Why is adding a loopback rule needed for internet to work when your iptables policy on INPUT is set to DROP?
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0
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I don't quite get why it's needed. Say you have the following iptables policies:
INPUT (DROP)
-m conntrack --ctstate ESTABLISHED -j ACCEPT
FORWARD (DROP)
None
OUTPUT (DROP)
Rules to allow 443, 80, 53
Trying to access any site with the web browser results in no success with this ruleset but if you add the rule
-i lo -j ACCEPT
to INPUT it starts working.
Basically, I want to know the architectural/design reason as to why this has to be a thing.
linux networking firewall iptables
asked Dec 3 at 7:35
Mystes
11
add a comment |
up vote
0
down vote
favorite
I don't quite get why it's needed. Say you have the following iptables policies:
INPUT (DROP)
-m conntrack --ctstate ESTABLISHED -j ACCEPT
FORWARD (DROP)
None
OUTPUT (DROP)
Rules to allow 443, 80, 53
Trying to access any site with the web browser results in no success with this ruleset but if you add the rule
-i lo -j ACCEPT
to INPUT it starts working.
Basically, I want to know the architectural/design reason as to why this has to be a thing.
linux networking firewall iptables
asked Dec 3 at 7:35
Mystes
11
1
The loopback device is for network connections between local applications (HTTP proxy, DNS proxy). If you disallow any of those with a default policy of DROP, but if your browser needs any of these, it won't work. To see what actually goes over the loopback device, run Wireshark etc. on it and have a look, then you'll know what it is that the browser requires.
– dirkt
Dec 3 at 7:49
Thank you very much for that answer. Guess I'll start digging next week XD I'll get back to you and hopefully come back with conclusive results and by then, I hope you put your comment as an answer so I can mark it as the solution.
– Mystes
Dec 5 at 3:53
It would be nice to have an answer which explains which traffic was actually going over the loopback interface in your case, which is why I just did a comment. BTW, you can also answer your own question. If you want to cut&paste snooped traffic into the question/answer, usetcpdumpinstead ofwireshark(if there's a good textual export from wireshark, I somehow missed it).
– dirkt
Dec 5 at 6:58
add a comment |
up vote
0
down vote
favorite
up vote
0
down vote
favorite
I don't quite get why it's needed. Say you have the following iptables policies:
INPUT (DROP)
-m conntrack --ctstate ESTABLISHED -j ACCEPT
FORWARD (DROP)
None
OUTPUT (DROP)
Rules to allow 443, 80, 53
Trying to access any site with the web browser results in no success with this ruleset but if you add the rule
-i lo -j ACCEPT
to INPUT it starts working.
Basically, I want to know the architectural/design reason as to why this has to be a thing.
linux networking firewall iptables
asked Dec 3 at 7:35
Mystes
11
I don't quite get why it's needed. Say you have the following iptables policies:
INPUT (DROP)
-m conntrack --ctstate ESTABLISHED -j ACCEPT
FORWARD (DROP)
None
OUTPUT (DROP)
Rules to allow 443, 80, 53
Trying to access any site with the web browser results in no success with this ruleset but if you add the rule
-i lo -j ACCEPT
to INPUT it starts working.
Basically, I want to know the architectural/design reason as to why this has to be a thing.
linux networking firewall iptables
linux networking firewall iptables
asked Dec 3 at 7:35
Mystes
11
asked Dec 3 at 7:35
Mystes
11
asked Dec 3 at 7:35
Mystes
11
asked Dec 3 at 7:35
Mystes
11
asked Dec 3 at 7:35
Mystes
11
11
1
The loopback device is for network connections between local applications (HTTP proxy, DNS proxy). If you disallow any of those with a default policy of DROP, but if your browser needs any of these, it won't work. To see what actually goes over the loopback device, run Wireshark etc. on it and have a look, then you'll know what it is that the browser requires.
– dirkt
Dec 3 at 7:49
Thank you very much for that answer. Guess I'll start digging next week XD I'll get back to you and hopefully come back with conclusive results and by then, I hope you put your comment as an answer so I can mark it as the solution.
