MS Paint

I think the only thing I miss from Microsoft Windows, it is “paint”. I can’t find anything in Linux that is simple and reliable. I have used pinta, gimp and others but they break or they are too pro.

So, if you have internet connection. This is your friend

And if you want something a bit more pro like Visio, then this one

I recommend both a lot!

I know you have GIMP, but I just a simple tool to draw red squares!

Troubleshooting a DCHP Relay connection

Today I have had “fun” troubleshooting an issue that looked easy at first sight. A colleague was trying to PXE boot some server from a network that we haven’t used for a while.

When the server boots up, asks for an IP via DHCP. As we have a centralized DHCP server infrastructure, we have configured DHCP relay in the firewall facing that server to send that request to the DHCP server.

First, let’s take a look at how DHCP relay works. This is a very good link. And this diagram from the mentioned link it is really useful:

One think I learned is the reply (DCHP Offer) doesnt have to use as destination IP the same IP it received as source in DHCP Discover. In the picture, it is packet 2a.

Checking in our environment, we confirm that:

Our server is in 10.94.240.x network. Our firewall is acting as DHCP relay, and send the DHCP Discovery (unicast) to our VIP DHCP Server IP.

The DHCP offer, uses as source the physical IP of the DHCP server and destination is the DHCP relay IP (so it is – the firewall IP in 10.94.240.x network)

Ok, so everything looks fine? No really. The server receives the query, it answers… but we dont see a DCHP Request/ACK.

BTW, keep in mind that DHCP is UDP….

So, we need to see where the packets are lost.

This is a high level path flow between the client and server:

So we need to check this connection is three different firewall vendors….

The initial troubleshooting was just using the GUI tools from Palo/Fortigate. We couldn see anything…. but the server was constantly receiving DHCP Discover and sending DHCP Offer… I dont get it:

# tcpdump -i X udp port 67 or 66 -nn

14:58:06.969462 IP > BOOTP/DHCP, Request from 6c:2b:59:c1:32:73, length 300
14:58:06.969564 IP > BOOTP/DHCP, Reply, length 300

14:58:28.329048 IP > BOOTP/DHCP, Request from 6c:2b:59:c1:32:73, length 300
14:58:28.329157 IP > BOOTP/DHCP, Reply, length 300

Initially it took me a while to see the request/reply because I was assuming the dhcp request had source So I was seeing only the Reply but not the Request. That was when I went to clarify my head about DHCP Relay and found the link.

So ok, we have the DHCP Request/Reply, but absolutely nothing in the Palo. Is the palo dropping the packets or is forwarding? No idea. The GUI says nothing, I took a packet capture and couldnt see that traffic neither…

Doesnt makes sense.

Let’s get back to basic.

Did I mention DHCP is UDP? So how a next generation firewall (like paloalto) with all the fancy features enable (we have nearly all of them enable…) treats a UDP connection? UDP is stateless… but the firewall is statefull… the firewall creates a flow with the first packet so it can track, any new packet is considered part of that flow. But why we dont see the flows? We actually have only one flow. The firewall has created that session and offloaded to hardware. So you dont see anything else in the control-plane / GUI. The GUI only shows the end of a connection/flow. And as our flow DHCP Relay hasnt’ terminated (it is UDP) and the firewall keeps receiving packets, it is considered life (the firewall doesnt really know what is going on). So for that reason we dont see the connection in the PaloUI. Ok, I got to that point after a while…. I need to proof that the packet from the server is reaching the firewall and it is leaving it too.

How can I do that? Well, I need to delete that flow so the firewall considers a new connection and the tcpdump can see the packets.

This is the a good link from paloalto to take captures. So I found my connection and the cleared it:

palo(active)> show session all filter destination

ID Application State Type Flag Src[Sport]/Zone/Proto (translated IP[Port])
Vsys Dst[Dport]/Zone (translated IP[Port])
135493 dhcp ACTIVE FLOW[67]/ZONE1/17 ([67])
vsys1[67]/ZONE2 ([67])
palo(active)> clear session id 135493

And now, my packet capture in paloalto confirms that it is sending the packet to the next firewall (checking the destination MAC) !!!

Ok, so we confirm the first firewall in the return path was fine…. next one, it is fortigate.

BTW, we were checked and assumed that the routing is fine in all routers, firewalls, etc. Sometimes is not the case… so when things dont follow your thoughts, get back to the very basics….

