pip version 20.3.1; however, version 21.0.1 is available.
You should consider upgrading via the ‘c:\programs\anaconda3\python.exe -m pip install –upgrade pip’ command.

SCP From Solaris to RHEL?

Evidently you cannot just scp files from an old Solaris box when you’re on a RHEL/CentOS system … there’s an incompatibility between them that requires you to (1) install scp1 on the Solaris server {not likely in a prod environment} or (2) use sftp to transfer the files.

 

Server1: Red Hat Enterprise Linux Server release 7.6 (Maipo)
Server2: Solaris 5.9

lisa@server1:~/$ scp lisa@server2:/data/stuff/file1.txt ./input/
lisa@server2’s password:
scp: warning: Executing scp1.
scp: FATAL: Executing ssh1 in compatibility mode failed (Check that scp1 is in your PATH).

Fedora — Disabling IPv6

Since it’s the third time I’ve had to do this so far this year, I’m going to write down how I disable IPv6 in Fedora. Add these lines to /etc/sysctl.conf

[lisa@server~]# grep ipv6 /etc/sysctl.conf
net.ipv6.conf.all.disable_ipv6=1
net.ipv6.conf.default.disable_ipv6=1
net.ipv6.conf.lo.disable_ipv6=1

Then load the sysctl settings (sysctl -p) or reboot.

Without IPv6, if you do X-redirection, you may get an error indicating the redirection was refused. In journalctl, there’s an error “error: Failed to allocate internet-domain X11 display socket”. Evidently you’ve got to configure sshd to use IPv4 by setting “AddressFamily inet” in /etc/ssh/sshd_config

[lisa@server~/]# grep AddressFamily /etc/ssh/sshd_config
AddressFamily inet