Node Discover V4 compatibility issue: PING request parity doesn't reply with PONG

[illya-havsiyevych]

... and in a result parity might not be compatible with geth bootnode
AS geth bootnode doesn't respond back to FINDNODE and internally logs error "unknown node"

• Parity Ethereum version: v2.5.0-beta-b52ac20-20190408/x86_64-linux-gnu/rustc1.33.0
• Operating system: Linux
• Installation: one-line installer
• Fully synchronized: yes
• Network: private
• Restarted: yes

Issue might me related to code
https://github.com/paritytech/parity-ethereum/blob/master/util/network-devp2p/src/discovery.rs#L535

ip-10-101-2-4:$ bootnode -nodekey bootnode.key -verbosity 9 -addr :30301 -writeaddress

c6fea65e3dd8b8cd5102816d8a3f3e7c98e8952357adb12970c77c15144c151f7e2fa86bee7ae7a980cfbf04eda58a737800e55a44a357f966a473eef8b38c2e

ip-10-101-2-4:$ bootnode -nodekey bootnode.key -verbosity 9 -addr :30301

INFO [04-26|14:22:09.746] New local node record                    seq=1 id=9af0873613686cd1 ip=<nil> udp=0 tcp=0
TRACE[04-26|14:26:57.351] << PING/v4                               id=bed6bb55be4f44dc addr=10.111.2.225:30303 err=nil
TRACE[04-26|14:26:57.351] >> PONG/v4                               id=bed6bb55be4f44dc addr=10.111.2.225:30303 err=nil
TRACE[04-26|14:26:57.352] >> PING/v4                               id=bed6bb55be4f44dc addr=10.111.2.225:30303 err=nil
TRACE[04-26|14:26:58.219] << FINDNODE/v4                           id=bed6bb55be4f44dc addr=10.111.2.225:30303 err="unknown node"
TRACE[04-26|14:27:07.527] << FINDNODE/v4                           id=bed6bb55be4f44dc addr=10.111.2.225:30303 err="unknown node"
TRACE[04-26|14:27:17.438] << FINDNODE/v4                           id=bed6bb55be4f44dc addr=10.111.2.225:30303 err="unknown node"
TRACE[04-26|14:27:27.650] << FINDNODE/v4                           id=bed6bb55be4f44dc addr=10.111.2.225:30303 err="unknown node"

ip-10-111-2-225:$ parity --base-path=./node --config=./config/gateway.toml -l discovery=trace