We have exactly the same issue as in PaloAlto. I can’t see anything in the logs about receiving a dhcp offer from palo and forwarding it to the last firewall Cisco.

And again, we apply the same reasoning. We have an UDP connection, we have a next-generation firewall (with fancy ASIC). And one more thing, in this fortigate firewall, we allow intra-zone traffic, so it is not going to show anyway in the GUI monitor…

So we confirm that we have a flow and cleared it

forti # diag debug flow filter
vf: any
proto: any
Host addr: any
Host saddr: any
host daddr:
port: any
sport: any
dport: any
co1fw02 #
co1fw02 # diag sys session list
session info: proto=17 proto_state=00 duration=2243 expire=170 timeout=0 flags=00000000 sockflag=00000000 sockport=0 av_idx=0 use=5
class_id=0 ha_id=0 policy_dir=0 tunnel=/ vlan_cos=8/8
state=may_dirty npu synced
statistic(bytes/packets/allow_err): org=86840/254/1 reply=0/0/0 tuples=2
tx speed(Bps/kbps): 36/0 rx speed(Bps/kbps): 0/0
orgin->sink: org pre->post, reply pre->post dev=39->35/35->39 gwy=
hook=pre dir=org act=noop>
hook=post dir=reply act=noop>
misc=0 policy_id=4294967295 auth_info=0 chk_client_info=0 vd=0
serial=141b05fb tos=ff/ff app_list=0 app=0 url_cat=0
rpdb_link_id = 00000000
dd_type=0 dd_mode=0
npu info: flag=0x81/0x00, offload=6/0, ips_offload=0/0, epid=8/0, ipid=8/0, vlan=0x00f5/0x0000
vlifid=0/0, vtag_in=0x0000/0x0000 in_npu=0/0, out_npu=0/0, fwd_en=0/0, qid=0/0
total session 1
forti #
forti # diag sys session clear

In other session, I have a packet capture in the expected egress interface:

forti # diagnose sniffer packet Zone3 'host'
301.555231 -> udp 300
316.545677 -> udp 300

Fantastic, we have confirmation that the second firewall receives and forwards the DHCP Reply!!!

Ok, now the last stop, Cisco ASA. This is an old firewall, I think it could be my father or Darth Vader.

I dont have the fancy tools for packet capture like Palo/Fortigate…. so I went to the basic “debug” commands and “packet-tracer”.

First, this was the dhcp config in Cisco:

vader/pri/act# show run | i dhcp
dhcprelay server EGRESS
dhcprelay enable SERVERS-ZONE
dhcprelay timeout 60

And, the ACL allows all IP traffic in those interfaces… and couldnt see any deny in the logs.

So, I enabled all debugging things I could find for dhcp:

vader/pri/act# show debug
debug dhcpc detail enabled at level 1
debug dhcpc error enabled at level 1
debug dhcpc packet enabled at level 1
debug dhcpd packet enabled at level 1
debug dhcpd event enabled at level 1
debug dhcpd ddns enabled at level 1
debug dhcprelay error enabled at level 1
debug dhcprelay packet enabled at level 1
debug dhcprelay event enabled at level 200
vader/pri/act# DHCPD: Relay msg received, fip=ANY, fport=0 on SERVERS-ZONE interface
DHCPRA: relay binding found for client f48e.38c7.1b6e.
DHCPD: setting giaddr to
dhcpd_forward_request: request from f48e.38c7.1b6e forwarded to
DHCPD: Relay msg received, fip=ANY, fport=0 on SERVERS-ZONE interface
DHCPRA: relay binding found for client 6c2b.59c1.3273.
DHCPD: setting giaddr to
dhcpd_forward_request: request from 6c2b.59c1.3273 forwarded to

So, the debugging doesnt says anything regarding the packet coming back from Fortigate… Not looking good I am afraid. I wasnt running out of ideas about debug commands. I coudn’t increase an log level neither….

Let’s give a go to packet tracer… doesnt looks good:

vader/pri/act# packet-tracer input EGRESS udp 67 67
Phase: 1
Result: ALLOW
Implicit Rule
Additional Information:
MAC Access list
Phase: 2
Result: DROP
Implicit Rule
Additional Information:
input-interface: EGRESS
input-status: up
input-line-status: up
Action: drop
Drop-reason: (acl-drop) Flow is denied by configured rule

So, we are sure our ACL is totally open but the firewall is dropping the packet coming from fortigate. Why? How to fix it?

