net: Allow -proxy=[::1] on nodes with IPV6 lo only #30245

m3dwards · 2024-06-07T12:53:49Z

This is similar to (but does not fix) #13155 which I believe is the same issue but in libevent.

The issue is on a host that has IPV6 enabled but only a loopback IP address -proxy=[::1] will fail as [::1] is not considered valid by getaddrinfo with AI_ADDRCONFIG flag. I think the loopback interface should be considered valid and we have a functional test that will try to test this: feature_proxy.py.

To replicate the issue, run feature_proxy.py inside a docker container that has IPV6 loopback ::1 address without specifically giving that container an external IPV6 address. This should be the default with recent versions of docker. IPV6 on loopback interface was enabled in docker engine 26 and later (https://docs.docker.com/engine/release-notes/26.0/#bug-fixes-and-enhancements-2).

AI_ADDRCONFIG was introduced to prevent slow DNS lookups on systems that were IPV4 only.

References:

Man section on AI_ADDRCONFIG:

If hints.ai_flags includes the AI_ADDRCONFIG flag, then IPv4 addresses are returned in the list pointed to by res only if the local system has at least one IPv4 address configured, and  IPv6  addresses
       are  returned only if the local system has at least one IPv6 address configured.  The loopback address is not considered for this case as valid as a configured address.  This flag is useful on, for ex‐
       ample, IPv4-only systems, to ensure that getaddrinfo() does not return IPv6 socket addresses that would always fail in connect(2) or bind(2).

AI_ADDRCONFIG considered harmful Wiki entry by Fedora

Mozilla discussing slow DNS without AI_ADDRCONFIG and also localhost issues with it

DrahtBot · 2024-06-07T12:53:51Z

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Type	Reviewers
ACK	tdb3, pinheadmz, achow101
Stale ACK	vasild

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vasild

Concept ACK on allowing usage of [::1] on nodes with IPv6 lo only.

A workaround is to use -dns=0 (which will avoid AI_ADDRCONFIG but will make it impossible to use hostnames).

I can reproduce the problem on Linux. FreeBSD treats the loopback address as valid, so this problem is non-existent on FreeBSD.

I think there is a merit in this for our use cases of getaddrinfo() / Lookup*():

IPv4 addresses are returned in the list pointed to by res only if the local system has at least one IPv4 address configured ... (and same for IPv6)

But not this:

The loopback address is not considered for this case as valid as a configured address. This flag is useful on, for example, IPv4-only systems, to ensure that getaddrinfo() does not return IPv6 socket addresses that would always fail in connect(2) or bind(2).

This is even bogus to some extent - if the system has only [::1] configured and we want to connect(2) or bind(2) to [::1] that will work. So the "always" is too strong / untrue.

Now, can we have the AI_ADDRCONFIG behavior but get it to consider loopback addresses as valid? For example, after running getaddrinfo() with AI_ADDRCONFIG run it again without AI_ADDRCONFIG and append any loopback addresses from the second run to the results?

src/netbase.cpp

m3dwards · 2024-06-14T16:39:51Z

Now, can we have the AI_ADDRCONFIG behavior but get it to consider loopback addresses as valid? For example, after running getaddrinfo() with AI_ADDRCONFIG run it again without AI_ADDRCONFIG and append any loopback addresses from the second run to the results?

I like this idea, I'll have a go at implementing it.

src/netbase.cpp

m3dwards · 2024-06-18T10:28:39Z

Pushed new approach of calling getaddrinfo twice. First pass is unchanged from original behaviour, second pass adds local IP addresses should they have not been found on the first pass.

vasild

Approach ACK a7cbc27

src/netbase.cpp

m3dwards · 2024-06-21T16:10:03Z

Thanks for your review @vasild, should have addressed all of your comments.

vasild

ACK 60a753b

Thank you!

pinheadmz

ACK 60a753b

Tested in an ubuntu docker container with local ipv6 but not external ipv6:

# ip address | grep inet 
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host 
    inet 172.17.0.2/16 brd 172.17.255.255 scope global eth0

Confirmed the feature_proxy test fails without the patch. Added extra logging to confirm the error:

getaddrinfo: ::1 error: Address family for hostname not supported

With the patch, getaddrinfo is called a second time with flags=0 and the call succeeds.

A few questions below.

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src/netbase.cpp

pinheadmz · 2024-07-01T16:35:09Z

src/netbase.cpp

+        if (n_err != 0) {
+            return;


You could check specifically for EAI_FAMILY and then only recheck without AI_ADDRCONFIG in that condition. I think the code you have now will always check everything twice, but ignore duplicates?

