How to Unblock IP Address of Email Client in Linux iptables Firewall in Linux Mail Server

Subject: How to Unblock IP Address of Email Client in Linux iptables Firewall in Linux Mail Server

Good day from Singapore,

I am an IT consultant in Singapore. Today, Black Friday 27 November 2020, my client Donald Trump (fictitious name used) informed me that he cannot send and receive emails using his home Wi-Fi connection. But he could send and receive emails using his mobile phone 4G connection.

Apparently, the public IP address of his home internet connection was blocked. The IP address is 666.666.666.666 (fictitious IP address used).

I checked the Fortigate 100D firewall but it was not blocked there.

Then I proceed to check the Linux iptables firewall on my client's Linux mail server. I login/putty/ssh into mail.teo-en-ming-corp.com.sg, which is my client's Linux mail server.

I executed the following command:

# iptables -S | grep 666.666.666.666

The output is:

-A DENYIN -s 666.666.666.666/32 ! -i lo -j DROP

-A DENYOUT -d 666.666.666.666/32 ! -o lo -j LOGDROPOUT

I proceeded to delete the above 2 Linux iptables firewall rules which blocked the IP address 666.666.666.666, using the following commands:

# iptables -D DENYIN -s 666.666.666.666/32 ! -i lo -j DROP

# iptables -D DENYOUT -d 666.666.666.666/32 ! -o lo -j LOGDROPOUT

Then I run the following command again to make sure the IP address has been unblocked.

# iptables -S | grep 666.666.666.666

There should be no output, confirming that the IP address has been unblocked.

Now my client Donald Trump is able to send and receive emails using his home Wi-fi connection successfully.

The author, Mr. Turritopsis Dohrnii Teo En Ming, 42 years as of 27 Nov 2020 Friday, is a TARGETED INDIVIDUAL (TI) living in Singapore.

Reference Guide: How To List and Delete Iptables Firewall Rules

Link: https://www.digitalocean.com/community/tutorials/how-to-list-and-delete-iptables-firewall-rules

REFERENCES

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[1] https://marc.info/?l=netfilter&m=160646592704155&w=2

[2] https://lkml.org/lkml/2020/11/27/89

[3] http://lkml.iu.edu/hypermail/linux/kernel/2011.3/03852.html

[4] https://marc.info/?l=linux-kernel&m=160646564204070&w=2

[5] https://lwn.net/ml/linux-kernel/841285c108f960bee81fd107dbc3798f%40teo-en-ming.com/

[6] http://lists.linuxfromscratch.org/pipermail/lfs-chat/2020-November/029163.html