Month: October 2014

TFTP server on Debian

Setting up a TFTP server in Linux is easy. In this case example, I am using Debian. We’re going to install HPA’s TFTP server.

Before installing anything, you should always check if there are existing packages installed. The example below uses aptitude to find out if packages have been installed and it looks like I have tftpd installed with the “i” indicator.


root@tftp-server:~# aptitude search tftpd
p   atftpd                                                                      - advanced TFTP server
p   libnet-tftpd-perl                                                           - Perl extension for Trivial File Transfer Protocol Server
p   tftpd                                                                       - Trivial file transfer protocol server
i   tftpd-hpa                                                                   - HPA's tftp server

Another way is checking the /etc and /etc/default directory if there is anything related to TFTP. Next, check if you have a service related to tftp that is running. If you do, stop the service so you can uninstall it.


root@tftp-server:~# ps aux | grep tftp
root 4390 0.0 0.3 7832 884 pts/0 S+ 20:39 0:00 grep tftp

Tftpd did not work for me so I’m going to remove it.


root@tftp-server:~# aptitude remove tftpd
The following packages will be REMOVED:
libfile-copy-recursive-perl{u} openbsd-inetd{u} tftpd update-inetd{u}
0 packages upgraded, 0 newly installed, 4 to remove and 30 not upgraded.
Need to get 0 B of archives. After unpacking 302 kB will be freed.
Do you want to continue? [Y/n/?] Y
(Reading database ... 38720 files and directories currently installed.)
Removing tftpd ...
Removing openbsd-inetd ...
[ ok ] Stopping internet superserver: inetd.
Removing update-inetd ...
Removing libfile-copy-recursive-perl ...
Processing triggers for man-db ...

Let’s proceed with the install.


root@tftp-server:~# apt-get install tftpd-hpa
Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
  pxelinux
The following NEW packages will be installed:
  tftpd-hpa
0 upgraded, 1 newly installed, 0 to remove and 1 not upgraded.
Need to get 50.7 kB of archives.
After this operation, 145 kB of additional disk space will be used.
Get:1 http://ftp.us.debian.org/debian/ jessie/main tftpd-hpa amd64 5.2+20140608-3                                                                                        [50.7 kB]
Fetched 50.7 kB in 0s (403 kB/s)
Preconfiguring packages ...
Selecting previously unselected package tftpd-hpa.
(Reading database ... 31430 files and directories currently installed.)
Preparing to unpack .../tftpd-hpa_5.2+20140608-3_amd64.deb ...
Unpacking tftpd-hpa (5.2+20140608-3) ...
Processing triggers for man-db (2.7.0.2-5) ...
Processing triggers for systemd (215-17+deb8u5) ...
Setting up tftpd-hpa (5.2+20140608-3) ...
Processing triggers for systemd (215-17+deb8u5) ...
root@tftp-server:~# 

Check if the directory /srv/tftp is created. It should look like this:


root@tftp-server:~# ls -al /srv/
total 12
drwxr-xr-x  3 root root    4096 Nov 11 12:04 .
drwxr-xr-x 22 root root    4096 Nov  9 13:03 ..
drwxr-xr-x  2 root nogroup 4096 Nov 11 12:04 tftp
root@tftp-server:~#

If /srv/tftp directory does not exist or if it does but not does not have the right permissions, you can follow these steps

If /srv/tftp does not exist, create the TFTP root directory for your TFTP server


root@tftp-server:~# cd /srv
root@tftp-server:~# mkdir tftp

Adjust the permissions for the new directory. Open it for everyone.

root@tftp-server:~# chmod 777 /srv/tftp

I’m logged in as root when I created the directory or when I installed the package. This makes the owner root by default. Change the owner of the directory to nobody.


root@tftp-server:/srv# chown nobody:nogroup tftp
root@tftp-server:/srv# ls -al
total 12
drwxr-xr-x  3 root   root    4096 Nov 11 12:04 .
drwxr-xr-x 22 root   root    4096 Nov  9 13:03 ..
drwxrwxrwx  2 nobody nogroup 4096 Nov 11 12:28 tftp

Verify if the TFTP service is running.


root@tftp-server:~# ps aux | grep tftp
root 5847 0.0 0.0 14860 148 ? Ss 20:39 0:00 /usr/sbin/in.tftpd --listen --user tftp --address 0.0.0.0:69 --secure /var/lib/tftpboot
root 5877 0.0 0.3 7832 880 pts/0 S+ 20:41 0:00 grep tftp
root@tftp-server:/etc/default# service tftpd-hpa
Usage: /etc/init.d/tftpd-hpa {start|stop|restart|force-reload|status}
root@tftp-server:~t# service tftpd-hpa status
[ ok ] in.tftpd is running.

The installation of tftpd-hpa created a configuration file located in /etc/default.