This comment from @pinheadmz about checking for the specific error we care about makes sense to me as well. It might make for a significantly smaller code change and prevent extraneous calls to getaddrinfo.

I looked into catching specifically EAI_ADDRFAMILY errors but this only applies on GNU Libc and is not POSIX. I checked windows and they have different errors. Rather than try to catalogue every possible system and libc implementation, the code now retries on any error (assuming you first tried with AI_ADDRCONFIG in the first place).

tdb3

Concept ACK
Good find. Left a couple of comments.

tdb3 · 2024-07-02T02:19:11Z

src/netbase.cpp

+        if (n_err != 0) {
+            return;


This comment from @pinheadmz about checking for the specific error we care about makes sense to me as well. It might make for a significantly smaller code change and prevent extraneous calls to getaddrinfo.

src/netbase.cpp

DrahtBot · 2024-07-03T15:44:48Z

🚧 At least one of the CI tasks failed. Make sure to run all tests locally, according to the
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_{Debug: https://github.com/bitcoin/bitcoin/runs/27000088118}

m3dwards · 2024-07-09T08:52:12Z

@pinheadmz @tdb3 I should have addressed all of your comments.

@vasild the change since your review is rather than always check twice and discard duplicates, we now only check on error. I chose not to look for the specific error as it was not POSIX and was different on different platforms.

tdb3

I think this is really close. Left one more comment.

src/netbase.cpp

AI_ADDRCONFIG prevents ::1 from being considered a valid address on hosts that have a IPV6 loopback IP address but no other IPV6 interfaces.

tdb3

ACK 23333b7

Tested with a similar approach to #30245 (review)

Created an LXC container with an external and loopback IPv4 address, and with a loopback IPv6 address.

root@test30245:~/bitcoin# ip a | grep inet
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host noprefixroute 
    inet 192.168.10.81/24 brd 192.168.10.255 scope global eth0

Inserted some extra log statments:

2024-07-15T12:06:09Z =============First try
2024-07-15T12:06:09Z =============n_err != 0
2024-07-15T12:06:09Z =============Trying a second time
2024-07-15T12:06:09Z =============Second time worked!

Appeared to work well.

Also synced approx 8000 blocks on signet with a tor proxy set up to listen on [::1]:9050 (-proxy=[::1] with bitcoind).

m3dwards · 2024-07-15T18:26:10Z

@pinheadmz @vasild

Ready for you to take a second look if you have any time.

pinheadmz

ACK 23333b7

Reviewed updated code change and tested in linux VM with local ipv6 only.

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pinheadmz · 2024-07-15T19:44:37Z

src/netbase.cpp

+        if ((ai_hint.ai_flags & AI_ADDRCONFIG) == AI_ADDRCONFIG) {
+            // AI_ADDRCONFIG on some systems may exclude loopback-only addresses
+            // If first lookup failed we perform a second lookup without AI_ADDRCONFIG
+            ai_hint.ai_flags = (ai_hint.ai_flags & ~AI_ADDRCONFIG);


If you need to retouch, you might consider using a local addrinfo variable for the second call since it serves a specific purpose only inside this if condition. It doesn't matter because it's not like it gets used again later, just something I noticed.

Don't have strong feeling either way but there is an argument that doing it with one variable means any changes to flags passed to the first call get automatically added to the second call which is I think better default behaviour.

achow101 · 2024-07-18T21:41:31Z

ACK 23333b7

AI_ADDRCONFIG prevents ::1 from being considered a valid address on hosts that have a IPV6 loopback IP address but no other IPV6 interfaces. Github-Pull: bitcoin#30245 Rebased-From: 23333b7

DrahtBot added the P2P label Jun 7, 2024

This was referenced Jun 10, 2024

ci: move ASan job to GitHub Actions from Cirrus CI #30193

Merged

RPC wont bind without an IP address on a non-localhost interface #13155

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net: Allow DNS lookups on nodes with IPV6 lo only

23333b7

AI_ADDRCONFIG prevents ::1 from being considered a valid address on hosts that have a IPV6 loopback IP address but no other IPV6 interfaces.

m3dwards force-pushed the allow-dns-ipv6-lo-only branch from e047f4e to 23333b7 Compare July 15, 2024 10:08

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net: Allow -proxy=[::1] on nodes with IPV6 lo only #30245

net: Allow -proxy=[::1] on nodes with IPV6 lo only #30245

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achow101 commented Jul 18, 2024

net: Allow -proxy=[::1] on nodes with IPV6 lo only #30245

net: Allow -proxy=[::1] on nodes with IPV6 lo only #30245

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