Ok, get back to basics. Focus in Cisco config. It uses as DHCP relay server, (VIP). But the DHCP reply is coming from the physical IP….. maybe Cisco doesnt like that…. Let’s try to add the physical IPs as a new DHCP server:

vader/pri/act# sri dhcp
dhcprelay server EGRESS
dhcprelay server EGRESS
dhcprelay server EGRESS

Let’s check packet tracer again:

vader/pri/act# packet-tracer input EGRESS udp 67 67
Phase: 1
Result: ALLOW
Implicit Rule
Additional Information:
MAC Access list
Phase: 2
Result: ALLOW
Implicit Rule
Additional Information:
Phase: 3
Result: ALLOW
Additional Information:
Phase: 4
Result: ALLOW
Additional Information:
Phase: 5
Result: ALLOW
Additional Information:
Phase: 6
Type: VPN
Subtype: ipsec-tunnel-flow
Result: ALLOW
Additional Information:
Phase: 7
Result: ALLOW
Additional Information:
New flow created with id 340328245, packet dispatched to next module
input-interface: EGRESS
input-status: up
input-line-status: up
Action: allow

Good, that’s a good sign finally!!!

I think I nearly cried after seeing this in the dhcp logs in our server:

May 12 16:16:27 dhcp1 dhcpd[2561]: DHCPDISCOVER from f4:8e:38:c7:1b:6e via
May 12 16:16:28 dhcp1 dhcpd[2561]: DHCPOFFER on to f4:8e:38:c7:1b:6e (cmc-111) via
May 12 16:16:28 dhcp1 dhcpd[2561]: Wrote 0 class decls to leases file.
May 12 16:16:28 dhcp1 dhcpd[2561]: Wrote 0 deleted host decls to leases file.
May 12 16:16:28 dhcp1 dhcpd[2561]: Wrote 0 new dynamic host decls to leases file.
May 12 16:16:28 dhcp1 dhcpd[2561]: Wrote 1 leases to leases file.
May 12 16:16:28 dhcp1 dhcpd[2561]: DHCPREQUEST for ( from f4:8e:38:c7:1b:6e (cmc-111) via
May 12 16:16:28 dhcp1 dhcpd[2561]: DHCPACK on to f4:8e:38:c7:1b:6e (cmc-111) via

So at the end, finally fixed…. it took too many hours.


  • DHCP Realy: It is not that obvious the flow regarding IPs.
  • UDP and firewalls, debugging it is a bit more challenging.
  • Cisco ASA dhcprelay server IPs…. VIPs and non-VIPs please.

All this would be easier/quicker with TCP 😛

Bash: shell quoting

Another issue I had during the weekend that took me hours. Thanks that I have been reading a bit this book (1.6) and had some clues.

I was trying to test a repo to start an Arista lab using docker and I assumed that everything should work if I followed the instructions. My problem was the script trying to push some basic config to the switches.

This is was the initial function:

#!/usr/bin/env bash
function fast_cli() {
  docker exec "${1}" bash -c "echo -e ${commands} | FastCli -p15 -e

If you type that command in a bash shell directly is something like this:

$ docker exec DOCKER_ID bash -c 'echo -e "configure\n hostname sp01\n end\n write\n" | FastCli -p15 -e'

As you can see that differs with what we have inside the bash script. So from the bash script we need to put between ‘ the parameter for -c but inside the parameter we need to use “. So I had to make the change below:

-  docker exec "${1}" bash -c "echo -e ${commands} | FastCli -p15 -e"
+  # need to update this command as the quoting doesnt work in my bash
+  docker exec "${1}" bash -c 'echo -e '"'${commands}'"' | FastCli -p15 -e'

The books says Enclose a string in single quotes ‘ unless it contains elements that you want the shell to interpolate. So let’s divide the solution in parts so can be easier to digest (and remember for me in the future because this will bit me again for sure)

  • 1st part: ‘echo -e ‘
  • 2nd part:
  • 3rd part: ‘${commands}’
  • 4th part:
  • 5th part: ‘ | FastCli -p15 -e’

The ” need to be outside the ‘ region because the commands need to be between ” for the docker command. The 3rd part will expand the variable commands.

I guess the author is using a different version of bash? This is mine

$ bash --version
GNU bash, version 5.0.16(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later 

This is free software; you are free to change and redistribute it.