Loading config file from ./config/gateway.toml
2019-04-26 14:26:50 UTC main INFO parity_ethereum::run  Starting Parity-Ethereum/v2.5.0-beta-b52ac20-20190408/x86_64-linux-gnu/rustc1.33.0
2019-04-26 14:26:50 UTC main INFO parity_ethereum::run  Keys path ./node/keys
2019-04-26 14:26:50 UTC main INFO parity_ethereum::run  DB path ./node/chains/db/94cd4c55fa93fb4b
2019-04-26 14:26:50 UTC main INFO parity_ethereum::run  State DB configuration: fast
2019-04-26 14:26:50 UTC main INFO parity_ethereum::run  Operating mode: active
2019-04-26 14:26:51 UTC main INFO ethcore_service::service  Configured for using AuthorityRound engine
2019-04-26 14:26:57 UTC IO Worker #2 INFO network  Public node URL: enode://cf849dcf68ba78181c256af456d57d6a5323b22617adf4ea04803eca23d8daa224dfef74f81daf3c90208bd54f3ecc6d21e151988e08941ac92db4c3a37ce4d2@10.111.2.225:30303
2019-04-26 14:26:57 UTC IO Worker #0 TRACE discovery  Sent Ping to NodeEndpoint { address: V4(10.101.2.4:30301), udp_port: 30301 } ; node_id=0xc6fea65e3dd8b8cd5102816d8a3f3e7c98e8952357adb12970c77c15144c151f7e2fa86bee7ae7a980cfbf04eda58a737800e55a44a357f966a473eef8b38c2e
2019-04-26 14:26:57 UTC IO Worker #2 TRACE discovery  Got Pong from V4(10.101.2.4:30301) ; node_id=0xc6fea65e3dd8b8cd5102816d8a3f3e7c98e8952357adb12970c77c15144c151f7e2fa86bee7ae7a980cfbf04eda58a737800e55a44a357f966a473eef8b38c2e
2019-04-26 14:26:57 UTC IO Worker #2 TRACE discovery  Inserting NodeEntry { id: 0xc6fea65e3dd8b8cd5102816d8a3f3e7c98e8952357adb12970c77c15144c151f7e2fa86bee7ae7a980cfbf04eda58a737800e55a44a357f966a473eef8b38c2e, endpoint: NodeEndpoint { address: V4(10.101.2.4:30301), udp_port: 30301 } }
2019-04-26 14:26:57 UTC IO Worker #2 TRACE discovery  Adding a new node NodeEntry { id: 0xc6fea65e3dd8b8cd5102816d8a3f3e7c98e8952357adb12970c77c15144c151f7e2fa86bee7ae7a980cfbf04eda58a737800e55a44a357f966a473eef8b38c2e, endpoint: NodeEndpoint { address: V4(10.101.2.4:30301), udp_port: 30301 } } into our bucket 253
2019-04-26 14:26:57 UTC IO Worker #2 TRACE discovery  Got Ping from V4(10.101.2.4:30301)
2019-04-26 14:26:57 UTC IO Worker #3 TRACE discovery  Starting discovery
2019-04-26 14:26:58 UTC IO Worker #3 TRACE discovery  Starting round Some(0)
2019-04-26 14:26:58 UTC IO Worker #3 TRACE discovery  Sent FindNode to NodeEndpoint { address: V4(10.101.2.4:30301), udp_port: 30301 }
2019-04-26 14:26:58 UTC IO Worker #3 TRACE discovery  Starting round Some(1)
2019-04-26 14:26:58 UTC IO Worker #3 TRACE discovery  Completing discovery
2019-04-26 14:27:00 UTC IO Worker #2 DEBUG discovery  Removing expired FIND NODE request for node_id=0xc6fea65e3dd8b8cd5102816d8a3f3e7c98e8952357adb12970c77c15144c151f7e2fa86bee7ae7a980cfbf04eda58a737800e55a44a357f966a473eef8b38c2e
2019-04-26 14:27:00 UTC IO Worker #2 TRACE discovery  Requests to node NodeEntry { id: 0xc6fea65e3dd8b8cd5102816d8a3f3e7c98e8952357adb12970c77c15144c151f7e2fa86bee7ae7a980cfbf04eda58a737800e55a44a357f966a473eef8b38c2e, endpoint: NodeEndpoint { address: V4(10.101.2.4:30301), udp_port: 30301 } } timed out 1 consecutive time(s)
2019-04-26 14:27:07 UTC IO Worker #3 TRACE discovery  Starting discovery
2019-04-26 14:27:07 UTC IO Worker #0 TRACE discovery  Starting round Some(0)
2019-04-26 14:27:07 UTC IO Worker #0 TRACE discovery  Sent FindNode to NodeEndpoint { address: V4(10.101.2.4:30301), udp_port: 30301 }
2019-04-26 14:27:07 UTC IO Worker #0 TRACE discovery  Starting round Some(1)
2019-04-26 14:27:07 UTC IO Worker #0 TRACE discovery  Completing discovery
2019-04-26 14:27:09 UTC IO Worker #1 DEBUG discovery  Removing expired FIND NODE request for node_id=0xc6fea65e3dd8b8cd5102816d8a3f3e7c98e8952357adb12970c77c15144c151f7e2fa86bee7ae7a980cfbf04eda58a737800e55a44a357f966a473eef8b38c2e