Linux bridge SNAT with ebtables
Clash Royale CLAN TAG#URR8PPP
I was reading the following code (dot1x bridge bypass):
https://github.com/mubix/8021xbridge/blob/master/scripts/trans_bridge.prepop
A quick description, we have 2 interfaces, the first is connected to a switch ($SWINT) and the other to our target host. The goal is to
interact with the rest of the network, while making it look like the traffic is generated by the target host.
From the referenced code, what I am trying to figure out is why both of the below commands are needed:
# use ebtables to source NAT the $COMPMAC for traffic leaving the device from the bridge mac address
(A)ebtables -t nat -A POSTROUTING -s $SWMAC -o $SWINT -j snat --to-src $COMPMAC
(B)ebtables -t nat -A POSTROUTING -s $SWMAC -o $BRINT -j snat --to-src $COMPMAC
I don't get why it's needed to SNAT the POSTROUTING traffic of $BRINT. Since our goal is to avoid transmitting any frames towards switch side interface with source MAC different than the target host, shouldn't the first ebtables rule be suffice?
Or in other words, isn't $SWINT (switch side interface) the only way any frames are getting to the switch? Based on the diagram below, the rule (B) looks redundant since we only care for frames going towards eth0 (SWINT), and we already apply SNAT to eth0.
Some good resources I found on bridges (but still haven't answered my question) are:
Understanding Linux Network Internals
Anatomy of Linux Bridge
ebtables with Linux Bridge
https://doc.lagout.org/operating%20system%20/linux/Understanding%20Linux%20Network%20Internals.pdf Page:359
firewall bridge
asked Jan 13 at 20:21
gipgip
665
add a comment |
I was reading the following code (dot1x bridge bypass):
https://github.com/mubix/8021xbridge/blob/master/scripts/trans_bridge.prepop
A quick description, we have 2 interfaces, the first is connected to a switch ($SWINT) and the other to our target host. The goal is to
interact with the rest of the network, while making it look like the traffic is generated by the target host.
From the referenced code, what I am trying to figure out is why both of the below commands are needed:
# use ebtables to source NAT the $COMPMAC for traffic leaving the device from the bridge mac address
(A)ebtables -t nat -A POSTROUTING -s $SWMAC -o $SWINT -j snat --to-src $COMPMAC
(B)ebtables -t nat -A POSTROUTING -s $SWMAC -o $BRINT -j snat --to-src $COMPMAC
I don't get why it's needed to SNAT the POSTROUTING traffic of $BRINT. Since our goal is to avoid transmitting any frames towards switch side interface with source MAC different than the target host, shouldn't the first ebtables rule be suffice?
Or in other words, isn't $SWINT (switch side interface) the only way any frames are getting to the switch? Based on the diagram below, the rule (B) looks redundant since we only care for frames going towards eth0 (SWINT), and we already apply SNAT to eth0.
Some good resources I found on bridges (but still haven't answered my question) are:
Understanding Linux Network Internals
Anatomy of Linux Bridge
ebtables with Linux Bridge
https://doc.lagout.org/operating%20system%20/linux/Understanding%20Linux%20Network%20Internals.pdf Page:359
firewall bridge
asked Jan 13 at 20:21
gipgip
665
add a comment |
I was reading the following code (dot1x bridge bypass):
https://github.com/mubix/8021xbridge/blob/master/scripts/trans_bridge.prepop
A quick description, we have 2 interfaces, the first is connected to a switch ($SWINT) and the other to our target host. The goal is to
interact with the rest of the network, while making it look like the traffic is generated by the target host.
From the referenced code, what I am trying to figure out is why both of the below commands are needed:
# use ebtables to source NAT the $COMPMAC for traffic leaving the device from the bridge mac address
(A)ebtables -t nat -A POSTROUTING -s $SWMAC -o $SWINT -j snat --to-src $COMPMAC
(B)ebtables -t nat -A POSTROUTING -s $SWMAC -o $BRINT -j snat --to-src $COMPMAC
I don't get why it's needed to SNAT the POSTROUTING traffic of $BRINT. Since our goal is to avoid transmitting any frames towards switch side interface with source MAC different than the target host, shouldn't the first ebtables rule be suffice?
Or in other words, isn't $SWINT (switch side interface) the only way any frames are getting to the switch? Based on the diagram below, the rule (B) looks redundant since we only care for frames going towards eth0 (SWINT), and we already apply SNAT to eth0.
Some good resources I found on bridges (but still haven't answered my question) are:
Understanding Linux Network Internals
Anatomy of Linux Bridge
ebtables with Linux Bridge
https://doc.lagout.org/operating%20system%20/linux/Understanding%20Linux%20Network%20Internals.pdf Page:359
firewall bridge
asked Jan 13 at 20:21
gipgip
665
I was reading the following code (dot1x bridge bypass):
https://github.com/mubix/8021xbridge/blob/master/scripts/trans_bridge.prepop
A quick description, we have 2 interfaces, the first is connected to a switch ($SWINT) and the other to our target host. The goal is to
interact with the rest of the network, while making it look like the traffic is generated by the target host.
From the referenced code, what I am trying to figure out is why both of the below commands are needed:
# use ebtables to source NAT the $COMPMAC for traffic leaving the device from the bridge mac address
(A)ebtables -t nat -A POSTROUTING -s $SWMAC -o $SWINT -j snat --to-src $COMPMAC
(B)ebtables -t nat -A POSTROUTING -s $SWMAC -o $BRINT -j snat --to-src $COMPMAC
I don't get why it's needed to SNAT the POSTROUTING traffic of $BRINT. Since our goal is to avoid transmitting any frames towards switch side interface with source MAC different than the target host, shouldn't the first ebtables rule be suffice?
Or in other words, isn't $SWINT (switch side interface) the only way any frames are getting to the switch? Based on the diagram below, the rule (B) looks redundant since we only care for frames going towards eth0 (SWINT), and we already apply SNAT to eth0.
Some good resources I found on bridges (but still haven't answered my question) are:
Understanding Linux Network Internals
Anatomy of Linux Bridge
ebtables with Linux Bridge
https://doc.lagout.org/operating%20system%20/linux/Understanding%20Linux%20Network%20Internals.pdf Page:359
firewall bridge
firewall bridge
asked Jan 13 at 20:21
gipgip
665
asked Jan 13 at 20:21
gipgip
665
edited 2 days ago
gip
asked Jan 13 at 20:21
gipgip
665
asked Jan 13 at 20:21
gipgip
665
asked Jan 13 at 20:21
gipgip
665
665
add a comment |
add a comment |
0
active
oldest
votes
Your Answer
StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "106"
;
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function()
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled)
StackExchange.using("snippets", function()
createEditor();
);
else
createEditor();
);
function createEditor()
StackExchange.prepareEditor(
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: false,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: null,
bindNavPrevention: true,
postfix: "",
imageUploader:
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
,
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
);
);
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f494302%2flinux-bridge-snat-with-ebtables%23new-answer', 'question_page');
);
Post as a guest
Required, but never shown
0
active
oldest
votes
0
active
oldest
votes
active
oldest
votes
active
oldest
votes
Thanks for contributing an answer to Unix & Linux Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f494302%2flinux-bridge-snat-with-ebtables%23new-answer', 'question_page');
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