LVM 102: pvresize

Something very basic but took me several hours to workout. I had a VM that I wanted to increase a VG as I wanted to create a new LV. I increased the partition in the host server so the PV of the VG had the extra space, but then I couldnt see the increase inside the VM:

[root@HOST]# lvs
  LV      VG  Attr  LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  vm_data vg_os -wi-ao---- 300.00g

[root@VM]# pvs
  PV         VG      Fmt  Attr PSize    PFree
  /dev/vdb   vg_data lvm2 a--  <200.00g 1020.00m

"fdisk" was telling me the disk was already 300G...

[root@VM ~]# fdisk /dev/vdb

Welcome to fdisk (util-linux 2.32.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

The old LVM2_member signature will be removed by a write command.

Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0xd46fa2fc.

Command (m for help): p
Disk /dev/vdb: 300 GiB, 322122547200 bytes, 629145600 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xd46fa2fc

I did a pvscan... and nothing. What I was missing? just "pvresize".... and then I can see my extra 100G in the PV and in the VG. So I can create the new LV I wanted...

[root@VM ~]# pvresize /dev/vdb
  Physical volume "/dev/vdb" changed
  1 physical volume(s) resized or updated / 0 physical volume(s) not resized
[root@VM ~]# pvs
  PV         VG      Fmt  Attr PSize    PFree   
  /dev/vdb   vg_data lvm2 a--  <300.00g <101.00g
[root@VM ~]# 
[root@VM ~]# vgs
  VG      #PV #LV #SN Attr   VSize    VFree   
  vg_data   1   1   0 wz--n- <300.00g <101.00g

TCP Congestion Control and Recovery

I have reading this new post from Cloudflare about their congestion control implementations for QUIC.

Reading the article I wanted to check the TCP CCA (Congestion Control Algorithm) available in my laptop (Debian 1o Testing).

So I searched a bit and found a couple of useful links like this:

For checking your current TCP CCA:

# sysctl net.ipv4.tcp_congestion_control
net.ipv4.tcp_congestion_control = cubic

$ cat /proc/sys/net/ipv4/tcp_congestion_control

For checking the available TCP CCAs:

# sysctl net.ipv4.tcp_available_congestion_control
net.ipv4.tcp_available_congestion_control = reno cubic

As well, you can see via “ss” the CCA per connection:

$ ss -ti
tcp   ESTAB      0       0                                             
	 cubic wscale:7,7 rto:320 rtt:116.813/2.428 ato:40 mss:1448 pmtu:1500 rcvmss:1448 advmss:1448 cwnd:10 bytes_sent:4366 bytes_acked:4367 bytes_received:7038 segs_out:98 segs_in:183 data_segs_out:91 data_segs_in:93 send 991.7Kbps lastsnd:1260 lastrcv:1260 lastack:1140 pacing_rate 2.0Mbps delivery_rate 102.2Kbps delivered:92 app_limited busy:10632ms rcv_space:14480 rcv_ssthresh:64088 minrtt:113.391

If you want to change your TCP CCA, this is a good link:

Check the modules installed:

$ ls -la /lib/modules/$(uname -r)/kernel/net/ipv4

Check the kernel config:

$ grep TCP_CONG /boot/config-$(uname -r)

We can see that “cubic” is the default TCP CCA and we have for example BBR available as a module.

So let’s change to BBR (rfc, github, blog) based on this link:

Check the kernel supports BBR:

$ cat /boot/config-$(uname -r) | grep 'CONFIG_TCP_CONG_BBR'
$ cat /boot/config-$(uname -r) | grep 'CONFIG_NET_SCH_FQ'

Enable TCP BBR:

# vi /etc/sysctl.conf

Apply the changes:

# sysctl --system

And check:

$ cat /proc/sys/net/ipv4/tcp_congestion_control

So we have moved from CUBIC to BBR. Let’s see how is the experience in the following days.

Vim + Python Linters

I was reading an article about tools to write python using VIM with good formatting. I am not a pro-user of VIM neither a pro-python programmer but I would like to be more efficient and write better formatted python code.

So this is the link I was reading and ended here for the specific details.

At the end, my goal is to use more often inside VIM: splits (:sp), nerdtree (file browsing), autocompletion, git and be sure my code is formatted automatically if I make a mistake.