To allow upload of new files to the tftp-server, adjust the configuration file /etc/default/tftpd-hpa. Basically, insert -c into TFTP_OPTIONS and set the TFTP_DIRECTORY to point to the directory you created earlier. The configuration should look more like:


# /etc/default/tftpd-hpa
TFTP_USERNAME="tftp"
TFTP_DIRECTORY="/srv/tftp"
TFTP_ADDRESS="0.0.0.0:69"
TFTP_OPTIONS="--secure -c"

Restarting is done with service tftpd-hpa restart.


root@tftp-server:~# service tftpd-hpa restart
[ ok ] Restarting HPA's tftpd: in.tftpd.

Check the status again


root@tftp-server:/etc/default# service tftpd-hpa status
● tftpd-hpa.service - LSB: HPA's tftp server
   Loaded: loaded (/etc/init.d/tftpd-hpa)
   Active: active (running) since Fri 2016-11-11 12:14:57 CST; 28s ago
  Process: 3466 ExecStop=/etc/init.d/tftpd-hpa stop (code=exited, status=0/SUCCESS)
  Process: 3471 ExecStart=/etc/init.d/tftpd-hpa start (code=exited, status=0/SUCCESS)
   CGroup: /system.slice/tftpd-hpa.service
           └─3477 /usr/sbin/in.tftpd --listen --user tftp --address 0.0.0.0:69 --secure -c /srv/tftp

Nov 11 12:14:57 tftp-server tftpd-hpa[3471]: Starting HPA's tftpd: in.tftpd.
Nov 11 12:14:57 tftp-server systemd[1]: Started LSB: HPA's tftp server.
root@tftp-server:/etc/default#

Most of your questions can be answered by checking the manual

root@tftpd-server:~# man tftpd

If you are using iptables for your firewall, you will need to add support for TFTP. Following is a simple example


IPTABLES=/sbin/iptables

#Load the modules that support TFTP
modprobe ip_conntrack_tftp
modprobe  ip_conntrack_ftp

#Allow TFTP requests from 192.168.1.0/24 network
$IPTABLES -A INPUT -s 192.168.1.0/24 -m tcp -p tcp --dport 69 -j ACCEPT
$IPTABLES -A INPUT -s 192.168.1.0/24 -m tcp -p udp --dport 69 -j ACCEPT

Try testing by creating a test.txt file and see if you can download that from your Cisco router and upload its IOS image, for example.

Filed under: Debian, Linux, Ubuntu

TCP header compression on a Cisco router

TCP header compression is used to compress TCP headers in a network to save bandwidth on a link. However, TCP header compression comes at a cost in terms of processor time (delay/serialization delay).

Conditions: must be configured on both ends of the network to compress and decompress packets.

cisco_router1(config)# interface serial0/1
cisco_router1(config-if)# ip address 172.16.10.1 255.255.255.252
cisco_router1(config-if)# ip tcp header-compression

cisco_router2(config)# interface serial0/0/1
cisco_router2(config-if)# ip address 172.16.10.2 255.255.255.252
cisco_router2(config-if)# ip tcp header-compression

cisco_router1# show ip tcp header-compression

cisco_router2# show ip tcp header-compression

efficiency improvement factor = (bytes saved + bytes sent) / (bytes sent)

Filed under: Cisco, IOS, QoS

Check Debian version

There are a few ways to check what version of Debian or Debian-based Linux (Ubuntu, etc) that you are using.

cat /etc/debian_version

cat /etc/issue

lsb_release -a

This file may not be available.
cat /etc/lsb-release

Output will look like this

root@unknown_server:~# cat /etc/debian_version
7.5

root@unknown_server:~# cat /etc/issue
Debian GNU/Linux 7 \n \l

root@unknown_server:~# lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 7.5 (wheezy)
Release:        7.5
Codename:       wheezy

Filed under: Debian, Linux

Disconnect SSH session on a Cisco ASA


asa# show ssh sessions

SID Client IP       Version Mode Encryption Hmac     State            Username
2   192.168.35.6    2.0     IN   aes256-cbc sha1     SessionStarted   elton
                            OUT  aes256-cbc sha1     SessionStarted   elton
3   204.16.58.6     2.0     IN   aes256-cbc sha1     SessionStarted   admin
                            OUT  aes256-cbc sha1     SessionStarted   admin


Notice the SID 2 and 3. Session ID 3 belongs to the one logged in as admin. Let’s drop the hammer.


asa# ssh disconnect 3

Verify.


asa# show ssh sessions

SID Client IP       Version Mode Encryption Hmac     State            Username
2   192.168.35.6    2.0     IN   aes256-cbc sha1     SessionStarted   elton
                            OUT  aes256-cbc sha1     SessionStarted   elton

asa# show logging
Oct 03 2014 11:22:00: %ASA-5-111008: User 'enable_15' executed the 'ssh disconnect 3' command.
Oct 03 2014 11:22:00: %ASA-5-111010: User 'enable_15',running 'CLI' from IP 199.48.158.6, executed 'ssh disconnect 3'
Oct 03 2014 11:22:00: %ASA-5-611103: User logged out: Uname: admin

Yeah, fuck that guy. If that wasn’t anyone you know, time to change your passwords.

Filed under: ASA, Cisco, Firewall