Postfix Cheatsheet

Check the queue

mailq – Check emails queue (then with ! are on hold)

Move all deferred messages to the hold queue

postsuper -h ALL deferred

Release 200 emails from holding back to deferred

mailq | awk ‘ /^[0-9A-F][0-9A-F]/ {print $1}’ | grep ! | head -n 200 | tr -d ‘!’ | postsuper -H –

Delete messages from the hold queue sent by ‘’

mailq | awk ‘ /^[0-9A-F][0-9A-F]*.*domain\.com$/ {print $1}’ | tr -d ‘!’ | postsuper -d –

Delete messages from the hold queue sent by ‘’

mailq | awk ‘ / example.domain\.com$/ {print $1}’ | tr -d ‘!’ | postsuper -d –

mailq | grep -E “^[A-Z0-9]+” | grep | cut -c1-12 | postsuper -d –

postqueue -p | tail -n +2 | awk ‘BEGIN { RS = “” } / example@domain\.com/ { print $1 }’ | tr -d ‘*!’ | postsuper -d –

Move 100 messages in the hold queue back to deferred

mailq | awk ‘ /^[0-9A-F][0-9A-F]/ {print $1}’ | grep ! | head -n 100 | tr -d ‘!’ | postsuper -H –

Block an IP/Subnet from sending email

Add a /etc/postfix/access file with content like the following. The first line blocks a single IP, the second blocks a subnet


Run ‘postmap /etc/postfix/access’ to create the lookup table.

Add the following line into /etc/postfix/

smtpd_client_restrictions = check_client_access hash:/etc/postfix/access, permit

The ‘permit’ statement at the end is very important to allow other email.

Reload postfix to apply the block.

Cisco CCENT Notes

Please find my notes below for CCENT exam.

  1. OSPF (Open Shortest Path First) uses cost as metric.
  2. Dotted-decimal equivalent of
    1. /15 is
    2. /16
    3. /24
    4. /32
  3. OSPF  is NOT called an advance distance-vector protocol
    1. EIGRP , RIP1, RIP2, Babel is an advanced distance-vector protocol
    2. OSPF OR IS-IS is link state router protocol 
    3. Link-state protocol is performed by every switching node in the network. The concept is that every node construct a map of connectivity to the network, in the form of graph, showing which nodes are connected to which other nodes. each node then independently calculates the next best logical path from it to every possible destination in the network. The collection of best paths will then form the node’s routing table.
    4. Distance-vector routing protocols, which work by having the each node share it’s routing table with it’s neighbors. in the link-state protocol the only information passed between nodes is connectivity related. The link-sate algorithms are sometimes called characterized informally as each routers “telling the world about it’s neighbors”.
  4. Area border routers are the routers which are in a least two areas in the same time. Any router that is member of two areas in an area border router, or ABR, by definition.
  5. show processes cpu – can be used  to determine a router’s capacity to generate debug output.
  6. Source and destination addresses, source and destination ports and protocol is used in the IP extended access list for permitting or denying packets.
  7. Backoff on a an Ethernet network is the retransmission delay that’s enforced when a collision occurs. n a variety of computer networks, binary exponential backoff or truncated binary exponential backoff refers to an algorithm used to space out repeated retransmissions of the same block of data, often as part of network congestion avoidance.
  8. show access-lists – allow to view the entire content of all access list.
  9. SSH – application layer protocol sets up a secure session that’s similar to Telnet.
  10. Only 1 collision domains  exists when a hub is used.
  11. L type is /32 mask in routes in the routing table.
  12. bandwidth 64 – command sets the bandwidth metric of Frame Relay connection to 64000 bps.
  13. FCS section is the Error detection in the ethernet II Frame
  14. Incrementing of runts on the interface is caused by collisions 
  15. False. Layer 2 switches and bridges are faster than routers because they don’t take up time looking at the Data Link layer header information.
  16. The logging synchronous command causes the system to return you to your previous promt after displaying