So I enabled most of the plugins from the article although I made some tweaks for me normal usage (I already had enabled some pluggins for jinja2). This is my .vimrc:

set nocompatible              " be iMproved, required
filetype off                  " required

" set the runtime path to include Vundle and initialize
set rtp+=~/.vim/bundle/Vundle.vim

set splitbelow
set splitright
"split navigations

" Enable folding
set foldmethod=indent
set foldlevel=99
" Enable folding with the spacebar
nnoremap  za

" Automatic formating for tab, whitespace and max 80 chars per line, etc
au BufNewFile,BufRead *.py
    \ set tabstop=4 |
    \ set softtabstop=4 |
    \ set shiftwidth=4 |
    \ set textwidth=79 |
    \ set expandtab |
    \ set autoindent |
    \ set fileformat=unix

highlight BadWhitespace ctermbg=red guibg=darkred
au BufRead,BufNewFile *.py,*.pyw,*.c,*.h match BadWhitespace /\s\+$/

set encoding=utf-8

let python_highlight_all=1


call vundle#begin()
" alternatively, pass a path where Vundle should install plugins
" call vundle#begin('~/some/path/here')

" let Vundle manage Vundle, required
Plugin 'VundleVim/Vundle.vim'
" added nerdtree
Plugin 'scrooloose/nerdtree'

Plugin 'ctrlp.vim'

Plugin 'Jinja'

Plugin 'tmhedberg/SimpylFold'

Plugin 'vim-scripts/indentpython.vim'

Plugin 'vim-syntastic/syntastic'

Plugin 'nvie/vim-flake8'

Plugin 'tpope/vim-fugitive'

Plugin 'Lokaltog/powerline', {'rtp': 'powerline/bindings/vim/'}

" Keep Plugin commands between vundle#begin/end.
" All of your Plugins must be added before the following line
call vundle#end()            " required

filetype plugin indent on    " required
syntax on

nmap  :NERDTreeToggle

" To ignore plugin indent changes, instead use:
"filetype plugin on
" Brief help
" :PluginList       - lists configured plugins
" :PluginInstall    - installs plugins; append `!` to update or just :PluginUpdate
" :PluginSearch foo - searches for foo; append `!` to refresh local cache
" :PluginClean      - confirms removal of unused plugins; append `!` to auto-approve removal
" see :h vundle for more details or wiki for FAQ
" Put your non-Plugin stuff after this line

au BufNewFile,BufRead *.lmx set filetype=xml
au BufNewFile,BufRead *.dump set filetype=sql
au BufNewFile,BufRead *.j2 set filetype=jinja

"for case-insensitve searches"
set ignorecase

"Override the 'ignorecase' option if the search pattern contains upper"
"case characters.  Only used when the search pattern is typed and"
"'ignorecase' option is on."
set smartcase

" I want to be able to resize the splits quickly so I want the mouse on
set mouse=a

" Always show statusline - This makes powerline always on
set laststatus=2

" autocmd vimenter * NERDTree

BTW, a quick reference for NerdTree (file browser) here.

Let’s see how I get on with these changes.


I am reading The General Theory of Employment, Interest and Money of John Maynard Keynes. The intro is really good and give you a good glimpse of Keynes’ life. The book itself is quite hard for me to digest. It is not my field but I am courius about Economics/Finance. One day, I would like to understand how money really works…. I think I bought the book after references from other books from Yanis Varoufakis.

Things I have learned he was against the Treaty of Versailles as the conditions over Germany were too abusive and that could cause an economic disaster for future generations. Well, you have created the easiest scenario for a crazy guy to play with the feelings of a proud nation to raise and get back what it was theirs…. Yeah, very simplify, but it works in my head. He wrote about it in “Consequences of the Peace”.

I think we have seen similar scenarios (without war involved thanks x (x = choose your god’s religion)) with Greece and Argentina.

Then after WWII, he helped to create the “International Monetary Fund” and “World Bank”. And attended Bretton Woods Conference. One of the important points of that conference was the fix rate gold/dollar and the lack of banking crisis as a consequence of this agreement. As well, I think the book from “The Signal and the Noise” says that during this conference, the American president couldnt understand Keynes ideas as he couldnt explain them easily. That conference is important because when the deal was cancelled in 1971 it caused some interesting economic shocks (books from Varoufakis are good) as countries couldn’t change dollar/gold as before and the banking crisis started to roll.

As per my understaning, Keynes believed that economic balance (mainly low unemployment) was the basic of a peaceful world. If you have most of your needs satisfied, I dont think you will pick your rifle whenever a lunatic tells you to do so.

As well, he was pro-European…. look now, we are in Brexit times…

…look now… it seems unemployment is going to skyrocket all around the world…. and we have too many dumb presidents in too powerful countries…. look now… how far right has been gathering support in the last years….

All the ingredientes for the cocktel of W..