– Mystes
Dec 5 at 3:53
It would be nice to have an answer which explains which traffic was actually going over the loopback interface in your case, which is why I just did a comment. BTW, you can also answer your own question. If you want to cut&paste snooped traffic into the question/answer, usetcpdumpinstead ofwireshark(if there's a good textual export from wireshark, I somehow missed it).
– dirkt
Dec 5 at 6:58
add a comment |
1
The loopback device is for network connections between local applications (HTTP proxy, DNS proxy). If you disallow any of those with a default policy of DROP, but if your browser needs any of these, it won't work. To see what actually goes over the loopback device, run Wireshark etc. on it and have a look, then you'll know what it is that the browser requires.
– dirkt
Dec 3 at 7:49
Thank you very much for that answer. Guess I'll start digging next week XD I'll get back to you and hopefully come back with conclusive results and by then, I hope you put your comment as an answer so I can mark it as the solution.
– Mystes
Dec 5 at 3:53
It would be nice to have an answer which explains which traffic was actually going over the loopback interface in your case, which is why I just did a comment. BTW, you can also answer your own question. If you want to cut&paste snooped traffic into the question/answer, usetcpdumpinstead ofwireshark(if there's a good textual export from wireshark, I somehow missed it).
– dirkt
Dec 5 at 6:58
1
1
The loopback device is for network connections between local applications (HTTP proxy, DNS proxy). If you disallow any of those with a default policy of DROP, but if your browser needs any of these, it won't work. To see what actually goes over the loopback device, run Wireshark etc. on it and have a look, then you'll know what it is that the browser requires.
– dirkt
Dec 3 at 7:49
The loopback device is for network connections between local applications (HTTP proxy, DNS proxy). If you disallow any of those with a default policy of DROP, but if your browser needs any of these, it won't work. To see what actually goes over the loopback device, run Wireshark etc. on it and have a look, then you'll know what it is that the browser requires.
– dirkt
Dec 3 at 7:49
Thank you very much for that answer. Guess I'll start digging next week XD I'll get back to you and hopefully come back with conclusive results and by then, I hope you put your comment as an answer so I can mark it as the solution.
– Mystes
Dec 5 at 3:53
Thank you very much for that answer. Guess I'll start digging next week XD I'll get back to you and hopefully come back with conclusive results and by then, I hope you put your comment as an answer so I can mark it as the solution.
– Mystes
Dec 5 at 3:53
It would be nice to have an answer which explains which traffic was actually going over the loopback interface in your case, which is why I just did a comment. BTW, you can also answer your own question. If you want to cut&paste snooped traffic into the question/answer, use
tcpdump instead of wireshark (if there's a good textual export from wireshark, I somehow missed it).– dirkt
Dec 5 at 6:58
It would be nice to have an answer which explains which traffic was actually going over the loopback interface in your case, which is why I just did a comment. BTW, you can also answer your own question. If you want to cut&paste snooped traffic into the question/answer, use
tcpdump instead of wireshark (if there's a good textual export from wireshark, I somehow missed it).– dirkt
Dec 5 at 6:58
add a comment |
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1
The loopback device is for network connections between local applications (HTTP proxy, DNS proxy). If you disallow any of those with a default policy of DROP, but if your browser needs any of these, it won't work. To see what actually goes over the loopback device, run Wireshark etc. on it and have a look, then you'll know what it is that the browser requires.
– dirkt
Dec 3 at 7:49
Thank you very much for that answer. Guess I'll start digging next week XD I'll get back to you and hopefully come back with conclusive results and by then, I hope you put your comment as an answer so I can mark it as the solution.
– Mystes
Dec 5 at 3:53
It would be nice to have an answer which explains which traffic was actually going over the loopback interface in your case, which is why I just did a comment. BTW, you can also answer your own question. If you want to cut&paste snooped traffic into the question/answer, use
tcpdumpinstead ofwireshark(if there's a good textual export from wireshark, I somehow missed it).– dirkt
Dec 5 at 6:58